GroupDIY

Project Specific Discussions => Preamplifiers => Topic started by: jsteiger on May 06, 2012, 06:10:45 PM

Title: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 06, 2012, 06:10:45 PM
***ATTENTION!! The VP28 kits are now shipping with Grayhill switches that have factory set stops. This means there will be no stop-pins or stickers shipped or required. A quick glance at the switch should give this away as there are no holes to put the stop-pins in!!***

Since the kits have started shipping, it's time for the support thread!  :)

**This is a somewhat complicated build. Harder than a VP312DI but easier than a Love Child. I recommend reading completely thru the Assembly Aid before starting on anything. There are some points made in the doc that can save you much time and prevent crucial errors during the build.

--------------------------------------------------------------------------------------------------------
September 21st, 2015 Update:
All support docs for this project can be found on the recently added Support Docs (http://capi-gear.com/catalog/support_docs.php) page at www.capi-gear.com
--------------------------------------------------------------------------------------------------------


--------------------------------------------------------------------------------------------------------
March 18th, 2016 Update:
Rev B boards are now shipping for the VP28. To easily identify, the Rev B boards are green and they have the CAPI® label affixed. To find out what has been changed, please read the Rev B Addendum (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Addendum.pdf). Like with all projects, it is crucially imperative that you follow the BOM that matches the revision of the PCB that you are building.
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Some details can be found here http://capi-gear.com/catalog/product_info.php?products_id=318 (http://classicapi.com/catalog/product_info.php?products_id=318)

(http://www.capi-gear.com/catalog/images/gallery/capi_temp/temp-VP28.jpg)

Cheers, Jeff
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 06, 2012, 06:11:03 PM
Sorry for the low quality of the following pics. I know 'Chung's work will make me want to take these down but until he gets them done, these will have to do.

Just click the thumbs to enlarge!!  :)

Finger tight hardware on the 553F-Sub-HPF PCB, top view.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-1.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-1.jpg)

Finger tight hardware on the 553F-Sub-HPF PCB, bottom view.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-2.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-2.jpg)

553F-Sub-HPF PCB, lightly snugged into position on the main PCB.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-3.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-3.jpg)

The important thing in this crappy pic, is to note the outside tooth lock-washer placed between the VP28 L-bracket and the small Keystone bracket.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-4.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-4.jpg)

The 4-40 x 1/8" undercut flat head screw tightened thru the front of the VP 28 L-bracket.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-5.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-5.jpg)

After final adjustments and a tightening of the HPF board's fasteners, it's time to solder the 5-pin header thru the access slot in the L-bracket.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-6.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sub-HPF-hardware-6.jpg)

Recommended bending and installation of the four LED's on the main PCB.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Four-LEDs.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Four-LEDs.jpg)

Recommended bending and installation of the signal present LED onto the long 2-pin header.
(http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sig-pres-LED.gif) (http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/pics/Sig-pres-LED.jpg)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: chunger on May 06, 2012, 06:11:47 PM
OK. . . as is customary in the Chinese Labor Camp, I waited a few weeks after receiving this package from jsteiger. . . you know.  To keep people guessing about delivery time-frames.  But, at some point, you need to begin cryptic communications in broken english allegedly for technical clarifications. . . then, of course, ignore those clarifications and do what you were going do anyways. . . ok.  It may be a bit too late in the evening to be typing so I'll just start posting pictures  :o

Let's see what's in the box, shall we?

(http://studio939.zenfolio.com/img/s11/v31/p791294435-4.jpg)

Another fine kit from Classic Audio Products of IL.  In keeping with the new no-nonsense anodized black look, this kit looks well laid out and intuitive to assemble.

(http://studio939.zenfolio.com/img/s11/v33/p752592215-4.jpg)

This build will feature the stepped grayhill gain switch option, so we begin by sorting the resistors for the gain switch and fader.

(http://studio939.zenfolio.com/img/s3/v41/p932589790-5.jpg)

Carefully bend the leads and install resistors to their designated positions.

(http://studio939.zenfolio.com/img/s3/v43/p989712091-4.jpg)

Bend the leads to hold resistors in place, flip the board over onto a nice, flat surface and solder from the back side of the PCB.

(http://studio939.zenfolio.com/img/s11/v34/p868887612-4.jpg)

. . . and trim the leads when finished.

(http://studio939.zenfolio.com/img/s3/v39/p596887611-4.jpg)

My aim with heat settings on my temperature-regulated iron is to limit the time the iron is actually on the component/PCB. I also try to  apply just the right amount of solder  to flow a smooth bead  that carries through a little bit to the top side of the PCB. . . all in the shortest amount of time practical so as not to damage the components.

(http://studio939.zenfolio.com/img/s3/v23/p628963692-4.jpg)

Next, sort the remainder of the resistors for the main PCB as well as the transistor and diodes.

(http://studio939.zenfolio.com/img/s1/v49/p138286483-5.jpg)

It's worth noting even though all of the small diodes on this PCB are the same that the tiny markings on the 1N914 diodes read:

91
4B

It doesn't hurt to confirm.
(http://studio939.zenfolio.com/img/s1/v19/p775789131-3.jpg)

Populate the resistors,

(http://studio939.zenfolio.com/img/s3/v38/p94591274-4.jpg)

(http://studio939.zenfolio.com/img/s1/v46/p471308580-4.jpg)

(http://studio939.zenfolio.com/img/s3/v38/p535610363-4.jpg)

and install diodes.  Orientation is important on the diodes.  Make sure the side marked with a solid stripe corresponds to the arrow screen-printed on the PCB.

(http://studio939.zenfolio.com/img/s3/v38/p449490756-4.jpg)

Diodes are populated.  Double check to confirm orientation is correct on all of these.

(http://studio939.zenfolio.com/img/s1/v47/p91854723-4.jpg)

Install Q1 transistor next.

(http://studio939.zenfolio.com/img/s3/v45/p128573896-4.jpg)

Next, locate the bag that contains hardware.

(http://studio939.zenfolio.com/img/s10/v16/p975531190-4.jpg)

And locate the Millmax sockets for the 2 DOA's.  Watch out.  They like to roll on the floor.

(http://studio939.zenfolio.com/img/s1/v19/p612716117-4.jpg)

Insert these from the back side of the PCB.

(http://studio939.zenfolio.com/img/s2/v53/p764894441-4.jpg)

Set soldering iron temp a bit on the hotter side and solder.  We want a little bit of solder to bleed over to the front side

(http://studio939.zenfolio.com/img/s1/v46/p601792772-4.jpg)

but not quite as much this. . . oops.  It's still ok. . . just not as pretty as it should be for an instruction set.

(http://studio939.zenfolio.com/img/s1/v48/p1064636930-4.jpg)

Next, sort the capacitors for the main PCB.

(http://studio939.zenfolio.com/img/s3/v45/p784387212-5.jpg)

and watch out for the markings on this little bugger.  68pF.

(http://studio939.zenfolio.com/img/s3/v45/p787588576-4.jpg)

And populate the capacitors.  I start from the short ones and work my way up to the tall ones, but it doesn't really matter what order.

(http://studio939.zenfolio.com/img/s1/v21/p578734400-4.jpg)

Before I got to the larger electrolytics, I pulled the IC socket.

(http://studio939.zenfolio.com/img/s2/v50/p195084987-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p291599270-4.jpg)

Next, install the electrolytic capacitors.  Polarity is important on these. The solid line indicates the "-" side of the capacitor.

(http://studio939.zenfolio.com/img/s3/v43/p495819353-4.jpg)

on the radial caps, the longer of the legs is the "+" side.  Also, the solid line indicates the "-" side.

(http://studio939.zenfolio.com/img/s1/v48/p198881582-5.jpg)

Caps installed.

(http://studio939.zenfolio.com/img/s2/v53/p17486801-4.jpg)





Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: chunger on May 06, 2012, 06:11:59 PM
Next, pull these switches.

(http://studio939.zenfolio.com/img/s1/v47/p396753335-4.jpg)

And install onto the PCB

(http://studio939.zenfolio.com/img/s3/v43/p33195728-4.jpg)

Solder one pin and then check alignment with the screen printing on the front side of the PCB.  Make sure the switches are sitting flush to the PCB and are aligned as straight as possible.

(http://studio939.zenfolio.com/img/s2/v53/p164345028-4.jpg)

(http://studio939.zenfolio.com/img/s1/v49/p207958885-5.jpg)

Solder in the rest of the pins after verifying alignment and making adjustments.

(http://studio939.zenfolio.com/img/s2/v50/p317477388-4.jpg)

repeat for the other 3 switches.

(http://studio939.zenfolio.com/img/s2/v53/p50157258-4.jpg)

Next, pull this toggle switch.

(http://studio939.zenfolio.com/img/s2/v52/p425777670-4.jpg)

SW2 fits on the back side of the PCB.

(http://studio939.zenfolio.com/img/s2/v53/p220017668-4.jpg)

IMPORTANT!  Before soldering in these pins, clip the leads close to the PCB.  The high pass filter board that goes directly above this switch will interfere otherwise.

(http://studio939.zenfolio.com/img/s2/v51/p274956165-4.jpg)

Solder after trimming the leads close.

(http://studio939.zenfolio.com/img/s3/v45/p331724523-4.jpg)

Next, pull the 2 grayhill switches.

(http://studio939.zenfolio.com/img/s3/v43/p34608117-4.jpg)

And install.

(http://studio939.zenfolio.com/img/s1/v49/p467191070-4.jpg)

Solder 1 pin only and confirm that the switch is positioned tight to the PCB and properly aligned.

(http://studio939.zenfolio.com/img/s3/v23/p402015233-4.jpg)

Install the 2nd grayhill switch and do the same.  Solder 1 pin and adjust for perfect alignment.

(http://studio939.zenfolio.com/img/s2/v53/p526582912-4.jpg)

(http://studio939.zenfolio.com/img/s3/v42/p140354419-4.jpg)

When satisfied with the alignment, solder in the rest of the pins.  Be careful not to use too much solder and bridge the contacts.  If necessary, verify with a magnifying glass that the solder joints are clean.

Next, pull the stop pins and retaining stickers.  Be careful, if these pins fall on the floor, they may very well never be recovered.
The Grayhill switches now have factory set stops. There is no need for the stickers or pins.

(http://studio939.zenfolio.com/img/s1/v46/p220638540-4.jpg)

(http://studio939.zenfolio.com/img/s2/v50/p400895034-4.jpg)

Install stop pins at the 12:00 position.
The Grayhill switches now have factory set stops. There is no need for the stickers or pins.

(http://studio939.zenfolio.com/img/s3/v42/p407292101-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p137026114-4.jpg)

And install stickers to retain the pins.
The Grayhill switches now have factory set stops. There is no need for the stickers or pins.

(http://studio939.zenfolio.com/img/s1/v49/p413136204-4.jpg)

Next, pull the EA2622 input transformer.

(http://studio939.zenfolio.com/img/s1/v48/p35489124-4.jpg)

Note the dot on the sticker on the transformer.  This indicates pin one.  Align pin 1 with the dot silk-screened on the PCB.

(http://studio939.zenfolio.com/img/s1/v49/p199652547-4.jpg)

Install a piece of double-stick foam tape.

(http://studio939.zenfolio.com/img/s2/v51/p138742287-4.jpg)

And install the transformer.

(http://studio939.zenfolio.com/img/s3/v44/p461373371-4.jpg)

Next, pull the hardware to install the 2 output transformers.

(http://studio939.zenfolio.com/img/s1/v48/p207673396-4.jpg)

And pull the output transformers.

(http://studio939.zenfolio.com/img/s3/v44/p370589465-4.jpg)

put a flat washer on 2 of the long screws.

(http://studio939.zenfolio.com/img/s2/v50/p506992195-4.jpg)

Install screws on the transformer.

(http://studio939.zenfolio.com/img/s1/v48/p128037712-4.jpg)

flip the transformer over and install another flat washer which will sit between the PCB and the transformer.

(http://studio939.zenfolio.com/img/s2/v51/p449833032-4.jpg)

And place the transformer on the PCB.

(http://studio939.zenfolio.com/img/s2/v51/p23062364-4.jpg)

Install lock washers.

(http://studio939.zenfolio.com/img/s1/v46/p150515559-4.jpg)

And then nuts.

(http://studio939.zenfolio.com/img/s2/v50/p458001930-4.jpg)

Tighten from the top side with a phillips head screwdriver.

(http://studio939.zenfolio.com/img/s3/v45/p310494026-5.jpg)

And repeat for the 2nd output transformer.

(http://studio939.zenfolio.com/img/s2/v52/p373815489-5.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p283959566-4.jpg)

Align both transformers with the screen printed boxes on the PCB and tighten them down.

(http://studio939.zenfolio.com/img/s1/v48/p179460339-5.jpg)

Carefully trim the leads to length, strip and tin the tips, and organize with some short strips of shrink tubing.

(http://studio939.zenfolio.com/img/s3/v45/p318927929-4.jpg)

And begin installing the transformer leads to their corresponding labelled terminals on the PCB.

(http://studio939.zenfolio.com/img/s1/v47/p101271502-4.jpg)

(http://studio939.zenfolio.com/img/s2/v50/p734912231-4.jpg)

(http://studio939.zenfolio.com/img/s2/v50/p861186041-4.jpg)

and repeat on the other transformer.

(http://studio939.zenfolio.com/img/s1/v47/p700565917-4.jpg)

(http://studio939.zenfolio.com/img/s2/v52/p805597173-4.jpg)

iron is installed.  IMagine how many kits Jeff could sell if there could just stay in stock  ;D

(http://studio939.zenfolio.com/img/s3/v42/p543742248-4.jpg)

Next, locate this long header.

(http://studio939.zenfolio.com/img/s3/v42/p999536018-4.jpg)

And install onto the PCB.

(http://studio939.zenfolio.com/img/s2/v53/p929355217-4.jpg)

Solder 1 pin and align tight to the PCB and perpendicular.  After everything looks aligned, solder the other pin.

(http://studio939.zenfolio.com/img/s2/v52/p796276218-4.jpg)

Next, sort the components for the HPF PCB.

(http://studio939.zenfolio.com/img/s2/v52/p540148853-5.jpg)

Install the L-header first.

(http://studio939.zenfolio.com/img/s2/v51/p692615761-4.jpg)

I used a clamp to secure the header while leaving one pin exposed on the side.

(http://studio939.zenfolio.com/img/s1/v47/p976293751-5.jpg)

And solder that exposed pin first.

(http://studio939.zenfolio.com/img/s1/v47/p712104993-4.jpg)

Then, shift the clamp to the other side and solder the opposite pin.

(http://studio939.zenfolio.com/img/s1/v48/p1035821172-4.jpg)

With the header tightly secured, solder the remaining pins.  OK. . . the method may be over-complicated, but it gets the job done.

(http://studio939.zenfolio.com/img/s2/v53/p769252670-4.jpg)

Pull push-button switches for the HPF board.

(http://studio939.zenfolio.com/img/s2/v52/p1038349874-4.jpg)

And place onto the PCB.

(http://studio939.zenfolio.com/img/s2/v52/p803661890-4.jpg)

Solder 1 pin on each switch.

(http://studio939.zenfolio.com/img/s2/v51/p908572716-4.jpg)

And verify alignment on the front.  Adjust as necessary before soldering in the rest of the pins.

(http://studio939.zenfolio.com/img/s1/v46/p1049592930-4.jpg)

(http://studio939.zenfolio.com/img/s1/v49/p843221392-4.jpg)

Next, populate the electronic components.

(http://studio939.zenfolio.com/img/s2/v51/p634517385-4.jpg)

(http://studio939.zenfolio.com/img/s1/v48/p881329648-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p547309329-4.jpg)

Pull mounting hardware for the HPF PCB.

(http://studio939.zenfolio.com/img/s2/v53/p1065280830-4.jpg)

And install the brackets finger-tight only at this point.

(http://studio939.zenfolio.com/img/s1/v49/p616526251-4.jpg)

Lock washer goes on this side.

(http://studio939.zenfolio.com/img/s2/v53/p542338315-4.jpg)

Followed by the nut.

(http://studio939.zenfolio.com/img/s1/v47/p897526962-4.jpg)

Repeat on the other side.

(http://studio939.zenfolio.com/img/s1/v48/p639452394-4.jpg)

And the HPF PCB is assembled.

(http://studio939.zenfolio.com/img/s1/v49/p882493705-4.jpg)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: chunger on May 06, 2012, 06:12:12 PM
Next locate the caps for the pushbutton switches.

(http://studio939.zenfolio.com/img/s3/v43/p813059182-4.jpg)

The pair of black ones go on the high pass filter PCB that was just assembled.

(http://studio939.zenfolio.com/img/s1/v48/p1030477851-4.jpg)

(http://studio939.zenfolio.com/img/s3/v43/p636649762-4.jpg)

And the rest go on the main PCB.  Red (48v phantom power), gray, white gray in that order.

(http://studio939.zenfolio.com/img/s1/v46/p580947450-4.jpg)

(http://studio939.zenfolio.com/img/s2/v53/p631655237-4.jpg)

(http://studio939.zenfolio.com/img/s1/v48/p704049382-4.jpg)

Next, pull hardware to mount the main PCB onto the L-bracket.

(http://studio939.zenfolio.com/img/s1/v49/p1041379994-4.jpg)

Start by inserting the screws from the bottom of the L-bracket.

(http://studio939.zenfolio.com/img/s3/v44/p763691624-4.jpg)

Tighten the rear standoffs all the way and install the front 2 half way only so the PCB can slide over and into position.

(http://studio939.zenfolio.com/img/s3/v44/p682278055-4.jpg)

(http://studio939.zenfolio.com/img/s1/v49/p903019152-4.jpg)

Pull Hardware to set the high pass filter PCB onto the main PCB.

(http://studio939.zenfolio.com/img/s1/v47/p701091397-4.jpg)

Install the lock washer onto the screw.

(http://studio939.zenfolio.com/img/s2/v52/p667510283-4.jpg)

Set the HPF PCB onto the main PCB.

(http://studio939.zenfolio.com/img/s1/v47/p992046542-4.jpg)

And attach from the bottom with the screw/lock washer.  Finger tight will do right now.

(http://studio939.zenfolio.com/img/s1/v47/p752418553-4.jpg)

Carefully slide the main PCB assembly onto the L-bracket.

(http://studio939.zenfolio.com/img/s1/v49/p762740056-4.jpg)

Pull hardware to set the main PCB.

(http://studio939.zenfolio.com/img/s2/v53/p766201430-4.jpg)

Align the front of the PCB snug and square to the L-bracket and install rear star lock-washers and nuts.

(http://studio939.zenfolio.com/img/s2/v52/p609588712-4.jpg)

(http://studio939.zenfolio.com/img/s2/v51/p770141067-4.jpg)

(http://studio939.zenfolio.com/img/s2/v51/p1073296460-4.jpg)

With the PCB in position, we can now tighten down the front standoffs.

(http://studio939.zenfolio.com/img/s1/v46/p896009807-4.jpg)

And install the small lock washers and nuts onto the front screws.

(http://studio939.zenfolio.com/img/s1/v46/p907679446-4.jpg)

Verify that the main PCB is square to to the L-bracket and tighten all 4 nuts.

(http://studio939.zenfolio.com/img/s2/v53/p844588191-4.jpg)

Pull hardware to mount the front of the HPF PCB onto the L-bracket.

(http://studio939.zenfolio.com/img/s1/v49/p562111426-4.jpg)

Insert the star lock washer between the L-bracket and the HPF PCB assembly like this.

(http://studio939.zenfolio.com/img/s2/v53/p984297369-4.jpg)

And insert small screw from the front and tighten down.

(http://studio939.zenfolio.com/img/s1/v49/p996513207-4.jpg)

(http://studio939.zenfolio.com/img/s1/v49/p1063367388-4.jpg)

Locate the front panel and install.

(http://studio939.zenfolio.com/img/s1/v47/p680683943-5.jpg)

Align the front panel to the 4 main PCB pushbuttons and to the L-bracket.  Then, install the 2 grayhill switch nuts (discard the Grayhill switch lock washers).

(http://studio939.zenfolio.com/img/s2/v52/p846883663-4.jpg)

(http://studio939.zenfolio.com/img/s2/v52/p1052233172-5.jpg)

Next, adjust the HPF PCB so the pushbuttons are aligned with the front panel.  The screws holding this in should only be finger-tight at this point so the board can move.  When happy with the alignment, tighten down these 3 screws to lock the HPF PCB into position.

(http://studio939.zenfolio.com/img/s1/v46/p829506208-5.jpg)

(http://studio939.zenfolio.com/img/s2/v53/p585062499-5.jpg)

(http://studio939.zenfolio.com/img/s2/v51/p820391736-5.jpg)

From the cutout on the back side of the L-bracket, solder the HPF PCB header to the main PCB.

(http://studio939.zenfolio.com/img/s2/v53/p997686912-5.jpg)

Next, pull the 5 LED's.

(http://studio939.zenfolio.com/img/s2/v52/p760665849-4.jpg)

Cut and bend the leads like this.  I chose to retain one short pin and one long pin when I cut the LED just for personal reference.

(http://studio939.zenfolio.com/img/s2/v51/p978545259-4.jpg)

Carefully position one red and 3 yellow LED's onto the main PCB from the bottom of the L-bracket.

(http://studio939.zenfolio.com/img/s2/v51/p546885776-4.jpg)

(http://studio939.zenfolio.com/img/s8/v9/p764394820-4.jpg)

Notice the short leg of the LED's correspond to the silk-screened arrow mark.

Once happy with the LED positioning, solder the leads to the main PCB pads.

(http://studio939.zenfolio.com/img/s1/v49/p661338723-5.jpg)

(http://studio939.zenfolio.com/img/s1/v49/p566612407-4.jpg)

Pull the 2 clear sections of heat-shrink tubing.

(http://studio939.zenfolio.com/img/s2/v52/p746068427-4.jpg)

cut these to reveal about 1/8" and slide them over the tall jumper near the front panel.

(http://studio939.zenfolio.com/img/s1/v46/p1059666512-4.jpg)

Next, connect the green LED to this jumper.  Some bending will be involved.  Please excuse my mis-matched shrink tubing.  Shortly after taking the previous photo, I must have accidentally flung the supplied tubing piece across the room when putting the camera down and I could not find it, so I had to use another one from my kit which was not the same diameter.

(http://studio939.zenfolio.com/img/s8/v10/p581449958-4.jpg)

The LED will end up looking something like this.

(http://studio939.zenfolio.com/img/s8/v10/p540796405-5.jpg)

Make sure the short end of the LED corresponds to the silk-screened arrow mark on the PCB for this LED just like the others.

Carefully position the green LED and make any final tweaks necessary for a tidy fit.

(http://studio939.zenfolio.com/img/s2/v52/p677794546-4.jpg)

And solder into position.

(http://studio939.zenfolio.com/img/s8/v10/p742873606-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p780309118-5.jpg)

Next, I used some water on a Q-tip to clean off some of the residue left on the faceplate from installation.

(http://studio939.zenfolio.com/img/s1/v48/p649975468-4.jpg)

Pull knobs and shaft adapter.

(http://studio939.zenfolio.com/img/s1/v46/p924151495-4.jpg)

Use pliers to position both knobs in the 12:00 position (pointing straight up), and position the shaft adapter so the gap is facing down.

(http://studio939.zenfolio.com/img/s8/v11/p985764448-4.jpg)

Press the adapter onto the Grayhill switch shaft until somewhat flush with the top of the post.

(http://studio939.zenfolio.com/img/s8/v11/p553143808-4.jpg)

Install knobs.

(http://studio939.zenfolio.com/img/s1/v47/p596472635-4.jpg)

Next, pull the 2 colored stickers that go on top of the knobs.

(http://studio939.zenfolio.com/img/s1/v46/p687640079-4.jpg)

Peel off the paper backing and install.

(http://studio939.zenfolio.com/img/s2/v1/p865520473-4.jpg)

(http://studio939.zenfolio.com/img/s2/v52/p682302058-5.jpg)

For this build, we will need 2 opamps.  I chose to use Gary Barnett's GAR2520 op amp kits for this build.  These are fast becoming my go-to op-amps for API-type builds and sound phenomenal.

(http://studio939.zenfolio.com/img/s1/v49/p862851650-4.jpg)

Pull the pins from the kits.

(http://studio939.zenfolio.com/img/s2/v51/p149248189-4.jpg)

and insert into the opamp sockets of the main PCB.  This positions the pins perfectly for soldering and also loosens up the sockets that can be extremely stiff the first time an opamp is inserted.  It is much easier to insert the pins one at a time like this.

(http://studio939.zenfolio.com/img/s2/v1/p365954996-4.jpg)

(http://studio939.zenfolio.com/img/s8/v11/p262063099-4.jpg)

Place the GAR2520 PCB's onto the pins and solder in place.

(http://studio939.zenfolio.com/img/s2/v52/p116346179-4.jpg)

Pins are perfectly aligned and the op-amps are ready to be populated.  The documentation provided with the GAR2520's is VERY thorough and walks you step-by-step through the process.  A steady hand and an eye toward detail are required.  I will populate a pair to the best of my current ability.

(http://studio939.zenfolio.com/img/s1/v47/p265724066-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p311115191-4.jpg)

(http://studio939.zenfolio.com/img/s1/v46/p335779515-4.jpg)

(http://studio939.zenfolio.com/img/s2/v1/p239965264-4.jpg)

(http://studio939.zenfolio.com/img/s2/v53/p475967697-4.jpg)

(http://studio939.zenfolio.com/img/s8/v9/p155640111-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p515875232-4.jpg)

(http://studio939.zenfolio.com/img/s8/v11/p143924925-4.jpg)

and opamps complete.

(http://studio939.zenfolio.com/img/s2/v53/p501593375-4.jpg)

(http://studio939.zenfolio.com/img/s1/v47/p475041548-4.jpg)

(http://studio939.zenfolio.com/img/s8/v11/p88392839-4.jpg)

Next, pull the IC that drives the green signal LED.

(http://studio939.zenfolio.com/img/s8/v10/p263947075-4.jpg)

and carefully insert into the socket on the main PCB making sure to align the dot with the notch on the socket.  Do not press the IC into position until you are absolutely sure all of the delicate pins are properly positioned in the socket.

(http://studio939.zenfolio.com/img/s1/v47/p30990011-4.jpg)

Note the IC's orientation.

(http://studio939.zenfolio.com/img/s8/v9/p275207808-4.jpg)

Next, install the 2 GAR2520 opamps or the +, -16V opamps of your choosing on the main PCB.

(http://studio939.zenfolio.com/img/s8/v10/p302477476-4.jpg)

The VP28 is now complete.  Humans win. . . again.  If all went well, it should run right out of the gate.

(http://studio939.zenfolio.com/img/s1/v48/p508029591-4.jpg)

(http://studio939.zenfolio.com/img/s1/v48/p200759318-4.jpg)

(http://studio939.zenfolio.com/img/s1/v46/p15083809-5.jpg)

(http://studio939.zenfolio.com/img/s2/v52/p319296452-5.jpg)

I sincerely hope you've enjoyed this installment of "Build Jeff's Stuff' as much as I have and find this photo documentary helpful  ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: bigevil on May 06, 2012, 09:55:24 PM
park place & boardwalk...

this comment box is now for sale.

Thanks for looking...






to the mods, this is only a joke.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 06, 2012, 10:23:39 PM
park place & boardwalk...

this comment box is now for sale.

Thanks for looking...






to the mods, this is only a joke.
Ouchy ouch!! Man that was Big....and Evil

So how much you want for it?

Haha, also a joke  ;) ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: michal_k on May 07, 2012, 06:50:03 AM
wow, love the metalwork, especially the undercut screw hidden behind the front panel. impressive.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: J Adams on May 08, 2012, 05:40:52 AM
Got my first one built.  Took around 4 hours but I think I can do my other one faster.  I built the 4 opamps while I was waiting on the 28s to get in stock so that helped.  Anyhow, just tested everything on it, and everything works, first try!  Won't try it out on some tracking till probably next week though   :(   I think the hardest part for me was getting the LEDs in there right, but even that wasn't bad.  Might just be bc its 5 in the morning and I need to go to bed.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 08, 2012, 10:06:30 AM
...Anyhow, just tested everything on it, and everything works, first try!
Excellent news! Congrats!  8)

Quote
...I think the hardest part for me was getting the LEDs in there right, but even that wasn't bad...
They can be a little tricky the first time. After building a few dozen, I fly thru them in no time.  ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: J Adams on May 08, 2012, 03:24:11 PM
I hit  2 very minor snags building the second one (my own fault), but still got it done in about 3.5 hours.  Again, everything works on it, first try.  Great design on the boards Jeff, can't wait to see how these fair in my sessions next week.  Even though they take a lot longer than the 25s and 312s I built, I think it was more fun as well.

Oh, and the reason I have both big knobs on one is because I don't have a small enough allen wrench for the small knobs but I wanted to at least test them both out.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 08, 2012, 04:10:46 PM
Looking good man!  8)

What were the snags? Maybe it could help someone in the future not fall in the same rabbit hole.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: gevermil on May 08, 2012, 05:53:58 PM
When is the Entire Bundle avalible
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 08, 2012, 06:05:28 PM
When is the Entire Bundle avalible
They were available late last week but I was cleaned out of 2623-1's in a about a day. I have another batch being shipped out this Thursday so I should have them in hand Friday. That will put the VP28's "in stock".
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: J Adams on May 09, 2012, 02:07:18 AM
I really don't remember, other than thinking some things were taking me longer than they should have bc I was tired.

What size allen wrench does the top knob use?  I have a ton of allen wrenches here but none small enough.  Stopped by WalMart after work tonight (only place open at 1AM) and they did't have anything smaller than what I have. 

These preamps (and gar2520s) are the only thing I have ever built or soldered.  I am glad I started with the 25s/312s before attempting the 28, and I thought even though the assembly guide was not as thorough as the other one you did, between that and the BOM I was perfectly fine and felt very comfortable with everything.   
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: toby-e on May 09, 2012, 04:36:28 AM
What size allen wrench does the top knob use?  I have a ton of allen wrenches here but none small enough.  Stopped by WalMart after work tonight (only place open at 1AM) and they did't have anything smaller than what I have. 

Most set screws on knobs are 1/16" – it was a ***** for me to find one of those in Europe since it's right between 1.5mm and 2mm, and none of them will work  >:(

Nice set of preamps you have there :) Can you describe the sonic differences?

-toby
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 09, 2012, 11:18:41 AM
...What size allen wrench does the top knob use?...
The small 3/8" blue pointer requires a .05" Allen wrench. The larger 1/2" clear pointer is 1/16". You will probably need to visit an Ace Hardware or something more "specialized" than Wal-Mart.  ;)

I am surprised that Imperial sizes are not readily available in the UK. I can go to any hardware store here and buy Imperial or Metric anything. Bastards.  >:(

BTW, nice lunchbox!  8)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: GregNey on May 10, 2012, 03:55:28 PM
Both of mine worked on the first try too. If you read the manual first and take your time it's very straight forward. The only "mistake" I made was using the wrong type of washer on some screws. Cheers Jeff- Thank you for your hard work!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 10, 2012, 03:59:11 PM
Thanks Greg. I think any of my VPxxx preamps make a good DIY foundation for the VP28 build.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tronax on May 10, 2012, 07:01:24 PM
Great job on this one, Jeff!  Successful build, took a few evenings of an hour here and there.
(only 1 leftover washer, and couldn't seem to find two lock washers, but I probably shouldn't build these things late at night!!)

In the studio now, comparing it to my other pre's... Enjoying it so far.
Thank you thank you thank you for the signal indicator LED!!!

-Chris

EDIT:  *love* it on guitar. driving the preamp gain hard results in some happiness. :-)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on May 15, 2012, 01:41:10 PM
Hi Jeff,

is there a rough availability time for the  "VP28 Entire Bundle" (I must make my lovechildren become channelstrips :D)?

Thank you in advance,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 15, 2012, 02:01:35 PM
Udo, the 2623-1's should arrive tomorrow or Thursday. It will be like a family reunion in your rack!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on May 15, 2012, 02:11:48 PM
Udo, the 2623-1's should arrive tomorrow or Thursday. It will be like a family reunion in your rack!
Oh yes,that´s what I wanted to read Jeff ;D.Definetely in for 2 sets!

Udo ;)

Edit:2 full sets ordered and paid-YIPPPPEEEEHHHH!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Winetree on May 17, 2012, 01:57:29 AM
Speaking of channel strips, Would there be a way to patch the "Love Child"
into the 528 between the preamp and output section?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 17, 2012, 10:02:34 AM
Speaking of channel strips, Would there be a way to patch the "Love Child"
into the 528 between the preamp and output section?
It could be done but would require some cleaver routing not allowed for in a normal 500 series rack. You would basically lift the primaries from the first 2623-1 (T2) and "insert" the LC or other EQ there.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: funkymonksf on June 01, 2012, 06:56:46 PM
Jeff,
I really love your aesthetic changes, especially the black pcbs and faceplates! You definitely have offered the whole DIY/recording community with some amazing projects. I hope to jump in on this project soon.

Brice
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 01, 2012, 10:31:22 PM
....I really love your aesthetic changes, especially the black pcbs and faceplates! You definitely have offered the whole DIY/recording community with some amazing projects. I hope to jump in on this project soon....
Thanks Brice. I just kinda went in the direction I was pulled in I guess. I really do like the brushed and anodized black too. Plus, the silkscreen shop does a super nice job. I have been thru a few hundred plates since I started using them and have not had one single reject due to the printing.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Cgaines on June 02, 2012, 06:45:54 PM
Hey can anyone help a green DIY guy?
I bought the vp28 and somewhere along the way something went wrong. Whenever I plug up the vp28 to my chameleon labs cps-501 I blow the fuse in it. The build was great. I thought I didn't have any problems. I double checked regularly and had a good experience. But soldering and finding the appropriate components from the BOM is the extent of my knowledge. Any suggestions or tests I can run with a multimeter?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 02, 2012, 06:50:50 PM
What is the opamp situation? Do fuses blow without opamps or only with? Which rail(s) goes down?

Maybe a few pics of the PCB top and bottom would help.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Cgaines on June 02, 2012, 08:26:42 PM
Good call! We're on the right path! I have a gar1731 and a gar2520. I took out both and no fuse was blown. I put back in the 1731 in the same location and no fuse was blown. I removed the 1731 and put the 2520 in its original location and blew a fuse. So I guess the problem is the 2520? I'll post some pics. I hope they'll help. If not I hope we can figure it out.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Cgaines on June 02, 2012, 08:27:48 PM
Here's the top side
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 02, 2012, 08:47:44 PM
At least you are narrowing it down.

Next, try the 1731 in the 2520 location. If the fuse blows, it is something on the VP28 PCB.

If not, then I would continue to look at the 2520. The most common errors are the larger BD transistors swapped with each other and/or they could be in backwards.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 02, 2012, 09:07:45 PM
Success again-2 nice pres done!
Here they are (just a quick shot):

(http://img545.imageshack.us/img545/3320/dunnit.jpg) (http://imageshack.us/photo/my-images/545/dunnit.jpg/)

Did only quick tests with a signal generator and an old MD421,but even this was nice to hear on headphones.
Will check them next week in the studio,I bet they´re killer!
I´ll hook up nice chains then too (VP28s/LC53As/LA500s) in my new 51x racks-very exciting :D

Easy to build (most time consuming was building the DOAs again),no issues except of 2 left-over parts (guess again ;D) and 1 splitlock washer which should have been a lock washer from what I understood.Oh,and the shrink tubes for the I/P signal present leds are too long........ ;D ;D ;D

Again:Thank you very much for this nice and great sounding gear Jeff,another very well done project indeed-CHAPEAU BAS!

Best from munich,germany,

Udo.

Edit:I know other guys did it already but THANK YOU for the isp-led!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Cgaines on June 02, 2012, 10:01:15 PM
That was it Jeff thanks! I had the transistors switched.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 02, 2012, 10:12:25 PM
That was it Jeff thanks! I had the transistors switched.
Hi,

Cool you got it sorted.
I wanted to say that some of the parts seem to be a bit far off from the pcb surface.
Beware of shorting things out by touching each other,the area around  R14 to R8 as well as the resistor and the diode arrangement D1A-B seem to be very close to contact in your picture.

Have fun,

Udo ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 03, 2012, 08:20:11 AM
Addendum:

I measured the current,loaded with gar2520s,boosting a -50dB signal to around unity gain.
Results are 59mAs on positive and 55mAs on negative rail.
Given around 60mAs on both one can drive more than 20 pres from a single 51x psu,without even touching the 24v rails,cool!
Sadly I can not measure the inrush current......

Just for info,

Cheers,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 04, 2012, 08:55:11 AM
You like? ;D

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 04, 2012, 09:02:10 AM
Oh Udo, "like" doesn't cover it.  ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 05, 2012, 07:30:24 AM
So I did some first tests with speech (narrator) and a male singer in our studios.
I used an U87Ai (like most of the times).
The vp28 has always a nice character,but you can get tons of sounds from it!What a great preamp!
It is never "clinical" sounding,always nice to dial in using both the gain and fader pots,a lot of variations can be achieved easily.
The filters work great and give a supercool control over the subs you want to remove or just flatten a bit.
After that I hooked up a channel strip using the modules as shown in the picture in my previous post (vp28-lc53a-la500).
People-this strip is a KILLER!!!
I prefer it over a m*n**y v*xb*x,no joke!

Now up to some more experiments and recording.

Have fun,

Udo.

P.S.:Jeff,I owe you one!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Pliplo on July 01, 2012, 02:52:22 AM
I just finished my first VP28 and without Chunger's documentation finished! Slowly recovering from my first assembly (and failure) with a 7-0 win streak!

Almost fried my monitors (and ears) when i pushed the MIC button with all knobs at full gain, but hey, you learn from experience xD

If i hear my voice that means i assembled both GAR1731's correctly and nothing's wrong? (it surprises me a little, plus im so bad with a DMM that every pre i finished goes directly in the lunchbox, testing is for noobs......)


=D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Seditionary on July 01, 2012, 09:02:50 PM
yes -- jeff's build guide on the vp28 is awesome !  very, very helpful.  and these channels sound incredible. 

i finished 2 of these with op amps that worked on first fire up.  the only thing i had a bit of a problem with is one of the LEDs... in the build guide jeff recommends bending the amber LEDs at 45 degrees... i'd recommend not bending those quite so much... maybe 30 degrees or so... as they lay pretty flat on the board... if they are bent at too much of an angle and you push them in slightly at an angle, SNAP... you'll lose an LED in the faceplate... (which i did)... so i recommend only slightly bending these to fit into the hole.  :)

the vp28 sounds amazing with all the op amps i've tried! !  but the op amps AND THEIR CONFIGURATION definitely change the tone.

i've tried the vp28 with gar2520's and a mix of gar2520 and gar1731... i think i prefer the mix of gar2520 and gar1731... but even their position makes a difference !  i originally chose the 1731 in the position near the input transformer as it was a fatter signal when i was testing vocals through it after i had completed the build... but i just did a test running mixes through with both op amp configurations. although they both sound great... there really is quite a difference ! the 1731 closer to the input transformer really warms things up... but kind of smears the highs a lot like hitting tape... trying the test again with the 2520 closer to the input transformer (which is how dr. bill has his op amps configured (thanks dr. bill!)), it still has the tape like saturation vibe, but without smearing the highs so much! so setting it up that way -- 1731 near the output transformers, gar2520 near the input transformer is brighter and clearer, which i think is better for mix buss. not as muddy, but still technicolor saturated. it sounds incredible! !

thanks so much to jeff who is such an amazing dude. besides offering these amazing sounding circuits, he is a teacher. he almost always responds immediately, which is mind blowing and super human... and he gives you just enough guidance in order for you to learn and figure out more on your own. jeff steiger is a diy guru teacher electronic rock star.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: chunger on July 09, 2012, 05:00:05 AM
Hey guys,

The photo build tutorial is now complete in broad strokes.  Please check for errors.  The preamp shown in the photos works right out of the gate, so I assume the build was at least mostly correct.  But, if any errors are spotted, please let me know immediately.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOF on July 30, 2012, 11:09:49 AM
Jeff, the VP28s sound wonderful.  Great design. And I must applaud both you and Chunger for the tremendous amount of work it took to put together the build process to help us.  With all of the components packed so clearly and Chunger's crazy-thorough video, it's a breeze to build these.  thanks very much-- loving them.
david
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Stone on August 09, 2012, 10:54:45 AM
Hey Jeff.  Just building 2xVP28s.  It is my first ever build and I'm a lot of fun doing it too! 

I have a quick question for anyone that may be able to help.  I'm having problems identifying the Ceramic Capacitors C3, C4, C6, C10/C11 (my multimeter isn't reading the correct values.)  Could anyone help me identify them?

102J/5A = As there are 2 of these I'm assuming this is C10/C11.  Is this correct?
82J/5A =
27J/5A=
221J/5A =

Thanks in advance for any help...it is greatly appreciated!

Stone
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 09, 2012, 11:07:09 AM
102J/5A = 1000pF/50V
82J/5A = 82pF/50V
27J/5A= 27pF/50V
221J/5A = 220pF/50V
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Stone on August 09, 2012, 11:44:56 AM
Thanks Jeff!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Stone on August 09, 2012, 08:33:13 PM
@Jeff.  Newbie question here!  Almost finished building both VP28's but accidentally cut one of Output Transformer wires too short.  Is it OK to solder the wire to make up the length and then protect it with either electrical tape or heat wrap?

Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 09, 2012, 11:16:43 PM
Yeah, just extend it with a cutoff piece and make sure the solder joint is good and solid. Then use a piece of heatshrink on it.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Stone on August 10, 2012, 08:33:03 PM
@Jeff.  Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Cgaines on August 12, 2012, 01:45:09 PM
Hey Jeff, I've had my vp28 built for awhile now and love it! I do have a question though. Is the vintage style gear noisier than new gear? I just built a u87 clone and it requires a lot more gain than my other mics. Since I had to turn up te gain I noticed a low level him I hadn't noticed before. Is that normal or should I check something?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 12, 2012, 03:35:46 PM
The pre should be very quiet. What you are describing doesn't sound normal to me. It sounds like maybe a grounding issue of some sort. Maybe something rack or cable related? Maybe mic related? If the pre is built correctly then it will not be from it. Is it like this with all mics? I have had guys use the VP26 with ribbons and gain wide open. They have commented on how surprisingly quiet the pre is a full gain. The VP28 will preform in a similar way with the fader set at 0dB. Increasing the fader level will of course increase whatever noise and/or hum may be in your signal already.

Dunno if this helps or not.  :-\
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Cgaines on August 12, 2012, 04:23:01 PM
Well after doing some more tests I've decided it has to be the mic... I only have the problem with that combination though. Any of my other mics plugged into your pre are extremely quiet with the input gain wide open. But my mic on other pres doesn't make any noise. Just a weird thing I guess. I'll go back and ground my mic every possible way I can and see if that rids me of my problem.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Stone on August 13, 2012, 06:17:31 PM
I just finished building 2xVP28's with a gar2520 closest to the input transformer and 1731 closest to output transformers.  This was my first ever DIY project, let alone first ever time near a soldering iron(!) and I couldn't be happier with the results. 

After completing the build I plugged them in and was blown away!  Not only did they work straight out of the gate but they sound absolutely glorious!  It's hard to describe in words but they make you feel like you are right in the room with the music, and not listening on speakers. 

Jeff I cannot thank you enough for designing and making available such a fantastic piece of gear.  I can't believe I lived without it for so long!  I also wanted to thank Chunger for his photo's and comments.  They were invaluable and I honestly don't think I could have completed this build without them.  Thanks for putting so much thought and time into them.

Anyway...just wanted to share my excitement!  I'm looking forward to testing some different op amp configurations but am loving the combination of 2520's and 1731's at the moment. 

Thanks again Jeff and Chunger!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 13, 2012, 06:41:46 PM
Excellent news and congrats on a successful build!! :)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: rCHESS on September 12, 2012, 04:55:29 PM
Really diggin the vp28's.  Just completed my second one and all I can say is they sound GREAT!!!   Thought I would add a photo of my Classic API stuff so far.  Thanks Jeff, Chunger,  and the list could really keep going (there is a lot of great info around here).  I have have learned a lot from this forum and am having a blast with the builds! 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: New Soul Rebel on September 28, 2012, 04:55:28 AM
Yahoo! About to build my first VP28's. Here goes... toward anarchy and oblivion!!!  >:( ::) :P ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: freestylemovement on October 20, 2012, 02:40:29 PM
Hi,
i'm a first time VP28 builder, i have signal, 48v etc.  sounds beautiful except a couple things.

in Mic mode the signal is very very very low.

in Line mode (when patching another pre or line level signal) the level is more acceptable and sounds very nice.

but still both gain pots are at maximum level which doesn't seem right.

but in either case the HP Filter needs to be in for signal to work.

i know it's a shot in the dark, but has anyone encountered this issue  before?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 20, 2012, 03:11:03 PM
The HP filter needing to be engaged sounds like maybe some cold solder joints on the HPF switches and/or 5-pin header. The signal is routed onto that small board and thru the switches even if not engaged. This could possibly be the cause of all your troubles.

As with any build like this, are both opamps known to be good, solid and working? Who built them? Did you follow the DOA install page on my site?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jmcnul on October 26, 2012, 02:01:26 AM
Hey all,

Just got finished with a 28 and I'm getting a crazy hot signal, like I need a pad on any source when mic is engaged. Is this normal? 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on October 26, 2012, 02:26:14 AM
Hey all,

Just got finished with a 28 and I'm getting a crazy hot signal, like I need a pad on any source when mic is engaged. Is this normal?
Hello and welcome to the forum,

can you explain your signal chain a bit more in detail?
What microphone do you use and what sigmal are you picking up?
What is connected after the mic pre,can you monitor it at this point?
At least we should know about what level we're talking when you say it's a "hot" signal.
Also interesting if the gain switch shows a normal action concerning the steps,maybe you've swapped gain resistors?

Best regards,hope to have helped,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jmcnul on October 26, 2012, 03:12:16 AM
Well, I tried a beta 52 on kick and got solid blocks in protools. I'm out of my lunchbox, line into my 002. It just seems like something's not right. I got my input and output as far down as they can go and am still clipping. This is with mic engaged though, when in line mode it seems alright. Thanks
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 27, 2012, 12:17:58 PM
It is easier and more accurate to judge with test tones and something to measure the output level. The line is easy enough since we can test for unity gain. If the line setting is at unity, the mic setting should be right. The only difference is the pad for line input.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jmcnul on October 30, 2012, 12:46:56 AM
I am pretty sure something is not right though. What could potentially be the cause?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on October 30, 2012, 03:00:31 AM
I am pretty sure something is not right though. What could potentially be the cause?
It is impossible to say when you write that "something is not right".
Let's start with the unity gain test as Jeff recommends.
Apply a sine wave,say with a 1 k frequency at  a known level,say 0dBu (=0,775 v or 775 mv) to the mic pre input.
Set the gain to "line".
Set output level to middle position ( to zero position so there is no boost or attenuation).
Filters to off for now.
The reading at the output should be the same as the input now,at least very very close to it.
You can measure with a level meter or a dmm set to ac.
If you measure with a daw be aware of the specs of your interface,you should know what level your dBfs refers to.
If you measure with a dmm then do it between pins 2 & 3 (hot and cold signal).
This is for checking the pre itself,so nothing else should be connected except of the signal generator on input and a meter of some kind on output.

Good luck,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: okgb on November 01, 2012, 04:56:07 PM
Just built up a pair , just with a quick voice test [ my own  ] they seem to have a nice " size " & openess
the HPF  " feels smooth too , nice to have options with that , who needs ultra low end on vocals ?

One question , I'm assuming the opamps closer to their respective transformers are the in and output stages ?
built the units with Gar 2520 & 1731  so used the 1731 for input [ at the bottom of the card ? ] and the 2520
closer to the out xfmrs .

thanks & regards Greg
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 01, 2012, 06:41:07 PM
Awesome Greg 8)

A1 is the preamp opamp and A3 is the post fader booster amp. For the folks using the gar opamps, most seem to prefer the gar2520 in the preamp position and the gar1731 in the post fader booster position. To each, his own though. I often tell folks who ask me about what to use where, there really are no wrong choices. Experiment and see what works best for you.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: okgb on November 01, 2012, 10:50:56 PM
Quote
Well, I tried a beta 52 on kick and got solid blocks in protools. I'm out of my lunchbox, line into my 002. It just seems like something's not right. I got my input and output as far down as they can go and am still clipping. This is with mic engaged though, when in line mode it seems alright. Thanks

Don't want to avoid the obvious but you are going into a Line input on the 002 correct ?
channels 5-8 are line with no mic [ gain  ] input options
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 02, 2012, 02:42:51 AM
Don't want to avoid the obvious but you are going into a Line input on the 002 correct ?
channels 5-8 are line with no mic [ gain  ] input options
Hello,

good point,one never knows......
Oh,I forgot to mention to look at the specs of the dmm if using this for measuring too because some of the cheaper ones won't "see" a 1khz sine wave.You must choose a lower frequency from your signal generator then!


..and @Greg:Congrats for your pres of course!Nice guys,no?I love mine,I have two gar2520 in them.
Cheers,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jmcnul on November 12, 2012, 08:17:55 PM
So, I switched inputs around and actually finished the pre (added green LED and IEC to power signal indicator) and that seemed to do the trick. Built another 28 and they both sound fantastic. Couldn't be happier except for the fact that I want more haha. I think the next purchase will be the 51x rack
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 13, 2012, 01:35:04 AM
So, I switched inputs around and actually finished the pre (added green LED and IEC to power signal indicator) and that seemed to do the trick.
:o
Sorry,but I don't get it.
What was the solution to your issue?
Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jmcnul on November 13, 2012, 03:00:31 AM
Haha, I'm not quite sure either. Could be a variety of things, but at this point I'm happy with a rocking pre
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 13, 2012, 04:28:51 AM
O.K.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on November 14, 2012, 12:02:32 AM
Momentary hijack questions, then i'm gone :) i've been thinking about selling my brand new seventh circle audio rig, i built 2 api style a-12s and 2 neve style n72's, with the new black chassis. I'm loving the a12's with the red dots, but i'm bummed i can't drive the input hard and attenuate the output , (pads are a drag) I found the capi stuff and I'm thinking i'm wanting the capi vp28's.
my questions are as follows;

1. Are they that much better sounding than the a12 with scott lieber red dots?
2. does the channel fader nob on the vp28 allow you to drive the input hard and cut the output enough to not overload your daw input?
3. would it be worth it to build a variable attenuator for the output of my a12 and forget the vp28 and 51x?
4. any other reasons i'm not aware of that make the 51x and vp28's a better option in the long run?

Thanx for your input fellas,
T
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Dylan W on November 14, 2012, 01:34:28 AM
Momentary hijack questions, then i'm gone :) i've been thinking about selling my brand new seventh circle audio rig, i built 2 api style a-12s and 2 neve style n72's, with the new black chassis. I'm loving the a12's with the red dots, but i'm bummed i can't drive the input hard and attenuate the output , (pads are a drag) I found the capi stuff and I'm thinking i'm wanting the capi vp28's.
my questions are as follows;

1. Are they that much better sounding than the a12 with scott lieber red dots?
2. does the channel fader nob on the vp28 allow you to drive the input hard and cut the output enough to not overload your daw input?
3. would it be worth it to build a variable attenuator for the output of my a12 and forget the vp28 and 51x?
4. any other reasons i'm not aware of that make the 51x and vp28's a better option in the long run?

Thanx for your input fellas,
T

I don't have the SCAs so I can only speak to #2. The channel fader on my VP28s is very useful for hitting the input hard and pulling back so you don't clip your interface. It's also got a pad (-20db, I think?) and a padded line input. They do sound noticeably thicker than API 512s I've used and are perhaps more useful on sources where the 512s would be often be too forward or "pokey," eg vocals. They don't seem to sit quite as far in front of the speakers as the 512--it's like they're pulled back a little bit in space, with a little more room around the sound. Not sure if that description makes sense. Mine have a gar2520 on the input and a gar1731 on the fader, btw.

They're also great for using on the 2 bus, either in line level mode or as makeup gain for passive summing. That versatility and the ability to throw them in line input on a kick or snare in the mix, for example, made them attractive and useful to me.

Dylan
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 14, 2012, 02:11:46 AM
Hello T.,

I love mine because they're really versatile.
You can get tons of different sounds pretty easy.
They have an isp led which is a good little helper in racks located a bit off your workspace.
One super feature is the excellent hpf with 3 corner frequencies and switchable slope at 6 or 12 dB.
This will help you on tracking without the use of an additional eq.
In line mode it is a perfect tool to bring back some life to those boring digital sounds.

Hope to have helped,

Udo.

P.S.:I have gar2520's in both postions.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: okgb on November 14, 2012, 09:43:26 AM
  Yeah , it's a good HPF  , Wish every pre had one

Don't overlook running multi stage pres with the outputs fully up or one notch away from top
and adjusting the in gain to where you like it , depending on where the control for the output stage is
you can get it working harder ,  keeping the output waay down can be like closing the door so you're not letting
much input gain through no matter where you have it set .  [ unless the attenuator is after the output transformer ,
then by all means run it low ]  ..............o.k. varied results on that depending on pre , so as always just listen !
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on November 14, 2012, 03:37:50 PM
Nice, the best info always comes from u guys at GDIY! thnx fellas, I'm now on the waiting list for 52x 500 and 4-vp28's ;D, now i have to figure out a worthy DIY 500 series replacement for my N72 neve style pre's :-\
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: okgb on November 14, 2012, 08:29:12 PM
Soundskulpter has an everything included  mp573 , I've personally build a couple of those
they sound great , Onlyme had a minimal kit on the white market i believe , both d.i.y.
maybe more check the W.M.
Oops I should have just pm'd ya instead of clogging up the VP 28 thread
classic api also one stop shopping with great support [ clear instructions & details ]
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: ben_allison on December 02, 2012, 12:42:19 AM
This build was a lot of fun!

Chunger your guide was ridiculously amazing and helpful.

One question: one thing I really liked about the VP26 guide was the testing procedure at the end. Is there a similar voltage checklist for the VP28? I couldn't seem to find one anywhere.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 02, 2012, 09:55:52 AM
I don't have any specific tests posted for this build but the main voltage measurements will be the same for any of the modules. Just make sure you have your +/-16V readings at the appropriate sockets.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on December 05, 2012, 11:48:07 AM
..just received some germanium opamps from Pier, and want to try them in the first stage. as they have an output offset, i have to install a 220-470u cap
after the amp.my question is if there is already a cap in the out ?
and will there be some space on pcb for this as it is already crammed a lot ?

axel
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 05, 2012, 12:06:57 PM
..just received some germanium opamps from Pier, and want to try them in the first stage. as they have an output offset, i have to install a 220-470u cap
after the amp.my question is if there is already a cap in the out ?
and will there be some space on pcb for this as it is already crammed a lot ?

axel
Hey Axel, yes the PCB layout for the VP28 is fairly dense but I already have a 470uF coupling cap after each DOA so you should be fine.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on December 05, 2012, 12:12:48 PM
 8) that was fast !
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on December 11, 2012, 03:21:43 AM
1 down, 3 to go. This is a really well engineered kit ??? very impressive, nice job Jeff
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on December 12, 2012, 02:53:35 AM
then there was 4 :o
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on December 15, 2012, 07:58:05 AM
..most nice boards i have ever seen, pure beauty..
but i have maybe done a little mistake, i have put in the opamp sockets from above and soldered them, so they are working but coming 2-3 mm hgher now
i dont think that will be a problem with the height later but if so, please somebody warn me !

axel
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on December 15, 2012, 09:13:20 AM
Hi Axel,

shouldn´t be a problem.
I´m re-arranging my racks at the moment,so I had them in my hands an hour ago.
Took a pic from both side by side with a ruler on the frontpanels,there´s roughly 9 to 10mm space (I have gar2520s in).
Hope that helps.

Have fun,

Udo ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on December 15, 2012, 09:26:17 AM
very nice, thanks , udo !
so happy soldering.. :)
greets
axel
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on December 17, 2012, 07:17:02 AM
uups, next one.. i skipped a picture of chunger while installing the output transformers...i now have mounted both 2623 facing downwards the pcb... >:(
so as everything else is build now, the unit is complete finished ...just wanted to try her out..
is there crazy interference with the both trannys magnetical fields, maybe cancellation somehow or is it just to keep the leads shorter what would be no issue i think because maybe now the wires are 1 cm longer than other way round.
so please somebody chime in (again.. ;))

btw, it seems that the screws of the blank aluminium knobs are a little bit bigger than my 1.5 mm inbuss srewdriver and smaller than my 2 mm inbuss one.
i havent found  a unit between them yet and would like to know which one i have to order, for example if it is some inch-scaled driver

thank you a lot

axel
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: mulletchuck on December 17, 2012, 10:23:55 AM
TX orientation shouldn't matter.  it just affects wire length.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 17, 2012, 10:49:09 AM
In this situation with the transformers, you will be fine Axel.

The small 3/8" pointer knobs require a .05" allen key.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on December 17, 2012, 11:30:55 AM
 :D great news
love this forum !

axel

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: voidar on December 19, 2012, 10:16:32 AM
Very nice kit jsteiger and very helpfull tutorial chrunger!

I just finished the kits I got a week ago. Some observations.

1. Instead of the double-sided tape chunger uses under the input I had to improvise with cutting up some foam from the kit.

2. The fader knob is of a different design and fits around the shaft from the start, which brings me to something that has me confused:

I am sure I installed the Grayhill switches correctly and you would think one of the nuts of the knob is supposed to rest against the flat cut-out of the shaft.
But the only way I could have the knobs face the right value is to have the nuts bite into the shaft of the switches. I am a bit worried if this will hold up and that I will have to reset the knobs all the time. Are they supposed to be like that?

3. The tip of the small LEDs break off easily. I had to super glue two of them. Thankfully the glue didn't dim them.

Also, another question.

Which of the op-amps is the fader/output amp?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on December 19, 2012, 01:02:41 PM
Very nice kit jsteiger and very helpfull tutorial chrunger!

I just finished the kits I got a week ago. Some observations.

1. Instead of the double-sided tape chunger uses under the input I had to improvise with cutting up some foam from the kit.

2. The fader knob is of a different design and fits around the shaft from the start, which brings me to something that has me confused:

I am sure I installed the Grayhill switches correctly and you would think one of the nuts of the knob is supposed to rest against the flat cut-out of the shaft.
But the only way I could have the knobs face the right value is to have the nuts bite into the shaft of the switches. I am a bit worried if this will hold up and that I will have to reset the knobs all the time. Are they supposed to be like that?

3. The tip of the small LEDs break off easily. I had to super glue two of them. Thankfully the glue didn't dim them.

Also, another question.

Which of the op-amps is the fader/output amp?

Strange on the Leds, i just built 4 with no problems at all. A good idea on the gray hill switches (for the stone hands type ;D), if your worried about stripping the hex to shaft contact, is to position the knob where you want it, pull 1 hex completely out, take a pencil and mark the bottom of the hex with plenty of "lead", screw it back in tight, loosen both hexes up, pull the knob off, it should leave a pencil mark on the gray hill shaft. Take your dremel and carefully flat spot the shaft, repeat for the 2nd hex. Ive done this on drill bits, works great, but for the vp28, i would just lighten your grip monkey fist! :o ;D ;)

my guess is the bottom opamp is channel fader, BUT DONT TAKE THAT TO THE BANK!!!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on December 19, 2012, 01:09:54 PM

3. The tip of the small LEDs break off easily. I had to super glue two of them. Thankfully the glue didn't dim them.

Wow,how did you manage that?Have never heard someone breaking an led before.....did you use a hammer? ;D

Cheers,

Udo ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: voidar on December 19, 2012, 01:16:03 PM
Well, the poiny plastic "nose" which is inserted into the holes are quiet tiny. I don't know how I managed  :D, but it's an annoying complication at the end of the build. I almost cried  :'(.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on December 19, 2012, 01:32:50 PM
i've been running my 1972 jazz bass through the vp and into my ampeg svt,...DAMN :o :o!!! I will use it my next bass gig for sure! Rear pickup only, fingers at the bridge, ladies and gentlemen, please welcome... Jaco Pastorius!, slower and fatter!,... yes,..no,....Its...welcome,... Jacob Pastorius! ??? Good enough for the girls i date!!! ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 19, 2012, 02:19:54 PM
The A1 DOA closest to the 2622 is for the preamp. A3 is the post fader booster.

I started stocking the knobs with the 1/8" hole for the fader switch to simplify things a bit. The set screws are not supposed to sit on the flat. It would be nice but impossible with the way each is made. BTW, I did get a quote from GH on a solid shaft. The price increase was insane so we have to deal with the flat. Tony's method is a good one if you can't get them correct without a flat. I have built many and not had trouble. The preamp gain is the same and so are all 6 switches on a LC53A build.

Ginger with those LED's!!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on December 19, 2012, 02:29:19 PM
The A1 DOA closest to the 2622 is for the preamp. A3 is the post fader booster,

I had a feeling if i suggest either, it would be or? ::) I hope you didn't go to the bank with that :-\
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: desun on January 04, 2013, 07:42:08 AM
Ordered one of these yesterday, looking forward to building it!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: desun on January 15, 2013, 01:10:15 PM
Got my VP28 finished yesterday, very easy to build (thanks to Jeff and Chunger, great instructions!), looking forward to giving it some use!

Edit: forgot to mention that i snapped the lens part of the Signal LED off whilst trying to get it into the hole and get the leads in the correct place, so just be a little more careful than I was with it! The LED still works, but doesn't sit in the hole anymore.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: greenhorn on January 16, 2013, 09:24:00 AM
Hey,

I thought I'd share the sorting sheets I've made for the VP28 builds. Heavily influenced by the ones Chunger have made in his pictorial, obviously  :)

They are made in the European A4 format. Hit me up for the excel version if you want to make it fit the american standard for printing.

VP28 gain and fader resistors.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: greenhorn on January 16, 2013, 09:25:30 AM
VP28 Main PCB and 553 HPF PCB Capacitors
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: greenhorn on January 16, 2013, 09:27:44 AM
Last, VP28 Main PCB resistors and diodes.

***updated sheet further down***
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Axelerator on January 16, 2013, 12:01:19 PM
just finished the second vp28, as always a sheer joy
gave them a small ride yesterday with mono beats from my machinedrum, one fitted with ppa germanium at input and  ppa 2050e in the out,
the other one with same opamp at output but with ppa 2055 fet input.
the vp´s really totaly blew me in a way i had not expected.. :o ;D
it sounds so nice to drive the first stage ... like having a really fat old console here..have to change my plans for my second 511x rack.. don´t need
more comps , just more vp´s !!
ok i have to test them out with the reddots and melcors  i have around, but so far the above combos are sounding very nice. the germ input is slightly more forward sounding with nice second (?)
harmonics, will keep two for drumbuss , the fet input is sounding a little more balanced to my ears, would like to try them on mixbuss i think,
for sure the vp makes more difference to the overall sound  than the opamps
thanks jeff for this cool units and congrats to be the inventor of modular 500/511x console sound in one box !
..now i have to earn some money for more of them ;)

axel
axel
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on January 16, 2013, 02:31:12 PM
Nice contribution Greenhorn! appreciated for my next units.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: greenhorn on January 17, 2013, 07:05:42 AM
***UPDATED VP28 Main PCB and 553 HPF PCB Capacitors sheet***
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: analogdays on February 07, 2013, 09:38:32 PM
We've built a few VP28's - they are indeed excellent sounding and have a great feature set that makes them extremely useful.

First, a quick question - is there any protection from phantom power for line devices if inadvertently connected? I can always check with my MM, but I'm not in my work room right now.

Bigger question - two of the most recent 8 we built have lower gain or output. Is this something that has a common cause? Ops are good. Checked that knob position was accurate and it was.

Waiting for replacement oscilloscope, so I'm a little hand-tied for the rest of this week, but solder sucker and iron are ready for action now. :-D

Thanks!

BTW Great Thread!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 07, 2013, 10:04:27 PM
....is there any protection from phantom power for line devices if inadvertently connected? I can always check with my MM, but I'm not in my work room right now....
No guardrails on this road. Proceed with caution. No different than caution with some ribbons.

Quote
....Bigger question - two of the most recent 8 we built have lower gain or output. Is this something that has a common cause? Ops are good. Checked that knob position was accurate and it was.
First, how much lower gain?

I would rule out rack slots, associated cabling, converter settings, patchbay, on and on. The next thing to look at are always the opamps. Swap them with one of the earlier units and see what happens. I would also look at 2623-1 wiring. Seems odd that both of the last 2 are equally lower (are they exactly equal?). Makes me also think it could be an accidental resistor swapping that occurred on both of the last 2 units.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: analogdays on February 07, 2013, 10:17:03 PM
You're fast!

That's what I'd guessed.

On the level question:
I'd guess 10-15dB lower. Useable, but not as much output as normal.  ???

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 07, 2013, 11:29:44 PM
Something is definitely wrong with the builds. Are the 2 latest ones the same as far as lower level?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: alexgriva on February 08, 2013, 01:51:05 PM
Just finished one vp28. Lots of gain! Big sound! Thanks jeff! Only one problem.... When i turn phantom on the led does not work and all the others go off. The green  (signal) led is not work too.. Any tips?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 08, 2013, 02:00:52 PM
Yes, the phantom LED is in backwards. All the LED's are in a series string. The phantom LED is first inn this series so when you engage the switch, the voltage to the string stops at the red LED because it's backwards.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: alexgriva on February 08, 2013, 03:30:35 PM
Thanks jeff! That was very basic..duuh. There is new question. I've done two 1731 simultaneously. One is ok and the other i had the R14 and R15 overheated, blown. Any tips?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 08, 2013, 03:46:08 PM
Yes, the best is to refer to the gar1731 help thread. I think you will find what you are looking for. It's probably either the diode orientation and/or the larger BD transistors.
Title: VP28 not amping signal
Post by: Mrosso on March 05, 2013, 06:22:36 PM
Symptom: Just built a VP28 in a DIY 51x 11-space rack w/ floor power supply. Aside from working LEDs, unit appears to be dead. No signal and signal LED is dark. I did get a little faint signal with lots of noise when I fed it a line level signal direct from my converters with the fader and gain all the way. Opamps are pre-assembled GAR2502s.

Troubleshooting so far:
Checked all solder joints for shorts, all look ok to me
Checked diode and caps for polarity, all good
Checked xfrm wiring, all good
Checked LEDs - phase LED backwards, now fixed
C1 Cap has a slight rounding on the top, like it was installed in rev

Any help would be incredibly appreciated. Please someone tell me I did something stupid I can fix easy. I don't want to be stuck with a $400 brick

Thanks

(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1334.jpg)
(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1345.jpg)
(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1343.jpg)
(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1338.jpg)
(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1337.jpg)
(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1336.jpg)
(http://i29.photobucket.com/albums/c257/mar629/VP28/IMG_1335.jpg)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on March 05, 2013, 06:58:57 PM
Mrosso,

I noticed you said "Checked all solder joints for shorts, all look ok to me", did you do a continuity test down the line? I've had "perfect looking" solder points be cold,..recently.

let us know
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on March 05, 2013, 10:07:00 PM
what happened mrosso? i could've sworn you posted you were going to try it?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mrosso on March 06, 2013, 09:49:31 AM
Yes that is correct, but can't find a schematic anywhere for this.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on March 06, 2013, 10:19:07 AM
First off, great job posting pictures - makes it easy for people to try to help.
So looking over the pics that cap does looks bad.
Not sure how it would have gotten fried though. Is your 51x rack and psu verified to be good?
You shouldn't have more than 25v even present on the vp28 to fry that cap, unless the psu was to hot.
I would plan to replace it, but I am guessing that is a DC blocking cap for the gain stage, so it shouldn't kill all your signal.

As you say it is hard to trace out without having a schematic, but, you can narrow things down pretty easily.
And the circuit is pretty simple so it is possible to do some tests.
Do you have any other DOAs you can swap, or do you just have the pair?
Verify the voltages from the psu?
When troubleshooting, I always try to be methodical and eliminate things that could be wrong.
What test equipment do you have?

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mrosso on March 06, 2013, 12:13:41 PM
DMP,
Thanks for the help. I dialed in the power supply voltages per Classicapi's documentation. The power voltages test good using Fluke 179 DMM. I have four other 500 modules all working fine in rack too. I have a couple of Hardy 990 DOAs I can swap from my Lola, although a bit worried about damaging them. Good advice all around - I'm trying to follow traces, etc., but I'm a real newbie at troubleshooting and electronics in general -- that's why I really prefer building the kit builds (SCA, hairball 1176, etc.). I'm going to replace the cap as soon as I can dig one up (probably have to order from mouser), but I'm not sure how it fits into the circuit.

Thanks again!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 06, 2013, 12:22:13 PM
Like DMP mentioned, that is the DC blocking cap before the gain switch. Hard to believe there would be enough V on that cap to cause that. Improper seating could cause some issues but don't think this would be a result.

Swap out the opamps with other known good ones just to rule them out. Gary burns them in and tests them before shipping to me so they should be fine but best to be positive.

You can desolder the blue and black leads from T2 and use those as a balanced output point to test the preamp section. That will remove the filter and 2nd stage from the equation. This will help narrow down where the issue lies.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on March 06, 2013, 12:28:37 PM
The orientation of the cap looks correct, based on the other pics by Chunger and the PCB pic on classic API.

First thing to do before swapping in other DOAs is measure the voltage at the +V and -V socket with respect to the C socket (ground).
If those are +16v and -16v your DOA should be safe. You can check the + - inputs and the O output also if you want to be REALLY safe.

As you are following traces it will help to know what the building blocks are. If you look at the API 312 schematic on Jeff's site, theVP28 is going to basically have something very similar as the first stage. Input at the card edge connector goes through the pad and into the primary of the 2622 input transformer. Then from the transformer secondary to the DOA gain stage and out to the first output transformer.

After that I believe it goes through the high pass filter and the fader, followed by the second DOA gain stage and out to the second output transformer. It looks like a lot of components but once you start breaking it down into different blocks, it will make a lot more sense.

If you can make up a test probe, I could suggest some places to probe the circuit to see where you are losing the signal.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mrosso on March 06, 2013, 06:48:29 PM
Quick update - measured DC volts between on each DOA socket +V to C and - V to C, all ok - both sockets give me between +16 and -16 on my DMM. Swapped John Hardy 990 DOAs and still nothing. Rechecked rack voltages again for sanity sake, still all correct, incl +/- 24v rails. Will try to trace per dmp's suggestion. When you say test probe, I'm assuming that is something to trace the AC audio signal, no? How do I make one of these? Starting to get above my level of competency.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: mulletchuck on March 07, 2013, 12:42:39 AM
test probe means you run some AC signal (like a -8dbFS sine wave from your DAW) into the unit, and measure with your volt meter along all the PCB traces to see where the signal stops or disappears. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on March 07, 2013, 10:03:07 AM
You can also make a test probe that hooks up to a little amp/speaker. That way you can hear the signal (or not)
Basically you use a capacitor to probe different areas in the circuit with music hooked up to the vp28 input.
There's a picture here:
http://www.diystompboxes.com/pedals/debug.html
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: clemfuture on March 07, 2013, 11:25:14 AM
Hello everybody !

I just started diy (i only did cables before!!) and I just finished two vp26 with good results ;) they sound incredible ! Thanks Jeff !!!
I also ordered two vp28's and I just finished the first one. Takes much long than the 26... But it was fun.
I had some issues with it and maybe someone could help me to fix it.
I explain you my woes:
Everything was quiet all right unil the sw6 switch, it was very late in the night and I made wrong with it, when I tried to placed it, i broke a pin, then i continued my work without taking care of it. I finished the all thing as chunger did on his thread and finally tested it with a mic.
And unfortunatly, no sound came up !!! I noticed at that point that I made something wrong and discovered the issue.
So I'm wondering before going back on it if it's possible that I blast a component plugging it in the lunchbox with the broken sw6 ?
Then I moved away the broken switch and before putting a new one instead, do you think I damaged my vp28?
For information the green led never switch on too ! The overs have no issues.
Hope someone had the same trouble than me and will could help me with that, it's so frustrating ;)
Thanks
Clement
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mrosso on March 11, 2013, 08:12:27 PM
Update 2 - dmp, man thanks for the tip on the signal probe, that was a huge help. I have one of those ribbon-cable extender tab things from JLM, so I can have the module connected to the rack but outside of the rack case. I probed the circuit using the 312 schemo and a lot of guessing - basically flipping between vp28 bom, pcb, and 312 schematic. I was able to trace the signal bit by bit from input leads on the card-edge connector all the way to output leads. I got signal at pin 1 of the input transformer, O pins of the two opamps, green & brown leads on the rear output transformer, and finally the outs of the card-edge connector. I was excited and a little bummed - looked like I may have jostled something loose that was shorted - preamp may work but bummed I didn't ID a problem that may happen again. Next, re-packaged everything back up in the aluminum frame, attached knobs, etc. plugged into rack and .... NOTHING! Same f-ing problem. This time I got some strange thumping-like noise when I pegged the gain, but nothing else. This is so f-ing frustrating! Anyone else have any ideas?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 11, 2013, 08:25:19 PM
It's probably a cold solder joint somewhere on the pre.

I would pull the module out but leave it assembled and use the extender. Maybe you can get lucky and find the culprit.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mrosso on March 14, 2013, 02:33:34 PM
Success! Finally! It was that damn C1 330uf cap!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: HummingPumpkin on March 15, 2013, 02:09:31 AM
After a long wait on a chassis, I was finally able to test out my recent VP28 build this evening.  I'd like to say it worked the first time, but that wasn't the case... although very close.  I actually bought two kits, but still had yet to do that last pair of opamps.  Anyhow, I stuck the complete build in the chassis (I chuckled a little at the irony of it going into an actual API lunchbox...) and found that I had signal, but it was crazy distorted.  I took a step back and calmly cursed for a minute, and then decided to test the the other build with opamps to get an idea if it was the preamp or the opamps, potentially.  I popped the first opamp and quickly saw that there as a wire coming off a resistor that I had evidently forgot to clip.  The wire was actually pressing up against one of the pins that plugged into the main board.  Whoops... but actually an easy fix!!  I decided to proceed with the swap and popped the second opamp.  Guess what, I missed the same wire again!!!  It just hid right next to that pin.  Clipped it, put the preamp in and heard clean, wonderful tones!!  It was a quick/lucky find and a very easy fix.  I can see why one of the first recommendations offered up is to pull the opamps when debugging.  I swapped the other build back in and that worked as well.  Just need to finish the other two opamps this weekend and I'll have a stereo pair, can't wait!  The effects of those unclipped wires touching the pins on the mainboard were "fun".  The MIC button pushed in gave you crazy distortion, pushed out the signal was much cleaner, but obviously very quiet.  The PAD and 48 switches worked as expected.  When the signal light went on, all the other lights went off.  The lights were also kind of weak.  Clipped the missed resistor wires and it's all normal.

Even though it's my first post, I'd like to pass along a few comments on the build for any other newbs out there thinking of grabbing a VP28.  It was way more time consuming than I initially thought.  Sure it's just soldering a bunch things on a board, but each step takes so much time.  Sort, map, install, solder, clip, repeat and repeat and repeat...  Chunger has a great set of instructions in this thread.  I found them highly useful when I had any doubts, thank you, sir!  And by the way, Jeff S is super responsive and helpful.  I had one question before I bought and one question after I got them and Jeff answered via email maybe in an hour or less in each instance.  Anyhow, back to the build...  I recommend reading these instructions carefully before you buy (and also making sure you are confident with a soldering iron... if you don't know what a cold solder joint is, learn that first and practice).  I pre-made my resistor/cap/diode sorting sheets (as Chunger displays) which helped me get an idea of what I was getting into.  Once I got the units, I sorted all the parts first and taped (painters tape) them down the sheets to keep everything together and organized (it's amazing how those resistors can fly off the table just by walking by).  By the way, I only put tape on the ends that I knew would get clipped off.  That was day one.  The second night I did a marathon run and put the main board together.  The third night I put the opamps together side by side (gar2520s).  I actually would recommend against the marathon runs.  I see people say they did it in this or that time, and that's great, but if you're new, take your time.  It's so easy to get tired or impatient and make a dumb mistake.  It was definitely easier the second time around.  The opamps were not as bad as I thought they might be.  Between Chunger's pix and the included diagrams I was able to put things together with confidence.  You just gotta be careful to keep the soldering extra clean on those little boards.  It's definitely nice to have a "helping hands" type of holder to hold the boards for you.  I can't wait to spend some more time on them this weekend.  I'm looking forward to trying out Jeff's other products and all the other things out there in the DIY world.  Good stuff!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: biosynth on March 30, 2013, 06:55:50 PM
Hi all,
i've just finished my first VP28, such a beautiful piece!  :-*
faceplate is really high quality, just as grayhill pots (suiss watchmaking) and PCBS. It's very pleasant to work with great material.
Jeff you seems to like good quality, it's very sensible and we are grateful to you.

A little note with knobs, which are too small in my point of view. I (we?) like to have big knobs in your hands, especially if it rocks!
(do you know where to buy big API knobs?)

congratulations Jeff, and thank you.
phil  :D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 03, 2013, 07:18:31 PM
In the middle of the build of my first VP28. Very fun and relaxing thing to do before going to bed. Love the kit, and Chungers photo manual is just great.

I'm curious about that double-stick tape for the 2622. Is it necessary? What is it good for?

Cheers
Paul
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 03, 2013, 07:41:31 PM
Paul, I use it more as a spacer than anything. I don't pull the protective paper from the side that touches the transformer's belly. That makes the 2622 hard to remove if it needs to be done for any reason.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 04, 2013, 02:05:36 AM
Paul, I use it more as a spacer than anything. I don't pull the protective paper from the side that touches the transformer's belly. That makes the 2622 hard to remove if it needs to be done for any reason.
Thanks Jeff!
Spacer you say - heat related then?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: useme2305 on April 04, 2013, 06:05:54 AM
the spacer keeps the transformer housing away from the solder holes to prevent contact and shorting.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 04, 2013, 07:40:37 AM
the spacer keeps the transformer housing away from the solder holes to prevent contact and shorting.
Brilliant, thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 04, 2013, 07:36:08 PM
Ready! This was my third kit ever, and after Horvitz's chorus and a pair of eqs this was a breeze. I'm sooo addicted. These kits are so beautifully laid out and fun to do. Just love sitting at the table behind our TV sofa, breathing the soldering fumes, and felt almost disappointed tonight when the VP28 was complete. Such a nice pre-go-to-bed routine. Luckily another pair of TB550s are on the way in, and then LC53As for sure.
Gonna test this baby tomorrow (w GAR 1731/2520) and compare to my AMS/Neve reissues, and then decide if I'll have to order more VP28s... Kudos to Jeff and to Chunger for the photo manual.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 05, 2013, 06:30:03 PM
Everything seems to work here.  :) Less noisy, but slightly more hum than my AMS/Neves, but that could be my studio (very old building and unfortunately a small issue with magnetic fields). I took an SM7, used a Y-cable (which perhaps isn't kosher) and fed both pre amps. They sounded very very close which is pretty impressive regarding the price difference. Perhaps the VP28 sounded a tiny bit brighter and lack something in the low mids, but it was so close I believe I would have failed in a blind test. Look forward to try it on drums. The two stage design is just awesome.

Only question: My unit had clearly more hum when engaging the phase shift. Anyone else have this?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: qmp audio on April 06, 2013, 01:38:02 AM
Only question: My unit had clearly more hum when engaging the phase shift. Anyone else have this?

I cannot hear any noticeable hum at all on any of mine
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on April 06, 2013, 02:50:49 AM
Quote
I cannot hear any noticeable hum at all on any of mine
Same here......

@unit7:

Hello,

picking up hum can appears in quite a few ways,e.g. from strong magnetic fields,unbalanced wiring,
bad shielding,wrong wiring on patchbays,mixing up different groundings etc......and all combinations of that.Even a mic capsule can do that.

So for sorting the VP28s out from that just disconnect that 500 type rack which contains the preamps and monitor them directly with a phonesamp or mixer or..........
Don't use those Y-cables (most of times no good idea,could work but mustn't).
You can do some experiments concerning grounding in your 500 rack,don't know which one you have,but e.g. in a 51x-rack one has a lot of options:
The 0 volts from the circuit can be connected to the mains ground(earth) or the 0volts from the psu or even left open.This is done by jumpers inside the rack.All this for every in-and output!
Let the inputs of the preamps connected to one of the grounds since you will need it for running the phantom power correctly.
Try to change the output ground or leave it open,it's o.k. to do so,you'll have an "earth-free" balanced out on linelevel then.

Just a thought,

Best regards from germany,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 06, 2013, 05:22:47 AM

@unit7:

Hello,

picking up hum can appears in quite a few ways,e.g. from strong magnetic fields,unbalanced wiring,
bad shielding,wrong wiring on patchbays,mixing up different groundings etc......and all combinations of that.Even a mic capsule can do that.

So for sorting the VP28s out from that just disconnect that 500 type rack which contains the preamps and monitor them directly with a phonesamp or mixer or..........
Don't use those Y-cables (most of times no good idea,could work but mustn't).
You can do some experiments concerning grounding in your 500 rack,don't know which one you have,but e.g. in a 51x-rack one has a lot of options:
The 0 volts from the circuit can be connected to the mains ground(earth) or the 0volts from the psu or even left open.This is done by jumpers inside the rack.All this for every in-and output!
Let the inputs of the preamps connected to one of the grounds since you will need it for running the phantom power correctly.
Try to change the output ground or leave it open,it's o.k. to do so,you'll have an "earth-free" balanced out on linelevel then.

Just a thought,

Best regards from germany,

Udo.
[/quote]


Thanks for the tips Udo!

Re Y-cable, no worries, I almost never use it, just did it yesterday for a quick A/B w the AMS/Neve. And the very quiet hum I got was there when using straight cable too. I'm fairly sure it's a magnetic field or perhaps from another unit in my rack. I always have problems w el guitars using single coil, and also with certain old RCA ribbon mics. Not much to do because it's a very old building. Besides that I believe my cabling, patchbays etc is ok becuase I have a LOT of outboard and never have issues with hum, and I don't think this case with the VP28 even could be called an issue, but thanks anyway for your concern!

All the best from Stockholm
Paul
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tylerromo on April 08, 2013, 08:35:07 AM
Built 2 of these with no problems this weekend after getting the package saturday. Thanks for the walkthrough chunger! Amazing pack job as always, Jeff! One thing I'll add about the opamp kits, PUT THE MATCHED PAIR OF TRANSISTORS BACK IN THE BAG after you dump the pins out. I panicked thinking I mixxed them up towards the end of my first shot at a 2520, and then noticed the hfe matched transistors' pins are spread from the gain matching. I lucked out because Q1 and Q2 were both still in the parts box.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: emfrank72 on April 10, 2013, 11:06:36 PM
Just finished my first VP28.  It works and sounds great.  Only problem is that the 4 LEDs on the left don't work.  The signal LED works fine.  I am guessing that there must be a + and - on them.  How can you tell which way they go in?  Is there something else that could be wrong?

Thanks,
Eric
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on April 11, 2013, 03:47:05 AM
Just finished my first VP28.  It works and sounds great.  Only problem is that the 4 LEDs on the left don't work.  The signal LED works fine.  I am guessing that there must be a + and - on them.  How can you tell which way they go in?  Is there something else that could be wrong?

Thanks,
Eric
The way they came, one leg was longer. Chunger's manual suggests keeping the longer leg longer when cutting, and the shorter leg soldered to the side where the arrow on the pcb is pointing. I'm sure Jeff will chime in! I'm just a noob!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 11, 2013, 09:49:22 AM
The signal present LED is by itself and in no way associated with the 4 status LED's.

The 4 status LED's are in a constant current series string together. The signal enters the string at the phantom LED and ends at the top LED. To test them, first make sure all 4 pushbuttons are in the out position. Try them one at a time. Make sure you leave the switch in the outward position before trying the next one. Which ever ones do not illuminate are in backwards.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on May 12, 2013, 12:03:19 PM
Hello Jeff,

hope you're fine.
Just wanted to know when the preamps will be available again.I know you're lacking of transformers,but can you tell us roughly estimated timeframe please?

Thank you in advance,

best regards,

Udo ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 12, 2013, 07:05:35 PM
Hi Udo

We are waiting for the 2622's to come in. They were suppose to ship last week but we have not gotten a UPS update yet. They should come literally any day now.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on May 15, 2013, 02:28:29 PM
Hi Udo

We are waiting for the 2622's to come in. They were suppose to ship last week but we have not gotten a UPS update yet. They should come literally any day now.
Another six of those beasts ordered and paid-hurrrrrrraaaayyy! ;D

Cheers,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: rtroast on May 15, 2013, 07:07:43 PM
Greetings all... just finished 3 VP28s, all of which appear to work great (checked voltages, passed signal). The only exception is that on one of them, the green signal LED is lit at all times. Any suggestions? I checked and they appear to be installed identically on each of the three builds.

Thanks in advance!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 15, 2013, 07:23:26 PM
I would check orientation on all the 1N914 diodes and also the 4558 chip. Maybe the LED is backwards?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: rtroast on May 15, 2013, 10:27:57 PM
I just thought of the diode orientation as well, so will go check. Ditto on the 4558 chip but I'll double check. The LEDs appear to all have been installed the same way. Thanks for the response.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Thanatopsis on May 26, 2013, 11:24:24 PM
What size allen wrench does the top knob use?  I have a ton of allen wrenches here but none small enough.  Stopped by WalMart after work tonight (only place open at 1AM) and they did't have anything smaller than what I have.

I had one of those allen wrench Swiss Army knife type things that fit both perfectly so that might work for people that have a hard time finding the right fit.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: qmp audio on May 26, 2013, 11:47:09 PM
What size allen wrench does the top knob use?  I have a ton of allen wrenches here but none small enough.  Stopped by WalMart after work tonight (only place open at 1AM) and they did't have anything smaller than what I have.

I had one of those allen wrench Swiss Army knife type things that fit both perfectly so that might work for people that have a hard time finding the right fit.

0.050"
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sharpeleven on June 07, 2013, 02:33:37 AM
Greetings all... just finished 3 VP28s, all of which appear to work great (checked voltages, passed signal). The only exception is that on one of them, the green signal LED is lit at all times. Any suggestions? I checked and they appear to be installed identically on each of the three builds.

Thanks in advance!

I had a similar problem with the LED staying lit about 30 minutes into operation. Turned out to be an opamp issue in my case. Try switching opamps around and hopefully it might be just that.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 14, 2013, 05:08:04 PM
Oooooops (I did it again!)

Another 6 PreAmps:Ordered,paid,built,tested,ROCKS! (You like,Jeff? ;D)
GAR 2520s on all of them.

(http://imageshack.us/a/img705/5543/s566.th.jpg) (http://imageshack.us/photo/my-images/705/s566.jpg/)



Greez,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Trench Recordings on June 14, 2013, 08:18:20 PM
Udo, stop hogging all the transformers  :P

Other people want to build some too ! lol
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 15, 2013, 01:33:47 AM
No.

 ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 15, 2013, 11:08:29 AM
Oooooops (I did it again!)

Another 6 PreAmps:Ordered,paid,built,tested,ROCKS! (You like,Jeff? ;D)
GAR 2520s on all of them.

(http://imageshack.us/a/img705/5543/s566.th.jpg) (http://imageshack.us/photo/my-images/705/s566.jpg/)Wow Udo...awesome rack you've got there!!  ;D



Greez,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on June 15, 2013, 10:31:17 PM
Oooooops (I did it again!)

Another 6 PreAmps:Ordered,paid,built,tested,ROCKS! (You like,Jeff? ;D)
GAR 2520s on all of them.

(http://imageshack.us/a/img705/5543/s566.th.jpg) (http://imageshack.us/photo/my-images/705/s566.jpg/)



Greez,

Udo.

you know the only thing that would make that rack look better?? Another rack full of vp's and Lc's right next to it!!! It's a sickness!! ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 16, 2013, 01:07:31 AM
Money,money,money........will take a while,but I already thought this "direction".
What I already have is rack full of compressors that will reside under the preamps.
These will form a killer front-end for my next musical where I'll have 8 lead vox accompanied by a lot of backing vox on their radio mics (yes,I use my diy gear live,too!).
Budgets are getting lower,and I'm sure I will only have digital consoles for the next projects.
I don't like them soundwise,so I'll have VP28s and LA500s on the line-inputs and won't use their mic pres.
Best of both worlds:Consoles with snapshot- automation and my private gear for the real sound.Not bad.....

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sharpeleven on June 16, 2013, 11:47:15 AM
Just wondering if anyone else noticed a slight change in transient response between the -6 and -12 setting while all filters are bypassed. So even with with no cutoff freq selected, the -12 / -6 switch is somehow always active and seems to have a significant influence on transient response. I have built 5 units so far and they all behave the same way.

I'm not saying that anything is broken, I actually kind of like having yet another option available to shape the tone. Just haven't seen it documented anywhere and no mention of it.

The -6 setting (switch in upward position, filters bypassed) sounds slightly more 'in-your-face'. I like the -12 setting (filters bypassed) when going for slightly slower transient response, to move things backwards a bit.

With all the different options available for tone shaping the VP28 quickly turns out to be a Swiss-army-knife kinda device.  8)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on June 16, 2013, 10:57:06 PM
Just wondering if anyone else noticed a slight change in transient response between the -6 and -12 setting while all filters are bypassed. So even with with no cutoff freq selected, the -12 / -6 switch is somehow always active and seems to have a significant influence on transient response. I have built 5 units so far and they all behave the same way.

I'm not saying that anything is broken, I actually kind of like having yet another option available to shape the tone. Just haven't seen it documented anywhere and no mention of it.

The -6 setting (switch in upward position, filters bypassed) sounds slightly more 'in-your-face'. I like the -12 setting (filters bypassed) when going for slightly slower transient response, to move things backwards a bit.

With all the different options available for tone shaping the VP28 quickly turns out to be a Swiss-army-knife kinda device.  8)

I never noticed this, but i'll check on it tmrw and report back, it seems strange that would happen though?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 17, 2013, 02:13:28 AM
Hmmmm.....I can't remember having noticed anything like that,but haven't done more than vocalese stuff with them.Filters always in at 40 or 80Hz,12dB/octave.
Will monitor this,thanks for the hint.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 17, 2013, 10:25:46 AM
I am not so sure about this from a design aspect. There is no circuit change with the -6/-12 switch if both filter push buttons are out. When the toggle is in the -12 position, the signal makes a additional trip onto and around the HPF PCB but only thru closed switch contacts, no components. I don't have a VP28 here to test but really can't imagine a change myself.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 17, 2013, 12:09:30 PM
From a logical point of view I don't see any sense or cause either.
Mine are at my work place atm.,will run some Moog sounds with attacks set to zero (the fastest envelope generators ever)  as well as key-clicks of an organ sound through it asap,this should clear it up.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 18, 2013, 11:08:49 AM
So I ran the before mentioned tests.On all my 8 VP28s.Listening chain was MicPre o/ps directly feeding a passive Monitorcontroller.Speakers are non-forgiving Meyersound HD-1s and in the nearfield Neumann KH120As.
Result:Swapping the slope with no Frequency Bottons engaged does not have any audible influence.
One might change transient response when changing the gain structure,but that is related to things like transformer saturation,therefore a different story.
There might be a measurable difference when looking at the switch resistances in the milliohms area,but I don't think it to be audible.

Hope to have helped and still wondering what's going on,

best regards,

Udo.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Terryb on June 24, 2013, 09:44:25 PM
(https://www.dropbox.com/s/fxryiujxpzzjkpk/2013-06-24%2018.31.52.jpg)


So, I'm soldering in c7 and c8 with my iron at 750f.
Then I notice what looks like a fuzzy white spider web.  Did I just ruin the capacitor?

http://moodyandbored.tumblr.com/post/53812512491/wtf
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Terryb on June 24, 2013, 10:09:51 PM
And while I'm at it;

C16 and c20
Are 470 microfarad 16 v

Mine don't have a solid band.  Just arrows with minus signs.
I tried looking it up to be sure, but no luck.

I assume the end the negative arrows are pointing to is the cathode?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 24, 2013, 10:14:36 PM
I run nearly at 800F and have never had a problem with overheating a cap (or other component). How long did you heat it for? Not sure what the "spiderweb" is that you are speaking of?

On the 470uF BC's, yes, the arrows point to the negative end. The positive end has a groove running around the can.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Terryb on June 24, 2013, 10:50:32 PM
Only heated it for a few seconds.

I'm using kester "44" and it could just be a flux buildup.
It kinda looks like a small spider egg sack.
I tried posting a photo, but I don't think my dropbox account is sharing nicely.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Marc Duchesne on June 29, 2013, 02:09:48 PM
Here are mine, those pre's are very nice...  Still have to built those 2 GIX 51X tube pre's to put in.

Thanks Jeff !
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Marc Duchesne on June 29, 2013, 02:10:32 PM
Power supply inside...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 20, 2013, 10:26:50 PM
hey all, first time posting.  Just finished building 2 and both have the green led light on constantly.  I noticed another user had the same issue.  Anybody know what I'm doing wrong?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: .:On The Rock:. on July 22, 2013, 11:43:52 AM
Hi Guys,
I'm having a blast building my two VP28s. I have one question though:

I was given two 137k ohm resistors and zero 13.7k ohm resistors. One of the 137k resistors is for R10 and the missing 13.7k is for R8. I'm just wondering if this is in fact a mistake because the same mistake was made in both preamps.

Thanks,
Scott.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 22, 2013, 11:46:40 AM
Hi Guys,
I'm having a blast building my two VP28s. I have one question though:

I was given two 137k ohm resistors and zero 13.7k ohm resistors. One of the 137k resistors is for R10 and the missing 13.7k is for R8. I'm just wondering if this is in fact a mistake because the same mistake was made in both preamps.

Thanks,
Scott.
Scott we had a few kits go out like this. Long story but shoot me an email and I will get them out in a regular envelope for you. Make sure you tell me how many kits you have like this.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: .:On The Rock:. on July 22, 2013, 12:10:32 PM
Scott we had a few kits go out like this. Long story but shoot me an email and I will get them out in a regular envelope for you. Make sure you tell me how many kits you have like this.

No worries, that's too much trouble. I'll just run out to an electronics store and pick some up. I mean, the resistors will be cheaper than the postage. I'll get back to you if I can't get the exact ones.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 22, 2013, 10:12:28 PM
ok me again with the green led problem.

I've checked the orientation of the LED itself along with the 4558 opamp and the surrounding 914 diodes.  All is good there.  I tried flipping the orientation of the 4558 for kicks and then nothing would work. 

Both of my units are doing this.  Can anyone think of anything else I can look at?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 22, 2013, 10:25:23 PM
ok me again with the green led problem.

I've checked the orientation of the LED itself along with the 4558 opamp and the surrounding 914 diodes.  All is good there.  I tried flipping the orientation of the 4558 for kicks and then nothing would work. 

Both of my units are doing this.  Can anyone think of anything else I can look at?
Hello and welcome to the forum.

From your posts I can not see if you have listened to them .
What is attached to the pres,any signal source or cables?
Maybe there already is something entering the input,maybe you have a hum there or so.
Does the led stay the same when you bring the gain fully down?

Best regards,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 23, 2013, 08:12:48 AM
Hey thanks for the reply!  The green led stays on with nothing plugged in and gain down.  It also stays on with a mic plugged in as well. 

I'm using one of the new lindell audio 500 series racks if that makes any difference.  Unfortunately I don't have any other 500 series units to test it with. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 23, 2013, 02:14:16 PM
....but did you listen to it?

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 25, 2013, 04:38:01 PM
Yes sorry, everything works and sounds fine on it.  No hum or noise.  Just green led won't go off.  Strange. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 25, 2013, 04:40:05 PM
Post some build pics of around the IC opamp area.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 25, 2013, 04:44:51 PM
ok will do when I get home tonight!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2013, 05:00:23 PM
Could it be killed when reversing it 180 degrees?

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 25, 2013, 05:14:34 PM
I thought the same thing.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2013, 05:47:34 PM
Brothers in mind....... ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 25, 2013, 06:49:43 PM
nope, flipping the phase doesn't do anything.  Will take some pictures after I feed the pregnant wife before she kills me!  haha
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 25, 2013, 07:17:34 PM
nope, flipping the phase doesn't do anything.....
No, we meant when you turned the 4558 opamp 180 degrees.....
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 25, 2013, 09:33:29 PM
Yeah I tried that and none of the LEDs work then.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 25, 2013, 09:44:41 PM
picture 1.  Is there a way to post more than 1 pic on a post?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 26, 2013, 12:50:32 AM
@cookiesuttle

Found a pdf for the 4558,found this:

"Notes 1.Reverse connection of supply voltage can cause destruction."

Supply voltage is on pins 4 & 8,by reversing the chip exactly this has happened.

I would give it a try and swap them out.It's a standard part,cost per chip bought as single part is 20 cent (Euro).
You have the Preamps in working condition and you said they sound good,so it's most likely that just this part is fried.A cheap way to check.....

Good luck,

Udo. ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 26, 2013, 07:30:21 AM
K ill give it a shot!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbarrett92 on July 26, 2013, 11:49:34 AM
Does anyone else think that the diode to the right of the 4558 looks destroyed or is that a bad look form the camera?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 26, 2013, 12:13:12 PM
This is not related to the sig prez circuit but R10 looks like 150k in the pic not 137k? It could be the pic though. I would try replacing the 4558's like Udo and I were referring to. The rest of the R's for the sig prez look correct.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on July 26, 2013, 03:19:53 PM
ok so no 4558's available locally. . argh.  Will order from capi
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: New Soul Rebel on July 29, 2013, 02:59:00 AM
 >:(

Built a VP28 a couple of months back. Hardly used it to be honest.

It works fine in operation (sweet as in fact), but for some reason, ever since I built the thing, I get a big thump when I have phantom power on, and switch the mic line on. To be clear, that's... phantom power ON before I engage the mic switch.

Huge peaking thump. But then, like I say, it works and operates fine. Its obviously something not quite right there. I've another pair of these. They work perfectly, and I took great care on this build, so am a bit annoyed i've got this problem. :-[

Has anyone else experienced this on a new vp28 build? Any particular thoughts? (Jeff?)

Matt
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 29, 2013, 09:21:58 AM
Well, when you have phantom on, there is usually a pop or thump when engaging the standard pad. After all, when switching DC there will be some popping. It has always been a standard practice to engage phantom and the like with a pre's output muted by some means or not connected, It really can also be said for the mic/line switch.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 29, 2013, 09:55:25 AM
+1 on this,had a similar discussion why not to switch patterns on a ldc- mic while monitors are open... ::)
Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cookiesuttle on August 02, 2013, 11:31:16 PM
So had the chance to try my 2 vp28's out on a session -- well a session setup.

2 different problems:

1) The preamp with the LED that stayed on the whole time:  I switched out the 4558 and that didn't do anything.  Other than that, works perfectly fine.

2) preamp 2: green led works fine on this one and overall seems to work fine, but by Shure KSM 32's will not pass signal.  Any other mic I have including other phantom powered mics work fine, but these mics will not work on this particular preamp.  The Shure's both work fine on my OTHER VP28, but not this one.  Do you think it's a phantom power issue on this one?  Maybe it's not quite enough voltage on it? 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: New Soul Rebel on August 03, 2013, 02:34:29 AM
Well, when you have phantom on, there is usually a pop or thump when engaging the standard pad. After all, when switching DC there will be some popping. It has always been a standard practice to engage phantom and the like with a pre's output muted by some means or not connected, It really can also be said for the mic/line switch.

Except that there must be something wrong... as I don't get this thump at all on my other VP28's.

Its an almighty thump Jeff... I know what you mean about a pop or thump when engaging mic lines or pads, but this is an unusually big thump! Any other ideas what might be causing this. Like I say, the pre works perfectly in operation. But I want to resolve this.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 03, 2013, 12:58:46 PM
2 different problems:

1) The preamp with the LED that stayed on the whole time:  I switched out the 4558 and that didn't do anything.  Other than that, works perfectly fine.
Hopefully the chip was inserted the right way this time.I don't have schematics for the preamp,I can only guess that surrounding parts might have been damaged too.Maybe Jeff can clear this up?


2) preamp 2: green led works fine on this one and overall seems to work fine, but by Shure KSM 32's will not pass signal.  Any other mic I have including other phantom powered mics work fine, but these mics will not work on this particular preamp.  The Shure's both work fine on my OTHER VP28, but not this one.  Do you think it's a phantom power issue on this one?  Maybe it's not quite enough voltage on it?
It shouldn't be a too lowish voltage,the Shure KMS32 starts working at 11vdc,positive voltage on pins 2 and 3.Read this from their manual.At 48vdc it draws a current of 4,68mAs,so totaly standard stuff.
So yes,it seems there's something wrong.Check at least every part,maybe you have swapped resistors or so.Since you have a working identical mic preamp I would lay them side by side and use a magnifying glass and good light.Compare the resistor colours then.

But what confuses me most is that other phantom powered microphones do work on this preamp.....strange......


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Terryb on August 06, 2013, 08:43:38 PM
I lost one of the grayhill stop-pins.   Not sure how, but I did.
Can i just use a trimmed lead to replace it?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 06, 2013, 08:52:58 PM
Yes, exactly.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: George Adamski on August 08, 2013, 12:04:08 PM
Hey all,

I have two VP28s that I built at the same time, using GAR2520 opamps. They both work and sound great, but with one constantly being roughly 10dB quieter than the other.

here's what I've tried so far :
- swapping opamps between both pres (all four of them)
- checking boards on both sides for build error, they are fine.
- checking transformer leads.
- trying the modules in different 500 series racks with different input signals from different converters and mics
- checking that the gain goes up and down the amount its supposed to for every step of the gain knob and output fader.

no luck with any of that. If any of you guys has any idea of what I could try next, it would be truly appreciated.

thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dgbt on August 10, 2013, 01:59:31 AM
I didn't see anything about this in the thread, but are the VP28 resistances supposed to read as mentioned in the VP2X assembly guide between the op-amp pins (V+,V-, C), over 200k?  I'm getting around 106k, and its rising to this measurement...SLOOWLY.  I just wanted to make sure everything is fine before I plug in the opamps.

I should note the card is not connected to a power supply, nor are the opamps installed.

Hopefully someone has an answer to this.

Thanks
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 10, 2013, 03:10:56 AM
I didn't see anything about this in the thread, but are the VP28 resistances supposed to read as mentioned in the VP2X assembly guide between the op-amp pins (V+,V-, C), over 200k?  I'm getting around 106k, and its rising to this measurement...SLOOWLY.  I just wanted to make sure everything is fine before I plug in the opamps.

I should note the card is not connected to a power supply, nor are the opamps installed.

Hopefully someone has an answer to this.

Thanks
Hello,welcome to the forum,

first thing is that you measure in circuit.Can't compare to mine because I'm on holidays but it doesn't look bad so far.
Rising resistance is normal in circuit because caps are charging.Remember that your dmm supplies a voltage when doing measurements!
More important is to look at the voltages at the milmax sockets for the DOAs.If you get +/-16vdc (a bit less it will be) then you're good to go,plug those little beasts in.

Tell us what you get and have fun,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on August 20, 2013, 05:24:13 AM
My first VP28 build was going great... until I made a silly mistake. I soldered the 6db/12db filter switch on the wrong side of the board... In the process if trying to get the sucker of, I broke the switch and stripped a couple of PCB contacts / track. I'll attach the embarrassing photo.

So.... Firstly.  Can anyone tell where I need to connect the two pins shown in the photo - to which component, bypassing the PCB tracks?

And then.. I guess I'll need to get another switch off Jeff so I can complete the build on the 2nd VP28 as I've grabbed the switch from that kit to replace this one.

I'm kicking myself... But I'm sure I can recover from this hopefully without ordering a new PCB etc... :(

Thanks for your help in advance.


//////// EDIT //////////

OK - I seem to have traced where the poles go from the switch. So I'll mod this one a little(!) and ask Jeff for another switch for the other VP28.... Hopefully I've not buggered up any of the tracks around the switch and I'll post a lovely pic of the successful build in a day or so :-)

///////
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on August 21, 2013, 04:14:05 AM
OK completed my first VP28 but I cannot get the knobs to go onto the greyhill switches.  I've unscrewed both the set screws so that the knob goes over the shaft. But when I tighten them, it does not completely tighten up, there's a lot of play in the switch and you can just pull the switch off. I saw in Chunger's post that there was a collar adaptor and there doesn't look like there's room to put one on any way. Anyone else had the same issue / could offer some advice please?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 21, 2013, 09:57:55 AM
Craig, the shaft adapter is no longer needed since the knob holes are 1/8" and so are the Grayhill shafts. They must be aligned properly but they will tighten correctly.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jakmusic on August 23, 2013, 11:35:57 AM
Hey Guys,

Newbster McGee here, just finished testing the sockets on my VP28, the tests resulted as follows

Seated in my 500 rack, Powered.  The voltages read:
A1
O/V-  15.5V
O/V+ 15.6V
C/V-  15.4
C/V+ 15.7

A3
O/V-  15.9V
O/V+ 15.2V
C/V-  15.4V
C/V+ 15.7V

How close to 16V are the sockets supposed to read when powered?

thanks
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 23, 2013, 11:57:04 AM
This is pretty normal since there is a .7V drop over the cards protection diodes.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: George Adamski on August 25, 2013, 09:06:52 AM
Hey all,

I have two VP28s that I built at the same time, using GAR2520 opamps. They both work and sound great, but with one constantly being roughly 10dB quieter than the other.

here's what I've tried so far :
- swapping opamps between both pres (all four of them)
- checking boards on both sides for build error, they are fine.
- checking transformer leads.
- trying the modules in different 500 series racks with different input signals from different converters and mics
- checking that the gain goes up and down the amount its supposed to for every step of the gain knob and output fader.

no luck with any of that. If any of you guys has any idea of what I could try next, it would be truly appreciated.

thanks!

anyone?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 25, 2013, 10:29:31 AM
You are gonna have to inject a test signal and measure the level at a few different test points thru the circuit. This will narrow down where the issue lies. You will need to be able to power the module flat on a bench or table in front of you. Make sure all controls are set the same for each module and compare your results. If you use mic mode, inject a -43dBu signal. If you use line mode 0dBu or +4 is fine. You can measure at the test points with a DMM set to ACV if you don't have a scope. I would use 400Hz as the test signal if using a DMM otherwise 1kHz should be good.

Attached is a pdf with specific points to measure at. Unfortunately, I do not have a built VP28 here to tell you what a properly built unit will measure at these points. But in your case, comparing should should get us in the right area to look at.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on August 26, 2013, 01:31:43 AM
Hi Jeff - thanks.

I found there was a bur inside one of the holes that wasn't helping. Cleared that out and all is good in the world again! Onto the op amps now.. wish me a steady hand and en eagle eye..

Craig, the shaft adapter is no longer needed since the knob holes are 1/8" and so are the Grayhill shafts. They must be aligned properly but they will tighten correctly.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jakmusic on August 26, 2013, 05:24:48 PM
Hey Jeff,

I just tested my API 500VPR rack and found out that the +16V Rail is delivering +16.4V and the -16V Rail delivers -16.1V.  When I questioned one of the service techs at API about it he said don't worry about it.

The only thing is that on one of my preamps I'm having trouble getting the DOA sockets to read the correct voltages.  They are:

A3
O/V+ 14.8V
O/V- 16.5V

A1
O/V+ 15V
O/V- 16.25

The C/V+ C/V- voltages all seem fine, within 15.5-15.8V 

I've gone over the main PCB and HPF PCB numerous times, trying to make sure there are no solder blobs, and checking it over for cold joints.  I've double checked all my resistors to make sure the right values are in the correct places. 

Any ideas or tips of where to start?

thanks much,

James

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 26, 2013, 06:02:48 PM
I would probe at the 1N4004 diodes and see what you have. On the rack side of them, you should have readings that match measuring inside the rack's card edge connectors. On the module side of the diodes, you should have appx .7V less (closer to 0V) than on the rack side. You can check around Q1, C2 and R1-R3. Also make sure A2 is in properly. You can compare readings between the 2 preamps.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jakmusic on August 26, 2013, 08:48:53 PM
I measured and compared between one of my preamps that has the correct voltage reading on the sockets and the one in question. 

I put one probe on the hole that connects to the ground pin and the other on each connection for the following components
CR1
CR2
R1-3
Q1
C2
Everything matched within between the two preamps to .02V 
A2 was a little loose so I pushed it further in, but that didn't change anything.

Anything else I should do or check?

Thanks Jeff
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 26, 2013, 09:03:52 PM
So the 2 pre's match now? I thought they did not match. If they are the same I would say to proceed and install the opamps.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jakmusic on August 26, 2013, 09:22:48 PM
I must be breathing too much solder.  I forgot to say they match except the one preamp in question still has incorrect voltages at the A1 A3 sockets.  The voltages at those sockets hasn't changed since my first post.

A3
O/V+ 14.8V
O/V- 16.5V

A1
O/V+ 15V
O/V- 16.25
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 26, 2013, 09:28:23 PM
Remove A2 (4558 chip) from that pre and measure again.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jakmusic on August 26, 2013, 11:10:25 PM
Jeff I'm going to ask another question about measuring with my dmm before I reply about removing the 4558 chip. 

Here's something I just did, and I don't understand why it happens.
My DMM reads
O/V- 16.5
O/V+ 14.8
If I hold the probes on the O/V- it will read 16.5V then drop as long as I hold the probes there. I held the probes there until my DMM  read around 14v Then I switched the probes to O/V+ and the voltage was around 17V  (but it was around 14V when I first measured it minutes ago).  By holding the probes on O/V-, am I changing the amount of electricity stored up in a capacitor and that's causing the O/V+ voltage reading to change? 

 :o
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Terryb on August 29, 2013, 02:42:51 PM
Jeff, can you recommend a bench psu for testing preamp, opamps, etc?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 29, 2013, 02:48:24 PM
Terry, I have a JLM 5 rail Powerstation that I use.

I also have a new 51x extender card kit that I need to post to the store. You can use it with any 500 or 51x series rack. I have everything here for it, just need to get pics and get it set up. I already sent one to 'Chung so he is gonna get a few build pics even though it is a simple as can be. I posted a pic on FB a few weeks ago.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 29, 2013, 03:23:31 PM
Hi Jeff,

yes,a powerstation I had left over was used on my little testbox too.
This way one has the "main" 5 voltages handy all the time.
Ideal for testing and adjustment,perfect when doing lots of modules,hahaha.......
For my racks I have yours and Volker's "real" psu stuff (Floorbox and 2 dual psus).
About the new extender card you're talking about,is there a link you can give us non-FB- members?

Thanks in advance,

Udo. ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 29, 2013, 03:51:41 PM
Here it is Udo.

(https://sphotos-b-ord.xx.fbcdn.net/hphotos-prn2/971915_699031866789865_311009338_n.jpg)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 29, 2013, 04:04:27 PM
Great stuff Jeff!
The first "real" extender card,very nice indeed!
When I did my first extender cables I used the protoboards,but they were hard to plug in when say just 1 module slot was open.

The switch and resistors,are they for cmrr (hard to read this printed instructions on the board)?

Cheers,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 29, 2013, 04:16:33 PM
When I did my first extender cables I used the protoboards,but they were hard to plug in when say just 1 module slot was open.
Yes, this one is easy with only 1 slot open. Plus it's simple for 15 or 18 pin card edge connectors.

Quote
The switch and resistors,are they for cmrr (hard to read this printed instructions on the board)?
Yes, exactly. Simple instructions for CMRR calibration are on the board. All parts will be included with the kit. There is a status LED for the CMRR switch that can't been seen in this pic.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 29, 2013, 04:21:15 PM
Wow,cool!

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on September 06, 2013, 08:30:58 PM
Hi Guys - just built my 2nd VP28 and I have a strange issue. I wonder if anyone else has come across anything similar so I know where to troubleshoot.

Only the Phase LED lights up when buttons are pressed. And - with the phase button pressed and the LED lit, it you press any of the other buttons - 48v / Pad etc - the Phase LED goes out.

Anyone had a similar experience / thoughts on the likely culprit?

Thanks in advance. Craig
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 06, 2013, 08:35:08 PM
All of those status LED's are in a string so if some are backwards, this will happen. Only the working one is the right way around.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on September 06, 2013, 08:58:00 PM

All of those status LED's are in a string so if some are backwards, this will happen. Only the working one is the right way around.

Hi Jeff - thanks for that. I've just plugged in a Dynamic mic and something is not happy. I've attached a link to an MP3 - First part is the VP28 that's fine. 2nd part is the one with the non-lighting LEDs.

https://soundcloud.com/orange-orb/vp28-issue

I'll take a look at the LEDs now but I'm pretty sure they're in the correct way.

Thanks for your support.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 06, 2013, 09:01:26 PM
Well, the direction of the LED's will have nothing to do with the audio path so there must be something else going on.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on September 06, 2013, 09:09:29 PM
Well, the direction of the LED's will have nothing to do with the audio path so there must be something else going on.

Yeah - checked the LEDs and they're all in the correct way. Any likely areas I should be checking on the board that could cause this type of issue Jeff? - I'll take the metal work off and take a quick look for solder bridges etc, but this was a cleaner build that the first one that works ironically!  :)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 06, 2013, 09:25:25 PM
Well, the LED thing does not make sense to me. The string starts at the lowest LED which is phantom and ends with the polarity LED. When a switch is out or disengaged, it is merely shorting out that particular LED. When the respective switch is pressed, the LED is introduced into the string. So, with the last LED (polarity) illuminated, if pressing any of the lower switches causes the polarity LED to go out, they are in backwards. I can't think of any other reason that this could happen.

As for the audio, start with the usuals. Rack slot, cabling, converter settings and then to swapping the opamps and general trouble shooting that is above in this thread.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on September 06, 2013, 09:58:02 PM
Well, the LED thing does not make sense to me. The string starts at the lowest LED which is phantom and ends with the polarity LED. When a switch is out or disengaged, it is merely shorting out that particular LED. When the respective switch is pressed, the LED is introduced into the string. So, with the last LED (polarity) illuminated, if pressing any of the lower switches causes the polarity LED to go out, they are in backwards. I can't think of any other reason that this could happen.

As for the audio, start with the usuals. Rack slot, cabling, converter settings and then to swapping the opamps and general trouble shooting that is above in this thread.

Hi Jeff - found the issue I think.

Firstly there was some solder from the LEDs that had bridged a couple of the pushbuttons. Fixing that solved the LED issue.

And I swapped the op-amps from the working VP28 and it looks like a 1731 was causing the distortion. So just need to have a look at that 1731 and see what the issue could be.

Thanks again for your help. Nearly there!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 06, 2013, 10:00:40 PM
Cool. Glad you are making some progress!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on September 06, 2013, 10:19:36 PM
Cool. Glad you are making some progress!
Yes! progress but then another glitch. Built the replacement 1731 and now the 2nd VP28 works perfectly. However when swapping the opamps out of the first VP28 (Which was working!) to try them in the 2nd something must of happened. As the 1st VP28 lights up but there is no signal passing through it whatsoever. Don't even get the quick green flash of the signal LED with switching mic in and out while the phantom is engaged or when the lunchbox powers up.

I thought there may be a cold joint on the opamp pins on the PCB so re-did though but nothing.... bummer. I've got a Sound Skulpture Kit on its way and I ordered an extender cable as I guess this will be the only sensible way I can check for cold joints in the signal path.

Anyone know the signal path through the board that I could check to see if I have dodgy joint? - I can't see any damaged components.

Oh well - Enjoyment and frustration at the same time. I tracked some vocals today with the working unit and it was amazing. Can't wait to get this one up and running (again) now.

Cheers
Craig
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: New Soul Rebel on September 15, 2013, 04:56:55 AM
Well, when you have phantom on, there is usually a pop or thump when engaging the standard pad. After all, when switching DC there will be some popping. It has always been a standard practice to engage phantom and the like with a pre's output muted by some means or not connected, It really can also be said for the mic/line switch.

Except that there must be something wrong... as I don't get this thump at all on my other VP28's.

Its an almighty thump Jeff... I know what you mean about a pop or thump when engaging mic lines or pads, but this is an unusually big thump! Any other ideas what might be causing this. Like I say, the pre works perfectly in operation. But I want to resolve this.

Hi Guys

 :-[

I STILL haven't managed to resolve this thump that happens when engaging the mic button.

the vp28 works perfectly once switched... but it is an almighty thump. not a bump that you get like when switching on phantom by mistake. this is a thump.

the peak meter on the motu HITS the roof when I switch the damn thing on.

I've got two good opamps in the unit. so its definitely not an opamp problem.

Matt
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: iacntspell on September 15, 2013, 10:18:58 PM
Hey guys, quick question.  I built two VP28s (well, I built one and a friend built the other).  Everything appears to be in working order, but when I was testing them out, I decided to really try to crank them and was getting some weird results.  I was using a pair of condensers, so phantom was on and mic level engaged.  I wasn't using the pad, and I had the input gain cranked to almost max and the output gain pulled way back.  At low signal levels, things were alright, but as soon as I got loud or there was a transient (clap or tapping the grill of the mic) I got a really nasty chirping sound.  Zoomed in on the waveform and I've got a very nasty looking square wave going on.  Is this just the preamp getting overdriven because I've got it cranked so hard, or is something wrong with the build?  This is happening on both preamps which were built by different builders, so I'm hoping its not a failure in the builds.  At more standard operating levels, particularly with the pad engaged, things seem to be fine (but more testing is in order).  I wasn't clipping at the converter stage, to my knowledge. I suppose there could've been some inter-sample peaking going on, but I was being pretty careful to not overdrive the converters.  Thanks, guys.  Fingers crossed!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on September 16, 2013, 04:21:09 PM
Hello and welcome to the forum.
Looks about right to me.When cranking the input gain with a hot signal from condenser mics you'll definetely overdrive the input stage.When a signal overdrives it starts to flatten out at its' peaks (these look like square waves then),after that (god beware) it even turns to dc.
It is unlikely that both preamps have been built wrong,it's more a matter of level matching.
E.g. an U87Ai will be good for speech and moderate singing at a gain of say 35 dB.
The input gain of these guys goes to +60dB,that's a lot and too much for a such a signal.
Do a short test and listen to it while increasing the gain,the distortion amount should follow your gain meaning starting clean.then start to gently overdrive up to heavy distortion..
Bringing down the output level won't help because-as said- the distortion has already taken place at in the input stage(s).

Best regards,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: iacntspell on September 16, 2013, 09:27:21 PM
Hello and welcome to the forum.
Looks about right to me.When cranking the input gain with a hot signal from condenser mics you'll definetely overdrive the input stage.When a signal overdrives it starts to flatten out at its' peaks (these look like square waves then),after that (god beware) it even turns to dc.
It is unlikely that both preamps have been built wrong,it's more a matter of level matching.
E.g. an U87Ai will be good for speech and moderate singing at a gain of say 35 dB.
The input gain of these guys goes to +60dB,that's a lot and too much for a such a signal.
Do a short test and listen to it while increasing the gain,the distortion amount should follow your gain meaning starting clean.then start to gently overdrive up to heavy distortion..
Bringing down the output level won't help because-as said- the distortion has already taken place at in the input stage(s).

Best regards,

Udo.

Thanks, Udo!  Unfortunately I didn't get any time to do more testing tonight because I ran into a snag with my interface.  While I'm not intimately experienced with API-style preamps, I feel like I understand the concept of analog saturation/clipping well enough to feel worried.  The issue I'm having isn't that I've got a single, sinusoidal wave-form that's been shaved/flattened/clipped at the top; what's happening is that I'll have a loud transient (say, a clap), and about 50 ms or so (very rough guess) AFTER the initial transient, the waveform looks like it's going into a feedback loop before it spikes up in a very unusual/digital-looking nasty triangle-wave-meets-square-wave-meets-random-wave spike.  The sound that accompanies it is a very strange tweeting/chirping noise... almost like a high-pitch blip or beep.  Would something weird like this be expected from a properly built unit?  I had in mind that things would start to distort gradually and just become a more granulated version of the original signal, but this is something altogether different.

As soon as I get the time, I'll try some more testing.  All I can do is hope that the builds were successful and that if they weren't I can get these babies working properly ASAP!

Thanks again for your help.  I'll report back.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on September 17, 2013, 01:40:23 AM
O.k., a very simple thought comes up......are you sure it is really coming from the preamps?
I mean could it be that your microphone has an issue?

Just my 2ct,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Craig32768 on September 19, 2013, 07:24:01 PM
Looks like the issue on the one VP28 that wasn't working was some solder that had flowed on the underside of a capacitor and bridged it. Fixed and both units are working great now.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: iacntspell on September 19, 2013, 08:50:27 PM
Just checking back in.  It appears that I had some really weird stuff going on in my session and the only thing that I can figure was the session was corrupt.  I did some line-level tests and the analog saturation appeared to be behaving exactly as expected.  I also took the pair in to a local pro studio I've got an affiliation with and did a comparison with an API 512c for some simple voiceovers, and let's just say I was very pleased with the results (as were the owner and engineer at the studio).  I'll check things out with the original mics I was using in a new session, but for now I think I'm good to go.  Thanks for giving it your consideration, sorry if it was just a red herring. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on September 20, 2013, 01:31:51 AM
No problem.glad it seems to work o.k.!

Have fun,

Udo. ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Dylan W on September 26, 2013, 04:11:07 PM
I need to get the front panel off my VP28. Does anyone have a trick for removing the 1/8" to 1/4" knob adaptor? Seems to be pretty well on there and I don't want to damage the Grayhill.

EDIT Got it off by using pliers to carefully twist it against the end of the Grayhill's rotation. Made me hesitant but the switch seems to be fine.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 26, 2013, 07:15:36 PM
An IC puller works a treat for this.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: maq3396 on October 02, 2013, 05:57:22 PM
Thanks for a great build Jeff-quiet and clean and a fun construct!
Can this preamp be dropped into a 51-X box or does it need to be modified in some way?
If so, what modifications would be required?

Thanks
Mac
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: qmp audio on October 02, 2013, 05:58:40 PM
Thanks for a great build Jeff-quiet and clean and a fun construct!
Can this preamp be dropped into a 51-X box or does it need to be modified in some way?
If so, what modifications would be required?

Thanks
Mac

No modifications needed.
edit:  will work fine in a 51x box.   it will run as standard VPR spec within the 51x box.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: maq3396 on October 06, 2013, 09:37:05 PM
Thanks Gunpoint!

Cheers
Mac
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: mulletchuck on October 16, 2013, 03:38:09 PM
Built up 3 of 'em in a day :-)  2 for me, one for a buddy.   Only headache was that one unit was sounding all glitchy, so I took it apart and reflowed all the solder joints.  I put it back together and tested again only to find out that it was my interface that was spitting out glitchy audio, not the preamp.   Restarted the comp, tested again, boom, done!   easy breezy!   

Also, jeff's card extender test rig made it very easy to test 'em out without having to load them into the racks :-D   
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mkr on October 30, 2013, 04:21:50 PM
Hey chunger

Thanks for the great build threads. May I just check please do you tin the legs of the resistors and caps and diodes etc?
 
Or no need now a days ?

Thanks
Mkr
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: ipagel on October 31, 2013, 08:09:10 PM
Just Finished two here. Worked out the block. Great guide by Chunger.
Sounds great and a wonderful project.  :)
The only thing that could improve it for me being in the UK is if I could have paid a little extra for the small Allen key as they are a bugger to get here. The only place I could find it was in one of the trade delivery shops which have a twenty pound delivery minimum.  :-\
I ground down a precision screwdriver in the end as I got impatient.
Thanks to Jeff for this project and I will be definitely trying out some of the other CAPI stuff when they come in.

Mark C

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on October 31, 2013, 11:45:22 PM
+1 for the hex key (had the same trouble here) & congrats for your builts.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on November 06, 2013, 04:41:12 PM
Hello everybody ! I'm french and I'm the happy owner of 10 vp 28! I already built 4 of them, but my first one is still not working properly.
I have a very low level, without the low frequencies, as If the hpf was engaged. I can use the mic and line modes, i can use the pad on the signal, and use the hpf, even If the original sound Is already filtered. All this with a very low level. The switches are working properly too.
I have to tell that i tried this first preamp with my first gar2520, that was not good, because i switched the bd139 and 140. So, the oamp heat a lot, and i wonder If there was no damage because of this try.
I checked the voltage on the oamps pins, and compared the r values with the other vp28 i built. All the values seems to be the same between the bad one and the good ones.
If someone can help me, it would be great!
I have to say that this amp is impressive, and I'm very happy that we can built a such beautiful preamp!
Thanks a lot.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on November 06, 2013, 04:54:34 PM
I'd like to add that the green led is not flashing (because of the low level I think). I tried with several oamps, discrète and ic.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on November 06, 2013, 05:40:52 PM
Last thing: If it can helps, on the link below, a PDF file i did, that help me to check and classify the resistors for the vp28.
https://db.tt/9z8T69oi
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 06, 2013, 07:28:08 PM
I have a very low level, without the low frequencies, as If the hpf was engaged. I can use the mic and line modes, i can use the pad on the signal, and use the hpf, even If the original sound Is already filtered. All this with a very low level. The switches are working properly too.
Hello and welcome to the forum.
A thin or lacking low frequency signal at a low level indicates a balancing issue most of times.
I'd check transformer wirings and functionality first,seems like a "hot" or "cold" signal part is missing.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sandreatta on November 06, 2013, 07:33:48 PM
Jeff and guys,
I finished my first 2 28's i couldn't be happier, it's an amazing kit with an amazing sound.
Just a question, I am measuring with a 1khz tone thru both units and I find a return signal with a difference of .3 db, this is really tiny but it's possible to improve it? .1 or .2 or something like that.

Cheers from Argentina!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 06, 2013, 10:17:39 PM
Do you mean the 2 pre's are off from each other by .3dB or the output level differs from the input by .3dB?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sandreatta on November 06, 2013, 11:03:20 PM
I mean .3 db from each other, but i did the test with the HPF engaged in one module...such an idiot. :double facepalm: Now the difference is just .1 db. I builded them slow to avoid mistakes...I was tired so...
Everything is ok!  8) Thanks for the reply Jeff! More will be coming :)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on November 07, 2013, 04:21:47 AM
Hello and welcome to the forum.
A thin or lacking low frequency signal at a low level indicates a balancing issue most of times.
I'd check transformer wirings and functionality first,seems like a "hot" or "cold" signal part is missing.

Best,

Udo.

Thanks Udo, I'll check tomorrow night and back to you.

Luc
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on November 07, 2013, 08:29:26 AM
Hello and welcome to the forum.
A thin or lacking low frequency signal at a low level indicates a balancing issue most of times.
I'd check transformer wirings and functionality first,seems like a "hot" or "cold" signal part is missing.

Best,

Udo.

Quick check during lunch break! Good idea Udo, I have an open circuit between the Black and the Brown wire of the T3 Transfo = dead transformer I think.
I tried to swap with another Transfo that I have, but still the same issue.
I'll follow this idea as soon as I have time.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on November 08, 2013, 01:36:48 PM
Hello and welcome to the forum.
A thin or lacking low frequency signal at a low level indicates a balancing issue most of times.
I'd check transformer wirings and functionality first,seems like a "hot" or "cold" signal part is missing.

Best,

Udo.

Udo,

When I changed the TR yesterday, I only plugged the VP28 in my lunchbox, and saw that the green led didn't flashed.
Or, in addition of the TR issue, the A2 was dead.

Thanks for your help, the VP28 is working well now.

Thanks for helping me!

Luc
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 08, 2013, 06:47:27 PM
Hello Luc,

glad you got it sorted!
For the A2,I remember at least one person here had it inserted the wrong way around.
Doing this kills that ic,it doesn't "like" reversed supply voltage.

Have fun with your VP28,I really love mine (I have eight),

best regards and have a nice weekend,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: aDrummer68 on November 12, 2013, 03:22:36 AM
Hey! I just need some advice:

I have the 51x rack (extra 24V), and now I am not sure which version of the VP's to get.
For the time being, I won't be making use of the 24V capabilities of the 51x preamps version.

Is it possible to get the 51x version VP's, but just putting in gar2520's and plugging them into the 18pin sockets?
From the pictures here, I've seen that there is a spot on the PCB labelled 16V and 24V and you solder (jump?) in a certain way (see attachment - bottom right corner)
What would I have to do if don't need the 24V, but have the 18pin PCB and the 51x rack?

I like to give myself options, even though i might not use all of them at the moment, so if it is possible to use the 51x version preamps, with 16V DOA in a 51x rack out of the box, I'll get them, but if i have to 'mod' outside of Jeff's recommended boundaries, I'll just get the VPR version then, bc when it comes to electronics, I 've only soldered cables so far :P

Thanks for the help!

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 12, 2013, 09:42:27 AM
Yes it is possible. That is what the switch is for on the VP312 PCB. Just set it to 16V and you will be fine with a 2520 style opamp. If you ever decide to get a 24V capable opamp, install it and flip the switch to 24V. Pretty straight forward.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: wadesey on November 12, 2013, 01:05:09 PM
I recently ordered+built two kits and caught that the grayhill switches are now fixed 12 o'clock (nice!), but noticed there were no shaft adapters included for the knobs. I figured maybe they weren't necessary anymore either, but the knobs are definitely falling off...Is a shaft adapter still needed, or am I just not installing them well?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 12, 2013, 01:29:40 PM
The shaft adapters are not needed as the knobs have an 1/8" hole for the GH's 1/8" shaft. Sounds like they need to be tightened more stoutly.  ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: PVJesper on November 12, 2013, 07:21:28 PM
I am just finished on my first Vp28 (ordered two).

However, on the finished one I just blew up the IC. Plugged it in and it blew instantly when i turned the electricity on.
This is what it looks like now, the top blew off.

http://cdn.imghack.se/images/ddbebde498ce582d4eb74fb7527d1145.jpg

Is there an obvious mistake i have made? If not, do you have any pointers for where to start troubleshooting?
And where can i get a new one?

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 12, 2013, 07:44:04 PM
I would guess it was installed backwards? Kinda hard to tell now. I have them in stock at the store. Just search for 4558 and you will find them.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: PVJesper on November 12, 2013, 08:18:53 PM
It didn't even occur to me that it had a direction. I was already finished since yesterday, tried and remembered, "uh yeah, i got to add the IC thingey". Thats what you get for rushing. I do have the blown off top piece left, so i'll see which way it fits and check. Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: wadesey on November 13, 2013, 06:38:04 PM
The shaft adapters are not needed as the knobs have an 1/8" hole for the GH's 1/8" shaft. Sounds like they need to be tightened more stoutly.  ;)

That's what I figured/hoped...I'll tighten them up good this time! Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on November 17, 2013, 11:59:34 AM
Built up a pair yesterday. Great design and really fun to build!
I have a problem with one of them. Distorting severely. From the waveform in the DAW, it looks like it is clipping on the negative swing of the waveform. Sounds fine with really low levels.
So I'm assuming there is a bias or supply voltage problem on one of the opamps.
Here's what I've done:
- All the features  (leds) work fine. No difference between good and bad units under careful visual inspection
- Swapped opamps between the good and bad unit.  Problem does not follow a opamp
- Checked V+/V- for DOAs, look good
- Checked voltages on Q1, same between good and bad unit.
- Removed IC - still have problem.

Any ideas?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: rjuly on November 17, 2013, 05:01:07 PM
I have managed to get one of my pair of vp28s built and working. Very pleasing build. However, I am having some difficulties with one of the gar2050 kits - as soon as I power on, the -16 volt power rail fuse blows. Can anyone direct me to the gar2050 support thread?

Regards,
rjuly
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on November 17, 2013, 06:45:56 PM
You'd have found it if you searched for 2520 instead of 2050 :)    http://groupdiy.com/index.php?topic=33970.0
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: rjuly on November 18, 2013, 03:18:06 AM
You'd have found it if you searched for 2520 instead of 2050 :)    http://groupdiy.com/index.php?topic=33970.0

Doh! Much obliged :-)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: ferrochrome on November 18, 2013, 08:38:01 AM
I have a question regarding connecting the input on the vp28. While waiting for my vp28 kits to arrive I'm deciding how to patch them up in my setup later.

I would like to easily be able to patch the vp28's in my setup for use as:

- inserts
- mic amps for phantom powered mics
- mic amps for dynamic microphones
- line amps for balanced signal
- line amps for unbalanced signal

I'm new to using 500/51x hardware, as far as I can see the modules only have one input circuit each. If this is the case (please correct me if I'm wrong), I would like to connect each vp28's XLR input from a TRS patchbay. Plugging in one of the above (TRS for mic/balanced line or mono-jack for unbalanced signal from synths etc). What happens if 48V phantom power accidentally is engaged with line or unbalanced line plugged in? Can the vp28 or input hardware possibly recieve damage from this?

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on November 19, 2013, 09:58:07 AM
I traced the signal back to before the input transformer, seeing it clipped on the scope. If I engaged the line pad, the signal was fine on the outside of the 4.99k resistors, but distorted on the other side. I reflowed all the solder joints in the area and checked for any bridged connections.
The transformer measures similar DC as the other one.
At this point I thought I was really getting close to finding the problem, and my scope died.
This is a really strange problem.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: ipagel on November 19, 2013, 06:35:53 PM
Just an update.
Have been testing for a couple of weeks and must say how wonderful these things really are.
They sound great for all duties.
I was using them to record a female singer using a lewitt lct 640 and everyone just stopped and was amazed how great it sounded. Only issue I have is the the led for polarity on one unit is not working, but other than that they are working perfectly. May have a look at it later.
Thanks again to Jeff for making this project possible.

M C
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 19, 2013, 06:47:39 PM
Only issue I have is the the led for polarity on one unit is not working
That LED is most likely in backwards or there may be a solder bridge around the switch pins. You can look closely at the metal innards of that LED from the cutout area of the L-bracket. See if it looks exactly like the other LED's or not.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on November 27, 2013, 10:33:45 AM
Well I tracked down the problem with mine. It turned out to be a problem at the phase switch. I had to pull components out to find it. With that switch out and the connection jumpered, the pre is working great now. Destroyed the switch unfortunately.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: bartacusad on December 26, 2013, 11:18:24 PM
Hey all, just finished building my new VP28 and it sounds amazing! Just have one issue though, only the green Signal LED is working. I can't get the others to work. I've flipped them around just on the off chance I got the polarity wrong and still nothing. What have I done wrong here?! I did break the red 48V LED but installed the rest of them while I'm waiting on the replacement to come in. It's not necessary to have all four LEDs in (1 red, 3 yellow) for them to work is it??
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 26, 2013, 11:25:38 PM
.....It's not necessary to have all four LEDs in (1 red, 3 yellow) for them to work is it??
Yes it is. They are all in a series string.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: bartacusad on December 27, 2013, 11:37:07 AM
Thanks so much Jeff! You rock my friend! Can't tell ya how much it means to have the designer on here answering questions. It definitely goes on a long way in me having the confidence to encourage others to build these amazing preamps!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: erol on January 03, 2014, 09:52:52 AM
Hi,

I am planning to build a VP28 with a gar2520 Full Kit Bundle.
This will be my first DIY project (with a lot of help from a good friend).

I've noticed there's a VP2x step-by-step guide on de CAPI website:
http://www.classicapi.com/catalog/images/gallery/VP26/VP2x-Assembly-Guide.pdf

But it seems this is all about the VP25 and VP26 am I right?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 03, 2014, 10:04:31 AM
Go back to the beginning of this thread. All of the support docs are there along with 'Chunger's build pics.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: choolder on January 04, 2014, 09:25:35 AM
hi guys,

I finished 2 VP28 units a few weeks ago? Both working perfectly, only minor issue was the pad led that broke (I still have to replace that).
I didn't use the vp28's for a week and today I wanted to use them and one doesn't work at all. No signal coming in.

Where do I have to start to debug the unit?


thanks!



Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: choolder on January 04, 2014, 09:37:15 AM
hi guys,

I finished 2 VP28 units a few weeks ago? Both working perfectly, only minor issue was the pad led that broke (I still have to replace that).
I didn't use the vp28's for a week and today I wanted to use them and one doesn't work at all. No signal coming in.

Where do I have to start to debug the unit?


thanks!


I switched the opamps and it works again, bad contact I think!

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 04, 2014, 10:07:47 AM
Are the opamps fully seated? http://classicapi.com/catalog/DOA_Install.php (http://classicapi.com/catalog/DOA_Install.php)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: verydeepwater on January 12, 2014, 12:16:53 PM
Jeff, Chunger, anybody, I just completed my first VP28, a lot of fun, however it doesn't pass any signal. The LED's are lit, a minor consolence........ I can't seem to find the trouble shooting page on Jeff's site, any suggestions? Thanks much, Rich
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 12, 2014, 01:10:06 PM
All of the help is in this thread here. I would suggest scanning thru it from the beginning.

The first thing to rule out are the opamps. Who built them? If they are prebuilt they should be fine. If they are kits, they need to be checked in another module. In a VP28 there 3 main potential issues. The 2 opamps and the preamp itself.

Also, make sure the opamps are seated properly. This page is linked a few times earlier in this thread but here is the link again.
http://classicapi.com/catalog/DOA_Install.php (http://classicapi.com/catalog/DOA_Install.php)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: orenradio on January 20, 2014, 05:09:08 PM
Hey Jeff,

Are the pads for T3's Green and Brown leads bridged together?  For some reason after trying to put the brown lead into the PCB i noticed there is a very straight, almost purposeful bridge between Gre and Bro pads.  See the picture below.  I have a solder sucker and a desoldering wick, but nothing seems to clean it up. 

Hopefully its a purposeful bridge:
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 20, 2014, 05:37:39 PM
Yes there is a track connecting them. I don't mean to be mean but this is the VP28 thread. You should be posting in the VP2x support thread.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Sansodor on January 20, 2014, 06:07:46 PM
Hey guys!

I just finished building my first VP28. I was wondering if it's normal to hear a slight popping sound when engaging the mic switch? Mine gives about -20dbfs pop when pressed (gain at full, fader at unity). The same thing happens when engaging the pad but not as loud. No biggie, but just wondering if it's normal behavior?

Otherwise everything seems to be working correctly on the first try. I built my a 1731 and a 2520 for the op amps myself.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: orenradio on January 21, 2014, 03:28:03 PM
Not a problem Jeff,

I wasn't sure if it was specific to T3 on VP28 - as the vp26 only has T1 and T2 and may have been traced differently on the PCB.  I can write a quick "fyi" post there to highlight the existence of the trace/bridge for future users.

Thanks again for your quick response, 3 Vp26s and 3 Vp28s later I'm an extremely happy DIYer; can't wait to build more
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: brent on January 22, 2014, 11:53:12 PM
Hey guys!

I finished building a couple VP28's a few days ago and they sound really great.  I'm using gar1731's on the input and gar2520's on the output, I found it to be a mellower combo for my needs than the reverse.  Before I built the op amps, I was using the 2520's from my API 2500 to test the units and those opamps actually sounded pretty good in this circuit! It's great to have multiple op amp choices and flavors.  I need to get some red dots next...

I botched up one unit temporarily when I put the 2 switches for the HPF on the wrong side of the PCB... ( not enough sleep apparently!)  I thought I was going to have to replace that PCB, but I managed to get them both out by taking the tops off, and pulling out one pin at a time while heating the respective pad.  Phew!  Ordered a couple new switches from digikey and I was back in business!  Jeff you probably have an email buried in your inbox from me about that fiasco.. you can safely disregard!

Great kit! Great guide too chunger!

Now off to contemplate some love childs...

B
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Pliplo on February 02, 2014, 12:55:01 AM
I assembled 4 VP28's the other week and one of them has a weird issue... Super low gain, almost as u were connecting a mic in line mode... Also kinda of a Low Cut effect all the time, and mutes completely when i engage the polarity switch... Phantom works but same issues with a condenser... Any ideas?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 02, 2014, 01:20:18 AM
Hmm, could possibly be something with the first stage output trafo. I would desolder all the leads from T2 and check DCR from one lead to all the others.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Pliplo on February 02, 2014, 03:48:33 AM
Hmm, could possibly be something with the first stage output trafo. I would desolder all the leads from T2 and check DCR from one lead to all the others.

Ok, not exactly sure how to check that or if the reading would be accurate with my DMM but... I checked all against the green lead and every one gives me 0 except the blue lead with 24ohms... also measurable resistance between black and brown leads (around 23.2)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 02, 2014, 04:10:46 AM
Ok, not exactly sure how to check that or if the reading would be accurate with my DMM but... I checked all against the green lead and every one gives me 0 except the blue lead with 24ohms... also measurable resistance between black and brown leads (around 23.2)
Hello,

Here's a pdf from Jeff's homepage:
http://www.classicapi.com/catalog/images/gallery/2623/2623-1-specs.pdf
Look at the pairs and measure them,your secondaries seem to be correct.
Primary is red & orange.Should give a reading too.
After that measure connections between the windings,there should be no connection,the dmm will go out of range so infinite resistance.
I think your dmm showing "0" was meant as no readings possible,right?
If it shows a steady 0 or 0,xx ohms you will have a short,please refer to your dmm manual.
Just hold the probe tips against each other,this is how a short looks like.

Hope to have helped,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Pliplo on February 02, 2014, 04:56:14 AM
Thanks, it sure helped! I have a Fluke 115, trying to locate the manual right now... No resistance between red and orange, shouldn't i get a reading from the primaries?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 02, 2014, 06:00:49 AM
Thanks, it sure helped! I have a Fluke 115, trying to locate the manual right now... No resistance between red and orange, shouldn't i get a reading from the primaries?
Yes! ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 02, 2014, 10:18:45 AM
Thanks, it sure helped! I have a Fluke 115, trying to locate the manual right now... No resistance between red and orange, shouldn't i get a reading from the primaries?
Pliplo, send me an email and we'll get this taken care of.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitol678 on February 13, 2014, 12:24:11 PM
Hi I just finished Building the vp 28 last week and love it! However, last night I was trying to use my condenser mic and could get a signal. The 48v red LED comes on, and I tested the mic on another unit and it was fine. Any help would be appreciated!

Eric
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 13, 2014, 07:58:00 PM
The LED is not powered from the 48V rail so it can illuminate even if phantom power is not being applied properly.

Like with everything, eliminate the rack slot and all associated cabling first. The return path from the mic is the shield of the mic cable which is pin 1 of the XLR. If that is floating somewhere for some reason, your phantom powered mic will not work.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitol678 on February 15, 2014, 01:15:47 PM
Thanks for the reply Jeff, sorry it took me so long to get back. I tested different card slots,cords etc and the 48v appears to not be working. When I tested on another unit I have the mic/cord combo works. Everything else, as far as dynamic mic recording, works great.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 15, 2014, 02:21:20 PM
Please post the VP26 issue in the VP2x build/support thread. Things get to confusing otherwise....although you are missing R1.

As for the VP28 build, do you have a way to power and test a module outside of the rack? If not that will be the first step. I have Extension Jigs at the store for this purpose.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitol678 on February 15, 2014, 03:54:13 PM
I dont have a way to test it but I'll look into the test jig, thanks for the reply!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: theokbird on February 19, 2014, 04:51:30 PM
hey there, i'm new to the DIY builds, wondering if anyone can offer advice..I've built 2 VP28 kits, and i'm stuck on a problem:  when i rack them without opamps they appear functional(led's light correctly), correct voltage, but when DOA's are inserted the green LED is on constantly and a low hum is passed as the signal, the other led's do not illuminate when the respective button is pushed.  any suggestions on where to start?  would this be a DOA problem?   or am i perhaps measuring the voltage incorrectly and there's a short on the board..i assumed the voltage measurement would be taken from the DOA socket labeled (+) and (-).... thanks in advance! 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 19, 2014, 05:56:30 PM
Sounds like an opamp(s) problem to me. First, are they seated properly? http://classicapi.com/catalog/DOA_Install.php (http://classicapi.com/catalog/DOA_Install.php)
 Next, who built them? If you scroll backwards in this thread you will see similar situations. Maybe looking at those will help you as well.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dherdesouth on February 21, 2014, 05:15:06 PM
(http://i57.tinypic.com/y13c7.png)

So after reading through a few pages I could not find a problem like mine.
This was my first ever soldering project, I knew it was a lot but read this build guide at least 100 times before submitting my order of 4pres. The only problem I ran into was using the multimeter and sorting all the pieces.
I built 2 within a couple days, first one was kinda weird but everything "looked"good (definitely placed a couple of the capacitors in the wrong spot). The second build felt great, after getting my technique right I flew through this one.

Then I received my lunchbox......
First test I ran a synth into each pre, got signal so that was a sigh of relief... but quickly turned into a different kind of sigh. I noticed the signal was extremely weak, when pushed to its max the volume was very low. Then tested the second one and it had the exact same problem!
Then came an actual frown when I plugged a sm57 in. No signal..... Then tried a tube mic with its own power source...no signal

Now here I am, wanting to throw my soldering station I'm the trash and having someone build my other 2 modules and fix the first 2.

Any advice guys?
Cheers
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 21, 2014, 06:28:20 PM
I see you have red dots so no need to worry about those. Rule out any outside possibility like cabling, converters and so on. This happens more than you can guess. I would then skim back thru this thread for a PDF that shows some test points. There will be some tips there about injecting signal and probing to see where you signal stops.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: qmp audio on February 21, 2014, 08:00:25 PM
Any advice guys?
Cheers

Check the polarity on your caps.   Looks like c16 is backwards from the photo.
Also,  hard to tell from the photo but looks like you may have some cold solder joints.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Patrick from Davis on February 21, 2014, 08:19:58 PM
I know this seems like a bonehead comment, but be sure the mic/line switch is positioned properly.  Also, looking at mine, that cap is backwards (The big blue guy between the green and red WIMA's)
Patrick

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 21, 2014, 09:34:02 PM
I see R's that are placed incorrectly. The 6k8's and 4k99's are definitely in the wrong locations.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 21, 2014, 10:43:31 PM
Except of wrong parts in wrong places I think gunpoint recording is on track too,nearly all solder joints I can see seem cold or bad,at least on the filter boards.Guessing joints are from the same quality on the mainboards I would at least do a good reflow job (after all parts are in their correct positions of course).Maybe you can show us some clear pics from the solder side(s) too?

Best regards and don't give up-these preamps are fantastic (I have eight! ;)),

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Ack on February 22, 2014, 02:14:39 AM
(http://i57.tinypic.com/y13c7.png)


Hi,

Your C16 and C20 were soldered in reverse direction.

BTW, I've just finished my first 02 VP28s, both at the same time, same stage from 10AM to 4AM and mis-positioned the Q7 and Q8 on 2 GAR2520 and 1 GAR1731 because I thought they are the same >.< I then had to de-solder and pulled them half way out, then trimmed the legs, swapped back to correct positions and soldered on the up-front pads. I tried to work as fast as I can, and both VP28s are working good now, but I still don't know if there's any heat affect to the transistors' operation.

I also had a reverse Phase LED which makes the other LEDs off but I figured it out.

Big thumps Up to Jeff, very excellent designs and very big, nice, detailed sound with these VP28s !

P/S: I have a VP312, too, not as big but sharper sound than VP28 :)

Regards,

Linh.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Kimster on February 23, 2014, 05:56:49 AM
Hi!

Does anybody here know if it would be possible to put NTP M100 DOA in the VP28? I have a couple of NTP 100 and it would be interesting to try them in the VP28, but there are a lot more pins on the ntp 100. Does anyone know the pins to use and if I have to do anything more (short some pins etc.). Also I think the NTP 100 are spec. to +/-15V, and maybe that´s also a problem?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: GregNey on February 23, 2014, 07:14:59 PM
Hello, I recently purchased a VP28 locally that someone tried to build but couldn't (I've built 3 successfully). Long story short, it didn't pass signal at all when I got it. I took it apart, cleaned it, re-heated some pads and now it's passing a very small amount of signal. I ran a 1k test tone through it and I'm getting good signal at points 1, 2, 3, but I have a pretty substantial level drop at 4. My natural instinct is to replace the components on the sub-board and Q1 (due to it's proximity) but just wanted to run it by the board before I did anything.

Any help is greatly appreciated.

Thank you, Greg
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 23, 2014, 07:54:24 PM
Greg, if things are good at #3 but low at #4, that indicates it's something with the fader switch and related R's.

If it was good at #4 and low at #5, I would say to look at the HPF board.

Q1 is part of the constant current source for the LED's so don't do anything with that.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: GregNey on February 23, 2014, 08:51:52 PM
Weirdly, the fader switch is the best looking part of this build... Not exactly sure what I'm looking for here...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 23, 2014, 09:32:28 PM
Dbl check the values for all of the fader R's. Maybe some got switched around.

Next, turn the fader all the way up, fully CW. Probe the right end of the row of fader R's just below the #3. The level should decrease as you go down the row.

These points will also match what is happening at test point #4 if you turn the switch with each point down the row. Test point #4 is equal to the switch wiper.

So,
+12dB switch setting = TP#3 = wiper = TP#4
+10dB switch setting = right end of RF11 = wiper = TP#4
+8dB switch setting = right end of RF10 = wiper = TP#4
and so on....
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: GregNey on February 28, 2014, 11:11:31 PM
Weird stuff happening.... My setup:
-EL500 rack with 2 VP28's in it. I built one and it is working perfectly, has been for over a year. The other I bought locally as non-working.
-Red Dots in both positions for both preamps.
-When I first bought the preamp it did not pass signal. I've cleaned alot up (it was not built very well) and I just plugged it back in to test. Things looked good at first.
-At line unity level and 0db on the fader, the 2 units track within .1db of each other.
-At line unity level and -20db on the fader, the 2 units track within .2db of each other (great so far.... or so I thought)
-At line unity level and +2db on the fader, I switched my build and I saw an appropriate level increase. When I switched the new build to +2db on the fader I got an incredibly HOT signal in BOTH channels... ??? I just turned it off as I didnt want to subject anything to what looks like a dangerous signal. Here I am now.

Thank you for your help! There is definitely something wrong at the fader switch.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 28, 2014, 11:39:35 PM
Hello,

I didn't get what the "level increase on both channels" means,are they linked somehow?
Anyway,an immediate increase leads to something is going wrong on the resistance curve/array on the switch.
I'd look at three points:
1.:Are all resistors in the correct place?
2.:Any shorts at the grayhill pins?
3.:Are the stop pins set right?

Start with nothing else attached to the rack,apply a signal,say 0dB at 1Khz,set to line mode and unity gain.What do you get on the output,is there any offset between in and outputlevel?

Let us know,good luck,

Udo.



Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: technoteo on March 01, 2014, 10:11:07 AM
Hi everybody
I need some help beacuse I'm stuck!!  >:(

I just finihed my first VP28 kit and everything was smooth and easy.  :) (BTW Thanks Jeff Steiger, the kit is awesome!!)

(http://i827.photobucket.com/albums/zz193/technoteo/vp28-finished_zps5eb8fdcb.jpg)

I'm assembling the 2 GAR2520 but realized that I used one of the matched BC550 for Q5 ....   :( :( :(  :o :o :o .....

https://www.dropbox.com/s/6w3vl2xoic2hrh4/bc550-2.jpg (https://www.dropbox.com/s/6w3vl2xoic2hrh4/bc550-2.jpg)

Now I'm trying to desolder the Transistor to put it in the right position, I try not to heat the transistor too much, used a pump and also some copper solder remover, but I still have a lot of tin on the silk screen side and there's no space to work on....

https://www.dropbox.com/s/5p832j4j8r1kx3b/bc550-1.jpg (https://www.dropbox.com/s/5p832j4j8r1kx3b/bc550-1.jpg)

Is the matched pair vital for the 2520 to function properly?

Any suggestion how to take away the remaining tin on the transistor? SOLVED!  ;D

I spent almost 2 hours to desolder the transistor trying to warm it as little as possible, and making it slowly not to damage the PCB

now my question is : do you know if the BC550 is sensitive to temperature? I mean, may I have damaged heating it too much?
Do I have to buy another matched pair of BC550 ?

thanks everybody
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 01, 2014, 11:04:55 AM
They are sensitive to too much heat. I would just try it and see if it works.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: technoteo on March 01, 2014, 11:15:16 AM
They are sensitive to too much heat. I would just try it and see if it works.

OK, I don't want to get crazy again in the case they're gone and since I have another pair of gar2520 I will change the transistors and order a new matched pair from you.  :)  I will wait for one of the kit to be completed.....

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: mbjr on March 05, 2014, 09:46:21 AM
Hi everybody,
first time builder from Denmark. Built 2 VP28's fairly successfully, they pass audio and all buttons work incl. the HPF.
I made a mistake with the DOA opamps: 4 gar2520. I switched the direction of Q7 & Q8 so the text is facing the wrong way.
They did pass audio, all though very distorted so now I'm going to desolder the Q7 & Q8 and turn them the right way but I'm not quite sure whether they have damaged them or any of the other parts of the opamps?
Jeff mentioned in a earlier post that they are quite heat sensitive and I'm having a hard time desoldering and removing them so I'm going slowly and try to apply as little heat as possible and still desolder them.
The question is whether the opamps can be salvaged if I manage to desolder the Q7 & Q8 or if rest of the components have been damaged so I just should build some new ones without making mistakes?
How do I test whether the Q7 & Q8 or any of the other components have survived my desoldering?

Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: qmp audio on March 05, 2014, 11:06:51 AM
For all you folks de-soldering... i can recommend the hakko 808.  With proper use, it makes de-soldering a quick and easy task.  I got one a while ago and i can say it was worth the investment.... it has paid for itself with time saved.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: GregNey on March 05, 2014, 08:28:09 PM
I didn't get what the "level increase on both channels" means,are they linked somehow?

There was a crosstalk issue that has been remedied and ALL IS WELL!!! Thank you all for your help!!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on March 06, 2014, 01:49:05 AM
There was a crosstalk issue that has been remedied and ALL IS WELL!!! Thank you all for your help!!
Cool,congrats and have fun ;)

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: erol on March 18, 2014, 04:24:22 PM
Just finished my first DIY VP28 and everything works!!
BUT...

I've got some problems installing the knobs on the grayhill switches.
First of all, what's the mm Allan key I need for both knobs?

just did a test, when I put the channel fader on +4, I can not go further. When I go back in value, the volume is getting softer so it looks like +4 is actually +12.
But this is the only way the knob fits on the greyhill switch?

Maybe someone here could explain to me in which position the grayhill switches has to be before installing the knobs. (flat side down and knob up??)
Thanks again
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on March 18, 2014, 10:38:16 PM
Just finished my first DIY VP28 and everything works!!
BUT...

I've got some problems installing the knobs on the grayhill switches.
First of all, what's the mm Allan key I need for both knobs?

just did a test, when I put the channel fader on +4, I can not go further. When I go back in value, the volume is getting softer so it looks like +4 is actually +12.
But this is the only way the knob fits on the greyhill switch?

Maybe someone here could explain to me in which position the grayhill switches has to be before installing the knobs. (flat side down and knob up??)
Thanks again
Hello.

Yes,looks like the pot caps are mounted wrong.
The answers are all on the first page,please read through the thread,it's not too long.
Page 4 points 15 & 16 of the assembly guide describe the how-to,and our friend chunger even made fantastic pics in his documentary (to be found on the first page in this thread too).
And about the allen wrench keys-they're not metric but imperial,again to be found on the first page,reply #16:
http://groupdiy.com/index.php?topic=48610.msg614212#msg614212

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: erol on March 19, 2014, 03:35:57 AM
Just finished my first DIY VP28 and everything works!!
BUT...

I've got some problems installing the knobs on the grayhill switches.
First of all, what's the mm Allan key I need for both knobs?

just did a test, when I put the channel fader on +4, I can not go further. When I go back in value, the volume is getting softer so it looks like +4 is actually +12.
But this is the only way the knob fits on the greyhill switch?

Maybe someone here could explain to me in which position the grayhill switches has to be before installing the knobs. (flat side down and knob up??)
Thanks again
Hello.

Yes,looks like the pot caps are mounted wrong.
The answers are all on the first page,please read through the thread,it's not too long.
Page 4 points 15 & 16 of the assembly guide describe the how-to,and our friend chunger even made fantastic pics in his documentary (to be found on the first page in this thread too).
And about the allen wrench keys-they're not metric but imperial,again to be found on the first page,reply #16:
http://groupdiy.com/index.php?topic=48610.msg614212#msg614212

Udo.

Thanks.
Any idea were I can find both Allen keys? (In Europe)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: erol on March 19, 2014, 04:56:42 AM
1,3 mm (0,05")
1,6 mm (1/16")
http://www.conrad.nl/ce/nl/product/824038/Wiha----263-13005-X-40-PicoFinish-Griff----00521----Inbus-schroevendraaier----1?

http://www.conrad.nl/ce/nl/product/407101/BernsteinInbussleutel-116-16-mm-6-852?

right?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Patrick from Davis on March 20, 2014, 01:32:37 PM
I tend to lose things like allen keys very fast.  I have an assortment of rarely used and odd sized ones that come with guitar bridges and cheap furniture, but those don't seem to be useful for anything other than their originally intended purpose.  So I got a couple of these (http://www.newegg.com/Product/Product.aspx?gclid=COr2rYfQob0CFUNhfgod_wYATQ&Item=05A-0006-00015&nm_mc=KNC-GoogleAdwords&cm_mmc=KNC-GoogleAdwords-_-pla-_-Hex+Keys-_-05A-0006-00015&ef_id=Uk3t5AAABf0vNiLJ:20140320172146:s).  This one linked is for the small sized imperial hex keys, there is one for the larger ones as well.  They color code the handle yellow for imperial, and red for metric, so it is easy to tell from a distance.  I am unsure if you have somewhere (Hardware store) that would carry something like this in stock, but I have saved quite a bit more time (and frustration) than the 5-10 dollars it cost me is worth.  Another one of those tools that doesn't get much use, but man o man does it save time when I need it.
Patrick

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on March 20, 2014, 02:35:41 PM
1,3 mm (0,05")
1,6 mm (1/16")
http://www.conrad.nl/ce/nl/product/824038/Wiha----263-13005-X-40-PicoFinish-Griff----00521----Inbus-schroevendraaier----1?

http://www.conrad.nl/ce/nl/product/407101/BernsteinInbussleutel-116-16-mm-6-852?

right?
Looks about right.
I know the WIHA stuff,pretty good for the price.
Once I bought a complete set in imperial size from the "magic ring"-series,very good but a bit expensive.But since I wanted to build a lot of modules I went for them.You can't beat good tools.
Just don't use too cheap tools,most of them are weak and might break.Be sensitive when tightening.
Hopefully you haven't damaged the screws using the wrong keys.

Best regards and good luck,

Udo. ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: antoniowas on April 03, 2014, 11:04:17 PM
Hello folks,

I've been getting into DIY audio gear and I found this thread. I went to the api website but I wasn't sure on how to buy the kit. I think it might be sold out, is that true?

Cheers,

Antonio
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 03, 2014, 11:11:30 PM
The VP28's are currently out of stock. I am waiting on 2623-1's to arrive. They should be here any day now.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: erol on April 07, 2014, 03:04:46 PM
To be sure,
I just build 2 VP28's,
I only use stereo line signals so I need to activate "PAD" and set the preamp gain to LINE UNITY right?
Could someone please explain what line unity means?
Thank you!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on April 07, 2014, 03:18:33 PM
I only use stereo line signals so I need to activate "PAD" and set the preamp gain to LINE UNITY right?
Hello,

no,just leave the mic switch in the off position.This is the line mode.
Unity gain means the in and output level will be equal then.E.g. if you feed a say +4dBu signal the output will be the same in line mode set to unity gain.
This "zero"gain point (no boost,no loss) normally is the reference level that audio units work at.
Mic and line modes are different in their input impedance.You use the pad when an incoming signal is too hot.

Best regards,

Udo.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: erol on April 08, 2014, 11:21:29 AM
I only use stereo line signals so I need to activate "PAD" and set the preamp gain to LINE UNITY right?
Hello,

no,just leave the mic switch in the off position.This is the line mode.
Unity gain means the in and output level will be equal then.E.g. if you feed a say +4dBu signal the output will be the same in line mode set to unity gain.
This "zero"gain point (no boost,no loss) normally is the reference level that audio units work at.
Mic and line modes are different in their input impedance.You use the pad when an incoming signal is too hot.

Best regards,

Udo.

Thanks mate
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on April 09, 2014, 10:41:49 AM
Hi All,

On one of my VP28, the C9 exploded when switching the phantom power for the first time.

Do you think that I only have to replace it, or must I to check something else before?

Thanks

Luc
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 09, 2014, 10:47:48 AM
Luc, replace C9. It was installed backwards the first time.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: OOstudio on April 09, 2014, 11:38:45 AM
Thanks for your answer ! You 're right !
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: orenradio on April 09, 2014, 12:59:28 PM
Hey Jeff,

I want to thank you so much for your products, I have 8 working modules and cannot be happier with my rack setup and the sound of my project studio.

I do, however, have one oddity that I just can't wrap my head around.

The first two VP28s I built are working flawlessly, however the last 3 all have the same problem and I am stumped.  The 3 modules sound beautiful both pushed and left with a cleaner gain-stage, however on all three the Signal Presence indicator is constantly lit at Unity Gain and below on the preamp gain knob at the top.  Once I switch the preamp gain knob past unity gain the signal presense indicator turns off and works as intended when signal is present and sent through the preamp. 

I thought it may had been a fluke on my third module, but after the next two were built with the same issue, I figured I must be doing something else.  There actually is no signal at the input or output stages when the signal presense LED is lit at or below unity gain so the issue appears cosmetic at worst.

Hopefully it is something that wont damage the pre in the long-run, could it be something with the standoff/LED attachment?  there are no shorts on the PCB that I can tell, and the plastic sheath adequately coveres each leg. I also thought it may have been a bad solder joint in the first few resistors, but they all work as intended sonically and none of the joints look disturbed.

Hoping to get some advice here, the issue is inaudible so either way I can live with the fault on my part if I can't figure it out.

Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 09, 2014, 03:28:37 PM
Is it possible some R's got flipped with each other around the 4558 chip?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: orenradio on April 09, 2014, 07:03:46 PM
Is it possible some R's got flipped with each other around the 4558 chip?

Its certainly possible. Let me check all of them again tonight and see if I can spot a mixup. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: lawrenceames on April 19, 2014, 12:07:36 PM
Hi guys,

First time building and I've run into a couple problems

Had a few components fry on me from incorrect placement, so I've replaced those.

My VP28 has really low output in both mic and line modes, could you have a look at the attached picture and see if theres anything noticeably wrong?

Thanks

Lawrence
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jswagler55 on April 27, 2014, 02:51:34 PM
So I have completed 4 of these guys, 3 of them work 100%. 1 of them the Signal light only comes on basically when the signal is clipping, everything else seems to be working fine. Changed out the 4558 and op amps it still has them same issue.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 27, 2014, 03:53:41 PM
Triple check the components around the IC opamp.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: j_fraser on May 06, 2014, 02:26:14 PM
Hi all - So I finally found some time to assemble the pair of these I purchased back last year, and firstly thank you to Jeff for putting together yet another great kit.  The assembly went smoothly and one of the units behaved perfectly on first power up, unfortunately the other unit is not so happy and is posing a bit of a problem, that I was hoping someone around here would be willing to give me a few pointers on.

Essentially the faulty unit heats up the DOA installed in A1 to an alarming level, and beyond testing the voltage at the DOA pins I have been reluctant to leave the unit powered up with DOAs installed for more than a minute or so for fear of cooking them.  Each of the DOAs installed for testing were known to be good at the time of testing, although the same is not so true of them now.  The DOA installed is A3 doesn't heat up in the same way and appears to operate normally from what I can tell.

The voltages at the DOA pins for the faulty unit are as follows:

A1 - Gets very hot when powered

C to +V = +15.28v
C to -V = -15.44v
C to O = +1.06v
+ to - = +1.06v

A3

C to +V = +15.28v
C to -V = -15.44v
C to O = -0.49v
+ to - = -0.49v

The voltage between the signal pins alarms me somewhat, especially as the working unit measures:

A1

C to +V = +15.29v
C to -V = -15.44v
C to O = +0.15v
+ to - = +0.15v

A3

C to +V = +15.29v
C to -V = -15.44v
C to O = +0.01v
+ to - = +0.01v

From this I assume I must have a short somewhere to account for the voltage in the signal path without DOAs installed on the faulty unit, but I'll be damned if I have managed to find it thus far.

Any pointers anyone can offer on where to start looking would be most appreciated.

Many thanks in advance…

Justin
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 06, 2014, 06:12:03 PM
Have you moved opamps to another unit to see if the problem stays with the pre or follows the opamps? Are the opamps seated properly? http://classicapi.com/catalog/DOA_Install.php (http://classicapi.com/catalog/DOA_Install.php)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: j_fraser on May 07, 2014, 10:01:17 AM
Hi Jeff - Thank you for the swift reply.  Having thought I had eliminated the op amps when testing previously, I thought it best to take another look, and test them again…  Schoolboy error discovered.  These Gar 2520s were made up a while ago, and I don't recall testing them, and it would turn out I didn't, they simply got moved from the "to test" section of the bench to being marked "tested" by mistake. 

As a result, it would seem two op amps from my stockpile have the same silly fault of the BD139 and BD140 swapped, hence the issue with the VP28.  Seems like I shall be ordering a couple more Gar 2520s from you, which isn't an issue except for dealing with the shipping and import to the UK.  C'est-la-vie…

Good news is the VP28s are working perfectly, down side is that I currently can't use them both at the same time without "borrowing" op amps from another piece of gear.

On another note, in testing the phantom on the VP28s it seems the current is carried by the hot conductor with the screen or cold forming the return, it also only measures about 28v.  Where as with the phantom my VP312DIs the current seems to be carried on the screen with the hot or cold forming the return and it is the full 48v (which in my understanding is correct). Is this normal behaviour for a VP28?

Once again many thanks for all you help and patience.

Best Wishes

Justin
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 07, 2014, 09:09:07 PM
The phantom part of both pre's is identical. The screen is always the return path but that is not exactly module related but rack related. I would maybe check your cabling.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Riinehart on May 12, 2014, 01:00:29 PM
Gigantic thanks to Jeff and gar for their work and to chunger and everyone who's posted such helpful information.

Just finished building a pair of these yesterday. Was quite difficult for a beginner like me, but they worked right off the bat and sound incredible.

Thank you all so much.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitol678 on May 16, 2014, 10:45:37 AM
I finished another VP28 and tested it with some other op amps (gar2520s) and it works great, but when I put in a 1731 I built the power transistors get really hot in a very short period even though it works just fine. I checked for shorts in the places indicated in the build guide and didn't find any.Any ideas what this could be? Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 16, 2014, 11:52:45 AM
Yes, visit the gar1731 support thread. These seem to be common problems. You will find the solutions there.
Title: Re: [BUILD] VP28~500 Series~2--- POWERING UP PROBLEMS
Post by: tctonn on May 16, 2014, 01:19:27 PM
Howdy world,

I just finished building my first VP28 and all looks good to the eye. However, when I plugged it into my Chameleon Labs CPS 501 chassis it killed the chassis. The power LED on the chassis came on, the signal LED on the VP28 flashed and then they both went out. Now the chassis power LED won't light up at all. I am waiting to hear back from Chameleon Labs but am note holding out great hope. I've done this with two of those chassis now and the same thing happened with both.

I'm no expert at this but I've done some pretty complicated builds before and had no issues. Anyone have any ideas where I might start troubleshooting?

TT

PS- thanks to Jeff for a great kit and Chunger for an amazing build doc. I can't wait to sort out my idiocy and get this guy fired up.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 16, 2014, 01:24:20 PM
Please scroll thru this thread and you will find things to look at starting with the opamps.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: zaiata on May 19, 2014, 03:55:02 PM
Complete my first two VP28s. Every thing worked, first try ....... The build was fun. But the sound is just awesome, Wooooooo!
Installed on a Purple Sweet Ten.

(http://groupdiy.com/gallery/0/54613-190514154338.jpeg)

(http://groupdiy.com/gallery/0/54613-190514154234.jpeg)

(http://groupdiy.com/gallery/0/54613-190514153630.jpeg)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: okgb on May 19, 2014, 05:30:39 PM
 Nice first post !
They're good enough and different enough that I think everyone should have a pair
in their arsenal !
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: zaiata on May 20, 2014, 11:10:52 PM
Thank you.
Now I realize... the photos are too big. 
I am waiting for Jeff to come back from Nashville to get some VP312......

I am reading so much good information on this forum... just awesome.
Need to decide on a good kit for a Neve flavor preamp.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tctonn on May 27, 2014, 06:53:36 PM
UPDATE-

No surprise that my problem was simple and Jeff was right on track: all works perfectly with two Red Dots from Scott Liebers.

I love this thing. All of the VP28s virtues have been well documented but to add to the praise- I use this primarily for long form narration and VO and was looking for a pre with a low sound floor and lots of clarity. I couldn't have asked for more. I think I like it more than the Hardy M-1 that we were using previously. Guitar tests coming today. 

For any novices who may have run into similar issues: it looks like I had a short on one of the GAR1731's that I built that somehow killed the Chameleon Labs power supply. Once I replaced those with the Red Dots and slotted it into my Lindell rack it worked flawlessly.

Next up- a pair ofVC528s.

Thanks again Jeff.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: j_fraser on May 28, 2014, 10:31:01 AM
So after making up a fresh pair of Gar2520s both VP28s power up, test and run perfectly.  The anomaly with the phantom, was as Jeff predicted, embarrassingly traced to a niggling earth fault in one of the multi-pin connectors behind the rack.

Have thus far only strapped the pair of VP28s across an existing mix, but wow!  I can see these units seeing a lot of use…

Once again, many thanks to Jeff for yet another great pair of mic amps, and also for his knowledgeable insight and support in getting this build up an running.  It is very much appreciated.

Many Thanks & Best Wishes

Justin
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: TheGiantLie on June 23, 2014, 03:29:03 PM
Hello, my signal indicator led isn't working. Everything else is perfect! I was hoping someone could give me an idea on where to check.

*its worth noting that I had originally had the ic chip in backwards. Of course it popped when I turned the unit on. I replaced the ic chip and the unit works perfectly other than the signal indicator.

Thanks in advance guys
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 23, 2014, 03:37:21 PM
Welcome to the forum,

Led is backwards too?

Best,

Udo.

P.S.:The chip does not like reverse supply voltage,so it is 99% sure it doesn't survive.Swapping it to a new one was correct.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: TheGiantLie on June 23, 2014, 06:05:52 PM
Thank you for the quick reply. When I say the ic chip popped, I mean it was in pieces. So I had no choice but to replace haha. The led tests fine, and I have double checked that it was in correctly.

I'm fine with leaving well enough alone for now (the pre works great) but if anyone has anything else that I should check I'm all ears.


ps: I have successfully built a second vp28, complete with a working signal indicator led!!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on June 24, 2014, 02:47:47 AM
Hello,

we don't have the schematics here,but with a working second unit you can do some comparisons.
Here's the chip datasheet taken from Jeff's site:

http://www.classicapi.com/catalog/images/gallery/opamps/IC/RC4558-datasheet.pdf

Best to use extension cables or - jigs and lay them side by side.Feed them with the same signal and compare the signals entering and leaving the chip,I thinks it's somewhere around there.

As always check for cold solder joints and/or shorts.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on June 28, 2014, 01:53:36 PM
Hey guys.
Just finished my first VP28, and 2 x gar1731's.

Plugged it in, and no signal comes in. Powers up fine, but it appears the input isn't getting in. When I switch phantom on, I can see a flicker in the converters which hints the output stage is ok, so I was thinking perhaps my issue is something to do with the input stage?

I have no idea what I'm talking bout really, so any advice on where to start checking would be lovely.

Thanks!
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 28, 2014, 04:42:09 PM
Dave, I would suggest scrolling thru this thread starting at the beginning. Ruling out the opamps is always the first step.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on June 29, 2014, 03:09:55 AM
Hey Jeff.

I'll scroll through it all tonight, but I'll have a go at putting some reddots in it I already have.

Thanks!
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 03, 2014, 11:08:50 AM
Hi guys.

I tried the unit with a pair of red dots I use everyday, and no change on the no signal issue.
I've read through this thread start to end, and saw one person with a similar issue, ( no signal) but no solution. There was a mention to check the preamp section, but I confess, I'm not sure where to start..

All LEDs work, obviously not the green gain light, as I have no gain. The rack is a radial rack, and is all good. Swapping leads over to the preamp next to it works fine, so that eliminates leads  mic, and converters.

Thanks in advance.

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 03, 2014, 11:43:44 AM
Hello,

so you have just swapped the DOAs,right?
Next step as usual would be voltage checking at their sockets.But I don't feel like this is the issue.
You say the green light is not working.That leads me to something else.Look here,this is the description from Jeff's site:

The only green LED on the faceplate. This is a full wave rectifier circuit feed from immediately after the 1st EA2623-1.

So since this is prior to the second stage (fader/amplifier/output) it seems to happen somewhere in the preamp.

I'd check if either a signal enters the transformer as well as leaving it.If there's a signal at the input but not on its' output then you might have

a)a broken transformer
b)wires misplaced
c)wires shorted or open condition(s) within the windings
d)a combination of them

If so measure the transformer dcr,you can find the info for wire colours on Jeff's site.
If the transformer is o.k. you can check the in- and outputs of the driving DOA.

http://www.classicapi.com/catalog/images/gallery/2623/2623-1-specs.pdf

If all fails you might want to post some clear and well-focused pics from the pcb,both sides.

Hope to have helped and good luck,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 03, 2014, 11:47:25 AM
Also, if you scroll thru this thread from the beginning, you will find a pdf with test points and some instructions on how to try and isolate where your problem is.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 03, 2014, 10:36:15 PM
Hello,

so you have just swapped the DOAs,right?
Next step as usual would be voltage checking at their sockets.But I don't feel like this is the issue.
You say the green light is not working.That leads me to something else.Look here,this is the description from Jeff's site:

The only green LED on the faceplate. This is a full wave rectifier circuit feed from immediately after the 1st EA2623-1.

So since this is prior to the second stage (fader/amplifier/output) it seems to happen somewhere in the preamp.

I'd check if either a signal enters the transformer as well as leaving it.If there's a signal at the input but not on its' output then you might have

a)a broken transformer
b)wires misplaced
c)wires shorted or open condition(s) within the windings
d)a combination of them

If so measure the transformer dcr,you can find the info for wire colours on Jeff's site.
If the transformer is o.k. you can check the in- and outputs of the driving DOA.

http://www.classicapi.com/catalog/images/gallery/2623/2623-1-specs.pdf

If all fails you might want to post some clear and well-focused pics from the pcb,both sides.

Hope to have helped and good luck,

Udo.

Thanks udo.

I will order a extender thingy for the 500 rack from jlm ( I'm in Australia) to be able to test voltages and signal. Too hard to reach in there...

When talking transformers, are you talking about the output trafos? You mention wires, so I thought that might be what u mean to check.

I did notice aft testing it yesterday, one of the opamps came out warm. Not hot, just warm. Whilst the other was cold. Hmmm. Mabey I can reach in enough to check opamp pin voltages. Will give it a go tonight..

Thanks again for your help.

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 03, 2014, 10:38:08 PM
Also, if you scroll thru this thread from the beginning, you will find a pdf with test points and some instructions on how to try and isolate where your problem is.

Hi Jeff.

 I just went back through it to find the PDF. Came up empty. I swear I'm not just being lazy. Can u direct me to the post or the page it's on?

Cheers
Dave

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 03, 2014, 11:22:27 PM
I will order a extender thingy for the 500 rack from jlm ( I'm in Australia) to be able to test voltages and signal. Too hard to reach in there...
Cool,a must-have when doing a lot of 500/51x stuff.
When talking transformers, are you talking about the output trafos? You mention wires, so I thought that might be what u mean to check.
Yes,because it is easier to reach.I just thought it was a good point to start and sort things out.You can of course start at the iput transformer too,but I haven´t heard from a broken 2622 so far,it is sealed while the o/p transformer has an more open structure with wires.Therefore easier to break something.The results of dcr measurings will lead you to where to look next anyway,either before or behind it.
I did notice aft testing it yesterday, one of the opamps came out warm. Not hot, just warm. Whilst the other was cold. Hmmm. Mabey I can reach in enough to check opamp pin voltages. Will give it a go tonight..
Be patient.Don´t try to do voltage checks when you can´t reach the pins safely.Too easy to cause a short.Wait for the extender cable.
About the opamps,it´s pretty normal that they get warm,especially the big o/p transistors.While it is not normal that they stay completely cold.....which one was cold,which one went warm?

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 03, 2014, 11:43:50 PM
I just went back through it to find the PDF. Came up empty.
It opens up in my case,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 03, 2014, 11:48:18 PM
Here it is as a jpeg (hope that´s o.k. for you Jeff?),

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 04, 2014, 04:55:23 AM
Here it is as a jpeg (hope that´s o.k. for you Jeff?),

Udo.

Thanks Udo.

Downloaded and stored...

Ive ordered the extention jig from capi, as the design seems cool. Not having to pull out other units to get it in seems great. Worth the extra week or so wait compared to jlm.

Is it worth while me posting photos of the pcb whilst i wait for the jig? If so, what part of the build would you like to see?

Cheers
Dave

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 04, 2014, 05:00:28 AM
Here it is as a jpeg (hope that´s o.k. for you Jeff?),

Udo.

Is there a page which has the corrosponding voltages for each test point?

Cheers
Dave

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 04, 2014, 08:41:59 AM
Is there a page which has the corrosponding voltages for each test point?

Cheers
Dave
I can't remember to have seen such a page,but at least with a  signal on input you can trace it and see where it stops or misbehaves.I'm sure Jeff then can tell you where to look at.

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 04, 2014, 08:46:04 AM


Ive ordered the extention jig from capi, as the design seems cool. Not having to pull out other units to get it in seems great. Worth the extra week or so wait compared to jlm.

Yes,I have got one too and assembeled a few days ago,cool stuff and very handy with the cmrr measuring  extras on board.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 05, 2014, 04:43:54 AM


Ive ordered the extention jig from capi, as the design seems cool. Not having to pull out other units to get it in seems great. Worth the extra week or so wait compared to jlm.

Yes,I have got one too and assembeled a few days ago,cool stuff and very handy with the cmrr measuring  extras on board.

Must do some reading on the measuring part of the jig...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 05, 2014, 05:55:02 AM
Dave,

the measuring part is just about cmrr,not more.The guide is printed even on the pcb itself,not joking.Pretty cool.But that's a part you can do later.First wait for the jig to get the module attached outside the rack.Way easier then.Once you have it trace through to the test points and see where the sognal stops.
Do you know how to do it?If not we will guide you through it,I'm sure you will make this baby work,it's really worth it (I know what I'm talking about because I have eight!).

Be patient and have a nice weekend,

best regards,

Udo. ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 05, 2014, 11:38:11 AM
Dave,

the measuring part is just about cmrr,not more.The guide is printed even on the pcb itself,not joking.Pretty cool.But that's a part you can do later.First wait for the jig to get the module attached outside the rack.Way easier then.Once you have it trace through to the test points and see where the sognal stops.
Do you know how to do it?If not we will guide you through it,I'm sure you will make this baby work,it's really worth it (I know what I'm talking about because I have eight!).

Be patient and have a nice weekend,

best regards,

Udo. ;)

Hi udo.

I don't even know what cmrr is at this point. Will work it out now!

I know we can get my vp going! I built a hairball blue stripe before, and had a few teething problems, so I have no fear it will be up and running soon. Been here before.. Ha..

I completely agree on the sound of the vp's.. As soon as I get four of them done, my API 3124 will be going. The vp totally destroyed my api's for me. Loved the vp so much more. It changed my view on what the API sound is. It now makes sense to me how API should sound. Should be rich, creamy, cool. The api3124 is not any of that. It is good, but very different. 3124 is bitey, forward, excited kid on Christmas morning.. VP wears a leather jacket, smokes out the corner of his mouth, and peers over the top of a pair of wayfarers..

I also got to use a VP 26 along side the 28. They sat perfectly together. I had a 28 on kick, and 26 on snare that session.. Also tried some hairball Lola's on same session. They were surprising things! Nice..

Waffle over..

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 05, 2014, 11:46:39 AM
Very well put Dave!
I have temoraily set up two monsterstrips in one "to-go-51x-Rack":
VP28---F76(with APP992 & Lundahl)---LA500---My beloved Lovechild(LC53)---Pultec EQP1S5.
A dual D.I. from Bo in the middle slot.
Totally killer!

Best,

Udo ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 06, 2014, 01:18:54 AM
Very well put Dave!
I have temoraily set up two monsterstrips in one "to-go-51x-Rack":
VP28---F76(with APP992 & Lundahl)---LA500---My beloved Lovechild(LC53)---Pultec EQP1S5.
A dual D.I. from Bo in the middle slot.
Totally killer!

Best,

Udo ;)

That looks like a chain I'd enjoy!
I think after getting 2x28, and 2x 26, I'll fill some other slots with the jlm opto comp x2. perhaps it's a budget way of doing what the shadow hills comp does on the opto side.  That's one sweet sound to my ears.. Heard great reviews too.
Love child has interested me too, but I gotta get some Lola's I reckon. I think I want to get 4 Lola's, and get rid of my vintech 473. The 473 has become my 'run outta pres' unit now.. The Lola's were like a cooler version of what the vintechs are to me.

That fills a 10 slot PS right there.. Sweet.

VPs first though!

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 06, 2014, 01:44:43 AM
O.K.,let's stop this here because it is the VP28 help & support thread.
Otherwise Jeff might kill me...... ;D

Tell us when you have results from the testpoints.
Btw.,best to have nothing attached to the module (e.g. a patchbay,other modules,A/D converters etc.).
Just a signal source on input and a dmm or scope or whatever you have to work with.
What you can do while waiting for the extension jig is to check back your dmm manual up to which frequency you can do ac readings.This is a very important point,otherwise you'll get wrong readings.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 07, 2014, 07:00:53 AM
O.K.,let's stop this here because it is the VP28 help & support thread.
Otherwise Jeff might kill me...... ;D

Tell us when you have results from the testpoints.
Btw.,best to have nothing attached to the module (e.g. a patchbay,other modules,A/D converters etc.).
Just a signal source on input and a dmm or scope or whatever you have to work with.
What you can do while waiting for the extension jig is to check back your dmm manual up to which frequency you can do ac readings.This is a very important point,otherwise you'll get wrong readings.

Best,

Udo.

Ok cool.. Back on track!.

So when checking the points outlined in the pdf, nothing but power it up, and put something like a 1KHZ tone through at 0DbU or so??

Cheers
Dave

Title: Fried the op amps on one of my two vp28s
Post by: guerrilladigital on July 10, 2014, 10:36:54 AM
Hi everyone. This is my first post on the forum.

I recently attempted to "fix" a vp28 build I did a few months back. When I used it for recording, I noticed it was tracking an audible, but distorted signal. I built a second vp28 simultaneously, and it worked correctly from the get go. Both have homemade gar2520 op amps in them. (Man, were those hard to build!)

When I plugged in my "fixed" vp28 last night, smoke started coming out of the top vents of my lunchbox. I immediately turned off the lunchbox and removed the unit. One of the two op amps smells really awful. As a noob (I've successfully built four vp26 and one vp28), I don't know where to start troubleshooting.

How can I test the resistance of the main PCB to rule that out? Jeff pointed me to this forum, but honestly, there's so much going on here, and I'm not exactly sure what search terms to put in. He said to "check the VP28 support thread at groupdiy.com for the DCR measurements". Does anyone have a link to the page where this info is listed?

If I put the op amps into a "known" working mic pre, will I risk destroying a properly working mic pre?

I'm really not looking forward to building two more gar2520s, but to match the other vp28 so I have a pair, it's gotta be done. If I can determine one to be good, and the other to be fried, at least I'll only have to build one more.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 21, 2014, 03:43:19 AM
I just built my first VP28. No Op Amps built yet. I think I got it right so far, though. This being my first build of this magnitude, is there some testing that I can do to confirm that it was built correctly? Like before I plug it in and have something melt?

Also, thanks so much to Mr. Chunger for that amazing detailed build pictorial!! And thanks to Jeff Steiger and the rest of you guys too!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 23, 2014, 03:57:03 PM
And another question....

The 1/16th hex key works for the bigger knob. Maybe I missed it somewhere, but what do you use to tighten the smaller knob? Not 1/16th hex, that's for sure...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 23, 2014, 04:04:53 PM
And another question....

The 1/16th hex key works for the bigger knob. Maybe I missed it somewhere, but what do you use to tighten the smaller knob? Not 1/16th hex, that's for sure...
0.050

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 24, 2014, 01:43:01 AM
Thanks, Udo. I read this thread a bunch of times prior to the build. Rereading today (the whole thing) I discovered the answer had been stated several times in the thread. I also noticed a lot of references to my initial question about testing, but I actually don't understand them. I wanted to know if there are tests I can do BEFORE I power these up (almost done with the second one. Two LED's and knobs to go...!)

I took my time building them. Fourteen hours for the first. Ten for the second. I don't understand you guys who build three in an evening! I was very careful, and I'm as positive as I can be that all solder joints are good and everything is oriented correctly. Do I just put them in the rack and see if something melts, or can I test without power first? And then how? I'm going to be searching for that PDF soon...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 24, 2014, 02:58:09 AM
Hello,

without powering up there is not too much you can do.
Just look over the solder joints for cold or shorted ones,especially at the grayhill switches because of the quite small pin pitch.Best done with a magnifying light,view from different angles.

The led driver must be inserted the right way,it will die immediately if reversed.

After that you might  measure the in- and output dcr with your dmm set to ohms,you should get readings there,no short or open condition.Same on psu gnd to positive and negative supply rail,under no circumstance should there be a zero (or short) reading.

The most tricky part is the DOAs when you build them for yourself.Especially the output transistors (the two big black ones) are either swapped or mounted reversed,nearly 90% of non-working DOAs have this issue.

After that it's time to power up,best with an extender cable (Jeff has a nice one in his shop).It's easier to reach the trim pot for cmrr setting anyway.

Hope to have helped,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 24, 2014, 03:11:25 AM
Thanks again Udo!

I did see some tips in section 12 of the Vpx build guide. I don't understand what a lot of that means, but I'm starting to figure it out. Is it safe to assume that it applies to the VP28 as well? Anything additional or different for the VP28?

Thank you so much for being so generous with your time and knowledge. Your patience with the initiated is awesome. I'm going to check out that extender cable now...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 24, 2014, 03:52:40 AM
Dave,

the measuring part is just about cmrr,not more.The guide is printed even on the pcb itself,not joking.Pretty cool.But that's a part you can do later.First wait for the jig to get the module attached outside the rack.Way easier then.Once you have it trace through to the test points and see where the sognal stops.
Do you know how to do it?If not we will guide you through it,I'm sure you will make this baby work,it's really worth it (I know what I'm talking about because I have eight!).

Be patient and have a nice weekend,

best regards,

Udo. ;)

Hi Guys.

Ive got my extention jig up and running, and ready to do the test points.
Do i need any signal going in for the tests? What do i use for ground when testing the points.
Where do the gain and fader knobs need to be?

Thanks!
Dave


Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 24, 2014, 04:10:05 AM
Thanks again Udo!

I did see some tips in section 12 of the Vpx build guide. I don't understand what a lot of that means, but I'm starting to figure it out. Is it safe to assume that it applies to the VP28 as well? Anything additional or different for the VP28?
You're welcome.

Had to read through this section 12 first.I "only" did the VP28s so far.
But yes,it seems to be right to do so.This is all about if the supply rails are o.k. so far.In case anything is shorted (zero ohms) or seems to be very lowish then something is definetely wrong.Fixing time then,don't try to attach the module(s) to the rack or a psu in this case.
The readings should be close to identical on both modules (you're building two,right?).

After powering up you can do a voltage check as described on the DOA sockets,so with none installed.

Again I highly recommend the extender cable,also for future use on other modules (you'll love I bet!).

Tell us what you get when done,

best regards,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 24, 2014, 04:25:07 AM
A few more developments...

Ive managed to get a signal through now.. Its a bit dodgy though.. I have to have the phase switch in for it to work.

If i press on the pad when its out of the chassis, so putting downward pressure 'towards' the PCB, the signal jumps to life. It seems to be when im pressing on the Pad switch more so, but pressure anywhere around there seems to work.

Line input works well, as long as i have phase switch in. It drops 30 db with phase set to normal. When i put pressure on the pad when all is well, it jumps in volume a whole bunch, and distorts and thins.

Mic Input seems to do the same. Needs to have the phase switch in to work, and need to put pressure on the pad switch toward the pcb..

Ill check solder points on switch again right now.

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 24, 2014, 04:43:39 AM
A few more developments...

Ive managed to get a signal through now.. Its a bit dodgy though.. I have to have the phase switch in for it to work.

If i press on the pad when its out of the chassis, so putting downward pressure 'towards' the PCB, the signal jumps to life. It seems to be when im pressing on the Pad switch more so, but pressure anywhere around there seems to work.

Line input works well, as long as i have phase switch in. It drops 30 db with phase set to normal. When i put pressure on the pad when all is well, it jumps in volume a whole bunch, and distorts and thins.

Mic Input seems to do the same. Needs to have the phase switch in to work, and need to put pressure on the pad switch toward the pcb..

Ill check solder points on switch again right now.

Cheers
Dave
Yes,that looks like either cold joints or a broken switch.Reflow the joints with a little fresh solder tin/-flux and try again.

If that doesn't help measure if the switch contacts are working.

Best,

Udo.

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 24, 2014, 05:42:12 AM
A few more developments...

Ive managed to get a signal through now.. Its a bit dodgy though.. I have to have the phase switch in for it to work.

If i press on the pad when its out of the chassis, so putting downward pressure 'towards' the PCB, the signal jumps to life. It seems to be when im pressing on the Pad switch more so, but pressure anywhere around there seems to work.

Line input works well, as long as i have phase switch in. It drops 30 db with phase set to normal. When i put pressure on the pad when all is well, it jumps in volume a whole bunch, and distorts and thins.

Mic Input seems to do the same. Needs to have the phase switch in to work, and need to put pressure on the pad switch toward the pcb..

Ill check solder points on switch again right now.

Cheers
Dave
Yes,that looks like either cold joints or a broken switch.Reflow the joints with a little fresh solder tin/-flux and try again.

If that doesn't help measure if the switch contacts are working.

Best,

Udo.

Udo.

Thanks udo. Done.
The pre is working almost as it should now.
Few things..
When I use condenser mics, so 48v on, the level drops off. The signal is clean and nice, just a volume drop compared to where it should be.(checked against other pres).
Also, when I engage phantom, there's one hell of a thump that goes through the system.

Any ideas?

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 24, 2014, 06:07:27 AM
Hi Dave,

always turn down your monitors when switching phantom on or off-I mean that!

What do you mean with the mentioned level drop "compared" to other preamps?How can you compare it with no technical readings/measurements or even no printed scale on the gain switch?
Physical pot positions are mostly very different on different preamps.
It also might be a question of the input impedance etc........

Can you confirm the unit has unity gain?And have you done the cmrr adjustment so far?
For unity gain check just apply a known signal,say a sine 1kHz at 0dBu,set the mic switch to "off",output fader to zero or middle position and the gain switch to "unity"(blue marking).
You should get the very same output signal.This will confirm the unit is working correct.

Happy you solved the switch issue btw.,best,

Udo.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 24, 2014, 10:02:35 AM
Hi Dave,

always turn down your monitors when switching phantom on or off-I mean that!

What do you mean with the mentioned level drop "compared" to other preamps?How can you compare it with no technical readings/measurements or even no printed scale on the gain switch?
Physical pot positions are mostly very different on different preamps.
It also might be a question of the input impedance etc........

Can you confirm the unit has unity gain?And have you done the cmrr adjustment so far?
For unity gain check just apply a known signal,say a sine 1kHz at 0dBu,set the mic switch to "off",output fader to zero or middle position and the gain switch to "unity"(blue marking).
You should get the very same output signal.This will confirm the unit is working correct.

Happy you solved the switch issue btw.,best,

Udo.

Thanks udo.

The level drop wasn't explained well. Sorry bout that. I was swapping between a dynamic and condenser, making sure all was well with both phantom and non phantom mics.
The re20 was louder than the C414 on the vp28 with equal gain, whereas on the API 4312 it was the other way round. I tried on the TG2 and Mercury m72 as well, as I thought the mic not be getting enough power.

I'll do the test again tomorrow, but test against all preamps I got.. the c414 isn't a mechanical switch for the pads and polar patterns. It has a led that shuffles across the options. It was kinda like the pad was stuck on.. I'll do more tests.

The white noise/ sub blast when switching on phantom isn't normal. I'm sure we all accidentally do it occasionally when monitoring at a decent level. With all my other pres, the spike is there, but not too scary.. With the VP28, it's a whole bunch louder, and takes a good few seconds to settle down. Not sure how loud it spikes, as my converters have a soft clip that hide the true peak of it.

It does run unity fine. I had a signal generator running outta tools, through VP, and back again. The levels were correct.

What is the cmrr adjustment? I can see it mentioned here n there, but struggle to work out what it is, what it calibrates exactly, and curiously, what it stands for. I'd love to know.

I'll post tomorrow after I've played more..

Thanks guys!

Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 24, 2014, 10:53:11 AM
Dave,forget about the cmrr ( which is common mode rejection ratio btw.).This Preamp doesn't seem to have a trimpot for it,my bad,excuse me.I think I swapped it with the LC53 or so.....

About phantom again:
Be aware of what you're doing.Don't switch it on if not needed,only with powered mics and d.i. boxes etc.
Of course it could happen but......never ever listen to it.
Basic audio rules,guess what might happen if you have bazillion kilowatts P.A. system attached instead of your monitors......

If and how loud it pops depends on the input load meaning what mic. or D.I is plugged in.,how the preamp input is designed,if it is a ramping phantom power etc.,and last but not least cmrr on input:If it is too bad a dc offset will accompain your audio.
Lots a variables,I know....but easy to handle with monitors down or off.
If it sounds right it is right,and you confirmed the unity gain test,pretty good.

If it is way too loud then something is wrong.My first spot to look at would be the two 6,8 k resistors and measure them if they are closely matched (or a resistor has the wrong value).They should be in a range of 0,1% or better.To do this lift one side of them each (R20 & R21 on the main board) to not have them in circuit and measure them.

I don't have my modules handy here nor schematics.At this point Jeff might chime in?

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on July 24, 2014, 01:57:53 PM
i've forgotten to turn down my monitors and gotten a pretty gnarly bump when engaging phantom, but i normally turn down no matter what preamp.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 25, 2014, 03:45:15 AM
i've forgotten to turn down my monitors and gotten a pretty gnarly bump when engaging phantom, but i normally turn down no matter what preamp.

Hey guys.

I agree it happens with all preamps. I do turn down my monitors when engaging phantom (most the time).. All my other pres (API, vintech, RCA, mercury, heritage, chandler), just make a click sound. It's loud, but it's just a click, so it's ok. 
VP28 is a full scale burst that hangs around for about 3-4 seconds. It just makes me feel like something isn't right. The noise is a sub blast, with a white noise burst. Even with the monitors quiet, it puts the fear in me.

Do all you guys have the same sound when engaging phantom on VP28?

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 03:59:16 AM
Dave,

can you tell us if this happens with nothing attached to the input or maybe the same microphone plugged in?
Also did you check the phantom resistor values?
It comes to my mind that it might be the load (your mic,the 414?) because there should only be a short(!) pop sound,not lasting for seconds and having a noisy tail.Looks like something is not charging correct.
Since it is a thing about time a capacitor could be faulty.
Try another condenser mic if you can and see if it is the same behaviour.This way we can sort out wether it is the mic or the preamp.

Tell us what you get,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 25, 2014, 04:07:26 AM
I ordered the extension card and in the meantime I built one 2520. I think I did well.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 04:15:35 AM
I ordered the extension card and in the meantime I built one 2520. I think I did well.
Cool,but don't you need two DOAs? ;D

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 25, 2014, 06:07:19 AM
Dave,

can you tell us if this happens with nothing attached to the input or maybe the same microphone plugged in?
Also did you check the phantom resistor values?
It comes to my mind that it might be the load (your mic,the 414?) because there should only be a short(!) pop sound,not lasting for seconds and having a noisy tail.Looks like something is not charging correct.
Since it is a thing about time a capacitor could be faulty.
Try another condenser mic if you can and see if it is the same behaviour.This way we can sort out wether it is the mic or the preamp.

Tell us what you get,

Udo.

Hey Udo..

So, it happens with both my 414's. Its alot more subtle with my other condensers.
With no mic plugged in, its completely quiet when I switch phantom on and off.

I also just put a multimeter on the mic lead going in with phantom engaged, and read 55.7volts in the hot and cold pins. And 0 in the earth..

55.7 seems a bit too far from 48v, or is that within normal tolerance.

Cheers
Dave

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 08:08:47 AM

Hey Udo..

So, it happens with both my 414's. Its alot more subtle with my other condensers.
With no mic plugged in, its completely quiet when I switch phantom on and off.

I also just put a multimeter on the mic lead going in with phantom engaged, and read 55.7volts in the hot and cold pins. And 0 in the earth..

55.7 seems a bit too far from 48v, or is that within normal tolerance.

Cheers
Dave
Hi Dave,

Holy cow!

55,7vdc is too high,definition for phantom power is 48 vdc +/-4.
It seems the mic tries to bring the overvoltage down,the noise tail might come from caps charging/discharging.

What kind of rack is it or what psu is the supply voltage?
Do you have specs for it or built it yourself?
Please measure the dc without any modules in the rack directly on a multipin connector.
Pins are 5 and/or 13 for 0vdc,pin 15 for +48vdc for phantom.
Here's the pinout data as a proof:

http://51xaudio.com/alliance/pinout.html

Be careful not to short pins doing it.
Easiest with an extension cable of course,but reachable in an empty rack.

So far I'd stop using any phantom powered gear on this rack/psu until this problem is solved.

Very curious now,

Best,

Udo.

Btw.,I don't believe in any issues with your VP28 anymore,seems up and healthy,cool!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tonycamp on July 25, 2014, 09:52:25 AM
just to reinforce, no matter the mic, my vps make a quick pop when i engage phantom with the monitors turned up, no where near 3-4 seconds, somethings not right there Dave, sounds like udo's got you steering in the right direction.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 10:01:30 AM
just to reinforce, no matter the mic, my vps make a quick pop when i engage phantom with the monitors turned up, no where near 3-4 seconds, somethings not right there Dave, sounds like udo's got you steering in the right direction.
Yes,should be a short spike only.I told that before,I have eight of them,all do behave this very same way.
Thanks for confirming!

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 25, 2014, 10:39:07 AM

Hey Udo..

So, it happens with both my 414's. Its alot more subtle with my other condensers.
With no mic plugged in, its completely quiet when I switch phantom on and off.

I also just put a multimeter on the mic lead going in with phantom engaged, and read 55.7volts in the hot and cold pins. And 0 in the earth..

55.7 seems a bit too far from 48v, or is that within normal tolerance.

Cheers
Dave
Hi Dave,

Holy cow!

55,7vdc is too high,definition for phantom power is 48 vdc +/-4.
It seems the mic tries to bring the overvoltage down,the noise tail might come from caps charging/discharging.

What kind of rack is it or what psu is the supply voltage?
Do you have specs for it or built it yourself?
Please measure the dc without any modules in the rack directly on a multipin connector.
Pins are 5 and/or 13 for 0vdc,pin 15 for +48vdc for phantom.
Here's the pinout data as a proof:

http://51xaudio.com/alliance/pinout.html

Be careful not to short pins doing it.
Easiest with an extension cable of course,but reachable in an empty rack.

So far I'd stop using any phantom powered gear on this rack/psu until this problem is solved.

Very curious now,

Best,

Udo.

Btw.,I don't believe in any issues with your VP28 anymore,seems up and healthy,cool!

Hey udo..

 So you feel the problem is more likely to be the power supply? Does the VP28 just directly route the 48 v from it's own power supply? I'll check the phantom power voltage on the heritage audio pre and see what's going on there. Silly I didn't check it before I left for the night. Will check tomorrow.

The power supply and rack is a 8 slot Radial workhorse.
I have the extender card, so can do all these tests asap.

Thanks for your help guys!

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 11:17:19 AM


Hey udo..

 So you feel the problem is more likely to be the power supply? Does the VP28 just directly route the 48 v from it's own power supply? I'll check the phantom power voltage on the heritage audio pre and see what's going on there. Silly I didn't check it before I left for the night. Will check tomorrow.

The power supply and rack is a 8 slot Radial workhorse.
I have the extender card, so can do all these tests asap.

Thanks for your help guys!

Cheers
Dave
Hi Dave,

I took a quick look at the Workhorse.So it has an external psu,right?
Don't know it o far,maybe it has a trim pot for phantom?
Seems like it is an smps....I don't trust them anyway due to bad experiences,I always build linear types.

When checking phantom on other preamps remember that without anything plugged to the input the measurement might be a tad higher because of the higher dmm input impedance.It should go down with a load,e.g. a condenser microphone.

Could happen that you must contact Radial Engineering.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 25, 2014, 11:49:56 AM
I ordered the extension card and in the meantime I built one 2520. I think I did well.
Cool,but don't you need two DOAs? ;D

Best,

Udo.

Well, FOUR, but I'm taking my time. Wanna get it right. Funny, the 2520 didn't really seem that hard. I just went slowly and carefully.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 12:12:45 PM
Well, FOUR, but I'm taking my time. Wanna get it right. Funny, the 2520 didn't really seem that hard. I just went slowly and carefully.
Great to hear.Not too hard to do,but patience is the key,same as working clean and carefully,having steady hands and some soldering experience is helpful.Personally I don't understand this competition about who is the fastest.Seems somehow at least a few guys here in the forum are those who do them wrong after "quick assembly".
Have done around 60 of this type,no issues except one that had a cold joint,was an easy fix then.
Can't remember how long it took,quite a while I think.Doing more than one at a time speeds it up of course.

Best regards and happy soldering,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 25, 2014, 01:10:15 PM


Hey udo..

 So you feel the problem is more likely to be the power supply? Does the VP28 just directly route the 48 v from it's own power supply? I'll check the phantom power voltage on the heritage audio pre and see what's going on there. Silly I didn't check it before I left for the night. Will check tomorrow.

The power supply and rack is a 8 slot Radial workhorse.
I have the extender card, so can do all these tests asap.

Thanks for your help guys!

Cheers
Dave
Hi Dave,

I took a quick look at the Workhorse.So it has an external psu,right?
Don't know it o far,maybe it has a trim pot for phantom?
Seems like it is an smps....I don't trust them anyway due to bad experiences,I always build linear types.

When checking phantom on other preamps remember that without anything plugged to the input the measurement might be a tad higher because of the higher dmm input impedance.It should go down with a load,e.g. a condenser microphone.

Could happen that you must contact Radial Engineering.

Best,

Udo.

Hi udo.

I did check just one other pre before I went home, but not one in the 500 rack. It was my API 4312, and that read exactly 48v with nothing more than a mic cable and the multimeter. The same way I tested the vp28..

Hmmm. I think ya could be quite right about the rack being out on the phantom, as I have a feeling the phantom turn on with the heritage may be a scary one too. Looking forward to checking it out tomorrow..

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 01:19:44 PM
Yep,do so.
What is "the heritage" btw.,the console (midas)?

Udo.

WAIT!!! DON'T TRY IT IF THE HERITAGE IS THE MICROPHONE FROM HEIL!!!
This is a dynamic mic!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 25, 2014, 02:08:59 PM
Yep,do so.
What is "the heritage" btw.,the console (midas)?

Udo.

WAIT!!! DON'T TRY IT IF THE HERITAGE IS THE MICROPHONE FROM HEIL!!!
This is a dynamic mic!

Heritage audio 1073.
http://www.heritageaudio.net/product-2-1073-500.php

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 02:21:39 PM
O.k.,got it.

About your question how the phantom power works until it reaches the microphone:
The +48vdc is a single rail coming from the psu directly.
It is then "split" to your preamp input via the before mentioned 6,8kOhms resistors to each xlr pin 2(hot) and pin 3(cold) with refernce to audio/psu ground (that pin 1 carries).
Nothing else is in between.It then reaches your condenser mic or active d.i. and is normally taken via a resistor combo (voltage devider) to supply the inside active electronics and to polarize the capsule.

Best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 25, 2014, 02:40:59 PM
O.k.,got it.

About your question how the phantom power works until it reaches the microphone:
The +48vdc is a single rail coming from the psu directly.
It is then "split" to your preamp input via the before mentioned 6,8kOhms resistors to each xlr pin 2(hot) and pin 3(cold) with refernce to audio/psu ground (that pin 1 carries).
Nothing else is in between.It then reaches your condenser mic or active d.i. and is normally taken via a resistor combo (voltage devider) to supply the inside active electronics and to polarize the capsule.

Best,

Udo.

That makes perfect sense. Thanks for that udo.
So if the power supply reads fine on the multimeter, it can only be those 6.8k resistors.
I am feeling the power supply is the issue, as u mentioned, but tomorrow will tell.

Cheers
Dave

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 25, 2014, 02:48:06 PM
You'll get a too high reading on your dmm,I'm 99,9% sure.

Udo ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 28, 2014, 07:29:37 AM
You'll get a too high reading on your dmm,I'm 99,9% sure.

Udo ;)

Hi udo..

You were spot on. The heritage is spitting out 55 v as well with phantom power. Radial rack off to tech.

So it seems I'm good to go then! One more VP28 alive and well in the world.

Thank you all who have commented and steered me right. You're a good lot.

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 28, 2014, 07:50:02 AM
Good to hear the VP28 is alive and healthy,sad for you the rack psu isn't.
May I suggest to build rack(s) and psu yourself in the future?
The stuff Jeff,Cemal and Volker have in their store are absolutely bullet-proof and easily top all industry stuff I've seen so far.And the biggest advantage is that you will learn a lot about how a psu works and then be able to service it.And the fun factor,it's a great feeling to see how fast it all grows,hahaha!Also the "51x-Advantage"(additional +/-24dc rails) will open up the possibility to build some of the more exotic or formerly non-500-series modules while it is always API/500-series-compatible.

Have fun,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: blackbird on July 28, 2014, 08:44:18 AM
Good to hear the VP28 is alive and healthy,sad for you the rack psu isn't.
May I suggest to build rack(s) and psu yourself in the future?
The stuff Jeff,Cemal and Volker have in their store are absolutely bullet-proof and easily top all industry stuff I've seen so far.And the biggest advantage is that you will learn a lot about how a psu works and then be able to service it.And the fun factor,it's a great feeling to see how fast it all grows,hahaha!Also the "51x-Advantage"(additional +/-24dc rails) will open up the possibility to build some of the more exotic or formerly non-500-series modules while it is always API/500-series-compatible.

Have fun,

Udo.

Hey udo.

Yeah, the racks been around for quite a while now. Before I was aware of the rack kits..
I've been eying them off, and will go that way once this power rack goes back home.

So is need to get a power supply kit, and the rack kit? Easy build?

Cheers
Dave
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 28, 2014, 09:16:08 AM


So is need to get a power supply kit, and the rack kit? Easy build?

Cheers
Dave
Hi Dave,

yes,the rack is super easy to build.
For the external psu please read through this thread:

http://groupdiy.com/index.php?topic=48959.0

To keep this thread (about the VP28) clean please post and discus over there.
Take your time reading and ask if necessary,

best,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on July 29, 2014, 03:19:12 AM
Hi Guys
I just made a pair of these with 1731/2520 combos in each and in use I'm realising one of the 2520's I built has a problem - as notes tail off there's a distorted sizzle. I've checked by swapping everything else around (glad I made a pair to make that job simpler!) and I'm pretty sure it comes down to the 2520. I just wondered if any of you had encountered this issue or could point me to which component can generate sizzly overtones (not just the nice drive they do when you ask them to)? I should re-wet each solder joint I guess. I did make the classic mistake of putting 139/140 power transistors in wrong way round so they've been overheated for sure during removal, just don't know if this is how that would fail. It's cheap to make a new one - I'm just in uk so bit of a wait for customs to decide if it's a DOA or an IED...
Any help v welcome
Cheers
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 29, 2014, 11:51:46 PM
Powered up my VP28's and listened to each for a second and both appear (so far) to be working perfectly! LED's, switches, all working (I think; cursory test, but had to come say "HOORAY!")

If I can do this, anybody can! Just take your time. My first one took 14 hours. The second one, ten. Op Amps about two hours each. Nothing was difficult, but I had to be diligent and careful the whole time. Before this I only soldered cables and one Group DIY reamp box.

Thank you Jeff, Chunger, Udo, Group DIY, the folks talking about this stuff on Gearslutz, etc. I don't know any of you, yet your generosity and skill have helped me a lot.

Six more slots in this rack and an empty wallet...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on July 30, 2014, 01:38:22 AM
Great to hear,you'll have a lot of fun working with them,promised!
Bazillions of sounds possible by just changing the ratio between input (gain) and output (fader) stages.My first choice for a lot of tracking jobs meanwhile.

Best regards,

Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on July 30, 2014, 04:03:26 AM
Got it all hooked up to my rig. Tested both a dynamic and a condenser mic. Everything works. TONS of gain.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on August 01, 2014, 04:48:14 AM

I just made a pair of these with 1731/2520 combos in each and in use I'm realising one of the 2520's I built has a problem - as notes tail off there's a distorted sizzle. I've checked by swapping everything else around (glad I made a pair to make that job simpler!) and I'm pretty sure it comes down to the 2520. I just wondered if any of you had encountered this issue or could point me to which component can generate sizzly overtones (not just the nice drive they do when you ask them to)? I should re-wet each solder joint I guess. I did make the classic mistake of putting 139/140 power transistors in wrong way round so they've been overheated for sure during removal, just don't know if this is how that would fail.

Just in case anyone else has had the same problem as I did, and is scanning through the thread for a fix, I thought I'd mention that re-wetting all the solder joints cured it and only took a few minutes. Couldn't see which one was bad but it's gone now. Thanks so much to Jeff and Chunger for the great kit and instructions respectively. Just finished first full track with everything going in through it and it's absolutely what I'd hoped for - I may never have to reach for the Decapitator again with drive like this available on the way in!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: martin bluenoise on August 23, 2014, 11:05:51 PM
Hi all, I guess this is my first post even though I've lurking around for quite some time now.

I finally went ahead and ordered 2 vp28. upon getting them, i was amazed at the overall quality of all the parts.

anyhow, just by being patient and thorough, i managed to get them both working at the first try!!

before installing the opamps i measured something incorrectly, and jeff was there to answer my questions and get me sorted right away. trully amazing service.

chunger's step by step build guide was also a great source of information and confidence. and so was this whole thread.

thus, to sum it up, thank you all, and cheers to an amazing product!!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Ultralag on August 27, 2014, 04:23:50 AM
Hi,
I registered just to say thank you!
The building of this kit was fun and quite easy following the step by step photo guide and the whole thread.

I never soldered before building this kit and never cared about resitors or capacitors, didn't even know what they where!
I've been using my pres and stuff without real attention for what was going on inside the box and was looking for a new pre to add to my lunchbox and ended here...I bought this kit and in two weeks it was waiting for me on my new workbench, new Hakko 888D soldering station and 60/40 Multicore solder. Everything new to me.
It took many hours of work just because I triple checked everything and learned using a multimeter during the build.

I have no other experience but the kit is very very well built and everything is where it should be, perfectly organized and top quality and in the end I'm glad to say it worked on first try with dynamic and condenser mics. Plus, the HPF is a brilliant choice and I can see me using it often in the future.
I grabbed a 57, hooked it in my lunchbox and the sound coming out of the speakers was a pleasure to hear...well...twice the pleasure because it was DIY.
It takes attention and patience in the process but if I could do it so could anyone!
I'm really proud of it.

Someday I'll get another vp28.  ;)

Ultralag
Happy Italian Capi customer
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on August 31, 2014, 11:12:29 AM
I have built a pair of VP28's and while they both work and sound great, one is consistently 3dB louder than the other. I have 2520/1731 combos in each and have switched them around to check, but strangely it's not the opamps. I've checked resistor values all over each board. All that leaves are transformers really. Any ideas? It's workable, but not ideal...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 05, 2014, 09:32:44 AM
There is most likely a resistor error in the gain switch or fader area. If you can't spot it that way, you need to be scientific. Inject signal and trace/measure it thru the pre to see where things are different. There is a pdf for test points earlier in this thread.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: SgtAlexx on September 28, 2014, 02:49:19 PM
Hi guys.

Just built my second VP-28. The first one works perfectly, but the second has an issue.

All lights light up when hitting the knobs, but there is no sound or no green signal light lightning up when feeding sound to it.

To be honest I have no clue what so ever how and where to start the troubleshooting so any help is highly appreciated!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: obigwan on October 03, 2014, 11:05:32 AM
Hello everybody.

This is my first post and i'm glad to entered the diy group with those very nice preamp.
I recently build 8 VP28 and finally had 2 defective units.
For the first one, the green led was on all the time. I found i had a bad strip between C13 and R14. This problem is now resolved.
For the second one, the sound goes in and out of the preamp but my gain knob have any impact on the preamplification. Only the fader stage seemed to work properly. I tried with other "good" opamp (2520 and 1731) but the result is the same. Any ideas?
Cheers

Arno
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 03, 2014, 11:14:04 AM
Arno

When you say "bad strip between C13 and R14" what do you mean exactly? PCB trace or a short? C13 and R14 should not directly connect to each other. I'm interested to see what you did here as a few other folks have had this same issue. I have never had the problem or been able to reproduce it.

For the other one, it sounds like you have an intermittent solder joint somewhere on the PCB. Sometimes the physical impact of the switching action can cause the bad solder joint/audio to go in and out.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: obigwan on October 03, 2014, 11:33:30 AM
Hi Jeff,
My "bad strip" was a short due to a bad solder from one point of R14. I also forgot to precise that i found another bad solder on the EA2622.
I will check all my solder joint and i will let you know.
Thanks for your help.

Arno
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: obigwan on October 03, 2014, 12:26:05 PM
Ok, so i checked my solder joint. I plugged my mic and...no sound...until i play with the grayhill knob (gain knob). I tried to "move" the knob and sometimes i have sound , sometimes not. I verified the solder and it's ok( it seemed ok). Now i have serious doubt on the knob. Do you think it's possible Jeff?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 03, 2014, 12:56:04 PM
This still sounds like a bad solder joint. I had the exact same thing on a proto build. I naturally thought it was the Grayhill but turned out to be a iffy solder joint on a coupling cap clear across the PCB. You could run signal thru and start touching components around the board with you finger and see if you can find the intermittent joint.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: SgtAlexx on October 05, 2014, 04:35:59 PM
So no reply for me  :-\
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on October 05, 2014, 04:49:14 PM
So no reply for me  :-\
I think Jeff once posted a guide how to trace the signal through specific points.
Must be somewhere in this thread.
When you've found it look where the signal stops.
Also you can do the basic measuring like if the suplly voltages are correct on the doa sockets etc.
Maybe you can trace it with the good module side by side?
Then report back.


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: SgtAlexx on October 05, 2014, 04:59:04 PM
Thanks.

@Jeff: Could you point me to the guide?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 05, 2014, 05:03:38 PM
SgtAlexx, many people post with the same problems. If you go back thru this thread, the advice on where to start never changes.

First, rule out any outside possibilities like rack slot, associated cabling, converter settings and so on.

Next rule out opamps. Swap with other known good working opamps and see if the problem follows the opamp(s) or stays with the pre. Are the opamps from kits? It is very hard to test a newly built pre with newly built opamps. That's too many variables. Also, are they seated properly? http://classicapi.com/catalog/DOA_Install.php

The next step is as Udo says. Find the test points PDF and follow your signal thru the circuit.

There is not always a magic bullet or free lunch when a kit doesn't work. Most every time it will require some systematic diagnostics by the builder.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: SgtAlexx on October 05, 2014, 05:09:27 PM
Wasn't looking for a free lunch, just a point in the right direction. These two beauties are the first ones I ever made and apart from checking rack slot, DOAs and so on I'm stuck on troubleshoot.

The test points PDF was what I was looking for. Thanks.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: obigwan on October 06, 2014, 09:04:46 AM
Hi everyone.

Just to let you know that i found the problem. The grayhill gain knob was the problem. I  changed it with one from another module and everything is going well now.
Bye

Arno
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on October 08, 2014, 11:18:14 AM
I've had my pair of vp28s for a few months and really love them. These are a must have!
Now I just need more.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on October 22, 2014, 10:39:07 AM
I'm getting closer to finding the level discrepancy between my pair - I just noticed it only happens in Mic mode, not line. So I just wondered which components are engaged when Mic button is pressed since I guess they're the culprits (or joints thereof)?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Gilgenberg on November 09, 2014, 05:04:30 PM
Hallo i have a problem:
I finished my VP28 and :
the HP does not work properly. :-[
when 80HZ is engaged the signal is muted.
40HZ and 6 + 12  works great
if 40 HZ is engaged and i add 80HZ signal is getting a bit lower, but not much.

The rest works great and i like the sound very very much
Also gain and impedance fit´s perfect to my Royer Ribon Mic
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 09, 2014, 07:38:59 PM
Sounds like a bad solder joint on that pushbutton switch or one of the caps on the filter board. I would try reflowing those.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Gilgenberg on November 10, 2014, 05:57:56 AM
 :) Thank you Jeff!

Did it!

Works great!

Andreas
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: weiss on November 10, 2014, 12:05:29 PM
Can't wait for my two vp26 to be delivered ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: novisplit on November 11, 2014, 10:45:39 PM
Hi. I’m building my 6th vp28! I’m getting no output. Green light shows signal, reflowed solder and I’ve switched opamps. Any advice on where I should be placing my anxieties?

Thanks.

David J.

Update: Checked all polarity and resistor values and nothing. I notice if I push my board I can hear signal that is faint. Preamp gain changes volume but fader knob has no effect. Also, if I pull opamp 2 out it has no effect. Could it be a bunk transformer?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 12, 2014, 06:07:38 PM
Jeff,

On one of my VP28 builds, with the HPF disengaged, the -6/-12 switch cuts the output when in the -12 position but not for -6. When the HPF is turned on, everything works fine. I didn't see a schematic for the preamp to aid in chasing down the culprit. I resoldered any suspect connections, but to no avail. Any thoughts?

Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 12, 2014, 07:15:25 PM
It is most likely a bad solder joint on the toggle or on the HPF PCB. Also reflow the solder on the 5-pin 90 degree header. The audio enters and leaves the HPF PCB twice. Once for -6 and again for the -12 position.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 12, 2014, 08:42:10 PM
How bout that. Apparently one of the switches on the HPF frequency selector switch had one of the pins not completely soldered to the HPF PCB. Usually stuff doesn't work because either my bad reading skills or intermittent dyslexia causes me to misplace components. Nothing like a bad solder joint to ruin your day.

Thanks for your help!

Side note, I did a live recording using an Earthworks SR40V on the vocal running through your VP26 with Red Dot DOA and it sounded fantastic. I was very pleased at how it could sit back in the mix without hardly any Eq or compression but still remain perfectly intelligible. I wished I had been building my own gear all these years and saved myself a ton of cash.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: novisplit on November 14, 2014, 06:13:04 PM
OK. So I checked all the test points and checks out good and getting the same readings from a good unit and a bad unit. The only thing that is different on the bad unit is that C20 negative wire doesn't seem to be connecting to the red and orange wires on t3 like they do on the good unit. I flowed some solder over to connect the negative lead and the orange wire but still no sound. Any ideas?

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 14, 2014, 06:22:45 PM
Desolder all of the leads for T3 and measure DCR.

Set your DMM to read less than 100 ohms and check
red to orange
brown to black
green to blue
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: novisplit on November 14, 2014, 06:59:56 PM
Looks like the board wasn't connecting the negative lead of c20 and the red wire from the transformer. Totally fixed now.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 14, 2014, 07:09:20 PM
The trace was missing? Can you send me a pic. I don't recall ever having that happen before.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: burn4ever on November 19, 2014, 12:43:40 PM
Thanks a lot for this extremly well documented manual  ;)

Are there any advices what I should check with a multimeter before I setup the preamp for the first time?
 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 19, 2014, 02:15:14 PM
Just make sure you have no direct shorts from either of the main voltage rails to ground and then follow the tips in the VP28 Assembly Guide from the first post of this thread.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: burn4ever on November 20, 2014, 02:38:23 AM
Hey Jeff,
thanks for the info. One last question.
Is there a circuit diagram or any info about how many volts the DOA sockets must have?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 20, 2014, 10:45:28 AM
Typical for 500 series racks, the sockets should have appx +/-16V on them respectively.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on November 23, 2014, 10:35:29 AM
Building a couple more of these, and me likes sorting sheets :)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: markphilpot on November 24, 2014, 01:54:49 PM
So I just finished my first pre and I, unfortunately, am dead in the water so far. I get a Signal light, intermittent LEDs from the other 4, and no actual signal when I turn up the gain…the polarity, 48V, and pad will make a popping noise, but that's it.

Being my first build, not sure where to start on trouble shooting this guy. Any thoughts ? Any help is appreciated.

MP
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 24, 2014, 03:50:01 PM
Did you build the DOAs or buy them pre built? If you built the DOAs, do you have any know working ones to do some testing? Also, double check all of your solder joints, even if you are experienced at soldering. I recently had a bad connection even though it appeared to be soldered correctly. And lastly, there is the tedious task of checking component placement. Meter all of your resistors and compare them with the BOM and PCB overlay. I've built a number of Jeff's preamps and every time I had an issue it was one of these things. These are some of the best kits available and double checking your component values should yield a fantastic sounding mic preamp.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 24, 2014, 04:23:17 PM
Did you build the DOAs or buy them pre built? If you built the DOAs, do you have any know working ones to do some testing? Also, double check all of your solder joints, even if you are experienced at soldering......
+1, exactly what I was about to say!

Also be sure the opamps are fully seated. I have said this numerous times in all of my support threads but it gets missed often for some reason.  http://classicapi.com/catalog/DOA_Install.php
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: markphilpot on November 24, 2014, 05:40:34 PM
 thanks…I've been reading through the thread…The transistor switching on the op amp was an issue, but I've fixed those…Not sure if this has any bearing,  but my LED for signal is always lit up, and the other 4 now don't light up. Still no actual signal coming through.

Checked the seating on the DOA, seems ok.  Don't have any other DOA's to use, though I'll probably by some assembled ones from the store to check.

I'll start running through component checks with the DMM and see if i can figure anything out there. At risk of displaying my noob soldering - here's a pic



MP
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 24, 2014, 07:11:49 PM
I've had ye 'ole transistor switched before and found it easier and more reliable to just build a new kit. The very tight spacing of the components leaves very little margin for error and unsoldering and resoldering significantly increases the chance of ruining the component you're swapping or even the board itself. Having a pair of pre built DOAs to do testing I believe is a must if this is your first time. Also, don't forget that LED's are directional (have polarity).

On another note, the VP28, as it says on the CAPI website, is a more challenging of a build. I actually had built several of the VP25's and VP26's before attempting the VP28. I'd recommend building a variable version of a VP25, 26, or 312 to get a better feel for these designs and hone your soldering skills. No, I don't get paid to promote CAPI's  products. But for myself being someone who not too long ago got into the DIY arena, this is probably the best advice I can give someone else who is recently new to all of this.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: markphilpot on November 24, 2014, 08:31:21 PM
thanks PC -
   I went back and checked the polarity on all LED's - they're correct - must be something with my soldering…resistors seem to check out. I knew I was biting off a lot for being a first build but thought I'd plow through and see - this may be where the rubber meets the road, eh.

Thanks - I may be out an expensive lesson, but I'm going to keep trouble shooting and see what I can find. Thanks again - and I'll likely invest in a 312 or 25/26 very soon!

MP
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: markphilpot on November 25, 2014, 06:54:06 PM
I've ordereed 4 DOA's from Gary and those should be in later this week…I built another 2520 to test b/c one was smoking (literally).  Here's an update:

I'm getting the signal lighting up when I clap/talk etc, phantom power, all LED's work, but signal is very low and distorted. I found the signal PDF referenced, but don't know how to actually test that…so I'll ask - Could any of you vets point me to a link/site that shows how to test signal? I've got a DMM and that's it.

Learning a ton - thanks for the help!
MP
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 25, 2014, 08:07:35 PM
You can inject a sine wave and use a DMM to measure the AC voltage. That is how the test points doc is written. The freq should be low like 100Hz or 400Hz.

The sig present circuit is at the output of the first or preamp stage so that could be OK, from your description.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: michaelwinston on November 27, 2014, 07:59:13 AM
Hi guys,

I'm building my first VP28 and am populating the board currently.  I've done SCA projects as well as a few pedals so it's not my first build by any means.

I have either misplaced or was not sent the TI RC4558ip and the socket.  I had a socket laying around and I also have a couple of different 4558's laying around from pedals... one is a JRC4558D and the other a TI RC4558P.  I'm assuming these are suitable replacements... but wanted to ask someone who might know for sure.

Thanks!
-Michael
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 27, 2014, 10:39:53 AM
Michael, either one will work fine. You don't really need to have a socket.

If you have misplaced those, you have also misplaced the 5-pin 90 degree header for the HPF board to main PCB connection. Those 3 items are all together on a small piece of pink foam. With the way we pack/sort the kits, it is impossible that we missed them. They are counted in 50pc lots by 2 different people.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 27, 2014, 11:57:11 AM
Long-haired carpet? ;D
That's the place a lot of little things go to.


Best,


Udo. ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: michaelwinston on November 27, 2014, 04:57:44 PM
I just got to the hpf so I haven't missed the 5 pin header until now...

My guess is they got thrown out or they possibly got mixed up in packing material of something sent off earlier this week...  :-[

Is it possibly for me to order that single piece?  Or add it to a future order?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 27, 2014, 05:01:33 PM
Yes no problem just remind me. They are always packing inside the pink bag with the resistors and diodes for the main PCB.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: markphilpot on November 29, 2014, 05:22:23 PM
All,
  I just got in 4 red dot DOA's from Scott so I could test the pre….holy crap - it works perfectly. I'm stunned. Turns out I just can't build the 2520's very well!


Thanks Jeff and Chunger, etc!

   I think the pins I have on the 2520's got bent a bit or something and maybe that's where the connection was shorting…

Anyway, I'm thrilled and excited to start on VP28 #2. I may be hooked.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on December 02, 2014, 04:23:58 AM
Jeff,

On one of my VP28 builds, with the HPF disengaged, the -6/-12 switch cuts the output when in the -12 position but not for -6. When the HPF is turned on, everything works fine. I didn't see a schematic for the preamp to aid in chasing down the culprit. I resoldered any suspect connections, but to no avail. Any thoughts?

Thanks!

I have this problem too on one of my recent builds (that I emailed about, but decided to bring it here instead). But only occasionally and the -6/-12 switch is 'scratchy'. I've reflowed the switch, all pads on the HPF and the 5 pin header and replaced op amps (just in case..) but still no change. HPF works fine btw.

I'm about to order some Marconi knobs so I wonder if it's possible add a new -6/-12 switch and a new 5-pin header to that order? If you think that's a good idea.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 02, 2014, 10:31:48 AM
Just remind me when you order and I'll include a new switch for you.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: michaelwinston on December 03, 2014, 11:44:58 AM
Jeff, after you mentioned the IC, Socket, and header were packed in a bag with some of the main components I knew it couldn't have gotten thrown out so I searched everywhere only to find the parts sitting on my work table.. (you know, the place I had looked about 10 times...).  So, that being said, I finished up the pre, a 2520 and a 1731 and fired it up.  It sounds great! Now I just have to figure out what I'm going to build next...
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on December 03, 2014, 12:11:03 PM
Now I just have to figure out what I'm going to build next...
Easy one:As many of them as you can,life is short! ;D


Welcome to the forum and congrats to your build!


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on December 30, 2014, 11:47:24 AM
Hello.

I just finished my first VP28. I did two! Great kits, a pleasure to do. Thank you Jeff!

Now I started to make four gar2520 opamps. But very soon I was made mistake: I pulled out to my table all pins AND ALL the Q1-Q2 matched pair transistors from four bags/kits!  :o :'(

Now I have all of the eight matched transistors in front of me now. Whether they are matched in pairs, or are they all matched each other?

What I can do now?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on December 30, 2014, 01:26:20 PM
Hello.

I just finished my first VP28. I did two! Great kits, a pleasure to do. Thank you Jeff!

Now I started to make four gar2520 opamps. But very soon I was made mistake: I pulled out to my table all pins AND ALL the Q1-Q2 matched pair transistors from four bags/kits!  :o :'(

Now I have all of the eight matched transistors in front of me now. Whether they are matched in pairs, or are they all matched each other?

What I can do now?
Can you measure hfe?
If so then use the best matching as pairs.


Cheers,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on December 30, 2014, 02:08:05 PM
Can you measure hfe?
If so then use the best matching as pairs.
No. My multimeter hasn't HFE measurement feature.
Situation is now this that I have all calibrated transistors together on the table. Do you know are they all matched each other or only per couple?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: weiss on December 30, 2014, 02:10:57 PM
No. My multimeter hasn't HFE measurement feature.
Situation is now this that I have all calibrated transistors together on the table. Do you know are they all matched each other or only per couple?

They are matched as a couple! that's the reason why you should get a multimeter with hfe measurement.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on December 31, 2014, 07:14:16 AM
My two first VP28 are soon ready for testing.  I would like to test these prior to use. I have read through this thread, and I do not find clear instructions. Also instructions from classicapi.com do not contain any information about the testing procedure.  :-[

I'm going to buy from classicapi.com "500 Series Jig Extension Bundle (http://classicapi.com/catalog/product_info.php?products_id=406)", but in addition, I would like to know that when I connected the VP28 to rack with this extension jig so what do I need to measure? What values must be found and where from pins?

It would be great if someone would make a step-by-step instructions how to test the VP28 after the assembly. I would really appreciate if such instructions will be available. The plan is to make a few more VP28 and maybe a couple of classic LC53A (Love Child) equalizers. I'm really interested in learning more, and to make more these fine products from classisapi.com.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on December 31, 2014, 02:10:38 PM
@drumski


Hello and welcome,


there's no "complete guide" existing,we all have to read through this thread.
Getting the jig from Jeff is a good idea,it also has a provision for cmrr adjustment.You don't need this for the vp28,but you want it for the lc53.Good to have it then.


First check the voltages on the doa sockets with the doas removed.
If all is good you can insert them and do some basic tests.If something is wrong stop doing anything else but solving problems first!


One of the most often happening thing are the doas themselves,try to have a pair known as working for testing and/or using.Make sure they are seated properly.


Then check if all leds work when switching the corresponding knobs.


Next will be audio tests,e.g. unity gain test in line mode,do the gain and fader switches work,does the hpf work etc.


Hope to have helped and a happy new year,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on December 31, 2014, 02:35:22 PM
First check the voltages on the doa sockets with the doas removed.
Thank you. This is just what I was thinking… but what voltage values from what pins? This is what I need to know…
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on December 31, 2014, 05:13:50 PM
First check the voltages on the doa sockets with the doas removed.
Thank you. This is just what I was thinking… but what voltage values from what pins? This is what I need to know…
Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.


Hope that's clearer now.


Will you have working (pre-built) doas then?


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on December 31, 2014, 06:15:28 PM
Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.

Hope that's clearer now.

Will you have working (pre-built) doas then?
Happy new year 2015 and thank you for your advice.  Now I know what I need measuring and from where.
My four gar2520 DOA Full Kit Bundles are now under construction. But first I need to go local electronic shop and buy multimeter with HFE-feature because…   :o :-[  :'(   (read my first post above).
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: weiss on December 31, 2014, 08:38:06 PM
take your time with the 2520's! they are beasts
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on January 01, 2015, 02:27:32 AM

Member weiss is right.
That's why I asked if you will have a known-as-working pair when you start testing.
Patience is the key!
They are a very dense build.Try to follow the build instructions exactly.
Non-working doas make about 90% of something going wrong here.
If you have any questions regarding them please ask over in the gar2520-thread:
http://groupdiy.com/index.php?topic=33970.0
Gary is a very helpful guy,same as Jeff here.
Have you ever built any doas?Or do you have a pair from another build for testing?
And about the hfe tests and matching,maybe you can borrow a tester from a friend or so.But in case you need a good or better dmm anyway then it's a good feature to have it of course.
Personally I bought a simple transistor tester in kit form for about 25 Euros or so,you'll find many of them on the internet.They are not precision instruments but do a good job for this purpose.
I always check all parts before assembly.A bit time consuming but keeps you away from running into serious trouble.


And now up to happy soldering,keep us posted,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on January 01, 2015, 06:10:02 AM
kante1603 and weiss, thank you for the kind assistance. Nice to know this gar2520-thread  :)

This is first time when I built doas. Yes, I can test with other gar2520 doas. They are in the one VP28 in our small studio of our church. So I can use these, if necessary.  I need now to wait over weekend when I get my new and better multimeter in HFE-feature. So my doas are now in a half way.

My feeling has been very positive to build these babies and my opinion is that these VP28 and gar2520 doas are not difficult to build. Jeff and Gary has really done a great job in creating these products. Of course, the gar2520 is very dense and it need very stable hand, good eyes and good soldering iron… I have these all  ;).  My faithful servant from late 80s, Weller WECP-20 with tiny soldering tip  8).
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on January 01, 2015, 01:52:35 PM
Great to hear you can use working doas!This is an immense help to sort things out.
And you have the Weller....Mine at home is even from the early 80s.....and still working like a horse!
So you are well prepared!


Cheers and have fun,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on January 03, 2015, 03:10:46 AM
Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.
Before I connect 16V DC to terminals 12 and 14 I need to make sure the location of the poles & order. See the attached picture. Have I understood correctly this order and reading direction? P.S. This is first time for me so I don't want to burn anything…  ;)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: weiss on January 03, 2015, 06:03:34 AM
just insert it and make sure the upper connectors go right in the upper socket. so if you don't use the additional voltages the lower two sockets are left empty!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on January 03, 2015, 06:12:09 AM
just insert it and make sure the upper connectors go right in the upper socket. so if you don't use the additional voltages the lower two sockets are left empty!
I do not have in my workshop  the system 500 rack. I would just like to make this above-mentioned test with laboratory power source and I want to be sure that I put DC 16V to correct terminals.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on January 03, 2015, 07:08:25 AM
Looks o k. to me!


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on January 07, 2015, 02:09:00 PM
Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.
Tomorrow I will go to the studio and to test these my first two VP28 preamp. But first, before I will put DOAs in place, I tried to do this test in my workshop but I get the multimeter readings as follows:
Black probe: C
Red probe: +V   (I got +0.59 VDC)
Red probe: -V   (I got -14.43 VDC)

What goes wrong? I put +16 VDC to pin 12 and -16 VDC to pin 14.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on January 07, 2015, 03:20:28 PM
Set your dmm to dc voltage reading.
Black probe to ground ("C"or "common").
Red probe on V+ should give around +16vdc,probe on -V should be -16vdc.
You might have a little drop on both rails due to the protection diodes (roughly -0,7v),this is o.k. then.
Test both doa locations.
Tomorrow I will go to the studio and to test these my first two VP28 preamp. But first, before I will put DOAs in place, I tried to do this test in my workshop but I get the multimeter readings as follows:
Black probe: C
Red probe: +V   (I got +0.59 VDC)
Red probe: -V   (I got -14.43 VDC)

What goes wrong? I put +16 VDC to pin 12 and -16 VDC to pin 14.
The supply setup is wrong.You need a dual rail supply with ground (0v).It will not work like this.


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on January 07, 2015, 03:31:53 PM
OK. I will made tomorrow this voltage testing (without all DOAs)  with our studio's 500 rack. If I get correct voltages from pins then I will put DOAs and IC to PCB. Thank you for your fast help!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on January 14, 2015, 12:17:18 AM
Woohoo. I just built four more VP28's. I had built two a while back. Those two came out perfectly. These four... not so much... Two seem to be fine. The other two have problems.

When I plugged one of the VP28's in, there was a real cool glow and a wonderful smell... Turns out I fried a few resistors in the Opamp in A3, a GAR 1731. Specifically, I can see that R15 is fried and possibly R8 next to it. (R8 might just have soot on it from being so close to R15.) All the way on the other side of the board R14 also fried. I can't see if anything else fried. So... how do I know why they fried? I don't want to stick another 1731 in there and have it fry if it's something on the VP28 that's causing it. I hope it's just the Opamp itself and I can just build another or even fix the one I have.

One of my other VP28's seems to be working well except that there's a low level distortion, like a kazoo, happening all the time. It's present when the knobs are all the way down and does not get louder as I turn the knobs up. It's subtle, but it's there. I've eliminated the microphone and cabling, etc., as the source of the problem. I will now attempt to post a link to a sound file:

https://dl.dropboxusercontent.com/u/1643668/Kazoo.mp3

I started with both knobs all the way down and then turned up just the top knob and then turned it all the way down again before doing the same with the bottom knob.

As always, I appreciate all the help from you guys!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on January 14, 2015, 12:41:43 AM
Found the answer to my 1731. I managed to reverse Q7 and Q8. I shall do the necessary repairs...

I'd still appreciate help with the kazoo sound, though.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: drumski on January 14, 2015, 02:44:09 AM
OK. I will made tomorrow this voltage testing (without all DOAs)  with our studio's 500 rack. If I get correct voltages from pins then I will put DOAs and IC to PCB. Thank you for your fast help!
Everything works perfectly!  ;D
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on January 15, 2015, 07:41:08 PM

One of my other VP28's seems to be working well except that there's a low level distortion, like a kazoo, happening all the time. It's present when the knobs are all the way down and does not get louder as I turn the knobs up. It's subtle, but it's there. I've eliminated the microphone and cabling, etc., as the source of the problem. I will now attempt to post a link to a sound file:

https://dl.dropboxusercontent.com/u/1643668/Kazoo.mp3

I started with both knobs all the way down and then turned up just the top knob and then turned it all the way down again before doing the same with the bottom knob.

As always, I appreciate all the help from you guys!

Trying to bump this to get a response... I also figure I should clarify that that is my awful voice in the link. If you listen carefully, however, you'll hear, especially when the volume is down, a bit of distortion. There is no distortion if there is no signal, hence the need for the silly vocalization.

Thanks again.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chuckles on January 19, 2015, 11:23:31 PM
With no load it is common for 48V supplies to rise above their rated voltage by 10 to 15%. 55V is nothing unusual. When a multiple output regulated power supply is used like in the Workhorse, the regulation is usually monitoring the + side of the lowest voltage output. In this case the +16V line.

This means that with no load or a light load on some lines, the other lines will be high or sometimes a little low.  Seeing 55V on a phantom output is OK as when the phantom powered loads are turned on, the voltage will fall to the correct range.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on January 28, 2015, 12:40:06 PM
Jeff,

On one of my VP28 builds, with the HPF disengaged, the -6/-12 switch cuts the output when in the -12 position but not for -6. When the HPF is turned on, everything works fine. I didn't see a schematic for the preamp to aid in chasing down the culprit. I resoldered any suspect connections, but to no avail. Any thoughts?

Thanks!

I have this problem too on one of my recent builds (that I emailed about, but decided to bring it here instead). But only occasionally and the -6/-12 switch is 'scratchy'. I've reflowed the switch, all pads on the HPF and the 5 pin header and replaced op amps (just in case..) but still no change. HPF works fine btw.

Just a brief follow up that after two failed attempts reflowing all pads around the HPF I've now replaced the -6dB/-12dB switch, and problem seems to be solved!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on February 24, 2015, 02:04:25 AM
Just built my first VP28.  Can't get any signal indication from the green led. 

I reflowed all my solder joints.  I have proper voltage in the opamp pins.  I double checked my resistors.  Everything seems fine.  So, I figure the problem is the DOAs.

I pulled the 990s out of my LOLA, and when those go in, none of the LEDs will light up.  I put the Gars(two 2520) in the LOLA, and it's LEDs illuminate very dimly.  Is there a difference between the two that I can't use the 990s to test the VP28?


Thanks!

Matt
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on February 25, 2015, 01:15:24 PM
Just built my first VP28.  Can't get any signal indication from the green led. 

I reflowed all my solder joints.  I have proper voltage in the opamp pins.  I double checked my resistors.  Everything seems fine.  So, I figure the problem is the DOAs.

I pulled the 990s out of my LOLA, and when those go in, none of the LEDs will light up.  I put the Gars(two 2520) in the LOLA, and it's LEDs illuminate very dimly.  Is there a difference between the two that I can't use the 990s to test the VP28?


Thanks!

Matt


The 990's that I have from Hairball work in my VP28s.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on February 25, 2015, 01:18:36 PM
Are your 990s for 24v or 16v?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on February 25, 2015, 01:48:06 PM
I used the resistors supplied with the kit for the 990s.  It says that makes 'em 16v.  I'm reading 15.8 across the common and v- v+ terminals on both opamp slots on the 28. 

All that is to say, I have no idea, but I think 16v.  Which I thought would be compatible with the vp 28.  But, the Gars at least let the vp 28 turn on.  No sound, or signal indication, but my two working 990s won't even give me lights on the other leds.  The gars do. 

Hmmmm.  Any thoughts?  I greatly appreciate the help.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: benediwa on February 26, 2015, 08:53:51 AM
Hi!

I have just built two CAPI VP28 kits but Im having trouble with one of the units.
It does not register the input signal though there is clearly power. I've tried changing the OP-amps and using the ones from the build that works, but its still not registering input.

Using the test point image from an earlier post I have measured the volts at the test points with the following results:

Test of CAPI VP28 does not register input signal:
(+/- 0.5 mV)

Point 1 : - 1,5 mV
Point 2 : - 14, 35 V
Point 3 : - 1,5 mV
Point 4 :  - 1,5 mV
Point 5 : - 1 mV
Point 6 : - 8,8 mV

Test of CAPI VP28 in working condition:

Point 1 : - 0.6 mV
Point 2 : -  9, 2 mV
Point 3 : - 0,5 mV
Point 4 :  - 14,5 mV
Point 5 : - 11 mV
Point 6 : - 8 mV

Im not sure where to go from here though.

Help would be greatly appreciated!

-B
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on February 27, 2015, 07:58:07 PM
So, I got a pair of red dots, and now the pre seems to be working.  Except no LEDs work.  I get signal.  But no light show.  I put in the gar2520s I built, and lights, but no signal. 

Has anybody experienced this?  Any thoughts?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 27, 2015, 11:06:08 PM
So, I got a pair of red dots, and now the pre seems to be working.  Except no LEDs work.  I get signal.  But no light show.  I put in the gar2520s I built, and lights, but no signal. 

Has anybody experienced this?  Any thoughts?
Hello,


seems you have multiple errors in your build.
So let's start with the audio first.You say "now the pre seems to be working".From your previous posts I can see you have tested GAR2520 and 990s.Did you build them yourself?
With red dots it seems to work you say,We must have this confirmed first to get it all started.
So please check this first,do a unity gain check.If it is o.k. check all switches for functionality,then the gain and fader positions.What do you get?


For leds not working,it's seems you might have one or some reversed.
For input led not working,might be a) led reversed,b) 4558 chip reversed,c)both.
Once the chip was reversed and powered up and will be dead forever.


But first do the audio stuff and stick with the red dots.


Best regards and good luck,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on February 28, 2015, 01:00:34 AM
Hey Udo,

Thanks for the response.  Definitely multiple errors.  :-[

All sonic functions are working correctly with the red dots.   Phase, mic/line, phantom, and the pad.  Hpf works.  Gain and output fader are operating correctly.  Unity is correct.  Signal is clean and fat and beautiful.   

No lights. 

When I put in the gars, all lights work properly.  No sound. 
I've double checked all the leds, they are in properly, although green was backwards initially, providing a constant beautiful green glow.  Now it's in th right way.   No love.

Ic and oc socket are in properly.  Double checked.   

My red dots only have 5 pins, no pin for common, which I assume is normal.   

Obviously my build on one of the gars was not right, but lights still come on with gars, but no sound.   Killer sound with red dots, no lights.   

Thanks for any thoughts or help. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Unit7 on February 28, 2015, 05:55:25 AM

I've double checked all the leds, they are in properly, although green was backwards initially, providing a constant beautiful green glow.  Now it's in th right way.   No love.

Obviously my build on one of the gars was not right, but lights still come on with gars, but no sound.   Killer sound with red dots, no lights.   

Not my intention to be Mr Obvious, but some things in your post makes me just want to check that you know that the signal led only should light up when the unit gets a signal strong enough.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 28, 2015, 06:19:33 AM
Not my intention to be Mr Obvious, but some things in your post makes me just want to check that you know that the signal led only should light up when the unit gets a signal strong enough.
Good point (Hi Paul ;) ).
I'm also confused about the other leds which just indicate the status of the switches,so not signal related at all.
Just to sum it up:The status leds work with GARs but not with red dots???
From what I understood this is just a led series-string.....how can a doa make them misbehave except of causing a short which will end up in a blown fuse in the psu? :o
The signal led is a different thing driven by the 4558.....


Strange.......


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on February 28, 2015, 02:24:24 PM
No problem with the obvious thing.  I am aware the green should only light up when signal is strong enough.  I just meant to say I know the green LED is able to work because I've seen it on, but no signal light at all.  Thanks Paul. 

Yes udo, none of the status lights work with the Red Dots.  When I put the gars in, they all function correctly. 

Everything sounds great, and I figure I screwed up one of my gars because I'm getting good signal with the Red Dots.  Still no lights though, including the signal LED, when I'm feeding a signal. 

 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on February 28, 2015, 04:34:47 PM
o.K.,we must ask Jeff then,I'm running out of ideas atm.
Still completely weird to me because I really wonder how different doas can affect even the status leds.
I wonder what would happen if the working GAR2520 is inserted together with one of the red dots......
You said you have messed up only one of them,so why not try it.......


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on February 28, 2015, 05:29:31 PM
Alright, so, now it's extra weird.

Udo, thanks for that very logical problem solving solution.  It makes complete sense, which is why I never thought of it.

I have two Gars that I built. 

One of the gars passes signal, and I get good sound.  No LED function.  That occurs in either opamp position. 

The other gar, I get no sound, but LEDs function properly.  That also occurs in either opamp position. 

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: MattRoss on March 04, 2015, 08:43:24 AM
Hi Guys,

It wasn't till I built my second VP28 that I realized something was definitely wrong with the first one. The first one has gotten worse for wear over the past year, it didn't always sound quite this bad, but I'm wondering if you guys can help identify the issue. I've linked to a recording of the 2 VP28s back to back, the healthy one, then the troubled one.

The only functions engaged in both are the phantom power and mic button, no HP on either. The HP does work, further thinning the sound. Both 28's use red dots op amps.

AA-CM47 > VP28 (A/B) > Duet XLR Line +4dBu > Pro Tools.

I've already reflowed solder and checked just about every joint I can find. The troubled VP28 only operates with the gain pushed to maximum, before that there is no signal recognition whatsoever.

I appreciate any feedback!

https://soundcloud.com/ross-matthei/vp28-the-good-the-bad
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jswagler55 on March 07, 2015, 03:55:03 PM
Matt this is strange because as of yesterday one of my vp28s is doing the same exact thing as yours.  I just recorded a sample if i needed it and sounds just like yours with the gain and fader maxed out.

Only with mine it  really hasn't shown any issues till now. I have 5 vp28s and all have been build for about 6 months or longer with no real issues till now.  Mine has worked randomly for about 10 seconds and then goes back to the distorted / high pass filter kind of sound. I switched out op-amps with another one and that wasn't the problem. Switched slots on the power supply nothing different. Looked over soldering nothing looked suspicious. 

The signal light does look like its functioning normally which leads me to believe its something on the output.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: MattRoss on March 09, 2015, 04:39:48 PM
Yes, the signal indicator is working fine and op amps have been switched. I have one flipped resistor, is that a necessary fix? It's my understanding they're not polarized other than for identification purposes.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on March 15, 2015, 03:00:09 AM
Why are resistors rf5, 6, and 7 not in ascending order of resistance on the output fader when all the rest are?  At this point, just a random question as a double and triple check everything.

Thanks!

Matt
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mbradford6288 on March 15, 2015, 04:06:11 AM
I've triple checked everything. 

Same deal as before. 

One of my Gars works for sound, one allows the LEDs to work.  No love at all from the signal LED.  Red dots great sound, no LEDs. 

It appears line level, when I ran it again, is actually 8 db above unity.  Tried it the first time on a buddy's interface and may have just had it set wrong.  Double checked it in my studio, controlled environment, and I have unity at the -8 channel fader setting, with the preamp gain set at line unity gain.

I've triple checked all resistors, and most of the caps.  All of the transformer wires.  I resoldered the LEDs.  Replaced C8, Radial electrolytic because the original appeared to be bulging slightly on the top. 

One odd thing, it appears I have no resistance between lugs 1-4, and no resistance between lugs 5-8 on the input transformer.  Couldn't find a schematic of the circuit.  Is that right?  Or did I potentially catch the housing with solder?   I tried to pull it, but I don't have the right tools I don't think.   :-[

Thanks for any advice.  Frankly, it's working and sounds pretty great.  I'd like to get a second to have a stereo set, but I can't pull the trigger until I have this one cranking right. 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on March 24, 2015, 12:00:50 PM
Hi all!
I built some VP28 till now. Latest stereo pair I've assembled showed different values on my DMM. One VP28 A3 gives me (on 0/+V) a value of 13.95V, instead of A1 giving 14.99V.  The second VP28 gives (on 0/+V) a value of 14.95 on both A1 and A3.
I thought: damn, something is not right. So I inserted a GAR1731 and a GAR2520 (pre tested) on both VP28 and put them inside the 500 series rack (a Lindell Audio unit). I used both mic and line signals and.. both worked perfectly, no discrepancy in the stereo image at every step of both input & output fader. Every switches, led work. I have tried to put a sine wave at 1Khz and the result was equal on both channel.
Why is it possible if the DOA values on the DMM were different and a bit distant from the normal 16V?!
Thanks for the help & infos you'll give me!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jswagler55 on March 25, 2015, 09:25:35 AM
Yes, the signal indicator is working fine and op amps have been switched. I have one flipped resistor, is that a necessary fix? It's my understanding they're not polarized other than for identification purposes.

Matt did you reheat any of the HPF board cause I believe that is where my problem was. I did the rest of the board and nothing changed, then I reheated some of the pins on the HPF board and everything works again.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: MattRoss on March 27, 2015, 06:42:53 PM
That did it :)
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Fenix511 on April 23, 2015, 04:28:13 PM
Ok question for capacitors C1 and C12. My kit came with a 330uf 16v instead of 330uf 25v for C1 and 1uf 63v instead of 1uf 100v for C12. I assume it is fine but since the values are different then the BOM I wanted to be sure. Thank you for your time.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 23, 2015, 05:55:23 PM
Ok question for capacitors C1 and C12. My kit came with a 330uf 16v instead of 330uf 25v for C1 and 1uf 63v instead of 1uf 100v for C12. I assume it is fine but since the values are different then the BOM I wanted to be sure. Thank you for your time.
Yes this is fine. I changed the 330uF to 16V since it should never see more than 100mV or so of DC.

Vishay/BC discontinued the 1uF/100V that is white and looks like the 33uF so I had to use the closest substitution from them. I have not been able to hear a dif but the other ones are unobtainium so sadly this is how we will move forward. This cap should also never see more than 100mV DC so the voltage rating here doesn't really matter.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Fenix511 on April 25, 2015, 04:16:27 PM
Thank you for that speedy replay I finished up them and honestly they are even better then I expected and I already had 2 VP25 pres. Now I just gotta get some more money to get some more rack space.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on May 09, 2015, 08:47:43 PM
Hi folks,

I have six VP28's, five of which work very well. The other one has some strange symptoms. I believe this is the one I posted about a while back. (Due to life situations and an historically hellish winter here, I've hardly done any recording at my studio, and no full band stuff where I'd need all my inputs, until now when I'm noticing this issue.)

First thing I did was swap out my DOA's with ones from a working unit, so I know it's not theDOA's. Too bad. That would be an easy fix. The symptoms are a HUGE pop when engaging the pad, a huge pop when engaging the phantom and a bit of a pop when engaging the mic/line switch. Phase button makes no pop. If that was it, I could live with it, but none of my other units make these noises and more important, there is a bit of hiss and hum in this one and possibly some signal distortion. It's not that loud, but it is audible, especially at high gain. Both knobs affect the volume of the noise(s). None of my other VP28's make these noises. As far as I can tell, everything is in correctly.

Any ideas?

Thanks, as always!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 10, 2015, 11:02:20 AM
Are the input conditions identical when you are checking this? What is the source? Maybe there is excessive DC at the input of the module for some reason?

Popping sounds when switching usually means there is DC present on the switch contacts somewhere. Maybe there is a solder bridge or short somewhere with phantom or something? If it where me, I would measure for DC at gold fingers #8 and #10 with phantom off. There should be none. Then check again with phantom on.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on May 14, 2015, 12:00:07 AM
Thanks, Jeff!

Where are gold fingers 8 and 10? On the card edge?

And I guess I'll need the jig to test this thing. I messed up the build on the jig twice! It's the receptacles on the cable; I keep breaking them when trying to put them on. This sounds crazy, but can I buy a length of cable with the connectors attached?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on May 14, 2015, 04:37:43 AM
Thanks, Jeff!

Where are gold fingers 8 and 10? On the card edge?

And I guess I'll need the jig to test this thing. I messed up the build on the jig twice! It's the receptacles on the cable; I keep breaking them when trying to put them on. This sounds crazy, but can I buy a length of cable with the connectors attached?
Hi,


pin 8 is before pin 9,pin 10 after it (couldn't resist  ;D ).
Here's the pinout for both 51x and 500 series.
With the module held in normal upright position (the way it is mounted in a rack) pin 1 is on top.


http://www.51xaudio.com/alliance/pinout.html (http://www.51xaudio.com/alliance/pinout.html)


How do you manage to break the receptacle (pics please)?


Best,


Udo.


Edit: If you're unsure on finding the goldfinger pins you can measure the dc on the corresponding female xlr (input) too.Just measure dc between pins 2 & 3.Make sure the module is fully and correctly seated,phantom off.
This is a very first step to sort things out,a working test jig is most helpfull,you'll need it for other modules anyway.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on May 14, 2015, 02:17:01 PM
Thanks Udo!

At the risk of sounded totally lame, that chart doesn't help me...  I'm trying to figure out where these things are on the unit... and I'm assuming these are the contacts on the back of the card that slide into the receptacles on the lunchbox... ?

As for the jig, I must not be squeezing the parts together correctly because I crack them every time. I'm sure this is also a very silly mistake. I tried very hard to duplicate what's going on Jeff's photos.

As I've stated before, I really appreciate all the help I get from you guys and I'm grateful to Jeff for putting these things out into the world. I've built six of these things with remarkable success (just the troubles with this one) because of the help of this forum. I am careful and follow directions well... however, I am still truly a neophyte in terms of understand exactly what goes on with this stuff, so some of my problems and questions may make me seem far more hopeless than I really am.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on May 14, 2015, 02:26:31 PM
Hahaha.....well,we all started as newbies,didn't we? ;)
O.K.,yes the goldfingers are the ones on the back of the unit,so the part that slides in the rack connector.
Btw,if you have Jeff's test extension jig it's easy to figure out which pin number is where- some are printed on the small pcb.


Best,


Udo.


Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: aaronnewberry on July 08, 2015, 10:42:38 PM
I have a simple question but has anyone encountered a kit where C12 is not the correct value?
The directions indicate a 1uf, 100 v and I receive a 1u0-m, 36v
Any help would be much appreciated.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 09, 2015, 12:32:14 AM
I have a simple question but has anyone encountered a kit where C12 is not the correct value?
The directions indicate a 1uf, 100 v and I receive a 1u0-m, 36v
Any help would be much appreciated.
Yes this is a forced change since BC discontinued the previous 1µF/100V. The replacement part is also from BC and is blue instead of white and is a 1µF/63V cap.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: hamlyf on July 12, 2015, 01:29:50 AM
Hey guys, just noticed I may have had a bridge in one of my Grayhill switches. Wondering if that has caused any issues with the rest of the circuit?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 12, 2015, 11:04:12 AM
Hey guys, just noticed I may have had a bridge in one of my Grayhill switches. Wondering if that has caused any issues with the rest of the circuit?
No. It will just cause that particular switch to not fully function.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on July 24, 2015, 10:17:50 AM
Hello everyone,

What is the testing procedure for the VP-28. Which values should I read and where?
I can't find any guide for that.

Thank you!
G.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 24, 2015, 11:44:26 AM
With no discrete opamps installed and no power connected, just check for a direct short from both V sockets to ground. I then would make sure your +/-V's are near 16V at the opamp sockets. That is done under power with no discrete opamps installed.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on August 01, 2015, 10:24:15 AM
Hello,

I finished my first VP28, test is done and I get around 15.8V at the +/- V sockets when powered on. I don't know how to check for shorts Jeff.
But I have a major problem : NO SOUND AT ALL, only a white noise / hum sound that increases from 0 when I turn the channel fader up and decreases by the same amount when I turn it down... The preamp gain switch has no effect on the level of that noise.
The green signal LED pops (lights for a microsecond) when I turn the module on and pops again when I turn it off but even when talking into a mic it doesn't light on again so I guess no signal is coming in. Also a crazy loud pop buzz sound when turning on/off.

I encountered one problem during the bulding process which was implanting C12 in the wrong direction, I fixed that before connecting the VP28 to DC but maybe I burned it down when de-soldering?

I also built the 501 power rack unit from Soundskulptor to power my VP28, tests on this unit all gave perfect voltages.
By the way I don't have another 500 module to test the power unit or another power unit to test the VP28 module. I don't have any other opamps either...

:(

EDIT : I switched the Opamps position (2520 / 1731) and now I get a signal ONLY with the 2520 in A3 and the 1731 in A1. SIG LED works. Still a tremendous amount of noise anyway...

Is it an opamp problem?

Thank you,
G
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 01, 2015, 12:32:06 PM
Yes this sounds like opamp(s) problems. It is nearly impossible to test a fresh preamp build (especially 2-stage) with freshly built opamps. There are too many variables and we don't know where to start looking. You either need to test the opamps in something else or you need 2 solid, good, known to be working opamps to test the VP28.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on August 01, 2015, 02:28:26 PM
Yeah I get the point, I should have thought about that before ordering but, hey, this was my first build ever ahah.
I see you provide assembled gar's with kits only, where can I order some otherwise?

Anyway I'd like to thank you Jeff for all the help I got from you from the beginning.
G
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: gar381 on August 01, 2015, 03:21:52 PM
Yeah I get the point, I should have thought about that before ordering but, hey, this was my first build ever ahah.
I see you provide assembled gar's with kits only, where can I order some otherwise?


These guys sell them  :)

http://www.customanalogservices.com/?t=doas

GARY
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on August 02, 2015, 07:49:29 AM
I finally identified the problem. The 2520 is faulty, I didn't check for NPNs / PNPs. De-soldering failed so I just ordered another 2520 kit from you Jeff. Everything should be fine this time.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on August 08, 2015, 07:27:33 AM
Alright I will stop spamming the thread after this message ahah.

My VP28 is finally perfectly working!

I received my 2520 replacement kit this morning and carefully built it right away. In fact NPNs / PNPs were not the problem, Q5 was soldered backwards on my first 2520!

Note to every first timers/DIY beginners like me : CHECK TWICE, SOLDER ONCE. These wise words are not mine, they're Jeff's. I made 4 stupid (but hopefully fixable) mistakes building this kit because I wanted to finish quickly and have fun recording... Take your time.

Shout out to Jeff, Gary, Chunger and everybody on the forum!
G
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on August 08, 2015, 12:41:04 PM
Hi everyone, I'm almost finished building my first CAPI VP28 but I broke the green LED (Panasonic LN382GPX). Does anyone know where I can get a replacement? I also need a couple pieces of the clear tubing. Thanks!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 08, 2015, 12:43:59 PM
You can get them at Rapid if you are in the UK. The are Kingbright 2mm Lighthouse LED's now since Panasonic discontinued the original line I used. If you are domestic, shoot me an email and I'll mail you one.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: discord on August 11, 2015, 01:08:42 PM
It seems these new C12 caps (the blue ones) have no positive or negative markings. Anyone know how to tell which way is which?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 11, 2015, 05:20:14 PM
It seems these new C12 caps (the blue ones) have no positive or negative markings.
This is not correct, they are marked in 2 ways. There are a series of arrows with a "-" inside each arrow running down the side pointing to the negative end. The positive end has a groove running around it. This is true for nearly all of the BC/Vishay axial caps.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on August 13, 2015, 05:12:31 PM
Hi, I wonder if anyone can point me in any direction...

After finishing my build, I have VERY quiet audio with a mic and no signal present LED at any time. 
All of the other LEDs appear to work correctly.  I have an audix i5 connected, and both pots have to be cranked to get anything.

I don't have a way to do the tests with power connected, but when testing the resistance on the DOA sockets, all measurements are correct except between v+ and c.  For both sockets, this resistance measures exactly the same: 4.06 k ohms.

I'm going through and re-flowing the solder everywhere and checking everything with a magnifying glass.  I hoped with my symptoms someone might be able to help me focus on a particular area of the board.

I also double-checked every diode and cap to make sure they were installed the right direction.

One mistake I may have made was soldering the transistor all in one shot.  Would a fried transistor show these symptoms?

I'm going to do the line level test now so I have actual numbers.

Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on August 14, 2015, 05:42:37 AM
Are you using known to be working opamps? Did you try switching their position? I had almost the same problem except I had NO audio at all. One transistor was backwards on the DOA used for input.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on August 14, 2015, 11:38:20 AM
thanks for the reply.

Yes sir, my two red dots came to me already assembled.  I do have an opamp in my tonebeast that I can pull out for troubleshooting though if it still seems like a good idea.

I reflowed the solder everywhere but the same symptoms persist.  I did remove the opamps and the IC before soldering, so they were re-seated before I tried again.  But after it was reassembled before I tried it out I still get that 4.06k ohm reading when I measure the opamp sockets between "+v" and "c".    And the reading is exactly the same for both opamp sockets.

I did not reflow the solder on the hpf board though.  I will do that as well.  I saw in another post that the audio goes through there no matter what, right?

Also, one point that I don't have a lot of confidence in is the soldering of the wires from the transformers.  I had a difficult time with these.  I will give them special scrutiny when I take the hpf board off.
I'm definitely planning on buying one of those diagnostic extension cables when the capi store is open again.

thanks for your help.

one more question actually - I am for sure a novice.  I only built a few simple things before.  I was trying to find a spec on a good temperature to be soldering this project.  It might seem like an obvious question but what is a good setting?  I was working most of the way through the project at 713.  I hope that wasn't stupid, and I was careful not to touch a spot on the board for more than about five seconds.  When I went back over the board I bumped it up to 730 so I wouldn't have the iron on the board too much..   
Thanks.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on August 14, 2015, 03:21:55 PM
I'm a novice too actually, I just built my first kit (VP28  ;D + DOAs + power supply) two weeks ago. The symptoms were the same.
If you ordered your red dots from CAPI I don't think opamps are the problem here tho it's worth nothing trying with other ones in different configurations. Maybe you could post HD pictures of the board? This could very helpful.

713°F is right for electronics. You can even go down to ≈ 650°F (as I did) if you're using quality lead solder.

Wait for Jeff to read your message, he will help you for sure.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on August 14, 2015, 10:14:49 PM
Thanks.

I went through and re-flowed the solder on the hpf board and the wires for the transformers... now i have sound at full volume and signal light! 
I was trying to test the hpf function with the line in but it doesn't seem to be working. Should the hpf work in line in mode?  My test consisted of changing my 1K test tone to 30hz and checking if the signal lowered with the hpf buttons in - which it didn't.

I also have a problem where my polarity flip button completely shuts off the signal, so I'll have to re-solder that I imagine.

Thanks for your help.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: discord on August 15, 2015, 11:22:36 AM
It seems these new C12 caps (the blue ones) have no positive or negative markings.
This is not correct, they are marked in 2 ways. There are a series of arrows with a "-" inside each arrow running down the side pointing to the negative end. The positive end has a groove running around it. This is true for nearly all of the BC/Vishay axial caps.

Ah, didn't realize that's what that was. Thanks!


Anybody tried 990c opamps in the VP28? Is it cool/worth trying? What's it sound like?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 17, 2015, 06:05:50 PM
Greetings!

I have just completed the VP28 kit, and am having some issues.  Firstly though, I would like to say thank you Jeff for a great, extremely well-documented, and fun kit!  And thank you Chunger for an excellent in-depth visual build guide!  I have done a few kit-based DIY projects (Hamptone, Five Fish, Seventh Circle, Hairball...) and this one was impressively straightforward with detailed documentation.  Had a blast.

Now my problem is, the preamp is passing a signal (a fine sounding one, at that) perfectly fine for dynamic microphones.  All of the LED's light up and the HPF works well to my ears.  Very badass with a good dynamic.  However, when I engage the phantom power switch, it ceases to pass a signal for a condenser mic - instead hissing a white noise.  There are some pops and bumps, and the signal LED lights up sporadically, but overall no signal and just fuzz.  Regardless of the MIC button being depressed or not - and when the PAD becomes engaged, the noise turns to more of a buzz.  I am using pre-assembled red dot opamps.

I tried the obvious - switched cables on all ends, switched microphones, changed interface channels for monitoring.  The rack I am using is the classic api vpr eleven space rack - and I just assembled the rack and power supply.  I double and triple checked the rails and have confirmed that the voltages are correct on all of the card edge connectors.  I tried swapping the VP28 to a different slot, same issue.  So I am thinking, somewhere along the line, the +48v supply is getting adulterated after hitting the receptor on the card edge.  I tested with my multimeter to confirm continuity between the traces leading from the bottom pin receptor, all the way to the phantom power switch. 

I am not really sure what the issue could be here - I am pretty novice when it comes to the theory and understanding of these circuits, and cannot find a schematic for this project.  This is actually the first project I have ever had to troubleshoot - all the rest worked well out of the gate - and frankly I'm excited to finally be "forced" to learn something deeper than proper placement and soldering/assembly technique.  That being said, I don't know where to start.  Attached a pic, in case anybody can spot something I have missed.  I'm pretty confident in the placement of the components.  Could this be caused by C9, or R15-R24?  I'm not even sure exactly how to confirm that the switch is working properly, mechanically.  I have scoured this support thread and can't seem to find anyone else with the same issue.  If there's anyone out there who might shed some light, any help is greatly appreciated! :D

Thanks!
Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 18, 2015, 09:29:37 AM
Hello and welcome to the forum.


May I ask if you have the ground jumpers set?
Phantom power uses the mic cable shield ( pin 1 on the xlr) as a return path,so it is absolutely necessary to have the jumpers set to ground for each slot where you will have phantom powered devices on.
Since the preamp seems to work o.k. with a dynamic mic it might be something simple like that.
Just a thought,


best,


Udo.


Edit:Some minutes ago I've seen that the rack design has changed,mine all have headers to place jumpers for any possible connection for the pin 1 on the xlrs,or to even leave them open.
You can still check continuity from the xlrs on the back to the corresponding pins on the edge connectors,best done with the rack unpowered to prevent shorting pins with the probe.
This way we can sort out wether you have a problem with the preamp or the rack/psu.
I'm on holidays atm and don't have a module or docs accessible,sorry.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 18, 2015, 02:00:39 PM
Udo - thank you for the reply!  I appreciate the reply while you are on vacation.  I understand Jeff is out of town as well so I am exercising my patience on this one  :)

Your edit is correct - my rack does not have jumpers for the XLR connectors.  Mine has a date stamp of November 2013 for the design.  However, I did not solder jumpers on the XLR pins when I assembled the rack.  It is something that now you mention, could likely be the issue - on all the other kits I have built, it has been necessary to solder a jumper between certain pins on the XLR jack.  I have checked continuity between XLR pins (to the microphone) and the pins on the card edge connector.  They are as follows:

XLR pin 1 - continuity ONLY to card edge pin 1

XLR pin 2 - continuity ONLY to card edge pin 10

XLR pin 3 - continuity ONLY to card edge pin 8

I apologize if this is turning into a troubleshoot for the rack - but should there be jumpers between certain pins on the XLR to achieve continuity between pins that I am missing?  Should pins 3 and 1 be jumpered? 

Thanks again!

Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 18, 2015, 04:10:07 PM
Hello Andrew,


your continuity check looks right so far.
But doesn't proof if there really is a connection from xlr pin 1 (input of course) to the psu/audio ground.
Can you measure continuity from pin 1 to pin 5 or pin 13 on the modules' card edge connector?
5 &/or 13 normally carry this ground.
Hard to tell from here,what makes me a bit confused is the white noise you described before.Was that with a connected microphone or without?How does the module behave without anything connected to the input,does it still have noise?
And another question,is there any assembly guide for your rack or are there any points to solder bridges on the back plane?
Still have a feeling it's just a simple grounding issue......but that's just a guess.


Udo.



Edit: Here's a link to the edac pinouts for your reference:


http://www.51xaudio.com/alliance/pinout.html
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 18, 2015, 07:25:26 PM
Hi Udo,

Thank you for the reference link.

I have confirmed there is continuity between pin 1 and both pin 5 and 13 of the card edge connector. 

So I plugged the module into an analyzer in my DAW - it looks like there is a low hum (60Hz and below) when i plug in an sm57.

With nothing plugged in, when engaging the +48v switch, WITHOUT the MIC switch, there is very significant hum at 60 Hz.  I suspect this is a ground loop hum...when I engage the MIC switch (still nothing plugged in) the 60 Hz hum drops about 30 dB, but is still there.  It is at this point that you can hear the "fuzz" or "white noise" (which through the analyzer is happening at about -95 dB, i can hear it through juiced headphones, which also have their own hiss).

The behavior is exactly the same with a condenser mic plugged in.

So I'm guessing this is a grounding issue - just not sure if it's on the chassis, or on the VP28 card itself...unfortunately this is my first 500 series module so I cannot plug a working one in to verify that the rack is working properly.  If the issue is on the module, is there an area of components I should be looking at, perhaps to reflow solder?  I keep checking for shorts but for the life of me can't find any...also, I have downloaded the test point reference overlay that Jeff posted way earlier in this thread - although I am confused about that as it does not show values of those test points.  I'm sorry if this is "no-brainer" stuff...like I said, I am a beginner when it comes to the theory and the "why" of these circuits...

The rack did come with an assembly guide, and with a thread on this forum, which I followed and cannot find any mention of bridging points on the back pane or with the XLR connectors.

Thanks again for your help and consideration.

Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 19, 2015, 09:53:19 AM
Hi Andrew,


the noise at -95dB(whatever) is normal,pretty low.
Still wondering about the hum,something is wrong there.
Do you have the ability to directly monitor the output of the VP28?
I don't know what else is connected to it like patchbays etc.......so let's create some "clean" conditions.
You can monitor it with any line level device the has a headphone amp like a little mixer,taperecorder in rec.-mode etc.
Are you sure all of your wiring is balanced?
Can you measure if the phantom reaches the cable end (xlr fm) on the microphone side?
And last but not least: Are you sure the module is properly seated in the card edge receptor?
What does the unity gain check have as a result (known line level source from a generator or daw ,e.g. +4dBu @1kHz,plugged to the module input with a balanced cable,gain set to unity,mic not engaged,fader to unity)?
Should be very close if not identical to the generator send level.


Not much more I can do from here,but as said:To find the issue(s) it is best to have clean conditions first.
It could be that there is one problem with the wiring and another in the phantom switch area.
Not too many parts working there,just the switch and two resistors (6k8 each).
Of course you can try to reflow that parts.


Highly recommended:Jeff's extension card,makes it all way easier,not only now but for your next builds too.


If all fails then you must wait for Jeff to come back,sorry.


Best and good luck,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 19, 2015, 06:57:52 PM
Thank you Udo.

The unit passes the unity gain check with a line level in, and monitored out.  I eliminated the patchbay and went straight into the interface (which I monitored both pre-conversion, and post-conversion to my DAW).  The .775 VAC signal I passed into the unit came out as exactly the same on the other side.  I am pretty positive all my wiring is balanced, as the microphone and cables I used for the test worked on all of my other units.

I tested for continuity between the card edge connectors and xlr pins on the back of the rack - all seems well (though I did measure 10 ohms resistance between pin 1 and XLR [mic input] pin 1 - not sure if this has anything to do with the grounding issue, as I expected a 0.000 ohm continuity measure).  I have pretty much ruled out the rack and PSU as the problem (all voltages at all card edge connectors are still correct, including +48v).

I am not getting 48 v at the XLR Input of the module's slot, when the VP28 has the phantom power switch engaged.  Between XLR (input) pins 1 and 3, I measure 162 mV when just the phantom switch is engaged, and when the MIC switch becomes engaged with that, the voltage rises slightly to 189.6 mV.  So I am definitely getting +48v to the card edge connector, but not through the module to the microphone's XLR.  I am pretty positive the module is properly seated in the slot.  I have the VPR rack and there's not much room for error, and I tried seating it multiple times and visually made sure the bottom plate and top plate on the board went right into the bottom and top card edge pins, respectively.

I re-soldered the 48v switch on the module's board, as well as other suspect solder joints.  Re-soldered the 6k8 resistors as well.  triple - close checked for shorts.  Same issues.  60 hZ hum with line input, terminated input, and microphone input.  Dynamics still work fine, line level works fine, phantom power is still a no-go.  So I'm wondering if there's a component I should look to swap out (probably have to wait for Jeff on this one)?  Perhaps I static-fried the IC chip?  or would the module not work at all without a working IC chip?  Double checked the seating of the opamps, and double checked the wiring and soldering on the transformers.

I will definitely be ordering the test extension jig.  Thanks so much for your responses and support - it is helping me understand more and I really hope to get this issue sorted soon, so I can populate the rest of this rack!   8)

Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 20, 2015, 03:03:10 AM
Hi Andrew,


the 10 Ohms seem to be correct,I guess it's the resistance between the audio and chassis ground which will "meet" in the psu.
From what I remember the only ic chip is the driver for the input present signal led,everything in the audio path is pure discrete parts.So no worries about it,you stated that it is working some posts before.


The only thing to check is the switch itself.Don't know if you can reach its' contacts.If so you can try if it is working with the module unplugged from the rack.It will work like this:Inside the switch there is just a metal plate (or two if it is a dual pole) connecting the adjacent pins,e.g. depressed will connect pins 1 & 2,pressed 2 & 3 on a row etc.
You can ohm this out easily.


As said,the extender card would help for the future because it is hard to measure things on the modules while seated in the rack.With that extension you can trace where the phantom power goes and where it stops.


Yes,best to wait for Jeff in this case.


Btw,hum is normal when you have inputs unloaded or. better said in open condition.From the "60 Hz" I guess you're in the u.s.,no?Maybe you want to add this in your profile,it's always helpful,just a thought.


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: discord on August 22, 2015, 09:57:46 AM
Just finished my first two VP28s. 1 sounds killer, but the other does not pass sound. I tried taking the opamps from the working one and put them in the non-working one to rule out the opamps. Still no sound. All the pushbuttons work (their lights come on). The green signal indicator works--just no sound. Any idea how I should go about troubleshooting this? I'm  kind of a newb, but I have a good multimeter.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 24, 2015, 05:17:12 PM
Udo -

here is what I have tried thus far.  I have been following traces on the board to find where the 48 v stops.  I was able to trace voltages while the unit was in the rack (carefully of course) and managed to reveal that the +48v switch is indeed working properly. 

When the switch is engaged, the side of R15 connected to the receiving contact of the switch is receiving 48.4 V.  It is zero at this point when the switch is disengaged.  (Bear with me, I'm learning this step by step...).  The voltage makes it across R15 to become about 45.8 V at the other side.  I noticed a small trace connecting R15 and R21 - the 45.8 V makes it to this point on R21.  It does not make it to the other side of R21, where the traces seem to intersect with the MIC switch traces and into the circuit.  I figured R21 was fried, so I un-seated it, tested for resistance and measured 6.8K - as it should be.  I re-soldered it back into place.  Unless I am incapable of soldering this specific point on this specific board (I've already re-soldered this point before this try), the voltage still does not make it across R21.

I did notice something about C9 - it receives 45.8 V from R15 when I test the positive end.  the negative end is just about 0.  I'm wondering if there could be something wrong with C9 wherein it is dumping everything to ground?  but then would R21 even get that 45.8 V?  or would C9 possibly be "stealing" this voltage from that junction at R15?  Am I going in the wrong direction?

Well, I already ordered more switches, but it seems like that's not the issue here...and I'm taking this piece-by-piece, so hoping for some more guidance before I try un-seating and replacing more components.  Somewhere at R21 the 48V is hitting a fence - and after un-seating R21 and testing and re-seating and re-soldering, my instinct is to rule out R21 being defective...?

As always, I truly appreciate the help.  Thanks.

Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 24, 2015, 05:45:15 PM
Welp - I un-seating C9 and it looks to me like it might be blown out, I'm comparing the bottom side to all the other spare electrolytics I have lying around and this one looks like, slightly bloated coming out the bottom.  Going to replace C9 and see what happens - stay tuned!

Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 25, 2015, 04:26:09 AM
Hi Andrew,


have reread the bom,R20 &R21 are the "responsible" resistors for phantom.Normally the +48vdc go to the switch,then split into two very closely matched 6k81 resistors,then both go to the input xlr pins 2 & 3 each.
So you should have the same values after r20 & 21.
Good news is that the switch is working fine,cool.
As said no module or schematics here,but if this cap looks odd then yes,swap it out!It's a lytic,yes?
Could be the culprit,sadly I can't say what it is doing from here,but you must swap it out anyway.
Watch polarity while reassembling.Caps can go short and will then load down anything close to it to 0v (psu/audio ground).


I guess you're on track!


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: discord on August 25, 2015, 12:20:46 PM
I still can't figure out why my VP28 does not pass sound. I performed all the tests in the VP2x-Assembly-Guide document. Everything checked out OK.  I checked the opamp sockets and then plugged it in and checked the power between each V-/C and V+/C and they read 15.74 volts. I'm guessing it's something to do with my output section since the input light comes on when I test with a mic. Can anyone give  an idea what else to measure to find the failing component? Is there a guide for something like this? Thanks!
(http://i.imgur.com/e9CrYtA.jpg)

Edit: I found the test points document. I compared values of my working VP28 to my non-working VP28. Not sure how to interpret the results. Here are the differences:

1->2 (R5->C16)
2.28MOhm on the working one
-1.4MOhm on the nonworking one (I'm guessing this is where the problem lies)

2->3(C16->RF11)
-2.2MOhm working one
1.4MOhm nonworking one

3->4(RF11->J1-1)
210R on both

4->5(J1-1->C12)
.2R on both

5-6(C12->C20)
-1M on working
-1.4 on nonworking

I'm not sure if I'm doing this right, but I'm guessing something is wrong between R5 and C16...
 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 27, 2015, 09:50:36 PM
Thanks as always for the reply Udo - unfortunately replacing C9 did not solve the phantom power issue, although it did change some things.  The underlying buzz is gone - the line and dynamic mic signal is totally clean now (was getting a buzz and 60hZ hum before).  Also, I should have mentioned this before, but before I replaced C9, I would lose the LED activity of the MIC, Polarity, and PAD switches when the 48V switch was engaged.  They would light up their respective LED's when pressed in, but when the 48V button was pressed, those LED's would not work any more.  After reading this entire thread I thought that was normal, as I read something about those LED's being in series with each other - but now that I have swapped out C9 for a new cap, all of those LED's work when their switches are pressed, when the 48V switch is pressed.  So changing C9 solved two issues - the slight buzz I was hearing as well as the LED activity of the other switches.

However, my 48V still does not make it past R20 and R21.  I tried a lot of things.  I wicked and re-soldered all of the joints close to those two resistors.  I took both R20 and R21 out of the board completely, checked them for resistance, measured a cool 6.8K on both, and switched their positions on the board (former R20 became R21 and vice-versa), and carefully re-soldered them in.  I have spent a few hours now scratching my head and checking solder joints.  When the 48 V switch is engaged, the voltage makes it to the front end of both R20 and R21, but does not make it across them - they see less than 200mV on the other side - it still seems to be a wall where the 48V stops.  Without a schematic, I am at a loss as to what to look for next - I'm now thinking there is some other errant component which originally damaged my first C9, or maybe the board is damaged in some way that I cannot see, but my theoretical understanding of this circuit so far ends about here.

Any insight or further ideas are greatly appreciated!  Perhaps someone has access to a schematic for this project?  Is Jeff out there to chime in at all?

Again, I really appreciate all the help.  And discord, I hope you are getting close to solving your issues - sorry if my replies are throwing you off when you receive notification.

Andrew
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 28, 2015, 08:35:32 AM
Hi Andrew,


so C9 was one culprit.Not too bad,because this is a huge improvement.
The phantom going to both R20 & R21 on one side but not to the other clearly indicates you have a short after them.Something is loading down the phantom here.
Since there won't be many parts from them to the card edge in between (I would even guess none!) you must have a short condition somewhere.Could be caused by a solder blob,a tiny cut-off piece of wire or parts-leg(!)..... maybe inside one of the card edge receptacles?.....or a non-balanced mic cable with one of either pin 2 or 3 on the xlrs shorted to the shield.
O.K.,please measure both sides from the two resistors to the modules'  corresponding card edge (the "gold fingers").If they are identical (2 times around some kilo ohms,two times zero ohms) we can expect the culprit to be outside the module.You can also measure these points refering to audio ground.


I hope we don't have a transformer issue here.


To check that please measure a known line level sine wave fed to the input,mic switch depressed,gain set to unity,fader to unity,no phamtom and measure the level (dmm set to vac) between the pins 2 & 3 on the output of the rack,nothing else connected please.E.g. if you feed a sine with 0dBu (or something like -18dBFS from your daw) will give you roughly 0,775 vac on the xlr.Don't use a high frequency,best to use something lower than 1kHz
because your dmm might not be able to read this properly.Try 400 Hz or so.What do you get?


About schematics,only Jeff has them,they are not public for some good reason,I'm sure you understand that.
But he'll definetely help you once he's back.


I'll be back home in one or two weeks and might be able to get hands on one of my own modules if the problem is not solved until then.


To not confuse Jeff too much with too many long long emails all he must know for now is that the phantom doesn't make it across those two resistors.Just as a "summary" if all fails.


Best,


Udo.


Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 28, 2015, 05:37:06 PM
Hi Udo -

so I am measuring some funky resistance between the far ends of R20 and R21, and the gold fingers for slots 8 and 10 on the rack.  R20 is measuring 28.5 ohms to (gold finger) slot 10, and 0.000 to slot 8.  R21 is measuring 29.7 ohms to slot 8 and 0.000 to slot 10.  the traces run through the MIC switch.  R20 hits R19, R19 hits the PAD switch, and that trace runs to the MIC switch.  R21 hits R17, and then that hits the MIC switch.  I followed the resistance up to the MIC switch, and it seems like theres about 28 or 29 ohms resistance between pins that the voltage should travel through (if you look at the switch vertically, the 5th pins down from the button on the left and right have that resistance between them - same with the 6th row, 8th row, and 9th row pins).  The pads that have a direct trace shot to the gold fingers for slots 8 and 10 have 0 ohms resistance, these are down towards the end of the MIC switch.  I'm currently trying to unseat the MIC switch (which is quite a task) to check for a short underneath where I cannot see.  I will keep posted.  Expecting not to find much, but who knows - if I do not solve the issue by checking the switch, maybe there is some component linked to this group of traces that is causing some resistance to happen there? 

Thanks again!!!  Your support is really helping me learn a lot, I really appreciate it!

Andrew

edit:  well, the switch is now in shambles, and I am still measuring a strange 25 ish ohm resistance across the pads where the 48V should hit.  R21 by way of R17 hits the 5th pad down from the LED of the MIC switch.  R20 hits the pad directly across that, by way of R19 and the PAD switch.  These two pads, the 5th pair down from the LED of the MIC switch, are measuring about 25 ohms resistance across them.  The traces of the MIC switch lead over to the R27-R32 section, where I measured 25 ohm resistance across R29.  Am I even going in the right direction?  I would assume there should be 0 ohm continuity all the way through for the 48V, but where it hits the pads of the MIC switch, there is resistance.  The 8th pair of pads down from the LED of the MIC switch have O ohm continuity through to the gold fingers for slots 8 and 10 on the rack.  I triple checked the placement of R27-R32, reflowed solder, and am now getting 24 ohms resistance across the 5th pair of pads on the MIC switch. 

I am assuming I need to wait to get a new switch in here before performing the line level test you recommended?  Although it did pass muster on this test when I did it before.  .775 VAC in and .775 VAC out.  I am assuming and hoping it is not a transformer issue in that case.  I feel like I'm going down a rabbit hole and have no idea what could be causing this resistance across the pads where the 48V hits.  It seems to match the resistance across R29.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 29, 2015, 01:48:05 AM
Hi Andrew,


o.k.,at this point I need to follow your readings on a working module to compare what is good and what is seems off.
The phantom -related resistances look fairly low but I can't proof it for now.They might be lower as their original value measured in circuit due to to other parts,but that low.....hmmmmm.....are you sure you have the correct values for r20&21 soldered in?Should be 6k8 both,you can lift one leg of them and measure them.
Sad to hear the mic switch is broken now,I did not ask you to do so,hahaha.....seriously,they don't like too much heat during assembly same as removing them.The circuitry around this switch is a pad resistor network that is used for line level mode (switch depressed).
The good news is that you say the unity gain check worked out fine,this means all transformers and the rest of the audio circuit are fine.


Please do me a favour and don't disassemble further parts for now,let's wait or Jeff to chime in.


S**t,this must be something totally ridiculous......


We're so close......


I still wonder why your former C9 went bad,what the cause was and which other parts might have gone bad.


Best,


Udo.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on August 29, 2015, 10:42:14 AM
Hi Udo -

yes, I feel like this is caused by something ridiculous that I have overlooked - but cannot figure out what it could be.  I promise not to disassemble any further parts!  haha I thought I was so cool, about to figure out the issue  ;)  I am going to use this time to set the module aside and order a new 6PDT switch, and hopefully clear my mind!

I am absolutely, 1000%, quadruple-checked positive that R20 and R21 are 6k8 ohms. 

I also have been wondering why C9 originally went bad - it was definitely seated in the correct orientation, I checked that more than a number of times and am looking at pictures I took and it's definitely been correctly oriented this whole time.

Thanks again for the help - I will patiently await Jeff's input. 

Andrew 
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on August 30, 2015, 03:43:19 AM
Patience is the key,heads up Andrew ;) ,


Udo.


P.S.:I'll have no internet access for a while now,maybe a week or so.When back home I'll try to follow your measurings.Meanwhile you might give Jeff a short summary via email?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on September 19, 2015, 02:29:03 PM
I finished my first CAPI VP28, installed it into my lunchbox, and powered it up....smoke  :'( :'( :'( I went very slowly to try to get everything right. Pretty bummed at the moment and I'm not sure where to start to try to fix it. Can anyone point me in the right direction? Should I post pictures? Thank you.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 19, 2015, 03:06:01 PM
It sounds to me like there is a short somewhere after the 6k8 phantom R's. This could be on the VP28 board or quite possibly after the board (rack, cable, mic, patchbay etc). Can you be more specific as to your test rig setup? It would be best if you have nothing connected to the module when first measuring for 48V on the input gold fingers of the preamp card. I am trying to eliminate any outside issues.

FYIW, the functioning of the status LED's has nothing to do with the 120µF cap. That is only some odd coincidence like a cold solder joint that got touched up or bumped into working mode.

Back to the phantom R's, they connect directly to card edge gold fingers 8 and 10 so there is no circuitry on the PCB that comes after them.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on September 19, 2015, 06:11:06 PM
I removed the op amps and tried again-- no smoke. Measured voltages at A1 and A3 with DMM and they're all just below 16V. So does that suggest the problem is the op amps? Especially considering that they're DIY.  Should I just purchase a couple pre-assembled op amps to throw in to the preamp? A top down picture is attached.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 19, 2015, 06:23:21 PM
I thought the main issue is no phantom power? Can you measure for phantom at the card edge gold fingers with nothing connected to the card except DC?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on September 19, 2015, 06:56:14 PM
I'm sorry Jeff, I think you might be confusing me for the previous poster.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 19, 2015, 07:07:18 PM
I'm sorry Jeff, I think you might be confusing me for the previous poster.
Oops. Right you are. I feel like a post was deleted.....

Sounds like you have already discovered that yours is an opamp issue.

FWIW, I will be selling "built by Gary gar's" soon. I just need to get the support docs page done first.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on September 19, 2015, 07:15:32 PM
I'm sorry Jeff, I think you might be confusing me for the previous poster.
Oops. Right you are. I feel like a post was deleted.....

Sounds like you have already discovered that yours is an opamp issue.

FWIW, I will be selling "built by Gary gar's" soon. I just need to get the support docs page done first.

Are there assembled op amps available for purchase now?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 19, 2015, 07:34:23 PM
I'm sorry Jeff, I think you might be confusing me for the previous poster.
Oops. Right you are. I feel like a post was deleted.....

Sounds like you have already discovered that yours is an opamp issue.

FWIW, I will be selling "built by Gary gar's" soon. I just need to get the support docs page done first.

Are there assembled op amps available for purchase now?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on September 19, 2015, 09:28:55 PM
Hi Jeff,

Andrew here with the phantom power issue.  Thanks for your responses.

I have disconnected everything except DC power to the module.  I took the rack apart to look for any shorts inside, reassembled it and it seems to me that there should be no issues with the rack.  So I am measuring with no input microphone and no output to a patchbay - just straight module.  This is the only module I have, so I have space to carefully measure with my DMM while it is seated in the rack.

For the record on this forum, as per our email exchange, I measured for phantom voltage between XLR pins to the microhpone on the back of the rack - and it seems like pins 1 and 3 are swapped, I get 187 ish mV between pins 1 and 3, and between pins 2 and 3.  I get a zero reading between pins 2 and 1.

On the card itself, I get 48.5 V DC at the bottom gold finger, and it makes its way onto the card.  I can follow the traces all the way to the phantom power switch, which receives the 48 V and gates it through to the 6k8 R's.  It is about 45.8 V at the base of R20 and R21.  I follow the board's traces after the 6k8 R's, which seem to hit the pins of the MIC switch, by way of a junction with one end of R17, and from the MIC switch further down (8th pair) I see the traces go to gold fingers 8 and 10.  I did find something at the MIC switch, right on the pin where the 48V trace hits - it looks like I must have knicked the board with my soldering iron right next to the solder pad.  I have attached a (rather crappy) picture to hopefully show you what I am seeing there - kind of a burnt dimple on the board which touches the solder pad of the 5th pin down on the MIC switch (5th pin, on the right, reading down from the LED3 marking).  It looks like a solder blob in the picture but it's actually the black screening of the board that got burnt a bit.  This is by far the messiest spot of my entire build, also considering I desoldered and replaced that switch (I believe this blemish was there before I replaced the switch).  Looks like I may have damaged the board here - and wondering if this is a fixable issue.  That specific pin measures about a 10ohm resistance to gold finger 1 of the card, which (I think?) tells me that pin is shorting to ground.  When I touch the "burnt dimple" itself with the probe, there is no measurement, but touching the 5th pins themselves or the solder pad itself, there is a low resistance straight to the ground gold finger 1.  Hopefully I'm explaining this well enough.  Wondering if those pins are supposed to have connection to ground, and if not, should I chalk this problem up to my own sloppy error?  Is there any way to fix this, or work around it to verify that this might or might not be the issue?  Other than phantom power, the preamp works wonderfully.

Thanks again for your support.

edit: I took the picture right after I wicked solder from the pad in question, in case you were wondering about the solder joint itself.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 20, 2015, 08:37:58 PM
It is impossible for "pins 1 and 3 to be swapped" in the rack.

With no power connected, module inserted in the rack, measure DCR between XLR input pins 1->2 and then pins 1->3. You are looking for a close to direct short so set the range on your DMM accordingly.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: sumyungdon on September 21, 2015, 02:04:21 PM
Jeff,

Here are my resistance measurements, with no power, and the module inserted:

XLR pins 1->2 - 4.97 K Ohms

pins 1->3 - 5 K Ohms

I'm not sure what this means, is this a normal reading?  If so, what should I look for next?

Thank you

Andrew
Title: Loss of gain in one of my VP28's
Post by: Sparqee on September 23, 2015, 06:32:03 PM
I have two VP28s with identical red dot DOA's from Scott L.  The two unit used to perform so identically as to be indistinguishable.  Recently one of the units has lost some gain. 

With both unit set to unity gain and filters disengaged I send a 1.5v test tone to each unit.  One unit outputs a signal of 1.67v while the other outputs a signal of only 1.01v.

Any suggestions on how to go about correcting this would be greatly appreciated.  :)

- Sparqee
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 23, 2015, 06:40:42 PM
I would firstly swap opamps around. Also make sure they are seated properly http://capi-gear.com/catalog/DOA_Install.php

Next rule out any outside possibilities like rack, cables, convertor settings yada yada
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Sparqee on September 24, 2015, 05:57:58 PM
I would firstly swap opamps around. Also make sure they are seated properly http://capi-gear.com/catalog/DOA_Install.php

Next rule out any outside possibilities like rack, cables, convertor settings yada yada

Bingo!  The DOAs all seemed to be seated properly but when I swapped them my gain came back.  Thanks for the advice.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 24, 2015, 08:19:48 PM
I would firstly swap opamps around. Also make sure they are seated properly http://capi-gear.com/catalog/DOA_Install.php

Next rule out any outside possibilities like rack, cables, convertor settings yada yada

Bingo!  The DOAs all seemed to be seated properly but when I swapped them my gain came back.  Thanks for the advice.
This is a tell tale symptom of a bad solder joint.......somewhere on the build. The jarring action of removing/inserting the opamps probably "made it good".....for a while.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on September 26, 2015, 05:05:18 PM
I've probed between the +V and C DOA sockets and the -V and C DOA sockets and got about 15.5V for both. Are there any other tests that I could do to make sure everything is good to go before installing a new functional op amp? I found the VP28 top overlay with test points highlighted (see attached), but I'm not entirely sure how to do it. Is anyone willing to explain how to use my DMM with these test points? Any help would be very much appreciated. Thanks!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 28, 2015, 12:21:03 PM
You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on September 30, 2015, 11:57:51 PM
If I neglected to install the piece of foam tape beneath the 2622 transformer would that potentially cause problems shorting something?

The issues I'm trying to track down are very low, noisy output (gain and fader both fully clockwise to get usable signal level) and my polarity switch stops all sound from passing when engaged.

 When I send a 1khz sine wave to the lunchbox at ~0db and I measure the hot and cold pins with my dmm, i get 4.5 at the input cable and 2.6 at the output.  The pad and hpf are not engaged, and the signal gets lower when they are engaged.

I bought pre-assembled DOA's. 

Also, thanks to northamrec for posting that test points image.  i had a hard time finding it.  can anyone hold my hand a little bit regarding how to test those points?  I'm very much a newb at this... i connect my vp28 to power, with the DOA's installed, sending a low signal sine wave to the input.  one probe goes on the test point, where does the other go?

Thanks very much for your help
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 01, 2015, 12:34:53 AM
Black probe goes to a 0V reference like the pad directly adjacent to gold finger #5 or #13.

These symptoms (like polarity cutting out signal) sound like bad/cold solder joints. The polarity flip is nothing more than the switch swapping the 2 primary wires of the first 2623-1. Bad solder joints or shorted pads on the switch would be a likely cause.

The foam tape is just for a spacer to keep the bottom of the can from shorting out to pads under it. Chances are, if it is shorted, you would have no signal passing thru the 2622.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on October 03, 2015, 04:30:32 AM
Hi,

I'm thinking about replacing the EA2623's for Litz's into my VP28 (more uniform highs and deeper bass? That's all I need on my mix buss!  ;D) but something got into my mind, what if I put Carnhill's or Cinemag's instead?

Did you try when designing the 28, Jeff? Or did somebody tried something crazy here?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 03, 2015, 06:54:17 PM
Hi,

I'm thinking about replacing the EA2623's for Litz's into my VP28 (more uniform highs and deeper bass? That's all I need on my mix buss!  ;D) but something got into my mind, what if I put Carnhill's or Cinemag's instead?

Did you try when designing the 28, Jeff? Or did somebody tried something crazy here?
No, I went with grandma's recipe when I originally came up with the VP28. I personally would not stray farther than the Litz 2623-1 but by all means, try out some other stuff!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on October 05, 2015, 12:44:29 PM
Thanks Jeff
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on October 11, 2015, 10:25:13 PM
I desoldered my 2622 to make sure it wasn't shorting...and i guess i wasn't careful enough as now two of the legs are loose.  they slide about 1 mm in and out of the transformer body.  is it shot?  is there a way to test with my voltmeter?

thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 11, 2015, 11:08:11 PM
Yes. Measure DCR between pins 5-->8, 1-->3, and 2-->4.
http://www.capi-gear.com/catalog/images/gallery/2622/2622-specs.pdf
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on October 11, 2015, 11:29:00 PM
Yes. Measure DCR between pins 5-->8, 1-->3, and 2-->4.
http://www.capi-gear.com/catalog/images/gallery/2622/2622-specs.pdf

thanks for the quick reply.

5-->8= no resistance
1-->3=55.4 ohms
2-->4=60 ohms
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 12, 2015, 12:17:38 AM
5-->8= no resistance
That is an open secondary so sadly this one is no good anymore.  :'(
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cbhris on October 12, 2015, 12:23:42 AM
5-->8= no resistance
That is an open secondary so sadly this one is no good anymore.  :'(

Okay.. i will order a replacement.. 
i put an order for another vp28 kit in yesterday... if i order this replacement 2622 tonight do you think you could ship them together?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 12, 2015, 10:13:40 AM
5-->8= no resistance
That is an open secondary so sadly this one is no good anymore.  :'(

Okay.. i will order a replacement.. 
i put an order for another vp28 kit in yesterday... if i order this replacement 2622 tonight do you think you could ship them together?
Yes they will ship together. Today is a federal holiday so no mail. All current open orders will ship out tomorrow.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: johnlukeyoung on October 18, 2015, 02:13:28 PM
Hello,

I have two VP28's, one of which is ~10dB quieter than the other. I swapped the op amps and checked that all my polarity was correct, and still nothing changed. I then purchased the extender rig and I am attempting to measure at the test points with a sine wave at 800hz and 0dB, but the measurements are super inconsistent. If I measure the same point multiple times, the readings vary drastically.
Though I am not sure what use they will be, here are the readings I took. Any advice would be very helpful!

Unit 1 (quieter):
#1=0.355 (0.480) (0.380) (0.551)
#2=1.114 (1.464) (1.370)
#3=0.677
#4=0.705
#5=0.475
#6=0.526

Unit 2:
#1=0.409
#2=1.000
#3=1.738
#4=0.604
#5=0.599
#6=1.112
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on October 18, 2015, 03:03:49 PM
You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

I set up a signal generator (-40db sine) in my DAW going out of my Lynx Aurora and into the lunchbox with my CAPI VP28 connected to an extender cable. I'm actually getting 0 V on all test points except #2, which initially measured .24 V. Clearly something is wrong! I'm still getting ~15.5 V at the DOA sockets.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 18, 2015, 09:23:48 PM
.....If I measure the same point multiple times, the readings vary drastically.....
Readings that vary can mean a poor connection for your probes or more likely bad solder joints.

If you are injecting a 0dB signal I assume you are in line mode? If that is the case, are you sure the preamp gain is set to the Unity position (and knob is properly located) and the output fader is at 0 (and knob is properly located)? With 0dBu going into line mode, you shook have appx 0.39V AC at test point #6 which is just before the 1:2 output transformer which is providing 6dB of "free" gain.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 18, 2015, 09:25:16 PM
You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

I set up a signal generator (-40db sine) in my DAW going out of my Lynx Aurora and into the lunchbox with my CAPI VP28 connected to an extender cable. I'm actually getting 0 V on all test points except #2, which initially measured .24 V. Clearly something is wrong! I'm still getting ~15.5 V at the DOA sockets.
Are you measuring ACV or DCV? Sounds like you are measuring DCV. Audio is ACV.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on October 18, 2015, 11:01:22 PM
You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

I set up a signal generator (-40db sine) in my DAW going out of my Lynx Aurora and into the lunchbox with my CAPI VP28 connected to an extender cable. I'm actually getting 0 V on all test points except #2, which initially measured .24 V. Clearly something is wrong! I'm still getting ~15.5 V at the DOA sockets.
Are you measuring ACV or DCV? Sounds like you are measuring DCV. Audio is ACV.

You're right. All the test points are between .7 and 1.0 mV AC, although the first one may be higher, i.e., ~ 1.5 mV AC.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: northamrec on October 19, 2015, 06:27:40 PM
I just installed my new pre-assembled op amps and everything works! I'm very, very happy with how it sounds. Thanks for the help! Looking forward to building the 3 VP 28s that I just got in the mail...  :D
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: johnlukeyoung on October 19, 2015, 06:52:04 PM
.....If I measure the same point multiple times, the readings vary drastically.....
Readings that vary can mean a poor connection for your probes or more likely bad solder joints.

If you are injecting a 0dB signal I assume you are in line mode? If that is the case, are you sure the preamp gain is set to the Unity position (and knob is properly located) and the output fader is at 0 (and knob is properly located)? With 0dBu going into line mode, you shook have appx 0.39V AC at test point #6 which is just before the 1:2 output transformer which is providing 6dB of "free" gain.

I am in line mode, and I've checked that both sets of gain knobs are properly oriented. I will try reflowing all accessible solder joints and see if that allows for more consistent readings. Also, should I test this with or without opamps installed? (I apologize if this has already been asked, I couldn't find it) Thank you!
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Zamoht on November 10, 2015, 03:59:27 PM

I reflowed the solder everywhere but the same symptoms persist.  I did remove the opamps and the IC before soldering, so they were re-seated before I tried again.  But after it was reassembled before I tried it out I still get that 4.06k ohm reading when I measure the opamp sockets between "+v" and "c".    And the reading is exactly the same for both opamp sockets.

I did not reflow the solder on the hpf board though.  I will do that as well.  I saw in another post that the audio goes through there no matter what, right?

I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 12, 2015, 07:06:02 PM
I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?
The A1 and A3 +/-V sockets are directly connected to each other so they will measure the same. I have a pair of properly working VP28's here and 4.06kΩ is correct for +V to C. The -V to C DCR is 6.4kΩ.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Zamoht on November 15, 2015, 06:02:29 AM
I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?
The A1 and A3 +/-V sockets are directly connected to each other so they will measure the same. I have a pair of properly working VP28's here and 4.06kΩ is correct for +V to C. The -V to C DCR is 6.4kΩ.


Ah ok, I got confused because the values given in the VP2x manual said it should be over 200KΩ.
-V to C gives me a resistance of around 100 KΩ (although it starts low when first measured, but steadily climbs). might be a bad solder joint somewhere? (or does DCR means it should be powered?)



Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 15, 2015, 09:31:43 PM
I am getting the same reading between the DOA sockets on both of my preamps. this can't just be a coincidence right?
The A1 and A3 +/-V sockets are directly connected to each other so they will measure the same. I have a pair of properly working VP28's here and 4.06kΩ is correct for +V to C. The -V to C DCR is 6.4kΩ.


Ah ok, I got confused because the values given in the VP2x manual said it should be over 200KΩ.
-V to C gives me a resistance of around 100 KΩ (although it starts low when first measured, but steadily climbs). might be a bad solder joint somewhere? (or does DCR means it should be powered?)
This is normal as its a cap charging. DCR is a resistance measurement and in this situation there should be no power applied. FYI, the results shown in the VP2x Assembly Guide are specific to a VP25 or VP26.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on November 18, 2015, 01:00:40 PM
I've been puzzling over my 4th VP28 build for hours now - the input transformer 1-3 and 2-4 are both measuring 2 ohms, then 5-8 is 1.6k and I'm getting very low output level but correct functionality across all controls. Are these readings odd?

A separate issue in same unit is that I stupidly put 4558 in reverse and it blew, replace it and now only 48v LED works. They're all in correct polarity - could IC blowing have damaged LED's?

Thanks for any advice
Nick
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 18, 2015, 01:05:10 PM
I've been puzzling over my 4th VP28 build for hours now - the input transformer 1-3 and 2-4 are both measuring 2 ohms, then 5-8 is 1.6k and I'm getting very low output level but correct functionality across all controls. Are these readings odd?
Are these measurements when the 2622 is soldered in circuit?

The LED's are in a string starting with the 48V and going towards the top of the module. They really won't have anything to do with issues regarding the chip opamp.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on November 18, 2015, 01:15:50 PM
Yeah it's in circuit still - only have a sucker so desoldering a bit of a worry...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 18, 2015, 06:42:15 PM
Yeah it's in circuit still - only have a sucker so desoldering a bit of a worry...
Well, I have recently found shorts on a few boards fresh out of the package. My gut tells me yours is one of them. Can you send me an email please?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nckptmn on December 07, 2015, 07:29:02 PM
hello all!
Sorry if this has been asked before, but i'm working on my first build of my VP28 and just finished populating and soldering the resistors. While i was double-checking everything with my multimeter, i found that R20 and R21, despite being labeled and color-coded for 6.8k, were returning a measurement of 3.7k each time. Is this normal or something I should be worried about?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 07, 2015, 08:51:39 PM
hello all!
Sorry if this has been asked before, but i'm working on my first build of my VP28 and just finished populating and soldering the resistors. While i was double-checking everything with my multimeter, i found that R20 and R21, despite being labeled and color-coded for 6.8k, were returning a measurement of 3.7k each time. Is this normal or something I should be worried about?
I assume you are trying to check after they are soldered in circuit? You cannot do this unless one lead is lifted. You will get all kinds of false readings.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nckptmn on December 09, 2015, 12:12:33 AM
thanks for the tip jeff! I'm trying to be as cautious as i can with this build but sounds like i was just setting myself up for confusion by checking the resistors in circuit.

another question -- i made the mistake of soldering in my EA2622 without placing a spacer underneath. Am i setting myself up for big problems when i plug this into my rack?

finished the main boards and assembly today, building the opamps tomorrow. this build's been a lot of fun to work through!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 09, 2015, 10:31:35 AM
-- i made the mistake of soldering in my EA2622 without placing a spacer underneath. Am i setting myself up for big problems when i plug this into my rack?
The idea behind the spacer is to keep the bottom of the can from shorting out to the pads. Most typically, it will be OK but I like the spacer to be certain. There is no DC there so its not like anything will blow up  ;)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nckptmn on December 09, 2015, 09:19:01 PM
quick update and one last question:
so after working for a while on my gar2520 kits, i got them to where i thought they needed to be, plugged everything into my lunchbox...and got the distinct smell of burning components, which i was able to confirm pretty quickly were the opamps. I don't mind being bested by them, i hadn't worked on a project that required that much precision before and i knew the risk going in, and i've got a pair of assembled units waiting in my shopping cart right now....but i wanted to ask, assuming that the issue is solely an op-amp issue, is there any risk of having done serious damage to the vp28 by powering it on for a minute as the op-amp began to burn out? with just a quick hand-check it didn't seem as though the signal IC or any of the transformers had gotten particularly hot, but of course, that can only tell so much.

Not sure if this question can even be answered conclusively without an experienced veteran taking a look at the unit itself, but figured i'd ask!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 09, 2015, 10:42:46 PM
so after working for a while on my gar2520 kits, i got them to where i thought they needed to be, plugged everything into my lunchbox...and got the distinct smell of burning components, which i was able to confirm pretty quickly were the opamps. I don't mind being bested by them, i hadn't worked on a project that required that much precision before and i knew the risk going in, and i've got a pair of assembled units waiting in my shopping cart right now....but i wanted to ask, assuming that the issue is solely an op-amp issue, is there any risk of having done serious damage to the vp28 by powering it on for a minute as the op-amp began to burn out?
The rest of the pre is most likely OK. Common mistakes on the opamp builds are swapping and/or turning the larger BD transistors. They ARE NOT interchangeable. Backwards diodes is the #2 error.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Zamoht on December 13, 2015, 09:45:00 AM
I'm kinda stuck with troubleshooting here.
I've got 1 working vp28 and 1 not (yet)

The signal I get in both mic and line mode is very very low (-50 db into the DAW with preamp and fader full open)


I already put it in the rackslot that worked with the other one.
I switched all combinations of op amps and confirm they are all ok.
The signal led works (and behaves exactly the same as the other working vp28)
so, the preamp section is also ok i think?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 13, 2015, 10:21:51 AM
I'm kinda stuck with troubleshooting here.
I've got 1 working vp28 and 1 not (yet)

The signal I get in both mic and line mode is very very low (-50 db into the DAW with preamp and fader full open)


I already put it in the rackslot that worked with the other one.
I switched all combinations of op amps and confirm they are all ok.
The signal led works (and behaves exactly the same as the other working vp28)
so, the preamp section is also ok i think?
Go back to around here http://groupdiy.com/index.php?topic=48610.msg767382#msg767382 and use the test points guide to trace signal thru both pre's and see where things change. I would start with the good one and use those results as a benchmark. Once we figure out exactly where the problem begins, we can isolate and solve it.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Zamoht on December 13, 2015, 11:39:42 AM
I'm kinda stuck with troubleshooting here.
I've got 1 working vp28 and 1 not (yet)

The signal I get in both mic and line mode is very very low (-50 db into the DAW with preamp and fader full open)


I already put it in the rackslot that worked with the other one.
I switched all combinations of op amps and confirm they are all ok.
The signal led works (and behaves exactly the same as the other working vp28)
so, the preamp section is also ok i think?
Go back to around here http://groupdiy.com/index.php?topic=48610.msg767382#msg767382 and use the test points guide to trace signal thru both pre's and see where things change. I would start with the good one and use those results as a benchmark. Once we figure out exactly where the problem begins, we can isolate and solve it.

Ok, I noticed that (any) opamp in the A3 position was getting really hot. Also this cap (C20) doesn't look to good either... I think this is the problem. any ideas on what the cause might be?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 13, 2015, 11:49:22 AM
Ok, I noticed that (any) opamp in the A3 position was getting really hot. Also this cap (C20) doesn't look to good either... I think this is the problem. any ideas on what the cause might be?
Yes that doesn't look good. 1st I would measure DCR from "O" socket of A3 to ground. Do this with no power applied to module. If there is no direct short or very low resistance, I would measure DC offset just after the A3 opamp. For this A3 needs to be in place and the module needs to be under power. Set DMM to read DCV. Red probe to the + end of that leaking cap and black probe to a solid ground reference. The pad adjacent to gold finger #5 is usually the easiest. Typically there should be no more than 100mV of DC present. Looks like the DC exceeded the voltage the cap is rated for indicating a problem with the opamp.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Zamoht on December 13, 2015, 01:05:17 PM
Yes that doesn't look good. 1st I would measure DCR from "O" socket of A3 to ground. Do this with no power applied to module. If there is no direct short or very low resistance, I would measure DC offset just after the A3 opamp. For this A3 needs to be in place and the module needs to be under power. Set DMM to read DCV. Red probe to the + end of that leaking cap and black probe to a solid ground reference. The pad adjacent to gold finger #5 is usually the easiest. Typically there should be no more than 100mV of DC present. Looks like the DC exceeded the voltage the cap is rated for indicating a problem with the opamp.

ok, I measured DCR 0 to ground. no connection,
Next I measured 101mV  between C20+ and ground, so that seems ok. (btw, I have removed the leaking cap before measuring)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 13, 2015, 01:20:58 PM
ok, I measured DCR 0 to ground. no connection,
Next I measured 101mV  between C20+ and ground, so that seems ok. (btw, I have removed the leaking cap before measuring)
OK this sounds normal. For a quick test, you can jumper over C20 to see if your output is normal. I would check the DC offset of your other opamps. Maybe one of the other ones was the problem and caused the cap to leak? I know you mentioned that you were moving them around. I would somehow mark the ones with low DC offset so you keep them straight.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Zamoht on December 13, 2015, 02:25:21 PM
OK this sounds normal. For a quick test, you can jumper over C20 to see if your output is normal. I would check the DC offset of your other opamps. Maybe one of the other ones was the problem and caused the cap to leak? I know you mentioned that you were moving them around. I would somehow mark the ones with low DC offset so you keep them straight.

tried to jumper C20, but still no output.
I measured all opamps in both preamps on the A3 location. The soundskulptor sk25's measured 95mV and 101 mV and the  1731's measured -15mV (which seems a bit odd? they do work in the working preamp....)



Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 13, 2015, 02:33:38 PM
tried to jumper C20, but still no output.
I measured all opamps in both preamps on the A3 location. The soundskulptor sk25's measured 95mV and 101 mV and the  1731's measured -15mV (which seems a bit odd? they do work in the working preamp....)
It is a different opamp design so not that surprising.

That said, its possible that the trouble is before the output of A3. I would suggest using the test points doc linked above to see where your signal stops. Time to be analytical about it. Start with the good unit and notate the signal at each point and then move to the bad one.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nckptmn on December 15, 2015, 12:35:04 PM
just wanted to follow up my opamp woes with some good news! i just installed a pair of pre-assembled gar2520s and my vp28 is up and running! i still have to do a line-level comparison but so far everything seems to be working perfectly! going to begin building my next vp28 soon and i'm gonna try my hand at the 1731 kits and hopefully put one of each op amp in my 2 pres. Thanks Jeff!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: darkus on December 18, 2015, 03:07:34 AM
Hi everybody. I just built two VP28´s and having problems with one of them.  Signal is very low, no matter how much gain applied, it looks like its clipping bad (and surprisingly sounds like that too) until i flip the phase switch and the preamp works like it should.

Is my switch bad or is there something else? Ive switched opamps between the units but that didnt help anything as the preamp works fine after the phase being switched.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 18, 2015, 06:43:44 PM
I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: darkus on December 21, 2015, 05:26:02 AM
I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

I checked the soldering in that switch and reflow some solder. All looks good, same problem still occurs. Where else should i be looking at? The signal level im getting with phase switch in normal position is somewhere around -60dB. once i flip the phase everything works like should

Oh and the signal LED wont light up either until i flip the phase switch.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: choskins on December 26, 2015, 12:22:48 PM
Built two VP28s.  First is absolutely superb in all respects.  Second works but has very low output.  I've emailed Jeff, but he's on vacation - I don't expect to hear from him any time soon.  I have no other test equipment except for a nice multimeter.  I've ruled out some things: not the opamps (used same pair in both units), not the microphone, not the rack or the slot in the rack, resistor position, diode position and orientation, cap position and orientation of polarized caps, input transformer position, Litz transformer leads identical to working unit, no solder bridges, no apparent inadequate solder connections.  Are there any measurements I can make that might narrow things down a bit?  Thanks in advance for any help, Carlo.

Update: Forgive me, but I do not know how to use this message board - I could not find a reply icon to respond directly to Potato Cakes from Nashville.  Thank you very much for your response.  Since I have no way to measure those voltages while it is powered up, I am going to contact Jeff today to get one of those extension test jigs to start running this down better.  But get this.  With the problematic unit in the rack, I can lightly rap on the face of it with my knuckle, and suddenly it works perfectly.  Then over the next minute or two it begins fading out all on its own - this is very annoying to say the least because it sounds **sooo luscious** on a guitar cabinet, acoustic guitar, and kick drum with my Royer 121.  Then it fades out.  Anyway, I rap on the face again, and it works again.  I began suspecting the DOA sockets, but all I could do to check was to swap the two to see if THAT made a difference - it did not - same exact problem.  Mystified here.

Final update - problem resolved.  I got to thinking about it some more and knew that the problem was not an assembly issue since it would function identically to the other momentarily.  I decided to touch up every single solder joint that didn't look perfect, and now the unit works perfectly.  Taught me a lesson: don't move to the next solder joint until the one you're working on is as perfect as you can make it.  Thanks again to the fellow who responded - I really appreciate it.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on December 27, 2015, 01:44:13 PM
Built two VP28s.  First is absolutely superb in all respects.  Second works but has very low output.  I've emailed Jeff, but he's on vacation - I don't expect to hear from him any time soon.  I have no other test equipment except for a nice multimeter.  I've ruled out some things: not the opamps (used same pair in both units), not the microphone, not the rack or the slot in the rack, resistor position, diode position and orientation, cap position and orientation of polarized caps, input transformer position, Litz transformer leads identical to working unit, no solder bridges, no apparent inadequate solder connections.  Are there any measurements I can make that might narrow things down a bit?  Thanks in advance for any help, Carlo.

Do you have the correct voltages at the each of the DOA sockets? Are any of the audio connections shorting to ground? Is it low output regardless of setting (i.e. gain up/down, fader up/down, HPF in/out/-6/-12, polarity in/out, etc)? On one of these I built, I had low output with the HPF engaged, and though it seemed that the soldering looked fine, I indeed had a less than stellar solder connection. Other times I have had a misplaced a component even after I had spent days going over it and thinking everything was in order, even if I had just successfully completed building the same thing. If you do feel that it's definitely not the construction, the only thing I think anyone here can suggest is to break out the schematic, send a 1k tone through both units at equal level, compare signal between the two at each component in line until you find where they significantly differ. I would do the same comparing the voltages downstream of the +/-16V rails. It's annoying and can be very time consuming, but without much more information, this is kinda the only way to find your problem with just a multimeter if you can't visually find any errors.

I'm interested to see if you indeed have a bad component as opposed to a seemingly good yet faulty solder joint.

Good hunting!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: coyotefacerecording on December 28, 2015, 06:28:57 PM
Hey guys!  First post

Just built 6 of these CAPI kits.  Love em already.

I have one VP28 that all the features works just fine on, except the Fader knob doesn't pass signal (or very little) unless it's wide open (aka turned all the way to the right).  Any suggestions on what the issue might be?  I scanned the PCB a few times with a magnifying glass to make sure everything was on there, with correct polarity and I don't see any glaring mistakes.  Could it be a solder joint?  Bad switch?

Thanks!

*UPDATE*
Tested all joints again.  Made sure all cups were sufficiently filled.  Double checked all resistor locations with magnifying glass.  Double checked leads on Litz transformers.  Made sure op amps were properly seated.  I'm kind of at a loss at what it could be.  Any advice would be helpful.

*UPDATE 2**
Swapped out op amps with a previously working unit.  Still no luck. 
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 06, 2016, 07:03:00 PM
You can try to bypass the switch for the problematic resistors values on the fader to see if you do indeed have a bad switch. It's rare a component is actually bad, but still possible.


Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: coyotefacerecording on January 13, 2016, 09:17:22 AM
Any thoughts Jeff?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 13, 2016, 10:26:36 AM
Any thoughts Jeff?
Did you find and use the test points jig? You will have to inject signal and trace it thru to see where it actually stops. It was posted a little ways back.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: darkus on January 14, 2016, 12:53:38 AM
I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

I checked the soldering in that switch and reflow some solder. All looks good, same problem still occurs. Where else should i be looking at? The signal level im getting with phase switch in normal position is somewhere around -60dB. once i flip the phase everything works like should

Oh and the signal LED wont light up either until i flip the phase switch.

I still have this problem, is anyone able to help me out?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 14, 2016, 05:48:02 PM
I would first guess its a soldering issue. The likelihood of it being a bad switch is very slim. Like 1 in 10,000.

I checked the soldering in that switch and reflow some solder. All looks good, same problem still occurs. Where else should i be looking at? The signal level im getting with phase switch in normal position is somewhere around -60dB. once i flip the phase everything works like should

Oh and the signal LED wont light up either until i flip the phase switch.

I still have this problem, is anyone able to help me out?
I would desolder the red and orange primary leads from the T2 output transformer and take a few DCR measurements. There is no need to connect the module to power or run signal into it. Use the pad near gold finger #5 as your ground reference for your black probe. With the polarity switch out, the orange pad should be connected to ground. The red pad should have some higher resistance to ground but not a short or low DCR. The red pad should be directly coupled to the negative end of C16. With the polarity switch in, the red and orange pads should swap their responsibilities. So, the negative end of C16 should never have a low DCR to ground no matter how the polarity switch is set.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on January 31, 2016, 11:48:40 PM
Hi there,

Once again after a long time of not messing around with my VP28's I still have two out of eight that need some love. I've found the sonic problem with the one that had some low level distortion; it needs a new 1731. This one also has a non functioning red LED. I swear it did work when I first built it, but stopped shortly thereafter. The pre does get phantom power, but the LED does not work. It is in correctly and I reflowed the solder just in case. Is it just the LED itself? If so, easy fix.

Next up is THE ONE.  My only lemon.  This one pops REALLY loud when you hit the pad button. It pops less loud, but still loud when you hit the mic button. It makes a very tiny noise when you hit the phase button. I put some OpAmps that I knew were working in it and tested it. You can listen to this mp3 of me blabbing into the thing and you'll hear about 53 seconds of noise before I hit the pad button. I dropped the pop by 15db so your ears and headphones don't explode, but it's still loud. After that, I engage the mic switch and you can (unfortunately) hear the dulcet tones of my voice babbling, describing what I'm doing.

Another strange thing about this pre is that I'd had the issues with it right after I built it; the pops and noise, but it was only tonight when I plugged it in that I fried the 1731 (R14), prompting me to test wit with known working DOA's.

Here's the link to the mp3:

https://dl.dropboxusercontent.com/content_link/IlUyVMsK3Q1fBkz1dQWoqWEPC30afPF0bPq0JezTflT5ePzTY2ipSIWuGr2Bm8p4/file

As always, thanks everyone for their help and thank you, Jeff, for not only making these available, but also guiding beginner DIYers through the building/troubleshooting process.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: mysticmerlin on February 02, 2016, 12:17:53 PM
Hi,
I have built two VP28's , one works fine and sounds great the other doesn't work at all
I have obviously checked to the best of my ability and there  is a weird 10.2 Ohm between C and V+ which seems quite abnormal except I can't find where that bridge is coming from. I checked R36 first since it's 10 Ohm but it seems to be properly soldered.
Any help would be most welcome

Thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 02, 2016, 05:54:01 PM
....I have obviously checked to the best of my ability and there  is a weird 10.2 Ohm between C and V+ which seems quite abnormal except I can't find where that bridge is coming from.....
Yes this is very odd. It must be a short or solder bridge. There are only a few parts that connect top the +V rail. I would investigate these areas.

~Socket for each DOA (obviously), make sure to remove both amps to rule them out.
~A2 IC, check orientation and maybe remove it? Check IC socket
~C8
~Right lead of C2
~Right lead of R2
~Right lead of R3
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 02, 2016, 06:05:45 PM
.....This one also has a non functioning red LED. I swear it did work when I first built it, but stopped shortly thereafter. The pre does get phantom power, but the LED does not work. It is in correctly and I reflowed the solder just in case. Is it just the LED itself? If so, easy fix.
Its possible that the LED is bad but unlikely. I would also reflow solder to the top 6 phantom switch pins. Basically there is current always flowing to the LED. The switch just shorts out the LED's leads when it is out. When it is engaged, there should be no contact between the LED's leads. This can be checked with a DMM when not connected to power. If you need an LED I can send you one no problem.

Quote
Next up is THE ONE.  My only lemon.  This one pops REALLY loud when you hit the pad button. It pops less loud, but still loud when you hit the mic button. It makes a very tiny noise when you hit the phase button. I put some OpAmps that I knew were working in it and tested it. You can listen to this mp3 of me blabbing into the thing and you'll hear about 53 seconds of noise before I hit the pad button. I dropped the pop by 15db so your ears and headphones don't explode, but it's still loud. After that, I engage the mic switch and you can (unfortunately) hear the dulcet tones of my voice babbling, describing what I'm doing.

Another strange thing about this pre is that I'd had the issues with it right after I built it; the pops and noise, but it was only tonight when I plugged it in that I fried the 1731 (R14), prompting me to test wit with known working DOA's....
Well, with phantom engaged, there will always be a pop when engaging the Pad switch. Since there is DC present on the contacts, it will happen. If this happens when phantom is off, I would check for DC at the input pins of the card, gold finger #8 and #10. A loud pop when switching most usually means there is DC on the audio switch. There shouldn't be any there with phantom off. Maybe its from an outside source? Whatever is plugged into the pre's input?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on February 02, 2016, 09:14:12 PM
[Its possible that the LED is bad but unlikely. I would also reflow solder to the top 6 phantom switch pins. Basically there is current always flowing to the LED. The switch just shorts out the LED's leads when it is out. When it is engaged, there should be no contact between the LED's leads. This can be checked with a DMM when not connected to power. If you need an LED I can send you one no problem.
Quote

Thanks! Reflowing the switch makes sense because, I do remember it working when I first built it.

Well, with phantom engaged, there will always be a pop when engaging the Pad switch. Since there is DC present on the contacts, it will happen. If this happens when phantom is off, I would check for DC at the input pins of the card, gold finger #8 and #10. A loud pop when switching most usually means there is DC on the audio switch. There shouldn't be any there with phantom off. Maybe its from an outside source? Whatever is plugged into the pre's input?

Yes, it pops no matter what the source is... although I haven't checked it with a line in, but with, for instance, a plain old 57 and the phantom OFF, it pops. Did you hear the noise also? That noise is there even with the volume all the way down.

And I beg for your patience again as I ask exactly how to check for DC at gold fingers 8 and 10. Do I do this with the unit powered up and in the jig? Seems difficult as the pins are in the jig. Obviously there is basic understanding that I don't have. What setting do I use for my DMM, and exactly where does each probe go and under what conditions (powered on, off, etc.)?

And then if there IS DC there, what does that tell me? Or shall I just cross that bridge when I get to it?

As always, thank you, thank you, thank you.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: mysticmerlin on February 03, 2016, 05:03:10 AM
....I have obviously checked to the best of my ability and there  is a weird 10.2 Ohm between C and V+ which seems quite abnormal except I can't find where that bridge is coming from.....
Yes this is very odd. It must be a short or solder bridge. There are only a few parts that connect top the +V rail. I would investigate these areas.

~Socket for each DOA (obviously), make sure to remove both amps to rule them out.
~A2 IC, check orientation and maybe remove it? Check IC socket
~C8
~Right lead of C2
~Right lead of R2
~Right lead of R3

Thanks so much , there was indeed a tiny solder leftover between C2 and R3 that was hard too see. I should probably lose the habit of soldering resistors on both sides of the pcb. It just makes it harder to spot small mistakes such as these .
Thanks Jeff, I was worried I would be stuck without a schematic to follow.
Great Sound I have to say.  I have expensive tube pres , neumann, 1073's, shadow hills, gml's  and the VP28 is the mainstay for our old fender basses , real clean solid low end , just special, (this is my 3rd build).  But I guess all the people in this thread already know this.
Cheers
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on February 04, 2016, 09:08:56 PM
OK...

I'm checking Jeff's instructions from last May (yes, this thing has been languishing for 9 months...)

Can anyone help me make sure I test it correctly?

It seems I am to have the VP28 in the jig, powered up and I am to put one lead from my DMM on gold finger 8 and the  other on gold finger 10 simultaneously. Both with phantom off and phantom on. As opposed to having one lead on gold finger 8 and the other on some mysterious ground or something and then testing gold finger ten the same way...?

Thanks, as always.




Are the input conditions identical when you are checking this? What is the source? Maybe there is excessive DC at the input of the module for some reason?

Popping sounds when switching usually means there is DC present on the switch contacts somewhere. Maybe there is a solder bridge or short somewhere with phantom or something? If it where me, I would measure for DC at gold fingers #8 and #10 with phantom off. There should be none. Then check again with phantom on.
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 05, 2016, 01:44:51 AM
OK...

I'm checking Jeff's instructions from last May (yes, this thing has been languishing for 9 months...)

Can anyone help me make sure I test it correctly?

It seems I am to have the VP28 in the jig, powered up and I am to put one lead from my DMM on gold finger 8 and the  other on gold finger 10 simultaneously. Both with phantom off and phantom on. As opposed to having one lead on gold finger 8 and the other on some mysterious ground or something and then testing gold finger ten the same way...?
To check for DC at the card edge inputs, connect the module the jig and under power, DMM set to DCV, red probe to gold finger #8 and black probe to ground. The gold pad adjacent to gold finger #5 is the easiest place to get a ground reference. Next check gold finger #10 to ground.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on February 05, 2016, 02:16:07 AM
Thanks! That's what I needed. Built one 1731 tonight and reflowed the solder on the phantom switch on the VP28 that's working (not the noisy one.) Didn't have a chance to test it, but will do that soon and check the DC on the noisy one.

Thanks again.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Golgoth on February 05, 2016, 01:41:48 PM
My channel fader knob is a bit loose, it's coming from the grayhill shaft and that's not really pleasant to use. Is there a solution to "fix" it without replacing the entire switch itself?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on February 07, 2016, 01:13:13 AM
OK... If I tested this right, and I'm not sure if I did, there is no DC present at gold fingers 8 or 10, either with phantom on or off.

So in order to get access to the gold fingers while the unit is in the jig, I had to take the whole thing apart. I am attaching a picture of the back of the unit with the jig on and just the smallest bits of gold fingers sticking out of the jig. Hopefully this is what I was supposed to do. I put the ground lead right on that little black pad on finger five. There are no fingers six or seven, so the next one down is eight. No nine so the next after eight is ten.  Holding the one lead on ground, I poked at gold fingers eight and ten, and even tried different ranges on the DMM (yes, checking DC.) The DMM never showed any reading at all. Not even zero. It showed where the decimal point was as I moved the range around. I was just using it to measure resistors as I built the other 1731, so I believe it is working. If anything is wrong, it's operator error.

Oh... I did this test with no DOA's in. Perhaps that's the problem? I'm going to take my time and not proceed until I have more of an idea of what I should be doing. What's my next step?

Thanks again.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on February 07, 2016, 01:59:38 AM
I was just working on my other troublesome VP28 that had a tiny bit of distortion that I had traced to a bad 1731. It also had a malfunctioning 48V LED. Reflowing the solder on the switch solved that problem. The noise that I was getting is gone... but I seem to have another noise. I don't know if it was present with the old 1731. When I tested this thing tonight, I switched through every step of each fader knob. When both are up really loud, suddenly noise kicks in. I recorded the sound it makes and twiddled the knobs a bit so you can hear how the noise changes. Both faders need to be on the highest two or three clicks to get this wonderful sound. None of my other six working VP28's do this. It's a lot of hiss that comes on suddenly, plus a little squeal that modulates at one particular setting.

https://dl.dropboxusercontent.com/content_link/SMTSvz4A3ExeQzbhW6dO6X9SUjeOzipgi0vPSJ8KweDDsOlKGUbvhxXoKNXjigIV/file
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on February 09, 2016, 10:12:52 AM
Hi there,

Hoping to make some progress on these tonight. Anyone have any suggestions?

Thanks.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: digitalblack on February 12, 2016, 02:43:36 AM
Hello, helpful internet folk.

So I just built a pair of these. One sounds fantastic and the other has extremely low output. I read in an earlier post that this could be caused by a bad input transformer? Mine is indeed wiggling on the pins just a liiittle bit. Is there any way to test my theory without desoldering the transformer?

Thanks in advance for your expertise.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: vitalelectric on March 10, 2016, 04:34:07 PM
Hey there, not sure if this thread is still monitored, but I have an issue with a VP28 I just finished a build on.

Everything seemingly works fine , however the Preamp Gain knob has no effect on the output level UNTIL you get the the maximum setting, and it overloads the converter like crazy.

The unit passes signal, and sounds like its working ok, but I obviously would like to have some control over this. Could be a resistor thing? I double checked all the solder joints, but have yet to do any testing.

Any help would be greatly appreciated, I wanna get this thing up and healthy!  :D :D :D
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: collinTHEbrewer on March 28, 2016, 07:35:41 PM
I've had this built for a while, and it was working to my knowledge though I was unable to do extensive testing at the time. Here's my issue:

My VP28 is acting as if there were a high pass filter on the output around 1K. I put some pink noise through it. See the images below. This happens at all gain setting in either MIC or LINE mode.

(http://s10.postimg.org/vqgcoai8p/VP28_Issues.png)

Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: collinTHEbrewer on March 29, 2016, 04:46:11 PM
Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

And I'm dumb. It was something weird going on with my 500-series rack...  :-X
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on March 29, 2016, 06:01:23 PM
Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

And I'm dumb. It was something weird going on with my 500-series rack...  :-X
Excuse me for being late to the party,but it looks or looked like a balancing/debalancing thing.
Often sounds like you described,thin and no bass.
What was the issue?


Udo.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: collinTHEbrewer on April 06, 2016, 02:13:25 PM
Any ideas on where to start troubleshooting? The actual high passes seem to work although the frequency content is so minimal in that range.

And I'm dumb. It was something weird going on with my 500-series rack...  :-X
Excuse me for being late to the party,but it looks or looked like a balancing/debalancing thing.
Often sounds like you described,thin and no bass.
What was the issue?


Udo.

Wow it's funny you say that. The original issue I had was with the DB25 connector on my 500 series rack being loose. I corrected it and the low end response returned. However, I noticed in some subsequent tests that I was having the same issues whenever I used my DIYRE L2P to patch my line outputs into my mic pres.

I think I just realized the issue. I have been using a Behringer PX2000 patchbay since that's what I had lying around. It turns out the PX2000, unlike the PX3000, is unbalanced. What a dumb thing to just now realize.  :-\
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on April 06, 2016, 02:40:16 PM

I think I just realized the issue. I have been using a Behringer PX2000 patchbay since that's what I had lying around. It turns out the PX2000, unlike the PX3000, is unbalanced. What a dumb thing to just now realize.  :-\
Better now than never,glad you got it solved!


Have fun,


Udo.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Buffstuffbuddha on April 10, 2016, 12:47:15 PM
I built a VP28 from CAPI and it works great but I am having one small issue. When phantom is engaged and I press the Line switch I get an audible pop. Not a suddle pop like when engaging a pad, it actually clips my converter. I checked for any bridges or otherwise weird looking spots but everything looks fine. Something has to be leaking DC then right?  A bad cap somewhere? I just really don't even know where to start troubleshooting this thing. Everything works on it other than that pop...I know this has been brought up on multiple forums but I have never seen anyone come up with a resolution other than some pins being bridged. I know that's not the case on mine so I don't really know where to start.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 10, 2016, 12:57:44 PM
This is normal thing since there is 48V DC present on the Mic/Line switch if phantom is on. There is only one solution and its simple. DO NOT press the Mic/Line switch when phantom is on. Not sure of the resason anyways. By doing so, you are applying 48V to the output of the device you are feeding the VP28 with. Its sorta like driving down the freeway and switching your transmission into reverse. Sure you can do it but its not a good idea  ;)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on April 10, 2016, 01:01:35 PM
Exactly what I wanted to say!


Since there's no button for it atm:


Like!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Buffstuffbuddha on April 10, 2016, 02:55:17 PM
So it should be a big enough pop to actually clip my converter? We're not talking about the same kind of pop you get from engaging the pad right? I may have to buy another one, you know, just to compare the two ;) 8)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 10, 2016, 02:58:48 PM
I would highly recommend not engaging the Mic/Line switch while phantom is on. Never. Not even to compare when you get a 2nd unit.  ;D
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Buffstuffbuddha on April 10, 2016, 03:16:38 PM
Haha I think I'll take your advice...now just to play devil's advocate. Is your mic/line switch circuit different than, say, an API 512 circuit? Just wondering what they do differently because there is only a very tiny pop when engaging the switch on a 512...I promise I will never engage that switch with phantom on again :D
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 10, 2016, 07:09:51 PM
Haha I think I'll take your advice...now just to play devil's advocate. Is your mic/line switch circuit different than, say, an API 512 circuit? Just wondering what they do differently because there is only a very tiny pop when engaging the switch on a 512...I promise I will never engage that switch with phantom on again :D
I honestly don't know what is happening with the 512C mic/line. I have never really looked. If its like the 1608, the line input goes straight into the 2520 as a balanced receiver so AFTER the input transformer. My line in goes into the 2622. Basically same path as a mic in except thru a u-pad. My mic/line switch is only switching in a 3 resistor u-pad.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: digitalblack on April 22, 2016, 06:22:34 PM
I'm measuring 11R across RF3 instead of 36.5R (it's the proper resister, measured when installing). Is it possible this is causing my low output issue?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tttbag75 on June 03, 2016, 03:39:38 PM
Hello !
I've just built my pair of VP28. And I have a problem with one of them. The first is OK, but with the second I have 444mV at TP3.
I don't know where does the problem come from.
Do you have an idea ?
Thank you for the answers !
Thomas
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tttbag75 on June 03, 2016, 03:54:56 PM
... I forgot...

- VP28 rev.B
- gar1731 v4.1
- gar2520 v4.1
- Litz transformers

Thomas
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 03, 2016, 04:32:49 PM
... I forgot...

- VP28 rev.B
- gar1731 v4.1
- gar2520 v4.1
- Litz transformers

Thomas
Everything is OK at TP2?

If so, double check your wiring for T2 (since it is Litz). If that is good, desolder T2's lead from the blue pad and measure the test voltage at that loose lead.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tttbag75 on June 04, 2016, 04:23:38 AM
Yes, everything is OK at TP1 and TP2.

My wiring for T2 are OK.

I have desolder the blue pad of T2, and now I have 462mV.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tttbag75 on June 04, 2016, 06:33:05 AM
... I have 694mV at the red pad of T2.

Thomas
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 04, 2016, 11:04:12 AM
You should next desolder all leads for T2 and check the windings for shorts. Make sure no leads are touching each other. Set your DMM for less than 200Ω.

You should only have resistance between the following leads:
Black to Red
Gray to Pink
Violet to Orange

Don't just check between the above sets but between all leads.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tttbag75 on June 07, 2016, 03:42:34 PM
After desolder :

- black to red : 15,7R
- grey to pink : 31,1R
- violet to orange : 31R

... And I have resistance between this leads :

- grey to orange : 31,1R
- grey to violet : 61,7R
- pink to violet : 31R
- pink to orange : 0,4R

My transformer is dead ?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: applespace on June 16, 2016, 04:35:40 AM
I would highly recommend not engaging the Mic/Line switch while phantom is on. Never. Not even to compare when you get a 2nd unit.  ;D

Could this blow PR1-4 on the main board? Just built 2 VP28s and when I plugged in one I accidentally had phantom and mic depressed. It fried those 4 resisitors. Any other ideas where to look if not?

Cheers

EDIT: I get the sarcasm now (idiot). Any ideas though? Bad OpAmp in the pre-amp section?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: geardude on July 02, 2016, 09:33:32 PM
Hey guys after tackling lots of vp 26's I dived into a 28. Fired her up and everything works like a charm! I do have one question however as when I go to shut off my snackbox/lunchbox I hear a loud squeal/laser beam noise.

Turn it on and everything operates fine- and again shutting it off I hear this again.

Doesn't happen with my other vp builds. Is this a known thing or is something a miss here?

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: geardude on July 02, 2016, 10:19:10 PM
Alright that's comforting to hear  : 8)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: achase4u on July 05, 2016, 04:04:38 PM
Curious, did you guys get flat washers with your kits for the X-formers?  I did not.  I am wondering if they have since been omitted as unnecessary?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 13, 2016, 09:45:39 PM
Curious, did you guys get flat washers with your kits for the X-formers?  I did not.  I am wondering if they have since been omitted as unnecessary?
The info is all posted on the support docs page with detail drawings  ;)
http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/VP28-New-Style-Hardware-BOM.pdf
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pachi2007 on August 30, 2016, 12:28:16 PM
Hi.
I had a perfectly working VP28 that now is giving me some trouble.
The  thing  now is I have noise ( even signal light shows it and it´s almost constant but it comes and goes  ) but it disappears when I switch the phantom on.
This is with a dynamic/condenser plugged or with nothing connected at all. Both mikes work great and there´s no noise with phantom but the noise comes back when I switch it off.
I can see a constant healthy +48 at the output though.

Any advice to get me started? Any help much appreciated.
Thx
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: BennyS on September 08, 2016, 05:20:39 AM
Hi There
Thanks for a nice build forum!

I recently bought a VP28.Rev B.1 Stepped Gain entire bundle. The VP28 works fine for dynamic mics with a large gain and clear sound. For Phantom power, however, there is no signal at the output.

I have measured 200mV between pins 1 and 2 on the Input XLR with Phantom enabled and that should be ok - It is anyway identical to the VP312 I also acquired which works perfectly! :-) The dynamic and condenser mics are both working fine on the VP312.

So the phantom power is there on the In-XLR of the VP28 but the signal is zero - again, the dynamic mic is working fine on the VP28.

While not having the schemo at my disposal, I would appreciate it a lot if anyone of you could give me a hint to where to start looking?

Ps. I saw a similar thread in this forum a while ago, posted by Andrew, but that thread ended abruptly - solution found or still open? Ds.

Best regards
Benny
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: BennyS on September 08, 2016, 06:12:00 AM
Hi

Short: Problem solved!
Long: After posting my question on the forum, I (again) looked over all solder points on the VP28. Did not find anything. Then I decided to resolder all points - and that was it - a bad solder point (somewhere).

I am happy that it now works fine !

Thank you!


Hi There
Thanks for a nice build forum!

I recently bought a VP28.Rev B.1 Stepped Gain entire bundle. The VP28 works fine for dynamic mics with a large gain and clear sound. For Phantom power, however, there is no signal at the output.

I have measured 200mV between pins 1 and 2 on the Input XLR with Phantom enabled and that should be ok - It is anyway identical to the VP312 I also acquired which works perfectly! :-) The dynamic and condenser mics are both working fine on the VP312.

So the phantom power is there on the In-XLR of the VP28 but the signal is zero - again, the dynamic mic is working fine on the VP28.

While not having the schemo at my disposal, I would appreciate it a lot if anyone of you could give me a hint to where to start looking?

Ps. I saw a similar thread in this forum a while ago, posted by Andrew, but that thread ended abruptly - solution found or still open? Ds.

Best regards
Benny
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 18, 2016, 11:49:19 PM
Hi there, folks,

It's been a while since I last attempted to fix my last (of eight) VP 28 build. Most of the others worked just fine right away. A couple had minor issues. I still don't know what to do about this one and I have some time in the coming weeks to put into it.

This thing seems really hot. Like loud. It's also noisy, like hiss. At a certain point, it squeals. For instance with the fader at +2, when the preamp gain gets all the way up, it squeals. I have attached an mp3 of exactly this sound. Couple of notes: I did this test with known working OpAmps from one of my other working VP28's. Also, if the fader is higher than +2, the preamp gain would squeal at a lower volume.

TURN YOUR VOLUME DOWN BEFORE HITTING THIS LINK:

https://dl.dropboxusercontent.com/u/1643668/Squeal.mp3

As always, thanks for the help.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 19, 2016, 12:00:05 AM
In reference to my post above, this is where I left off the last time I tried to work on this last VP28. It is very possible that I'm not testing it correctly, assuming that testing is the next step. I am confused by the fact that when in the jig, the gold fingers are almost completely covered by the jig. It is also possible that I'm not understanding the description of where to get ground.  These things are paint by numbers to me. If I am meticulous, everything is great. If I make a mistake, I have no idea what this dang thing is, nor where a mistake might be, etc.

Thanks again.

OK... If I tested this right, and I'm not sure if I did, there is no DC present at gold fingers 8 or 10, either with phantom on or off.

So in order to get access to the gold fingers while the unit is in the jig, I had to take the whole thing apart. I am attaching a picture of the back of the unit with the jig on and just the smallest bits of gold fingers sticking out of the jig. Hopefully this is what I was supposed to do. I put the ground lead right on that little black pad on finger five. There are no fingers six or seven, so the next one down is eight. No nine so the next after eight is ten.  Holding the one lead on ground, I poked at gold fingers eight and ten, and even tried different ranges on the DMM (yes, checking DC.) The DMM never showed any reading at all. Not even zero. It showed where the decimal point was as I moved the range around. I was just using it to measure resistors as I built the other 1731, so I believe it is working. If anything is wrong, it's operator error.

Oh... I did this test with no DOA's in. Perhaps that's the problem? I'm going to take my time and not proceed until I have more of an idea of what I should be doing. What's my next step?

Thanks again.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 19, 2016, 12:21:29 AM
Sorry for spamming with these questions. Looking at what I had tried to do, perhaps I don't know how to count gold fingers... Do we count only the ones on the card or do we count the spaces where there *could* have been more? I did the latter. It's not making sense to me now. Also, when Jeff says I can get ground at the gold pad next to gold finger five, I have no idea what gold pad he's talking about. I don't even remember where I stuck the ground lead when I tested this thing back in May.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 19, 2016, 12:11:41 PM
The VPR protocol has 15 gold fingers. They are number 1 thru 15 with 1 being at the top edge of the card. It makes no difference if some of the pads are left off. They are still numbered 1 thru 15 from the top to the bottom so if they are missing you need to count the spaces. There is a small thru hole via adjacent to gold finger #5 that makes for an easy spot to get a ground reference.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 19, 2016, 04:56:49 PM
Thank you! I actually understand what you're saying. I will retest this thing tomorrow night.

Another question:

Do the DOA's need to be in it when I test it?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 20, 2016, 11:44:53 PM
OK. I tested it again and got no reading either pad. I did notice that the green signal present LED blinked on every time I touched gold finger 5. Even if I held the probe in place, it would just blink at first contact and then go out. No blinding when testing gold finger 10. Not sure if this information is relevant, but it's the only thing that I noticed happening.

What's next?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 21, 2016, 02:06:41 PM
Not sure if I asked you for this before but measure DCR between the following gold fingers:
5 to 8
5 to 10
8 to 10

If you have a manual DMM be careful of the range setting. We are looking for a close to direct short.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 21, 2016, 03:15:51 PM
Thank you again for the help!

I just want to clarify in terms that a drummer can understand...

I did the tests with one probe on pad 5 and then moved the other probe to both 8 and 10. You want me to also put one probe on 8 with the other on 10? Does it matter which probe goes where? What range should y DMM be set to? When I tested last night, I moved through all the ranges. I first tested it  with black on ground and red on the 8 and 10, and then tried it the other way. No readings at all either way.

Oops, correction:  When I tested last night, the signal present light blinked when I touched pad 8 while also touching pad 5. Touching pad 8 alone did nothing. Touching pads 5 and 10 did nothing.

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 21, 2016, 03:23:40 PM
We are looking for a direct short here so the module should not be racked and not under power. Your DMM needs to be set to read DCR. Same as if you were measuring resistors.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 21, 2016, 03:50:37 PM
DOH!

Got it.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 22, 2016, 10:21:00 PM
Not sure if I asked you for this before but measure DCR between the following gold fingers:
5 to 8
5 to 10
8 to 10

If you have a manual DMM be careful of the range setting. We are looking for a close to direct short.

I get 8.58k between 5 and 8 and also between 5and 10.

I get 9.97k between 8 and 10.

What does that tell us?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 22, 2016, 10:23:42 PM
That tells me its in Line mode. I should have specified to put 'er in Mic mode.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 22, 2016, 10:30:41 PM
That tells me its in Line mode. I should have specified to put 'er in Mic mode.


I'm glad I refreshed the screen before shutting down for the night! Thanks for the quick reply!

5 to 8 and 5 to 10 are both now 3.61

8 to 10 is now 27.5
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 23, 2016, 09:06:40 PM
With the Mic button in and all others out, you should have 3k6Ω from 5 to 8 and 5 to 10. Between 8 and 10 will be around 29Ω.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 24, 2016, 02:19:00 AM
With the Mic button in and all others out, you should have 3k6Ω from 5 to 8 and 5 to 10. Between 8 and 10 will be around 29Ω.

This is what I have (almost.)  3.61k for 5 to 8 and 5 to 10 and 27.5 for 8 to 10.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on October 27, 2016, 03:30:32 PM
Bumping. Should be able to work on this more tonight. What's the next thing for me to do or check?

Thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on November 02, 2016, 02:23:59 PM
With the Mic button in and all others out, you should have 3k6Ω from 5 to 8 and 5 to 10. Between 8 and 10 will be around 29Ω.

This is what I have (almost.)  3.61k for 5 to 8 and 5 to 10 and 27.5 for 8 to 10.

Quoting and bumping again.  I may have some time to work on this tonight. What's my next step? What should I try?

Thanks again.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 27, 2016, 02:52:59 PM
Hi guys! I too have the green light problem happening. When I power up the unit, it passes signal perfectly with good tone, however the green light stays lit until the gain knob is turned to the center position. It then turns off and functions normally. Is there any way to remedy this? Thank you.


Hey Jeff and everyone else,

I too am experiencing a similar problem on a new build. I searched, but was not able to find any responses to this problem.  The green signal led stays lit when I first power up the module, but strangely, when I turn on phantom power, the signal led functions as normal. Turn phantom back off, the signal led goes back to constantly lit. Another odd behavior I have noticed is that around 30 minutes of use, the signal led begins to function as normal, regardless of the 48v phantom switch position. Kinda stumped on this one. Everything looks like its correct. Signal passes fine and the preamp sounds fantastic.

Things I have tried:
     Changing the op-amps for known working op-amps.
     Reheating connections that are in the path.
     = Same behavior.

Could this be a faulty chip? What would cause it to function normally once heated and running for 30ish minutes?
Any help is appreciated.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 28, 2016, 12:23:11 PM
Over the holiday I finished my second vp28. Looks like it's having the same problem. Signal led stays lit unless I push the 48v button. Then functions as normal. Yes this is with any type of mic.

Any ideas?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 29, 2016, 10:49:37 AM
Over the holiday I finished my second vp28. Looks like it's having the same problem. Signal led stays lit unless I push the 48v button. Then functions as normal. Yes this is with any type of mic.

Any ideas?
What board Rev are these? Black Rev A or green Rev B?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 29, 2016, 12:34:00 PM
What board Rev are these? Black Rev A or green Rev B?

Both are green Rev B.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 29, 2016, 01:37:14 PM
Both are green Rev B.
Hmm, OK this is the first issue I have heard of with the signal present on the new PCB. If both are acting the same, it is likely the same build error on both or possibly a rack situation? What rack are you using?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 29, 2016, 01:53:55 PM
Hmm, OK this is the first issue I have heard of with the signal present on the new PCB. If both are acting the same, it is likely the same build error on both or possibly a rack situation? What rack are you using?

Agreed it does suggest the same error. I am using a Radial PowerStrip 500 Series 3-Slot Power Rack. I do have 2 other lunchboxes I can try. An aphex usb 500 and a bento 6. But I have never had any other problems with the radial. Of course that doesn't mean it's not causing the problem.

I guess the question would be what feeds the ic when the 48v button is pressed vs not pressed? As this seems to be related somehow.

I followed the guides by both you and chunger in this thread. Used the BOM provided on the site and carefully separated each part and tested individually with a volt meter taping each part to a piece of paper similar to the way chunger does. Never had any discrepancies. All parts matched up to their respective pcb names. Many builds under my belt. First time I've had a problem.

That said I am a human. Weird that I would repeat the mistake unless there is something I repeated in the guides?? Maybe I am missing something. Thanks for your help.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 29, 2016, 02:09:56 PM
I did notice that the original guides are Rev A. Is there a difference somewhere in the guides between A and B that could lead to this problem?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 29, 2016, 02:18:58 PM
Did you follow the Rev B BOM?

Is there possibly low level noise where the signal light is actually correct?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 29, 2016, 02:25:49 PM
Did you follow the Rev B BOM?

Is there possibly low level noise where the signal light is actually correct?

I did follow the Rev B BOM. As far as noise, I have not noticed any. Seems very quiet. But I have only tracked electric guitars through it so far. If there is any noise what would that suggest?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on November 29, 2016, 05:15:11 PM
I think I mentioned this already. But the preamp works and sounds great regardless of the Phantom power button position. The only issue is the green signal led staying lit when 48v is not pressed in.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 29, 2016, 08:51:08 PM
I think I mentioned this already. But the preamp works and sounds great regardless of the Phantom power button position. The only issue is the green signal led staying lit when 48v is not pressed in.
Yeah this is not right. I have not witnessed this issue myself so not sure what to say. The 48V switch has nothing at all to do with the sig present LED. The phantom switch simply unshorts its respective LED and also allows 48V to flow to the 200Ω R which feeds the 6k8's. I am wondering if you can measure or see anything at the output when the sig LED is on before you engage the 48V LED?

To add, the sig present pickoff point is after the preamp opamp not before it. If the sig light is on, I would measure the module's output to see if there is something there.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on December 01, 2016, 02:43:06 PM
Yeah this is not right. I have not witnessed this issue myself so not sure what to say. The 48V switch has nothing at all to do with the sig present LED. The phantom switch simply unshorts its respective LED and also allows 48V to flow to the 200Ω R which feeds the 6k8's. I am wondering if you can measure or see anything at the output when the sig LED is on before you engage the 48V LED?

To add, the sig present pickoff point is after the preamp opamp not before it. If the sig light is on, I would measure the module's output to see if there is something there.

That is indeed strange then. To add to the strangeness, the first VP28 I built is no longer doing this. It now functions as normal. I did not change anything.

I did some testing and verified that there is no low level noise present when the 48v button is not pressed. But the second VP28 still has this problem. I will change the preamp opamp tonight and see what, if anything, happens.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on December 01, 2016, 11:18:40 PM
Switched opamps out with 4 different amps, same result. This is pretty strange. I did find that the VP28 with the problem has slightly lower input volume. When the 48v button is pressed in and the led is functioning "normally", the preamp gain takes one more click to light the signal led in the same way as the other VP28 I have. But I don't think this would be causing the signal led to stay lit constantly when the phantom power is not on.

Should I post some pictures? Any recommendations?  Frustrated... :-\
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 02, 2016, 12:33:31 PM
Well, due to part tolerances the level required to fire the sig present will vary about 1-2dB but should not be more than that.

In line mode (set to unity), it takes an approximate -16dBu signal applied to the input to illuminate the sig present LED. I have built many of these and seen it vary from -15 to -17. Those are worst case. Its usually closer to +/- 1 db around -16.

Do the pre's match in level or do the sig present LED's light at slightly different levels? Its two separate things actually.

The fact that the first one is working normally now makes me think a faulty solder joint somewhere. There is nothing here that needs to "break in" or anything.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on December 05, 2016, 05:12:20 PM
With the Mic button in and all others out, you should have 3k6Ω from 5 to 8 and 5 to 10. Between 8 and 10 will be around 29Ω.

Hey there. Still looking for the next step.

From a previous post:

This is what I have (almost.)  3.61k for 5 to 8 and 5 to 10 and 27.5 for 8 to 10.

What do I check for now?

Thanks again.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on December 05, 2016, 11:32:31 PM
Well, due to part tolerances the level required to fire the sig present will vary about 1-2dB but should not be more than that.

In line mode (set to unity), it takes an approximate -16dBu signal applied to the input to illuminate the sig present LED. I have built many of these and seen it vary from -15 to -17. Those are worst case. Its usually closer to +/- 1 db around -16.

Do the pre's match in level or do the sig present LED's light at slightly different levels? Its two separate things actually.

The fact that the first one is working normally now makes me think a faulty solder joint somewhere. There is nothing here that needs to "break in" or anything.

Thanks Jeff. I sent a test tone through both preamps into pro tools and using the same settings they measured  -10. So it seems to just be the signal led that is having the problem. I also recorded the same source and found no discernible difference between the two preamps.

I repeated the same tests after changing and swapping the op amps (using 8 different op amps, 2 from ADK). No difference. I then tried a different chassis. Again, no change at all. Exact same problem. So it seems the only problems I am having is that the signal led stays lit unless I press the 48v power button and when pressed, the signal led required more gain on the preamp (2-3 stepped notches higher) than the other to activate the led in the same manner. But again the audio levels are the same at matched settings when recording.

I do realize that nothing needs to "break in" and have tried reheating many connections following the signal path, but to no avail. My solder connections are very clean. I have been soldering for over 15 years. Not trying to say that i am infallible, I know its most likely human error, but I have checked multiple times now and all the parts are correct according to the documentation I have. I have been quite meticulous and possibly even anal... about my connections.  :o

Does this get us anywhere? Any ideas on where to focus my efforts in troubleshooting?

I really appreciate your help,

Jason
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AudioHak on December 13, 2016, 10:42:50 PM
Well Jeff, I am happy to report everything is working. But the fix still puzzles me. I was running a session and decided to the leave the 48v power on while tracking for 2 hours or so. I shut everything down for the night and came in the next day, powered everything up, switched off phantom and the led worked as normal. I have had one instance where the signal led came back on, but I did the same thing, left the phantom power on for about 30 mins, and everything was working again after that. Since then, everything has been working without any issues.

The preamps both sound incredible, compared to my 2 VP26's and my 2 VP312's ( and those sound fantastic). I hope this post helps anyone who might have this same problem, as it really was not a major issue, just a very minor annoyance.

Have a happy holiday everyone!!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: scratchyTheHobo on December 16, 2016, 02:11:24 AM
Evening, Gents! (morning?)

I've got a VP28 Rev B with blue dots and litz, and everything works minus the signal LED. Everything passes signal, the test points read out, but the signal LED won't light up. The LED is in the right direction, and I read no output voltage at the leads or the 4558. I also can't find VCC+ at the chip, so that's probably the issue.

I've tried a different LED, but not a new chip. The chip's in the right direction and always has been.

Any thoughts? I've been chasing around this thing for hours now  ???
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: byoung on December 19, 2016, 02:00:33 PM
I just finished 2 VP28's Plugged the first one in without opamps and checked voltage, everything seemed good +-16v(+16.07 and -16.04) at the opamps pins. So I powered it down and put in a gar1730 and gar2520. when I powered it back up I saw the green signal LED flicker and PR3 smoked. I saw a small glow(arc) on the gar1730 around the general area of R14/R11 before I got the power off, but that could have been PR3 going up in smoke possibly? I assume PR3 is a protection resistor and am hopeful it did it's job at preventing any further damage. I've checked the gar1730 for solder bridges visually and didn't see any or any smoked parts, but that doesn't mean something isn't dead. Does anybody have an idea what I should be looking for based on what happened? 

I'm cautious to plug in my second VP28 until I have this resolved since I populated them at the same time, if I screwed something up then it very well is on the second preamp/set of opamps too.

Also is there a breadboard circuit I can put together to test the opamps on their own?  to prevent any further hardware damage?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 19, 2016, 02:15:14 PM
When a PR smokes it indicates trouble on the respective discrete opamp. The most common errors are swapping and/or turning the larger BD transistors. The next thing is wrong facing diodes.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: byoung on December 19, 2016, 02:48:08 PM
Am I assuming correctly that PR3 would be for the fader booster opamp? I want to make sure I'm focusing on the proper one.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: byoung on December 19, 2016, 03:07:09 PM
Argh :-[ I swapped the BD139 and 140 on the 2520's,  seriously don't know how I missed that. Would this cause PR3 to blow? Also do you think they need to be replaced now, or just moved to the right position? I don't see any external damage to them, but not sure how robust they are.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 19, 2016, 04:01:25 PM
Argh :-[ I swapped the BD139 and 140 on the 2520's,  seriously don't know how I missed that. Would this cause PR3 to blow?
Yes indeed it would. You can just try putting them where they belong and see if it works ok.

FYI, PR1 and PR3 are for the preamp opamp. PR2 and PR4 are for the fader booster amp.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: byoung on December 19, 2016, 07:34:41 PM
Well I swapped the 140 and 139 in my 2520's and and put my second VP28 in the rack. Works great so far, going to let it burn in for a bit. I've got to get another 10r before I get #1 up running because I don't have any on hand, have hundreds of other values but no 10R.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: hecjrivera on December 27, 2016, 02:22:59 PM
I recently bought the vp28 Rev B.1 and noticed its different from this build. Does anyone know where I can find the right guide?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on December 29, 2016, 08:50:22 PM
I recently bought the vp28 Rev B.1 and noticed its different from this build. Does anyone know where I can find the right guide?
It is basically the same as the first Rev but you MUST follow the Rev B BOM.

All VP28 support docs can be found here:
http://capi-gear.com/catalog/support_docs.php#VP28
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Derrils on January 04, 2017, 04:54:43 PM
Hello,

I'm having a little trouble with a vp28. I built a kit with Litz wire transformers and the pre-built Red Dots. 3 main symptoms:

1) Upon first couple power ups, the green signal LED was constantly lit. I found that it would flicker when pressing the 48v switch. After running signal through it for a few seconds, it no longer does this and works as it should. I guess I'll call that solved?

2) Line mode passes a clean signal, but -10.58db lower than whatI send in. I use Pro Tools to generate a 400hz sine wave at 0dbfs and measured through the vp28 at unity,  then with the patch wire straight from the output, patchbay,  to the input of the converter (rosetta 800). Signal without the vp28 in the chain comes back clean at 0dbfs.

3) In Mic mode, I can see that it powers my 414xls and it passes a clean signal for a few seconds, then dies out to a very low signal.  There is a loud, low rumbling sound in the signal. I have a recording of it if needed.

I tried switching positions and re-seating of my op amps and switching to two of my home-built gar 2520's but the symptoms are identical.

I've switched slots of my Lindell audio 10 space rack but no difference.

I've reflowed every solder joint except the switches on the HPF board, since they were hard to reach once assembled. That will be my task tonight, but if anyone has ideas on something else to check as well, please let me know.

Thanks!

Edit: This is the Rev B, green pcb. I followed the proper BOM
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 06, 2017, 10:03:50 AM
I would check the windings on the output transformers to make sure nothing is internally shorted. If you scroll back thru this thread, it should be covered a few different times.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Derrils on January 07, 2017, 01:34:03 PM
I've done some more testing. I suspect that I may have previously had the preamp gain knob set incorrectly. Now, passing a line level signal at unity returns within 0.33 db.

After desoldering the hpf board and reflowing, the c414xls mic signal is much more consistent, but still problematic. It seemed to be working until I tested the high pass section. I eventually get loss of signal and a low frequency rumble.  Signal then seems to slowly swell and diminish.

Here are the results from the T2 and T3 leads as well as the test points. Thanks so much for the continued help. I can't imagine how frustrating it must be to remotely diagnose the problems of an internet moron like myself! It's truly appreciated.

Test Points:
From Pro Tools: 400hz Sine wave -15.8db, measured 1.271 Vac
TP1: 115.4 mv
TP2: 0.729 v
TP3: 1.29 v
TP4: 315.1 mv
TP5: 315.2 mv
TP6: 315.1 mv
TP7: 0.664 v

T3:
Orange-Violet: 31.6 ohms
orange pink: open
orange - grey: open
orange-red:open
orange-black: open

T2:
Orange-Violet: 31.1 ohms
orange pink: open
orange - grey: open
orange-red:open
orange-black: open
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Majestic12 on January 09, 2017, 05:13:57 AM
Also have a look at the gold fingers at the card edge connectors. I had one new Rev B pcb here at my place where a small residue of copper has shorted out some of the contacts when the module was plugged in.
I had similar symptoms (green led always on, signal fading out....) but was able to fix it with some gentle scratching along both angled edges of the connector.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jessegimbel on January 11, 2017, 09:46:28 PM
Just wanted to leave a quick comment to say I just finished my third VP28 (my first Rev B, the other two I've had for a bit are Rev As). Sounding great! I had an issue where the phantom power light wouldn't come on, and when the button was depressed it cut out the Mic light. I reflowed the solder for the phantom power switch, and all seems good! I've got two more waiting to build, and I'm sure I'll eventually make it 6, if not more. Love these, thanks Jeff!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Derrils on January 12, 2017, 10:21:04 PM
I believe I found the cause of my issue. My c414 mic is dead.  I thought it would be a great mic to test with because it has LED's to show that phantom power was working. 

I suppose it's possible that I had an issue with my vp28 build that killed the mic, which I later fixed after reflowing the solder joints. Otherwise it's just a coincidence that a known working mic suddenly stopped working upon the first test of this build.

Don't take any part of the scientific approach for granted!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 12, 2017, 10:38:45 PM
....I suppose it's possible that I had an issue with my vp28 build that killed the mic, which I later fixed after reflowing the solder joints. Otherwise it's just a coincidence that a known working mic suddenly stopped working upon the first test of this build.
Highly unlikely. I imagine it was a coincidence.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: haselnusse on February 28, 2017, 06:21:25 AM
Hi all,

i recently noticed a crackling noise and slight (DC?) hum on one of my two VP28s that i built in december.  This happens also on lower gain settings, equally in mic and line mode. The other VP28 is just fine. Im running them in a Lindell 510 Rack without any issues so far, they are the only two modules in there. Does anybody have an idea where that could come from?

best regards,

marius
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 28, 2017, 09:32:49 AM
I recently noticed a crackling noise and slight (DC?) hum on one of my two VP28s that i built in december.  This happens also on lower gain settings, equally in mic and line mode. The other VP28 is just fine. Im running them in a Lindell 510 Rack without any issues so far, they are the only two modules in there. Does anybody have an idea where that could come from?
I would try swapping the modules in the rack to rule out outside or rack issues. Next swap opamps around. Besides that I would look for bad/cold solder joints.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: haselnusse on February 28, 2017, 01:57:05 PM
Thanks Jeff, i will try that this week!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 04, 2017, 07:42:09 PM
I quickly put this in my API 500 rack and powered up. The rack was attached to a patch bay. I forgot that this portion of the patch bay was full normal as it was reserved for EQs. Everything seemed fine and powered up but when I engaged phantom power these power resistors had a meltdown. Is this the result of the normalled patch bay (which i believe fed the 48v back into the VP28 based on my understanding of normal) or is it something else. I suspect I did this to myself and have an order for some new resistors but just want to make sure.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 04, 2017, 11:04:14 PM
I quickly put this in my API 500 rack and powered up. The rack was attached to a patch bay. I forgot that this portion of the patch bay was full normal as it was reserved for EQs. Everything seemed fine and powered up but when I engaged phantom power these power resistors had a meltdown. Is this the result of the normalled patch bay (which i believe fed the 48v back into the VP28 based on my understanding of normal) or is it something else. I suspect I did this to myself and have an order for some new resistors but just want to make sure.
This indicates a problem on both opamp builds.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 05, 2017, 10:56:06 AM
Thank you Jeff. I guess you've seen it all.... 

Darn it! I was sure they were the least of my worries. With that said I ordered 2 new assembled DOAs as well as the replacement resistors. While building the Op amps I was thinking next time I would order assembled and tested just due to the tediousness of building them. Live and learn.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 12, 2017, 04:32:24 PM
I quickly put this in my API 500 rack and powered up. The rack was attached to a patch bay. I forgot that this portion of the patch bay was full normal as it was reserved for EQs. Everything seemed fine and powered up but when I engaged phantom power these power resistors had a meltdown. Is this the result of the normalled patch bay (which i believe fed the 48v back into the VP28 based on my understanding of normal) or is it something else. I suspect I did this to myself and have an order for some new resistors but just want to make sure.


I just had this issue minutes ago. It was working fine with the opamps in different positions but then when I swapped them they burned up.

It looks as though the +V connection point for the opamp was the instigator.

Does there appear to be anything wrong with the opamps?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 12, 2017, 04:33:47 PM
Here is the 2520
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 12, 2017, 04:34:25 PM
Here is the 1731.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 12, 2017, 07:37:10 PM
I was sure that my Opamps were cleanly built but I'll defer to Jeff's experience on these matters. I ordered a new 1731 and 2520   opamps prebuilt and tested with 4 x 10 Ohm resistors thrown in for the repair. They should arrive this Monday or Tuesday and I will let you know if it is the fix for me. I had the 2520 in the preamp position and the 1731 in the fader/booster position and took out the opposing  (lower) resistors.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 12, 2017, 08:17:12 PM
I was sure that my Opamps were cleanly built but I'll defer to Jeff's experience on these matters. I ordered a new 1731 and 2520   opamps prebuilt and tested with 4 x 10 Ohm resistors thrown in for the repair. They should arrive this Monday or Tuesday and I will let you know if it is the fix for me. I had the 2520 in the preamp position and the 1731 in the fader/booster position and took out the opposing  (lower) resistors.

Thanks - I'd appreciate that. Currently working on the 312s. Hopefully these go smoothly!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 13, 2017, 01:44:34 PM
If it is not already abundantly clear from my post, I should provide the following disclaimer: I have zero experience working with electronic components. The CAPI builds are my first venture into the field. I have wanted to do this for years and finally found the opportunity to. Please be patient with me.

I have a few questions for anyone that is kind enough to help out:

1) Is some sort of pad or double sided sticky tape required for the underside of the EA2622 input transformer? What happens if it is not used?

2) The circular gold ground plane of the +V socket of my preamp opamp slot has partially lifted from the PCB surface - Is a solder connection necessary between the pin and that gold ground plane or is it sufficient to have a soldered connection through the hole?

3) Any general or specific advice with regard to building the opamps? I thought I had them perfectly soldered yet one caused the PR2 and PR4 resistors to fry. The Q7 and Q8 transisters (black blocks with the holes) overheat on another.

I was so excited about building a pair of these vp28s and a pair of the vp312s.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 13, 2017, 07:54:58 PM
If it is not already abundantly clear from my post, I should provide the following disclaimer: I have zero experience working with electronic components. The CAPI builds are my first venture into the field. I have wanted to do this for years and finally found the opportunity to. Please be patient with me.

I have a few questions for anyone that is kind enough to help out:

1) Is some sort of pad or double sided sticky tape required for the underside of the EA2622 input transformer? What happens if it is not used?

2) The circular gold ground plane of the +V socket of my preamp opamp slot has partially lifted from the PCB surface - Is a solder connection necessary between the pin and that gold ground plane or is it sufficient to have a soldered connection through the hole?

3) Any general or specific advice with regard to building the opamps? I thought I had them perfectly soldered yet one caused the PR2 and PR4 resistors to fry. The Q7 and Q8 transisters (black blocks with the holes) overheat on another.

I was so excited about building a pair of these vp28s and a pair of the vp312s.

1) no (it only makes it easier to solder in)
2) Solder from the back and re-float the top.
3) You have to be careful with heat around those transistors when soldering and they can get damaged.

Despite my experience here in this topic I have successfully built many pieces. I got lazy and used solder that was to big and a tip that was to big. I have decent soldering skills and thought I could overcome the need for smaller solder etc. , I was wrong. I will only buy pre-made and tested DOAs as they took me and 1hr and a half a piece to build.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 13, 2017, 08:24:08 PM
1) no (it only makes it easier to solder in)
2) Solder from the back and re-float the top.
3) You have to be careful with heat around those transistors when soldering and they can get damaged.

Despite my experience here in this topic I have successfully built many pieces. I got lazy and used solder that was to big and a tip that was to big. I have decent soldering skills and thought I could overcome the need for smaller solder etc. , I was wrong. I will only buy pre-made and tested DOAs as they took me and 1hr and a half a piece to build.

Thanks for your response, Carl!

With regard to your answers:

1) I have watched/read a few videos/instructions concerning the building of these pres. Some people say having some insulation between the input transformer and the PCB is "critical" because the ground is now on the front side? Here is a link to one such opinion:

http://studio939.blogspot.ca/2012/08/vp26-microphone-preamp-build.html

2) I did exactly that. I won't know if it worked until I can fix at least one of the 6 opamps I have built! :S

3) Do you mean Q7 and Q8? If so, do you mean heat from the soldering pen, or heat caused by a solder bridge?

I think my opamps are having problems because of either one or more of the following:

a) bridged connections that I can't see by eye;
b) components touching on the top side; or
c) the maxpins not soldered correctly. I simply dropped a bead of solder on them from the top side. There is no solder on them on the bottom side (where the connectors stick out to attach to the PCB.

I also did not twist the leads on the two diodes. I put all 4 in individually with the black (negative) side on the PCB board matched with the silk screen circle, as per the instructions. Hopefully not going the twisting route was not what caused my demise.


By the way: your opamp- that caused your PR1 and PR3 resistors to fry - was it irreparably damaged?


Thanks again for your time. I am hoping I can figure these things out. I was pretty excited about all of this but at this time I am just feeling discouraged by my inability to build at least one correctly following the instructions.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 13, 2017, 08:56:19 PM
As to the tape thing with the input trans I can still see air under mine and have installed dozens of these style on PCBs without issue but  I like to gently hold the piece against the PCB when soldering 2 of the leads and never clamp it hard as to do damage to the coatings on a PCB or have any stress on the leads. Chunger does some of the best tutorials ever and teaches good habits especially to people new to DIY so I should say that it's a good habit that would guarantee you don't dig in to the ground plain.

Yes I removed the resistors and cleaned the board and no damage. No other damage that I can note or see. The odd thing is I measured the resistors and they were still close even after the meltdown.

In respect to the opamps, the issue could be any of the issues you mentioned. Although you save money building them, if you destroy the board or other components you are about even or worse. I'm building 3 of these and will only buy pre-made for the remaining 2 and now for the one I have now. Enjoy the builds and remove any situation that will cause you grief ( such as as the opamp build) that's hard to test outside the pre.

And don't get discouraged. There can be hiccups along the way but solving them makes it even sweeter when it's working and helps build confidence to attack the next project. This forum has some great advise and help when you ask for it. Hang in there!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 13, 2017, 10:30:15 PM
As to the tape thing with the input trans I can still see air under mine and have installed dozens of these style on PCBs without issue but  I like to gently hold the piece against the PCB when soldering 2 of the leads and never clamp it hard as to do damage to the coatings on a PCB or have any stress on the leads. Chunger does some of the best tutorials ever and teaches good habits especially to people new to DIY so I should say that it's a good habit that would guarantee you don't dig in to the ground plain.

Yes I removed the resistors and cleaned the board and no damage. No other damage that I can note or see. The odd thing is I measured the resistors and they were still close even after the meltdown.

In respect to the opamps, the issue could be any of the issues you mentioned. Although you save money building them, if you destroy the board or other components you are about even or worse. I'm building 3 of these and will only buy pre-made for the remaining 2 and now for the one I have now. Enjoy the builds and remove any situation that will cause you grief ( such as as the opamp build) that's hard to test outside the pre.

And don't get discouraged. There can be hiccups along the way but solving them makes it even sweeter when it's working and helps build confidence to attack the next project. This forum has some great advise and help when you ask for it. Hang in there!

Ya my input transformer does not have any gap - it is directly on the PCB. Still not clear on what the implication is. Hopefully someone can chime in.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 13, 2017, 10:42:42 PM
The implication is that when installing it you scratch through to the metal underneath (which is the grounds Plate). Most likely not your issue. If you followed the instructions closely you should be ok. When my new opamps solve my issue you can feel a little better and maybe try the same. Mose issues you will experience are bad solder joints or bad grounds. I'll let you know tomorrow.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 14, 2017, 11:26:57 AM
The implication is that when installing it you scratch through to the metal underneath (which is the grounds Plate). Most likely not your issue. If you followed the instructions closely you should be ok. When my new opamps solve my issue you can feel a little better and maybe try the same. Mose issues you will experience are bad solder joints or bad grounds. I'll let you know tomorrow.

Thanks, Carl. Hoping the opamps work out for you.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 14, 2017, 08:54:31 PM
Allecstatic, I replaced the (4) 10 ohm resistors first . Note that after that amount of heat it is easy to lift one of the solder pads on the pcbs so dismantle to the point where you can access from the back and carefully remove the resistor with whatever desoldering tool you have and not forcing anything (be gentle). Once you are replacing the resistors solder and make sure that front and back pads are soldered and that goes for your opamp receiver as well. I would also note that the already assembled 1731 and 2520 had been soldered from underneath with a generous amount of solder with the solder bridges that are in the instructions but I hesitated to do when building. Long and short of it has fixed the issue and the pre is passing sound and well. Hope this helps.
After seeing your pics I would recommend that if you have some extra wire from your transformer you strip it back and resolder and cover with heat shrink. Good luck!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 15, 2017, 11:50:47 AM
Allecstatic, I replaced the (4) 10 ohm resistors first . Note that after that amount of heat it is easy to lift one of the solder pads on the pcbs so dismantle to the point where you can access from the back and carefully remove the resistor with whatever desoldering tool you have and not forcing anything (be gentle). Once you are replacing the resistors solder and make sure that front and back pads are soldered and that goes for your opamp receiver as well. I would also note that the already assembled 1731 and 2520 had been soldered from underneath with a generous amount of solder with the solder bridges that are in the instructions but I hesitated to do when building. Long and short of it has fixed the issue and the pre is passing sound and well. Hope this helps.
After seeing your pics I would recommend that if you have some extra wire from your transformer you strip it back and resolder and cover with heat shrink. Good luck!


Carl - happy to hear it worked out! Ya I was reluctant to make the solder bridges they suggest can be helpful to secure the Milmax pins as well. Do the opamps fit snugly into the slot or are they loose?

I cleaned up and resoldered the 6 opamps yesterday. 4 of 6 work. I am debating lightly soldering one pin of the opamp to the preamp so that they don't move around so easily. They sound great! When I replaced the resistors I lifted one pad on the top of the pcb board. I just made sure it was soldered through and on the bottom, as well soldering the top as best I could. If there was a gap between the resistor and the circuit line I would have bridged it but it was fine.

I'm not sure entirely what you mean by resoldering the transformer wires. Are you referring to how some of the wiring is exposed? The first preamp I made turned out that way but the other 3 I did what most would consider properly.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 15, 2017, 05:24:03 PM
How are you guys securing the opamps in their positions? Some of mine will stay on their own while others just slide out.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Carl Cress on March 15, 2017, 07:36:10 PM
Bump
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on March 15, 2017, 08:24:23 PM
How are you guys securing the opamps in their positions? Some of mine will stay on their own while others just slide out.

If you use the same size pin as sold on the CAPI website,  they shouldn't slide out. I've build dozens of DOAs from pretty much anyone that sells the PCBs and I've never had any trouble with them falling out. Maybe hot glue if you its going to be semi permanent. That would work better than tape I think. You could just remove the pins that are too small and replace them the correct size.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 16, 2017, 06:13:55 PM
I don't mean to be rude but this has been cover a million times and I have a page at the site about it http://capi-gear.com/catalog/DOA_Install.php

As someone that is new to these builds I did my best to print out and organize the documents provided on the webpage. Even though they may appear to be straight forward and organized in a logical way, it was still overwhelming to an amateur like me. Accordingly, I will concede that there were certain aspects that I did not understand at first but I eventually figured them out on my own; however, there were some issues that I was unable to resolve on my own within a reasonable time. I deferred to this support thread as it was listed on the VP28 build page as a exactly that, a support page.

The issue with the pins, according to the shop I brought it into, happened to be due to the gold coating on the interior of one of the slots restricting the pin from going in all the way.

I am now trying to finish this build as I have been excited to use the preamps.  I was actually encouraged to buy these preamps and seek help in the forum if needed, as Jeff was supposed to be quite supportive. I will continue to look through these documents I have already printed to figure the remaining issue. This was a lot of fun and a great learning experience for me. I really enjoy the units I have finished. Thank you to all for the help!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on March 17, 2017, 10:10:07 PM
The shop you took it to is mistaken regarding the pins. Brand new sockets require the initial break in, then after that, swapping out and seating new DOAs is easier, but it still requires a little bit of force. Once seated they should never come out on their own if the proper size pin and socket is used.

Again: http://capi-gear.com/catalog/DOA_Install.php

Also, Jeff, along with all the wonderful people here, who offer these amazing design so others can build world class studio gear at a fraction of the cost and get some electronics education in the process, have been and are extremely supportive with the products they offer, not supposedly.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on March 20, 2017, 12:51:46 PM
The shop you took it to is mistaken regarding the pins. Brand new sockets require the initial break in, then after that, swapping out and seating new DOAs is easier, but it still requires a little bit of force. Once seated they should never come out on their own if the proper size pin and socket is used.

Again: http://capi-gear.com/catalog/DOA_Install.php

Also, Jeff, along with all the wonderful people here, who offer these amazing design so others can build world class studio gear at a fraction of the cost and get some electronics education in the process, have been and are extremely supportive with the products they offer, not supposedly.

Thanks!

Paul

Hey Paul,

Thanks for the input. I appreciate these builds, which is why I purchased them in the first place. I was not bashing anyone on that front. I also appreciate the assistance just as much as you or anyone else does. I was only addressing what appeared to be a bit of frustration with my question.

I clarified I was new and I had already viewed and printed all of the necessary documents. A friend that has built these units was adamant that as someone new to electronics, the capi pres were a good start. He also insisted that I defer to the forum and to Jeff if I had any problems. So I did. The first response I received was the one quoted above that has since been deleted.

I only responded to express my first impression and further, that my intention was clearly not to frustrate someone but rather to get help and finish these awesome pres. If someone wants to say I was wrong for asking questions here and further that I was wrong for being a bit discouraged with the first response I received, then go ahead.  I have no other experience with someone from capi.

As to the pins - I had put much force pushing the opamps into the sockets, like the others (which went in securely). These other ones would not go in, despite how much I pushed. In any event, the pins will be replaced. I have managed to fix the opamps that were not working (solder bridges) and ordered a vp26. I will only buy premade opamps now to be sure. Where I am in the US seems to take a bit for the order to get here so not sure when I will be able to work on the vp26. I look forward to one day trying the vp312.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 20, 2017, 01:15:35 PM
Wow that is strange. I have not deleted anything on this thread.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: geardude on March 25, 2017, 05:58:23 PM
Hey guys,

I'm having a problem with one of my 28's on this build go around.Having a slew of these under my belt, I've never encountered this error before  but here it is.

Apparently this unit is quieter when activated in line mode when sending a 1khz sine wave I'm getting a return output exactly 3 db quieter. Besides this the preamp is functioning just swell. I've attached some pics.

The greyhill knobs are in the correct orientation & using red dot op amps; possible cold joint somewhere in the line switch signal chain? I tried touching up the pins on that particular switch but no differences.

All capacitors are in their correct positions and orientations comparing them to other builds. And I can't seem to spot any solder brides anywhere.

http://s1067.photobucket.com/user/_Geardude_/embed/slideshow/
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 26, 2017, 10:49:46 AM
I would double check the location of all R's specifically the 160R's and 4k99's just behind the mic/line switch. Next I would check all R's behind the preamp gain Grayhill. After that look for a solder bridge on the preamp gain Grayhill. Exactly 3dB is far too suspicious since all but the lowest 2 gain settings are 3dB.

Are you 100% sure the preamp gain knob is orientated properly?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: geardude on March 26, 2017, 11:42:55 PM
I would double check the location of all R's specifically the 160R's and 4k99's just behind the mic/line switch. Next I would check all R's behind the preamp gain Grayhill. After that look for a solder bridge on the preamp gain Grayhill. Exactly 3dB is far too suspicious since all but the lowest 2 gain settings are 3dB.

Are you 100% sure the preamp gain knob is orientated properly?

Yes indeed, well stupid me, after thoroughly checking those and confirming everything is gravy I realize that my totalmix fx settings changed...and you guessed it...had the lineoutput 3db quieter. UGH go figure. Now line level is the same going out and as coming in.... however now after building trying out this test on a bt50 with the proper RME fx settings, & my line level return is -16.5 dbfs instead of -18 as it should be. again all knobs are in their correct orientation. I'll post this over in that forum
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on April 06, 2017, 11:30:13 AM
I just realized the light indicators on the 4 vertically aligned push buttons are not working even though the buttons themselves are. I resoldered to be sure and visually inspected the area - still not working. Anyone run into this problem or know what the issue may be?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 06, 2017, 04:16:31 PM
Polarity of LEDs soldered backwards?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on April 06, 2017, 06:13:47 PM
Polarity of LEDs soldered backwards?

That was my first thought. The polarity is correct as per the instructions as well as by way of confirming this with other vp28s I have with working LEDs. Thanks for the response!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 07, 2017, 10:48:11 AM
I just realized the light indicators on the 4 vertically aligned push buttons are not working even though the buttons themselves are. I resoldered to be sure and visually inspected the area - still not working. Anyone run into this problem or know what the issue may be?
My first thought was the same as Paul's, polarity of the LED's. The other thing I have seen is leads too long that hang off the back edge of the pad and short to ground. They are in a string so if one down stream is wrong then none will work. You can probe at the pads to make sure you have appx 16VDC starting with the anode pad/lead of the 48V LED and working towards the top of the module.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on April 07, 2017, 11:12:38 AM
My first thought was the same as Paul's, polarity of the LED's. The other thing I have seen is leads too long that hang off the back edge of the pad and short to ground. They are in a string so if one down stream is wrong then none will work. You can probe at the pads to make sure you have appx 16VDC starting with the anode pad/lead of the 48V LED and working towards the top of the module.

Thanks Jeff. I'll check that out. Is it possible that the LEDs can become defective if the leads end up shorting to the ground or if the polarity is swapped?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 07, 2017, 11:37:44 AM
Thanks Jeff. I'll check that out. Is it possible that the LEDs can become defective if the leads end up shorting to the ground or if the polarity is swapped?
No.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 10, 2017, 01:24:49 AM
OK,

I'm still trying to get my last two VP28's sorted out. I never got a response to my last few queries, so I decided to just try to reflow all the solder (or most of it) on one of my VP28's. I think I solved whatever problem I was having. And so now I have new problems, of course.

Before I was getting a squeal after certain volume was reached. Now, no squeal.

Before the VP28 was VERY loud, louder than the other six that are working properly.  This time, when I turned it on it was very loud, then it made a "pop" and it became closer to what I expect it should be.

Now, it makes a light crackling noise that's easier to hear at high volume and it also has a hiss that is audible even at low volume, but gets louder as you increase the volume. It IS passing signal.

Other details that may or may not be useful for diagnosing the current situation with this VP28:

I did not reflow all the solder. I figured I should rule some things out and skip them. I did NOT reflow solder to any of the switches, and in the case of the gain and output switches, I did not reflow any of the solder to any of the resistors in line with those switches. I also did not reflow the solder to the chip that controls the LED's, and I did not reflow any of the pins for the Op Amps. I did nothing with anything associated with the HPF.

Then, and this might be my hiss, I cleaned the board with alcohol and a plastic brush. I figured it was messier than usual after being reflowed, and it WAS messier than before. However, the board is now a sticky mess, even after being scrubbed several times. When I first plugged it in, it got no power. Then I unplugged it and cleaned the gold fingers with more alcohol and q tips and plugged it back in and it got power and worked, but just had these noises.

As always, thank you Jeff and everyone else. I have a little time this week to pay attention to these things. I'm hoping I can get them figured out this time.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 12, 2017, 12:27:43 AM
Did you compare voltages to your working VP28's? Seems like that would be the next step after checking soldering and component placement. And if your soldering now is messier than before you may have some new sweet bridging action.

Thanks!

Paul
Title: Re: [BUILD] VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 12, 2017, 12:54:35 AM
Hi folks,

I have six VP28's, five of which work very well. The other one has some strange symptoms. I believe this is the one I posted about a while back. (Due to life situations and an historically hellish winter here, I've hardly done any recording at my studio, and no full band stuff where I'd need all my inputs, until now when I'm noticing this issue.)

First thing I did was swap out my DOA's with ones from a working unit, so I know it's not theDOA's. Too bad. That would be an easy fix. The symptoms are a HUGE pop when engaging the pad, a huge pop when engaging the phantom and a bit of a pop when engaging the mic/line switch. Phase button makes no pop. If that was it, I could live with it, but none of my other units make these noises and more important, there is a bit of hiss and hum in this one and possibly some signal distortion. It's not that loud, but it is audible, especially at high gain. Both knobs affect the volume of the noise(s). None of my other VP28's make these noises. As far as I can tell, everything is in correctly.

Any ideas?

Thanks, as always!

So today, I went one step further and carefully cleaned the entire main board of this one last VP28 that's not working properly. Because it's sometimes hard to get a response, I'm rereading this thread again and searching this thread to see if anyone has had any similar issues. What I found is that the condition of this preamp hasn't changed in years! This thing is behaving exactly as it did when I described it  years ago... Even with the reflowing and cleaning.

Can anyone help me with this problem?

I did notice that someone else who was using a Radial Workhorse was having a similar issue, although not exactly the same.

I notice some strange behaviors that *might* point to the Workhorse, but I'm not sure. The loud pop that happens on this preamp when engaging phantom power shows up on other channels! But it's not that simple. I did some experimenting.

First, any and all linking buttons on the Workhorse are OFF.

There are no microphones plugged into anything.

I opened up Pro Tools and had a track set up for every channel of the workhorse (8 VP28's! 7 that seem to be working great.) I started with the preamp in question in the third slot of the Workhorse. When I hit the phantom button, there was a loud pop showing up in my DAW for that channel. A lesser, but still loud pop showed up in the forth slot. And a lesser pop showed up on the second slot. A similar pop happens wit the pad button and it also shows up to a lesser degree on the adjoining channels.

So I swapped slots two and three. The preamp behaves exactly the same and the neighboring channels behave the same as well. It does not matter what slot the preamp is in. I *think* this rules out the Workhorse.

One other symptom, however, might just help nail this issue:

No matter what slot the preamp is in, when I engage phantom on any of the other preamps, THIS ONE pops, not the ones that had phantom toggled. That's gotta be some kind of giveaway.

Thanks again, and someone please help me. I've actually been trying to make this one last preamp work for years!!!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 12, 2017, 12:58:17 AM
Did you compare voltages to your working VP28's? Seems like that would be the next step after checking soldering and component placement. And if your soldering now is messier than before you may have some new sweet bridging action.

Thanks!

Paul

Thanks Paul!!

You got your post in just before my last post. My soldering is pretty good on this. I'm hoping the last issue I listed in my post above is some kind of hint as to where the problem lies.

I wouldn't know where to begin comparing voltages. If it comes to that, is that something I do in the jig with the power on? I know there's a PDF somewhere of test points. Is that what you're talking about?

This is Rev A.2, BTW.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 12, 2017, 01:29:36 AM
Do you have an extension test jig for 500 series units? It is necessary to have when building modules.

As for voltages, you would start with making sure proper power is getting to the DOAs. Then you can use this as a guide:

http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Test-Points.pdf

It's for Rev B so you would have to email Jeff and ask which components  those test points connect to and just apply that to your Rev A. It should be close to the values listed in the test point guide.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 12, 2017, 01:40:22 AM
Do you have an extension test jig for 500 series units? It is necessary to have when building modules.

As for voltages, you would start with making sure proper power is getting to the DOAs. Then you can use this as a guide:

http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Test-Points.pdf

It's for Rev B so you would have to email Jeff and ask which components  those test points connect to and just apply that to your Rev A. It should be close to the values listed in the test point guide.

Thanks!

Paul

Thank you again, Paul.

I do have the jig.  Does the fact that this preamp pops when any other preamp in the Workhorse has it's phantom power switch toggled narrow anything down? And it happens no matter which slot this preamp is in.

I'm still digging around this thread. I found the Test Points PDF for Rev A.2. If it needs testing, I'm going to have to ask some silly questions first:

Which probe goes where on my DMM for this test? Now which one goes to ground on the VP28. And where is ground on a REV A.2?


I need instructions that a drummer can understand...

Thanks again...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 12, 2017, 03:58:08 PM
Com (-) to ground. + to test points. Look up API 500 series edge connector pinout for reference.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 12, 2017, 04:21:26 PM
I appreciate the response, but it's not down to drummer level...

First of all, I have no idea how I'm supposed to have something in the jig and probe the card edge at the same time.

I need to be told physically where to put the probes. The RevA.2 PDF tells me where to put one of the probes, but I'm not sure which one. The other probe goes to ground. Great. Where is ground? On the card edge that's in the jig? The one labelled "audio ground" or the one labelled "PSU Ground"? Makes no sense. There's got to be ground somewhere on the board or maybe somewhere else altogether.

But also, I'm really, REALLY curious to know if the one particular symptom where phantom power toggling on OTHER pres causes THIS pre to pop tells us anything at all about the nature of the problem.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 14, 2017, 12:15:12 PM
Bumping.

Need to know a few things:

Where do I put my probe for ground on the RevA.2 board? And do I do these tests with the DOA's in or out?

When I toggle phantom on any VP28 in my rack, this one, and this one only, makes a loud pop. Clipping. It does this no matter which slot it is in. It als pops when I toggle it's phantom and it's pad switch also makes a huge pop. Does this help narrow down the problem I'm having with this VP28?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Allecstatic on April 14, 2017, 02:53:25 PM
No.

Although the leads from the LEDs were not touching the board, one of the connections had a bit too much solder which I believe WAS touching the board. That solved the issue. Just wanted to post in case someone runs into this issue down the road. Thanks again for the help, Jeff!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 15, 2017, 01:31:27 PM
Hi there,

Still looking to understand a few things so I can finish this last VP28; the only one of eight that has had any serious issues. It has been languishing in a box for years. I haven't been able to get a complete response here for years.

I want to know if the fact that this pre not only pops really loud when engaging phantom and pad and also pops really loud when phantom is engaged on any other pre tells us what the issue might be.

Next, if I'm going to test this pre, I need to know how. Am I to test it under power? With DOA's in or out? I found the test points PDF for Rev A.2, but I actually need to know which probe goes to the test point and which one goes to ground, and I need to know where ground is.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 20, 2017, 03:12:51 PM
I've had this VP28 in near working condition for years now. When I have time, I start a conversation here to make progress on it and get a few responses, but never get all the information I need to take any steps to solve its issues. Then, after getting no response, life and work get in the way and months later, I start the process all over again.

I'm bumping this thread hoping that someone will understand what information I need in order to make some progress on this.

Thank you.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 26, 2017, 11:29:05 AM
help...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 26, 2017, 12:01:19 PM
help...
Remove the pre from the rack and measure DCR between gold fingers 8 and 10. Do this in mic mode with all other pushbutton switches out. Next do the same with the phantom switch engaged.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 26, 2017, 05:36:52 PM
Remove the pre from the rack and measure DCR between gold fingers 8 and 10. Do this in mic mode with all other pushbutton switches out. Next do the same with the phantom switch engaged.

Thank you!!!

I actually did this and posted the results a while back, and I did it in mic mode as you instructed, but I don't believe I A/B'd with the phantom button in.

The results with the phantom NOT engaged are:

This is what I have (almost.)  3.61k for 5 to 8 and 5 to 10 and 27.5 for 8 to 10.

I will be at the studio tonight and I will double check all of this and post the results. Please don't forget about me. Check back soon. I understand that it can be frustrating  guiding a neophyte through this process and I appreciate the help. I also really, REALLY appreciate that you're instructing me to specifically remove the unit from the rack as well as other instructions that may not be necessary for people who have a better grasp on electronics.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 26, 2017, 11:22:06 PM
And the results are:

27.6 no phantom

27.5 with phantom
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Johnkenn on April 29, 2017, 02:46:34 PM
Hey guys...I've built a VP28, but I'm running into a head scratching problem. The input gain works fine, but the Output fader is working in an "off and on" fashion." When the output fader is switched to 8, the signal passes. Before that, no signal. Turning it to 10 and 12 gets it no louder, it just seems to turn on at 8. I figured it could have been a cold solder or something on the fader, but I've touched up, resoldered, and I'm still getting the issue.

I've checked for continuity - I'm a newbie, so please cut me a little slack...I get readings on all the points, but some read really quickly and then go to open loop. Didn't know if that was common or not. I'd be shocked if it's a cold solder - I've checked it 20 times and they all look fine. Could it be a bad switch? I know that's pretty unlikely...
Anyway, any help would be appreciated.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on April 29, 2017, 08:45:02 PM
Hello,


as said in the other thread please check if the switch has stop pins or not.Easy to set them wrong when the shaft was not in the correct position while inserting.
Also check if all fader resistors are in the right place.Sometimes they are misread and swapped.


Best regards,


Udo.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Johnkenn on April 29, 2017, 08:52:25 PM
Hello,


as said in the other thread please check if the switch has stop pins or not.Easy to set them wrong when the shaft was not in the correct position while inserting.
Also check if all fader resistors are in the right place.Sometimes they are misread and swapped.


Best regards,


Udo.

Thanks, Udo. Don't believe there are any stop pins in the new builds.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 29, 2017, 10:48:11 PM
I appreciate the response, but it's not down to drummer level...

First of all, I have no idea how I'm supposed to have something in the jig and probe the card edge at the same time.

I need to be told physically where to put the probes. The RevA.2 PDF tells me where to put one of the probes, but I'm not sure which one. The other probe goes to ground. Great. Where is ground? On the card edge that's in the jig? The one labelled "audio ground" or the one labelled "PSU Ground"? Makes no sense. There's got to be ground somewhere on the board or maybe somewhere else altogether.

But also, I'm really, REALLY curious to know if the one particular symptom where phantom power toggling on OTHER pres causes THIS pre to pop tells us anything at all about the nature of the problem.

Hello.

As it has been said before on this forum by people far knowledgeable about life and electronics than I, there are several things that are assumed the end user already knows how do, one of which is how to use a multimeter. If not, one would need to do some Internet searches on the subject. It's not meant to be demeaning at all, but to encourage one to do research and search out information for one's self. I've asked numerous questions when I first started learning that went unanswered and I had to just stay at it till I found the answer to problems with builds.

As of for the popping, have you tried this preamp in a different rack? Do the rest of the preamps pop when you engage phantom on any other channel? The Radial boxes have the feed switch to make channel strips. Are any of those engaged? That would make the most sense in this scenario. If that is not the case, you will have to take your multimeter and look up API 500 series pinout and look at the VP28 overlay on CAPI's website. Pull the op amps. Negative/Common Probe on ground of the edge connector. Positive probe one at a time on the DOA connectors and see if there is any difference when you engage +48V on a different preamp. Do the same to the output on the edge connector. Same with DOAs installed. Another thing to look at is with just the extension jig plugged in and no VP28 measure to see if there is difference in voltage between In+ and ground and In- and ground. Somehow voltage is making its way to input or output from another slot. Your VP28 might be fine after all.

Good hunting.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 29, 2017, 10:59:07 PM
Hey guys...I've built a VP28, but I'm running into a head scratching problem. The input gain works fine, but the Output fader is working in an "off and on" fashion." When the output fader is switched to 8, the signal passes. Before that, no signal. Turning it to 10 and 12 gets it no louder, it just seems to turn on at 8. I figured it could have been a cold solder or something on the fader, but I've touched up, resoldered, and I'm still getting the issue.

I've checked for continuity - I'm a newbie, so please cut me a little slack...I get readings on all the points, but some read really quickly and then go to open loop. Didn't know if that was common or not. I'd be shocked if it's a cold solder - I've checked it 20 times and they all look fine. Could it be a bad switch? I know that's pretty unlikely...
Anyway, any help would be appreciated.

I've been shocked how many times I didn't think it was a cold solder joint/misplaced/mis-wired component and it turns out to be just that.

Get some solder wick, remove as much solder as you can and reapply. If you're not already, use the very small solder so it melts and flows easier. If you think it is the switch, you can measure continuity between the pole and each of the pins per switch position to verify it it. You will still have to remove some of the solder first to ensure you are making contact to the pin and not the solder.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on April 30, 2017, 11:52:36 AM

As it has been said before on this forum by people far knowledgeable about life and electronics than I, there are several things that are assumed the end user already knows how do, one of which is how to use a multimeter. If not, one would need to do some Internet searches on the subject. It's not meant to be demeaning at all, but to encourage one to do research and search out information for one's self. I've asked numerous questions when I first started learning that went unanswered and I had to just stay at it till I found the answer to problems with builds.

Thank you, Paul. The problem with not knowing things is that you don't know what you don't know. Of course I've looked up all sorts of things, but if I don't know what I should be looking to learn, it's hard to find it. As you can see, it's hard to even know what I'm asking for. What does help is when Jeff specifically tells me to test the thing out of the jig, with no power, or when you describe testing it with the DOA's out.


As of for the popping, have you tried this preamp in a different rack?

No, but I've tried it in different slots in this rack. All symptoms described are the same no matter what slot it's in.

The Radial boxes have the feed switch to make channel strips. Are any of those engaged? That would make the most sense in this scenario.

No, they are not. Yes, I double checked. And triple checked.

If that is not the case, you will have to take your multimeter and look up API 500 series pinout and look at the VP28 overlay on CAPI's website. Pull the op amps. Negative/Common Probe on ground of the edge connector. Positive probe one at a time on the DOA connectors and see if there is any difference when you engage +48V on a different preamp. Do the same to the output on the edge connector. Same with DOAs installed.

OK, bear with me a second... I can't tell if the unit is supposed to be powered for these tests. I can't probe the card edge while it's under power and engaging phantom on another unit won't have an effect if it's not plugged in.

And when you say "ground," in this case it's gold finger #5? Or is it #13? 5 makes sense to me because we're testing the audio path... *I think*...

(http://www.vsiaudio.com/VSI-API-Pinouts.JPG)


Now, when testing the DOA connectors, there are six for each DOA. Am I to probe all twelve of these or is it understood that I'm only probing certain ones?  It seems possible to me that I wouldn't be probing the power connectors and I'd just be probing the audio path, and that if this test is supposed to be done under power (which I still don't understand how I'd probe the card edge while under power) it seems that there could be a considerable amount of voltage difference between these connectors, and that could have implications for my DMM settings or safety. (This is the area where I don't know what it is I don't know, and I don't know how to find out what I don't know because I don't know what it is that I don't understand. I know that's a ridiculous sentence, but I'm trying my best to describe things as I perceive them so someone can identify where my deficit is.)

Next  question about probing the DOA's and the DOA connectors:

Do I do it from the underside of the board?  Does it matter? (Shouldn't as far as I can tell...) If I'm probing the card edge, the underside will be up and the connectors will be easily accessible from the underside.

What do you mean by "Do the same to the output on the edge connector"? Test from ground to the output or from each DOA connector to the output? And WHICH output? Reading the API pinout, I see three. I get that they correspond to the XLR pins, but I'm not sure which one is the correct one for this test.

Another thing to look at is with just the extension jig plugged in and no VP28 measure to see if there is difference in voltage between In+ and ground and In- and ground. Somehow voltage is making its way to input or output from another slot. Your VP28 might be fine after all.


This is testing the Workhorse? Hasn't that been ruled out when we know that the behavior is identical in multiple slots?

Thanks again and apologies again for questions that might appear to be stupid. Once I get a grasp on some this stuff that's obvious to others, I'll be able to ask questions from a better informed position in the future...







Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Johnkenn on April 30, 2017, 01:26:46 PM
I've been shocked how many times I didn't think it was a cold solder joint/misplaced/mis-wired component and it turns out to be just that.

Get some solder wick, remove as much solder as you can and reapply. If you're not already, use the very small solder so it melts and flows easier. If you think it is the switch, you can measure continuity between the pole and each of the pins per switch position to verify it it. You will still have to remove some of the solder first to ensure you are making contact to the pin and not the solder.

Thanks!

Paul

I solder-sucked every pin joint on the input and fader switch and re-soldered with Kester solder. Still the same thing. I did check for continuity by starting with the -20 pin and t4 test point. I switched the switch and moved consecutively. At each point I would end up with a 0 reading...which I think is correct, right? I was connecting the MM pole to the solder instead of the pin, so maybe I'll try again.
I've triple checked that every resistor is in the right place. Every cap is turned correctly. Output transformers are wired correctly (Double checked the Litz to 2623 configuration)...I'm at a loss.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on May 03, 2017, 01:50:56 AM
Thank you, Paul. The problem with not knowing things is that you don't know what you don't know. Of course I've looked up all sorts of things, but if I don't know what I should be looking to learn, it's hard to find it. As you can see, it's hard to even know what I'm asking for. What does help is when Jeff specifically tells me to test the thing out of the jig, with no power, or when you describe testing it with the DOA's out.

No, but I've tried it in different slots in this rack. All symptoms described are the same no matter what slot it's in.

No, they are not. Yes, I double checked. And triple checked.

OK, bear with me a second... I can't tell if the unit is supposed to be powered for these tests. I can't probe the card edge while it's under power and engaging phantom on another unit won't have an effect if it's not plugged in.

And when you say "ground," in this case it's gold finger #5? Or is it #13? 5 makes sense to me because we're testing the audio path... *I think*...

You can just use the Chassis for reference if you want. Or any of the other ground connections.

Now, when testing the DOA connectors, there are six for each DOA. Am I to probe all twelve of these or is it understood that I'm only probing certain ones?  It seems possible to me that I wouldn't be probing the power connectors and I'd just be probing the audio path, and that if this test is supposed to be done under power (which I still don't understand how I'd probe the card edge while under power) it seems that there could be a considerable amount of voltage difference between these connectors, and that could have implications for my DMM settings or safety. (This is the area where I don't know what it is I don't know, and I don't know how to find out what I don't know because I don't know what it is that I don't understand. I know that's a ridiculous sentence, but I'm trying my best to describe things as I perceive them so someone can identify where my deficit is.)

You need power to the unit to test voltage...

Leave your common probe connected to ground (which ever you chose). Take your +/V (how ever your meter is labeled) and test all the DOA with the op amps pulled pins and make sure you are getting +/-16V (or close to it) on the +/-V connections and then measure what you are getting for the rest of the pins (follow the DOA pinout on the VP28 Overlay). Then do the same thing with +48V. Then do it again while switching +48V on an off.

Next  question about probing the DOA's and the DOA connectors:

Do I do it from the underside of the board?  Does it matter? (Shouldn't as far as I can tell...) If I'm probing the card edge, the underside will be up and the connectors will be easily accessible from the underside.

What do you mean by "Do the same to the output on the edge connector"? Test from ground to the output or from each DOA connector to the output? And WHICH output? Reading the API pinout, I see three. I get that they correspond to the XLR pins, but I'm not sure which one is the correct one for this test.

This is testing the Workhorse? Hasn't that been ruled out when we know that the behavior is identical in multiple slots?

Thanks again and apologies again for questions that might appear to be stupid. Once I get a grasp on some this stuff that's obvious to others, I'll be able to ask questions from a better informed position in the future...

Doesn't matter if it on the underside. Which ever way is easiest.

Test all of the output connections. The two on the DOA and the two on the edge connector. Jeff's design doesn't use the third one (look at VP28 PCB and compare it to the API pinout).

I'd still test the workhorse. Just to be certain.

Like I said, if the VP28 channel is popping when you engage phantom power on a different module, voltage is some how getting to it's input to the the output XLR.

You could also measure the output XLR of the VP28 when +48V is engaged. You will have to do a search for how +48V connects to an XLR if you haven't already.

Good Luck.

Thanks!

Pau
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on May 03, 2017, 02:01:18 AM
I solder-sucked every pin joint on the input and fader switch and re-soldered with Kester solder. Still the same thing. I did check for continuity by starting with the -20 pin and t4 test point. I switched the switch and moved consecutively. At each point I would end up with a 0 reading...which I think is correct, right? I was connecting the MM pole to the solder instead of the pin, so maybe I'll try again.
I've triple checked that every resistor is in the right place. Every cap is turned correctly. Output transformers are wired correctly (Double checked the Litz to 2623 configuration)...I'm at a loss.

If you are getting 0 (some meter beep when selected for continuity) between the pole and each corresponding switch then the switch is fine. Are you getting continuity between any of the adjacent pins? If the switch is fine then either a connection between the switch position and it's resistor or the resistor to where ever it goes next (I can't remember if it goes to Q1 or the one of the op amps, you'll have to ask Jeff).  Don't give up just yet. I've had projects sitting around for a year that I couldn't find the problem and then it wound up being a simple mistake.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Commander Fluffypants on May 04, 2017, 06:57:13 PM
Thanks again, Paul. I've been reading your last response for the past two days and I almost understand it... or rather I understand most of it. Mostly, I just don't know where I'm supposed to put the black probe while I stick the red probe in all those places.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on May 24, 2017, 02:06:57 PM
If you look at any of the Chunger or Hairball build guides, they show you how to use a DMM. Use them for a reference.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Neutrino on July 28, 2017, 11:59:07 AM
Hi there,

I just put together my first (of four) VP28. I've been building several 500 series preamps before, but this is the first time I got the famous magic smoke once I put it in my lunchbox  :o

As far as I can see, I fried PR2 and PR4 and would like to ask for suggestions where I should start troubleshooting. I use gar1731 (V4.1) opamps which I also assembled myself.  The smoke appeared once I installed the opamps, i.e. without the opamps at least nothing obvious went wrong, though I didn't check any voltages. Apparently, this was too much self-confidence which was fatal   :-[

Should I try reconnecting without opamps and test any voltages or can I create further damage by doing so? Any help is appreciated!

Cheers,
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on July 28, 2017, 01:14:45 PM
If there was no smoke without DOAs, then smoke with DOAs.....


You should have some DOAs that you bought instead of built for this very purpose.

DOAs.


Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Neutrino on July 28, 2017, 01:24:23 PM
I have a pair of gar2520 DOAs from a previous build that worked flawlessly up to now. I could use those.  But before  installing them, I would like to make sure that there is nothing wrong on the main PCB, in order to avoid further damage.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on July 28, 2017, 01:56:30 PM
Replace any burnt out resistors connect without op amps and measure voltages on the DOA sockets. If those are fine then you should be safe to install your working gar2520s. The PR resistors are there to project anything downstream in case improper voltages going to inappropriate places inappropriately.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Neutrino on July 28, 2017, 04:51:51 PM
Should I check only the 16V supply voltages, i.e between C and +V/-V or anything else before installing the gar2520s?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Neutrino on July 29, 2017, 05:58:57 AM
OK, I swapped the fried resistors by new ones.
Before connecting to the power source, I measured the resistances between the DOA sockets as described in the VP2x Assembly Guide. Most of them seem too low:

Preamp DOA position:
-V to +V:  35.8 kOhms
-V to C: 31.8 kOhms
+V to C: 4.1 kOhms
C to O: fluctuating > 250 kOhms
+V to O:160 kOhms
-V to O: 11.4 MOhms

Fader DOA position:
-V to +V: 35.8 kOhms
-V to C: 31.5 kOhms
+V to C: 4.1 kOhms
C to O: out of DMM range
+V to O: out of DMM range
-V to O: out of DMM range

I conclude that the (or at least one) problem is on the main PCB. Any suggestions where to look first?

When connected to power (without DOAs installed) I measure +-15.8 V at all -V/+V terminals.
I also noticed that on one of the DOAs R14 and R15 (10 Ohms) are burnt as well, I that is useful info.

Thank you for your help!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on July 29, 2017, 10:40:56 AM
If the voltages are correct to The sockets, then you should be able to install properly working DOAs without blowing them up.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Neutrino on July 29, 2017, 11:44:52 AM
Good news: with the 2510s the preamp seems to work fine in a first test. I presume the culprit is one of the 1731 DOAs, probably the one with the burnt resistors. Any suggestions on how to further narrow it down?

And is there a way to test opamps without installing them? I would like to avoid a scenario like this for the upcoming builds (still 3 x VP28 and 2 x VP312 to do  ;D)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Neutrino on August 03, 2017, 10:26:40 AM
OK, I finished my 6 additional modules +12 DOAs. All of them are working except the one gar1731, which I still haven't figured out what the problem was. Luckily, I had ordered a few spare DOA kits  :)

One thing that I noticed though with the VP28 modules is that once I power off the lunchbox, the VP28 sends out a nasty sweep signal to my sound card. This happens with all four VP28 modules and with none of the VP312 (or any other 500-series preamp I own). I tested with a BAE 6-slot lunchbox as well as a 11-slot rack by VanDaal Electronics. Is this the normal behavior?

Apart from that, I'm up for testing now once all modules are installed  ;D
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: soundsofsirens on September 24, 2017, 09:30:06 PM
Hi guys,


i recently got a vp28 off someone online but when testing it, the output was super low and i had to crank it to almost full to get anything out of it and it also distorted. the waveform that appeared on my daw looked weird. is the a waveform of any significance to show what went wrong and how i can rectify it?

I'm really new to this but would like to learn more but don't really know where to start.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on September 25, 2017, 10:01:54 AM
I've built a few of these now for friends and keep a pair myself, but in helping a buddy out of a tight spot have landed myself with one that has an issue. ("Hey, don't worry dude - you've got a session booked in, I've got some spare time, I'll find the problem...") It's basically functioning properly but the whole sound has a low cut going on. When I engage filters I do hear a difference so it's not that they're always on - the whole bottom end is being sucked out somewhere. I read in a post from a while back that transformers are potential cause but my readings for all 3 of those are normal. I don't have an extension jig so haven't done voltage readouts - perhaps that should be next step? I'm in UK so all moves to order gear have to be done carefully as it all costs a lot. Are there any other components which are (a) always in signal path with (b) capability to suck low frequencies away while retaining otherwise normal functionality and pass signal? No capacitors look blown, but to test them I guess I'd need to desolder one by one, right?
Any advice welcome.
PS Have done full solder reflow, DOA swaps, transformer tests (out of circuit). That's where I'm at.
Cheers
Nick
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: c1bomber on October 05, 2017, 01:35:46 PM
On a newly built and seemingly totally functional vp28 I'm getting the following results from the VP2xx assembly guide final testing prodecures:

-v to +v = 32-35k at both op amp sockets (way below 200k)
-v to c = 29-32k at both op amp sockets (way below 200k)
+v to c = 4k at both op amp sockets (way below 200k)
o to c = above 40M at both op amp sockets
+v to o = above 40M at preamp and 35-38k at fader booster
 -v to o  = 32-39M at both op amp sockets

is this worrisome?

thanks!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Catrunes on October 05, 2017, 04:40:36 PM
Hey all, just finished up another pair of these guys and im getting some things happening that shouldnt be. one module isnt passing correctly but ill compare to my functioning builds but the other module is weird. when supplied power from 2 diff racks all the leds besides the signal led are lit at all times. any ideas?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: nicholasdover on October 09, 2017, 07:33:22 AM
The last 6 questions don't seem to have had any answers - has this thread gone cold...?
Or maybe the questions are kind of repetitions of old ones?

If anyone could advise which capacitors might be worth replacing on the off chance?

I really want to get this working again as I'm missing it on sessions and as far as I can tell it can now only be a fault with  (a) a capacitor (b) a resistor  (c) PCB trace, but it's so close to working correctly it's driving me mad!

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: TDJ on November 06, 2017, 06:16:26 PM
Hey guys! Not sure if this thread is being answered anymore, but I have a problem. I finished a couple of VP28s and I am having an issue with one. It doesn’t pass signal except for the helicopter ground noise. The green signal led is constantly lit. I measured 25mv on test point one. I moved over to see if the op amps were getting signal and then r29 smoked. Not sure if I accentally moved something that made it smoke up? Or if it’s part of my problem. Any help would be appreciated.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 06, 2017, 06:45:54 PM
Hey guys! Not sure if this thread is being answered anymore, but I have a problem. I finished a couple of VP28s and I am having an issue with one. It doesn’t pass signal except for the helicopter ground noise. The green signal led is constantly lit. I measured 25mv on test point one. I moved over to see if the op amps were getting signal and then r29 smoked. Not sure if I accentally moved something that made it smoke up? Or if it’s part of my problem. Any help would be appreciated.

Did you test voltages without op amps installed? Did you build your DOAs?

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 06, 2017, 07:02:52 PM
I've built a few of these now for friends and keep a pair myself, but in helping a buddy out of a tight spot have landed myself with one that has an issue. ("Hey, don't worry dude - you've got a session booked in, I've got some spare time, I'll find the problem...") It's basically functioning properly but the whole sound has a low cut going on. When I engage filters I do hear a difference so it's not that they're always on - the whole bottom end is being sucked out somewhere. I read in a post from a while back that transformers are potential cause but my readings for all 3 of those are normal. I don't have an extension jig so haven't done voltage readouts - perhaps that should be next step? I'm in UK so all moves to order gear have to be done carefully as it all costs a lot. Are there any other components which are (a) always in signal path with (b) capability to suck low frequencies away while retaining otherwise normal functionality and pass signal? No capacitors look blown, but to test them I guess I'd need to desolder one by one, right?
Any advice welcome.
PS Have done full solder reflow, DOA swaps, transformer tests (out of circuit). That's where I'm at.
Cheers
Nick

Check to see who over on that side of the world sell extension test jigs, because they are absolutely necessary building modules, as you are finding out. Sahib at Total Audio Control does and he's in Glasgow.

Troubleshooting someone else's work can be tricky, as you have to very carefully go over the BOM and the PCB overlay and make sure everything is in the correct place and oriented correctly, even if you already have you still may have missed something. That has happened to me several times.

Depending on how much roll off there is, it could be a bad solder joint on the input or output transformer or the wires for the output transformer are wired incorrectly. You could also have a very tiny solder bridge that is making an unwanted connection on the filter board that is hard to see. If all the components are good then something is either misplaced or a suspect solder joint is present.


Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 06, 2017, 07:07:35 PM
Hi guys,


i recently got a vp28 off someone online but when testing it, the output was super low and i had to crank it to almost full to get anything out of it and it also distorted. the waveform that appeared on my daw looked weird. is the a waveform of any significance to show what went wrong and how i can rectify it?

I'm really new to this but would like to learn more but don't really know where to start.

Go to the VP28 build guide on CAPI's site and read it step by step and compare what you see actually on the unit you have. It's either the DOA's, transistors, or any of the solder joints connecting them to the board.

The VP28 is not a good project if you are new to this. It's even harder if you are correcting someone else's work.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on November 06, 2017, 07:14:29 PM
On a newly built and seemingly totally functional vp28 I'm getting the following results from the VP2xx assembly guide final testing prodecures:

-v to +v = 32-35k at both op amp sockets (way below 200k)
-v to c = 29-32k at both op amp sockets (way below 200k)
+v to c = 4k at both op amp sockets (way below 200k)
o to c = above 40M at both op amp sockets
+v to o = above 40M at preamp and 35-38k at fader booster
 -v to o  = 32-39M at both op amp sockets

is this worrisome?

thanks!

The VP28 is different that the VP2x (VP25 and VP26).

This guy:

http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Test-Points.pdf

If you did have something actually wrong, it wouldn't work right, would have burnt out some resistors, or both. You're probably fine.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 06, 2017, 09:18:00 PM
Hey guys! Not sure if this thread is being answered anymore, but I have a problem. I finished a couple of VP28s and I am having an issue with one. It doesn’t pass signal except for the helicopter ground noise. The green signal led is constantly lit. I measured 25mv on test point one. I moved over to see if the op amps were getting signal and then r29 smoked. Not sure if I accentally moved something that made it smoke up? Or if it’s part of my problem. Any help would be appreciated.
I am subscribed to this thread but have not received any email updates. Seems to be a glitch in the site here.

Our friend Potato Cakes has posted some very helpful info above, thanks Paul!

R29 does nothing more than connect the chassis ground plane (gold finger Pin-1) to the PSU ground plane (gold fingers 5 and 13). I have never smoked this R nor can I think of a simple reason on the build why this might happen. Not sure what rack you are using but theoretically, these card edge pins should be connected inside the PSU at a star ground point. If there is no connection inside the PSU (error on many racks) some current can be dumped and if the VP28 is the only card in the rack with something connecting them, R29 could smoke depending on the situation.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: TDJ on November 06, 2017, 09:29:34 PM
Did you test voltages without op amps installed? Did you build your DOAs?

Thanks!

Paul

Yes, the op amps were in when that measurement of 25mv was taken at test point 1. And yes, I built them, but they worked inside of a different module. I don't have any 1/2w resistors on hand, so I'll post back once I can get a new R29 in. I'm thinking something must have moved when I was digging around to test around the DOAs. If you guys have any other suggestions of things I could check in the meantime, that would be great. Thanks in advance!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 06, 2017, 09:35:44 PM
Yes, the op amps were in when that measurement of 25mv was taken at test point 1. And yes, I built them, but they worked inside of a different module. I don't have any 1/2w resistors on hand, so I'll post back once I can get a new R29 in. I'm thinking something must have moved when I was digging around to test around the DOAs. If you guys have any other suggestions of things I could check in the meantime, that would be great. Thanks in advance!
Well, TP1 is immediately after the 2622 so you should have signal here with no amps installed. Its completely passive up to this point.

As per my email, in theory the pre should function just fine without R29. Its sort of a precaution to make sure those 2 points are connected. What rack are you using? With no modules installed you can easily check if card edge connector pin 1 has continuity to pin 5 and/or pin 13.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: TDJ on November 07, 2017, 08:41:48 AM
Well, TP1 is immediately after the 2622 so you should have signal here with no amps installed. Its completely passive up to this point.

As per my email, in theory the pre should function just fine without R29. Its sort of a precaution to make sure those 2 points are connected. What rack are you using? With no modules installed you can easily check if card edge connector pin 1 has continuity to pin 5 and/or pin 13.

Jeff & I e-mailed back and for for a while last night. For some strange reason, TP1 is now at proper voltage. However, I'm getting 0v at TP2 now. We've eliminated op amps. We popped in some different op amps that we know work (from a different VP28), and have the same symptoms - not passing audio, helicopter noise (grounding issue somewhere?), and green led stuck on. If anyone else has suggestions, I would more than welcome them at this point. I feel like i'm running out of ideas.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Paxkal on November 10, 2017, 10:59:21 AM
Hello,

I just mounted eight preamps CAPI VP28 and I have three that do not work after a transistor reversal on the 1731 and 2520 cards that burned me 10 ohm resistors. I solved the problem of the 1731 and 2520 cards and now I have three modules with problems. Here are the problems encountered:

-Module 1: low level at the output of the preamp
-Module 2: led signal (green) lit without any audio signal in input, I send signal in input but there is no sound in output
-Module 3: the signal enters the card correctly the led signal (green) works perfectly no sound output

Someone to help me?

thank you very much
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 10, 2017, 11:20:46 AM
Hello,

I just mounted eight preamps CAPI VP28 and I have three that do not work after a transistor reversal on the 1731 and 2520 cards that burned me 10 ohm resistors. I solved the problem of the 1731 and 2520 cards and now I have three modules with problems. Here are the problems encountered:

-Module 1: low level at the output of the preamp
-Module 2: led signal (green) lit without any audio signal in input, I send signal in input but there is no sound in output
-Module 3: the signal enters the card correctly the led signal (green) works perfectly no sound output

Someone to help me?

thank you very much
Welcome to the forum.
3 modules down,that's bad.
O.K.,first make sure the rack containing the modules should be disconnected from everything that belongs to audio.
Check voltages,as described in the build manual.
Depending on the version you have you can follow the test points guide (if you have rev B):


http://www.capi-gear.com/catalog/images/gallery/VP28/Build-docs/RevB/CAPI-VP28-Rev-B-Test-Points.pdf


Please only use DOAs that are definetely known as working!
I also highly recommend the use of extender cables or jigs,Jeff has them in his shop:


http://capi-gear.com/catalog/product_info.php?cPath=87_88&products_id=406


Depending on your location you can get extender cables from other sellers too.


Please post your findings here so others can participate or chime in,maybe even Jeff Steiger.


Best regards,


Udo.



Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 10, 2017, 11:30:23 AM
Paxkal, do you have any 2520 style opamps that are for sure good working amps? Something you have used and tested in another module? If not, you have broken the cardinal rule of building my (or similar) stuff. That is never try to test a freshly built module with freshly built opamps. If something isn't working right, we really have no way of knowing if its the pre or one or both of the opamps. At this point, I would suggest getting a pre-built pair of opamps somewhere. That way you can see if the pre's are truly not working.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 10, 2017, 11:37:43 AM
Paxkal, do you have any 2520 style opamps that are for sure good working amps? Something you have used and tested in another module? If not, you have broken the cardinal rule of building my (or similar) stuff. That is never try to test a freshly built module with freshly built opamps. If something isn't working right, we really have no way of knowing if its the pre or one or both of the opamps. At this point, I would suggest getting a pre-built pair of opamps somewhere. That way you can see if the pre's are truly not working.
Hi Jeff,


he wrote he has 8 modules "mounted",so -in case the other five are properly working- he should have a bunch of them…..


Cheers,


Udo.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 10, 2017, 12:07:27 PM
he wrote he has 8 modules "mounted",so -in case the other five are properly working- he should have a bunch of them…..
Indeed Udo! I missed that part! Time to start swapping out opamps.

The biggest failure with DIY builds are solder joints.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Paxkal on November 10, 2017, 01:55:37 PM
Actually the solder joints ....

Thank you all !!!!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on November 10, 2017, 02:00:18 PM
Actually the solder joints ....

Thank you all !!!!
Good to hear!
So all are up and running?


Best regards,


Udo.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Paxkal on November 10, 2017, 02:10:38 PM
Yes everything is ok !!

Thank you very much!! ;)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on November 15, 2017, 12:04:22 PM
Hi guys, got a issue with a VP28 that was made several months ago. Today the gain seems fixed. I mean, there's no change in gain using the Grayhill stepped pot. Gain is fixed. I tried to swap opamps but the issue is still there.  Pad works, HPF works, signal led is ok, Mic/Line switch works. Then, after a couple of hours, I decided to put a line signal and press MIC button. After this crazy volume rising the knob came back to work. What may have caused this?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 16, 2017, 08:46:58 PM
Hi guys, got a issue with a VP28 that was made several months ago. Today the gain seems fixed. I mean, there's no change in gain using the Grayhill stepped pot. Gain is fixed. I tried to swap opamps but the issue is still there.  Pad works, HPF works, signal led is ok, Mic/Line switch works. Then, after a couple of hours, I decided to put a line signal and press MIC button. After this crazy volume rising the knob came back to work. What may have caused this?
This sounds exactly like the last guys problem. Bad solder joint somewhere. Things changing here and there typically points to a bad solder joint. Can't really tell you where though but I would look at the resistor network behind the gain switch as well as the 200Ω R's and the 330µF cap.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: cubivore on November 20, 2017, 07:35:26 PM
hi everyone! just built two vp28 with red dots somewhat successfully. the first build was blotched here and there but i managed to fix a bunch of my mistakes and get it working 100%! the second build, while much more smooth and relaxing, has a minor problem. it seems like after walking on carpet (not every time, but most of the time) and simply touching the input gain knob will create a tiny burst of clicks. this doesn't happen touching the output fader or any other part of the module. i swapped mic cables, mics, output cables and it seems to only be this module. any ideas where i should start looking before i start dismantling the module?  thanks in advance! :D
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: BramK on November 21, 2017, 10:02:09 PM
Just successfully built 2 VP28s along with the 2520/1731 combos. Thanks Jeff for the great communication via email and everyone in this thread for sharing your experiences. For any who are thinking about building these, the biggest help for me was taking it slow, labeling and taping components down on a piece of paper, double (triple) checking each component value and placement before soldering, and using a magnifying light for the Op Amps. Check for correct placement and any solder bridges as you go along with the magnifier and you'll be up and running in no time!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Catrunes on December 02, 2017, 11:38:54 PM
This sounds exactly like the last guys problem. Bad solder joint somewhere. Things changing here and there typically points to a bad solder joint. Can't really tell you where though but I would look at the resistor network behind the gain switch as well as the 200Ω R's and the 330µF cap.

i ended up stupidly forgetting to solder the entirety of the push switches. derp. tends to happen when im trying to build while trying to mix a band live ha
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 23, 2018, 06:36:40 AM
Quick question: Somehow R7 went missing, a 59K MF. I do have an equivalent, a 56K3 MF. OK to sub?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 23, 2018, 02:12:20 PM
Quick question: Somehow R7 went missing, a 59K MF. I do have an equivalent, a 56K3 MF. OK to sub?
Yeah its OK temporarily but needs to be 59k. It will change the load on the input slightly and lower the operating level. Its very slight but you'll want it to match for any stereo use etc. Send me an email thru the store and I'll mail you one.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 30, 2018, 01:24:03 AM
I have an issue with the output fader grayhill switch. At the zero setting, there appears to be no attenuation and then one click to the right I end up where I'm guessing the lowest gain setting should be, then it slowly climbs to where I assume unity gain should be. I checked the resistors against the BOM and a working VP28 I have and they appear correct, also reflowed the RF resistors. It almost seems like the grayhill stops are off, have you ever experienced that? I have no previous experience with those switches, they are the variety where the stops are already set. Once the almighty desoldering tool arrives I'll redo that whole part of the PCB, just curious to hear if there have been any faulty grayhills around this thread.

I would love to know how I could troubleshoot a grayhill output fader issue with a DMM. Any pointers?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 30, 2018, 01:36:39 AM
I have an issue with the output fader grayhill switch. At the zero setting, there appears to be no attenuation and then one click to the right I end up where I'm guessing the lowest gain setting should be, then it slowly climbs to where I assume unity gain should be. I checked the resistors against the BOM and a working VP28 I have and they appear correct, also reflowed the RF resistors. It almost seems like the grayhill stops are off, have you ever experienced that? I have no previous experience with those switches, they are the variety where the stops are already set. Once the almighty desoldering tool arrives I'll redo that whole part of the PCB, just curious to hear if there have been any faulty grayhills around this thread.

I would love to know how I could troubleshoot a grayhill output fader issue with a DMM. Any pointers?

Have yet to encounter a faulty gray hill switch of any kind, but that doesn't mean it is impossible. To test turn the switch all the way CW or CCW, doesn't matter which. One probe of DMM on the pole (the connection by itself) and the other on one end of the row of pins, depending on your switch position. Once you find that out, turn the switch one click and you should have continuity on the next pin in on the row of pins and the pole. Rinse and repeat. You should not have continuity between any of the pins in the row of pins on a Grayhill switch (at least in this design).

And just to clarify, when you say the "zero setting," do you mean in the middle (0=unity gain=no attenuation) or do you mean fully CCW?

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 30, 2018, 07:47:15 AM
Have yet to encounter a faulty gray hill switch of any kind, but that doesn't mean it is impossible. To test turn the switch all the way CW or CCW, doesn't matter which. One probe of DMM on the pole (the connection by itself) and the other on one end of the row of pins, depending on your switch position. Once you find that out, turn the switch one click and you should have continuity on the next pin in on the row of pins and the pole. Rinse and repeat. You should not have continuity between any of the pins in the row of pins on a Grayhill switch (at least in this design).

And just to clarify, when you say the "zero setting," do you mean in the middle (0=unity gain=no attenuation) or do you mean fully CCW?

Thanks!

Paul

Thanks Paul! Yes, by zero I meant unity gain. So to be even more specific on this issue: imagine the unity gain position being the most gain available, then going CCW arriving at close to where unity should have been. So in essence, a mirror reverse scenario. Going one step CW from the unity gain position seems to be what the lowest gain setting of the switch should be, i.e. fully CCW on a working switch and then it increases normally CW to what I imagine the gain of the position just one step CCW from the unity setting should be. So it's like the throw of the switch is split in two and mirror reversed. Interesting indeed.
Not sure I'm explaining this well...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 30, 2018, 10:16:34 AM
As history has it, your issues are likely not the Grayhill switch. Can you post a pic of the bottom side of the board where the fader switch and resistors are?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 30, 2018, 09:45:30 PM
As history has it, your issues are likely not the Grayhill switch. Can you post a pic of the bottom side of the board where the fader switch and resistors are?

Sure Jeff, here is the bottom.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 30, 2018, 09:46:50 PM
Sure Jeff, here is the bottom.

And here's the top...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 31, 2018, 02:30:27 AM
OK guys, here it is:

Even though there's no evidence on this thread of a faulty grayhill switch, I have the dubious honor of finding the first! I desoldered and resoldered everything related to the switch, no change. I finally removed the switch (love the new Hakko FR300!!!)
Fully CCW, I measured continuity between pole and pin 7!! Not pin 1 seen from the bottom as in a functional switch I got. This confirms my hunch in the thread above. I will open up the switch and see what I can see.

Goes to show you that eventually every component can and will fail, even a brand new one.

Cheers and thanks for the pointers
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 31, 2018, 03:08:09 AM
Can you do a test of all the switch positions? It may just be the factory stop is in the wrong the place. The switch itself may be fine. You can actually fix that yourself if you want to as I have had to open up several of the continuous rotation versions and install my own stop pins.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 31, 2018, 03:19:04 AM
Can you do a test of all the switch positions? It may just be the factory stop is in the wrong the place. The switch itself may be fine. You can actually fix that yourself if you want to as I have had to open up several of the continuous rotation versions and install my own stop pins.

Thanks!

Paul

Sure Paul,  I suspect the factory stops as well...
Yes indeed, it was the factory stops. Now to open up the switch.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pkordel on January 31, 2018, 03:38:50 AM
Sure Paul,  I suspect the factory stops as well...
Yes indeed, it was the factory stops. Now to open up the switch.

Well, opening it was fairly easy, not sure if I'll be able to re-assemble it haha! Gotta work now, glad I had a spare switch to get back up and running.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 31, 2018, 04:44:35 AM
Well, opening it was fairly easy, not sure if I'll be able to re-assemble it haha! Gotta work now, glad I had a spare switch to get back up and running.

The trick to keeping all the wafers together for this and other rotary switches is to remove one of the bolts, re-insert it backwards with the threads facing forward, then do the same for the other bolt. It will keep it the wafers from falling apart, which I learned only doing it the hard way several times. When it's completely disassembled, the tiny bent piece of metal that makes contact between pole and the positions will fall out of place. That is when the real fun begins.

Glad you're up and running.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on February 10, 2018, 03:30:42 AM
A VP28 I built more than a year ago is showing this issue: when you switch it on, after for example a couple of days not using it, the input knob stays at fixed value and if you move the knob ccw or cw it doesn't change the gain. If you press the MIC switch one second and depress it to let it come back to Line mode the knob works perfectly and so it does for short periods of time. I checked all the solder joints and repassed everyone with fresh solder. Opamp have been changed with several types, it doesn't change the issue. Anyone could help me?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 10, 2018, 11:35:51 AM
A VP28 I built more than a year ago is showing this issue: when you switch it on, after for example a couple of days not using it, the input knob stays at fixed value and if you move the knob ccw or cw it doesn't change the gain. If you press the MIC switch one second and depress it to let it come back to Line mode the knob works perfectly and so it does for short periods of time. I checked all the solder joints and repassed everyone with fresh solder. Opamp have been changed with several types, it doesn't change the issue. Anyone could help me?
Classic signs of a cold solder joint somewhere on the build. Hard to say where exactly at this point. The physical jarring action of engaging the switch doesn't necessarily mean its one on that switch but it could be.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on February 10, 2018, 01:43:22 PM
Solved Jeff. Don't know how but the 330uF cap near the input trafo was a bit moving. Desoldered and resoldered did the job. Thanks!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: forgotteng on March 02, 2018, 07:42:09 PM
Does anyone know what the part number for the IC socket at A2 is I somehow can't find the socket and need to order another one.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 02, 2018, 08:14:20 PM
Does anyone know what the part number for the IC socket at A2 is I somehow can't find the socket and need to order another one.
The IC socket has not been supplied with the VP28 kit for a few years now. They are not required for operation. If you want to grab one, I have them at the store. The Mill-Max part # for the ones I have is listed on the product page here.
http://capi-gear.com/catalog/product_info.php?cPath=71_77&products_id=331
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: forgotteng on March 03, 2018, 01:10:10 AM
The IC socket has not been supplied with the VP28 kit for a few years now. They are not required for operation. If you want to grab one, I have them at the store. The Mill-Max part # for the ones I have is listed on the product page here.
http://capi-gear.com/catalog/product_info.php?cPath=71_77&products_id=331
That's awesome Jeff. That explains a lot. I was looking all over the place for that bugger. I think the toughest part for me has been keeping all the versions and revisions of the instructions and BOM's  separate and organized. It's been a learning process but so good. Thanks.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: adamhall202020 on March 17, 2018, 06:23:33 AM
Hey guys could you please help me a bit. Im struggling with the last thing. Is it placing op-amps about using quite a force to push them deeper? Even if im trying to push them a bit harder they wont go any deeper and the thing is that they feel loose im even afraid they could fall off.  Op-amps are quite high above board. I have pre-build red dots, only about to insert them
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: kante1603 on March 17, 2018, 07:01:49 AM
Hey guys could you please help me a bit. Im struggling with the last thing. Is it placing op-amps about using quite a force to push them deeper? Even if im trying to push them a bit harder they wont go any deeper and the thing is that they feel loose im even afraid they could fall off.  Op-amps are quite high above board. I have pre-build red dots, only about to insert them
Welcome to the forum,


have you read Jeff's instructions?


http://capi-gear.com/catalog/DOA_Install.php


Best regards,


Udo.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: IndietownRecording on March 17, 2018, 11:58:26 AM
Hey guys could you please help me a bit. Im struggling with the last thing. Is it placing op-amps about using quite a force to push them deeper? Even if im trying to push them a bit harder they wont go any deeper and the thing is that they feel loose im even afraid they could fall off.  Op-amps are quite high above board. I have pre-build red dots, only about to insert them

A good trick is to use a paper clip to insert in the sockets to "break them in" a bit. It helps.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: IndietownRecording on March 17, 2018, 12:04:16 PM
So, I've got a VP28 here with the mic signal present LED constantly on. These are the earlier black board versions. I got these from someone else, and cleaned up the build substantially. I went through the BOM and verified all components are correctly placed. Been looking at it for a couple weeks now and can't figure it out. Thought it's time to reach out to you guys on here to see if you might be able to give me some pointers on where to look.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 17, 2018, 12:10:14 PM
So, I've got a VP28 here with the mic signal present LED constantly on. These are the earlier black board versions. I got these from someone else, and cleaned up the build substantially. I went through the BOM and verified all components are correctly placed. Been looking at it for a couple weeks now and can't figure it out. Thought it's time to reach out to you guys on here to see if you might be able to give me some pointers on where to look.
I know this happened to a couple of folks but I've never seen one in person so not sure what the cause is. You can try putting an R around 1k to 2k across the pins of the LED header to see if that handles it.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: IndietownRecording on March 17, 2018, 05:57:16 PM
I know this happened to a couple of folks but I've never seen one in person so not sure what the cause is. You can try putting an R around 1k to 2k across the pins of the LED header to see if that handles it.

Just tried a 1k2 with no luck.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: IndietownRecording on March 23, 2018, 11:38:45 AM
Anyone have any other ideas before I throw in the towel on this one?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 23, 2018, 03:12:09 PM
Anyone have any other ideas before I throw in the towel on this one?
What are the opamps and who built them? Have you tried swapping them around? Maybe there is some noise or something present at the output of the first amp that is triggering the sig present circuit?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: IndietownRecording on March 23, 2018, 08:30:15 PM
What are the opamps and who built them? Have you tried swapping them around? Maybe there is some noise or something present at the output of the first amp that is triggering the sig present circuit?

Hey Jeff, they’re red dots. I’ll try swapping them.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 23, 2018, 08:48:26 PM
Hey Jeff, they’re red dots. I’ll try swapping them.
They should be fine.

This is the main reason why I simplified that part of the build for the Rev B version. A handful of folks had trouble and I could never reproduce the problem on our end.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: critterkllr on March 27, 2018, 04:33:19 PM
I just finished a VP28 and the green signal led is stuck on. Everything else works as it should. It’s not a big deal, but I’d like to fix it. I’m at the mercy of this thread since schematics aren’t available. What should I look at to fix the signal LED?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 27, 2018, 05:56:41 PM
I just finished a VP28 and the green signal led is stuck on. Everything else works as it should. It’s not a big deal, but I’d like to fix it. I’m at the mercy of this thread since schematics aren’t available. What should I look at to fix the signal LED?
Which Rev PCB is it? Green or black?

Have you made sure its not opamp related?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: critterkllr on March 27, 2018, 06:01:05 PM
Which Rev PCB is it? Green or black?

Have you made sure its not opamp related?

It’s the green pcb. I just received it yesterday. It doesn’t matter if the opamps are in or out. And I’ve also switched them around with known working opamps. There was no change in performance.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 27, 2018, 06:15:48 PM
It’s the green pcb. I just received it yesterday. It doesn’t matter if the opamps are in or out. And I’ve also switched them around with known working opamps. There was no change in performance.
I think this is the first report of this on a green PCB I have heard.

The circuit is comprised of
A2
CR3
LED5
C11
R17-R19, R27, R28

I would look for solder bridges,/cold solder joints on the amp or incorrect part placement of the above.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: critterkllr on March 27, 2018, 06:19:32 PM
I think this is the first report of this on a green PCB I have heard.

The circuit is comprised of
A2
CR3
LED5
C11
R17-R19, R27, R28

I would look for solder bridges,/cold solder joints on the amp or incorrect part placement of the above.

Great, will do. That’s for responding on your spring break, Jeff!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: MountCyanide on March 28, 2018, 07:14:16 AM
I'm just gonna detract from the support thread to give props to Jeff for a fun and supremely well designed kit. No hiccups. It fired right up and worked as soon as I shut off my iron. Nice! Seems it's gonna pair well with my AEA Ribbon.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: critterkllr on March 29, 2018, 09:33:03 AM
Great, will do. That’s for responding on your spring break, Jeff!

I guess it would help if I actually soldered in A2, huh? Sockets have made me lazy. Thanks again for the help!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: forgotteng on April 02, 2018, 01:33:56 PM
I'm having some issues with my build. The pre passes audio but doesn't seem to sound like my other VP28. It seems like there is a little frequency issues with the top end and the bottom. When I engage the HPF it really sucks the bottom. on both the 40 and the 80. It seems to drop out 500 and below. (Just a guess but suffice it to say it's a lot higher than 80Hz. It looks like the caps are in the right position but am I missing something?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 03, 2018, 10:25:02 PM
I'm having some issues with my build. The pre passes audio but doesn't seem to sound like my other VP28. It seems like there is a little frequency issues with the top end and the bottom. When I engage the HPF it really sucks the bottom. on both the 40 and the 80. It seems to drop out 500 and below. (Just a guess but suffice it to say it's a lot higher than 80Hz. It looks like the caps are in the right position but am I missing something?
The typical questions must be answered. What opamps are in there and who built them? They must be known good working amps. Short of that, I would look at all solder joints. A bad joint could easily cause this type of symptom.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: forgotteng on April 04, 2018, 02:35:20 PM
The typical questions must be answered. What opamps are in there and who built them? They must be known good working amps. Short of that, I would look at all solder joints. A bad joint could easily cause this type of symptom.

Thanks Jeff, I did take my working opamps out of my existing working VP28 and it was the same results. Should  I be looking at solders on specifically the HPF card or on the whole card. Forgive me if these are dumb questions I'm just trying to see if there is a strategic starting point.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 04, 2018, 03:07:50 PM
Thanks Jeff, I did take my working opamps out of my existing working VP28 and it was the same results. Should  I be looking at solders on specifically the HPF card or on the whole card. Forgive me if these are dumb questions I'm just trying to see if there is a strategic starting point.
Your best plan of attack is to use the published Test Points guide to see where things go bad. That will help narrow down where to look for trouble.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tydbowl on April 16, 2018, 09:47:26 PM
Hey guys... I've been trying to troubleshoot one of my VP28's that is -9db too quiet.  I checked my solder joints and re-soldered ones I thought might be questionable but I think they look good.   I have tested this in the same slot, same op amps, same converter, confirmed unity knob positions.  It sounds fine but just quiet.  I am using the Rogue Five Op Amps

I used my audio interface and a Fluke 115 Multimeter.  I tried to get it as close to 1.228 volts from the interface output but it was occasionally bumping 1.229

everything starting at TP3 and below is low

Here are my test point results...

TP1 - 111.6 mV
TP2 - .702 V
TP3 -  1.242 V
TP4 -  307.2 mV
TP5 - 307.2 mV
TP6 - 307.2 mV
TP7 - 643.4 mV

OUTPUT - .427V

Any help appreciated!
Tyler
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tydbowl on April 20, 2018, 04:26:58 PM
Bump  :)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 21, 2018, 01:08:30 PM
Hey guys... I've been trying to troubleshoot one of my VP28's that is -9db too quiet.  I checked my solder joints and re-soldered ones I thought might be questionable but I think they look good.   I have tested this in the same slot, same op amps, same converter, confirmed unity knob positions.  It sounds fine but just quiet.  I am using the Rogue Five Op Amps

I used my audio interface and a Fluke 115 Multimeter.  I tried to get it as close to 1.228 volts from the interface output but it was occasionally bumping 1.229

everything starting at TP3 and below is low

Here are my test point results...

TP1 - 111.6 mV
TP2 - .702 V
TP3 -  1.242 V
TP4 -  307.2 mV
TP5 - 307.2 mV
TP6 - 307.2 mV
TP7 - 643.4 mV

OUTPUT - .427V

Any help appreciated!
Tyler
I think the only one I would worry about is the output. How are you measuring the final output? It is floating with no reference to ground so you can only get an accurate reading between the +/- with your DMM probes or by checking the reading into PT's etc. Don't bother probing between + and ground or - and ground.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tydbowl on April 21, 2018, 10:21:42 PM
I think the only one I would worry about is the output. How are you measuring the final output? It is floating with no reference to ground so you can only get an accurate reading between the +/- with your DMM probes or by checking the reading into PT's etc. Don't bother probing between + and ground or - and ground.

I measured the output on pins 2 and 3 of the XLR coming out of the card slot like the daw output.  Is that correct?

Thanks!!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 22, 2018, 10:58:13 AM
I measured the output on pins 2 and 3 of the XLR coming out of the card slot like the daw output.  Is that correct?

Thanks!!
Yes.

You will need to verify that the transformer is good. The easiest way is with some DCR measurements. The following is for a Litz 2623-1. If you have the EA version, just follow the cross reference chart on the transformers datasheet.

For the Litz 2623-1 you should have the following:
Black to Red, 15.7Ω
Pink to Gray, 31.5Ω
Violet to Orange 31.5Ω

Make sure your DMM range is set to below 200Ω. Make sure all leads are floating and not touching anything including each other.

There should be no measurable resistance between leads except for the ones shown above.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: PerspectiveSound on April 22, 2018, 12:43:53 PM
Hey everyone, my first post here, I came looking for support as my first foray into diy audio has not gone as planned, there were clouds of smoke involved... I built a CAPI 511 rack and vp28 preamp, was pretty careful to follow the proper build and leave no obvious bad soldering points on either piece, but I finished the pre up, plugged it in the rack, and I started smelling smoke... 

 I immediately cut the power to the rack, I did however notice that the power link switch for that rack space was on which made me want to test a different rack space, after powered everything down I switched the rack space and the pre powered on, leds working, no smoke, but no audio signal either, just some faint noise that clicks when phantom power is engaged.

 It seems likely that the rack is fine and the pre was also fine until I set it up wrong. My question would be if there is a way to pinpoint which components might have failed and might be replaced, also to make sure the rack is OK and won't damage other gear since I planned to build other pieces.  I'm sure this looks like a newbie story, any help y'all could give me would be much appreciated!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 22, 2018, 12:48:59 PM
The obvious way is to look for charred parts. You never mentioned if the opamps were kits you built or pre-built from the store.

We should only handle VP28 questions here. Testing the rack is simple but should be handled on the respective rack support thread. Please skim it before posting as I am sure it has been covered before.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: PerspectiveSound on April 22, 2018, 01:03:32 PM
Oh yes I should have said, I built the opamps GAR2520, I didn't see any burned parts but I just checked and the transformers have a strong burnt odor, I will look through the power rack forum for testing as you suggest but is it possible the power link issue cause the transformers to burn out on an otherwise well built pre-amp? Thanks so much for your quick response
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 22, 2018, 01:11:36 PM
Oh yes I should have said, I built the opamps GAR2520, I didn't see any burned parts but I just checked and the transformers have a strong burnt odor, I will look through the power rack forum for testing as you suggest but is it possible the power link issue cause the transformers to burn out on an otherwise well built pre-amp? Thanks so much for your quick response
It will not be the transformers. It will likely be on the opamps or the 10Ω damping resistors that are inline for each opamp's power rails.

A golden rule has been broken...do not try to test a fresh preamp build with fresh opamp builds. There is truly no way of knowing where the problem is at. It could be one amp, both amps, the preamp or any combination of the above which is WAY too many variables.

There is no gold finger for pin-6 so the DC-Link switches in the rack are irrelevant to the VP28.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: tydbowl on April 22, 2018, 04:11:41 PM
Yes.

You will need to verify that the transformer is good. The easiest way is with some DCR measurements. The following is for a Litz 2623-1. If you have the EA version, just follow the cross reference chart on the transformers datasheet.

For the Litz 2623-1 you should have the following:
Black to Red, 15.7Ω
Pink to Gray, 31.5Ω
Violet to Orange 31.5Ω

Make sure your DMM range is set to below 200Ω. Make sure all leads are floating and not touching anything including each other.

There should be no measurable resistance between leads except for the ones shown above.

Thanks Jeff,

Here are my measurements on the output transformer leads...

Black to Red, 15.7Ω
Pink to Gray, 32.8Ω
Violet to Orange 31.1Ω

I went ahead and measured other colors together after you mentioning there should be no measurable resistance between the other colors. I am getting resistance between these other leads..

Grey and Orange 33.3
Grey and Violet 63.7
Pink and Violet 31.1

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 22, 2018, 04:27:02 PM
Thanks Jeff,

Here are my measurements on the output transformer leads...

Black to Red, 15.7Ω
Pink to Gray, 32.8Ω
Violet to Orange 31.1Ω

I went ahead and measured other colors together after you mentioning there should be no measurable resistance between the other colors. I am getting resistance between these other leads..

Grey and Orange 33.3
Grey and Violet 63.7
Pink and Violet 31.1
It looks like there is an internal short with this transformer. Send me an email thru the store referencing this post and I will get you a replacement.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: onss13 on April 22, 2018, 08:00:15 PM
Hey all/Jeff,  I’ve read through this thread because of a similar issue and was hoping to find the solution here.  I have two VP28s in a Capi 11 space rack and both sound perfect but one is having the signal present led issue others have had. It stays on with no signal actually present but turns off when 48v is switched on. Makes no sense.
I’ve ran a few pure sine waves  into it from a calibrated generator I borrowed from work. Both pres look damn near identical in PT.
Doesn’t have any noise on the output when not sending tones through it. The led seems to have no discernible effect on the sound whatsoever.
Switched the placement in the rack as well as having it in there by itself. No change.
It’s on with or without opamps installed.

For years I was a bench tech as well as building custom effects. I have appropriate tools and supplies but as I am human, I did reflow every things on the board as a precaution. Again, no change at all.

Anyone have some pointers. Or a partial schematic to start tracing.

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 22, 2018, 08:15:21 PM
Hey all/Jeff,  I’ve read through this thread because of a similar issue and was hoping to find the solution here.  I have two VP28s in a Capi 11 space rack and both sound perfect but one is having the signal present led issue others have had. It stays on with no signal actually present but turns off when 48v is switched on. Makes no sense.
I’ve ran a few pure sine waves  into it from a calibrated generator I borrowed from work. Both pres look damn near identical in PT.
Doesn’t have any noise on the output when not sending tones through it. The led seems to have no discernible effect on the sound whatsoever.
Switched the placement in the rack as well as having it in there by itself. No change.
It’s on with or without opamps installed.

For years I was a bench tech as well as building custom effects. I have appropriate tools and supplies but as I am human, I did reflow every things on the board as a precaution. Again, no change at all.

Anyone have some pointers. Or a partial schematic to start tracing.
What version of PCB is this?

The 48V switch has literally nothing to do with the signal present circuit.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: onss13 on April 22, 2018, 10:09:45 PM
I couldn’t think of any reason why either. I’ll try to post a video tomorrow if I can. It’s the darndest thing.
It’s a Rev B (green) pcb.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitone on April 28, 2018, 04:10:03 AM
Hello everyone!

Recently I built two VP28 and they both seems to be perfectly working, but one is out of phase ... :-[
I swapped slots and made some checks with cables and everything so I'm pretty sure one unit is phase reversed, when I flip the switch works as expected. No big deal, I could live with that but where did I go wrong?
Could it be something wrong while building an OpAmp?
If someone of you ever had the same problem please let me know, thank you very much!

Have a nice weekend

Roberto
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 28, 2018, 10:22:05 AM
Hello everyone!

Recently I built two VP28 and they both seems to be perfectly working, but one is out of phase ... :-[
I swapped slots and made some checks with cables and everything so I'm pretty sure one unit is phase reversed, when I flip the switch works as expected. No big deal, I could live with that but where did I go wrong?
Could it be something wrong while building an OpAmp?
If someone of you ever had the same problem please let me know, thank you very much!

Have a nice weekend

Roberto
Maybe you swapped some leads with one of the output transformers? Easy enough to correct.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitone on April 28, 2018, 11:12:55 AM
Maybe you swapped some leads with one of the output transformers? Easy enough to correct.

Hi, thanks!
Tried to swap the opamps from the other unit and checked the wiring of the (Liz) transformers and all seems to be correct but no luck so far, the same unit is out of phase...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 28, 2018, 11:40:00 AM
Hi, thanks!
Tried to swap the opamps from the other unit and checked the wiring of the (Liz) transformers and all seems to be correct but no luck so far, the same unit is out of phase...
You can swap the red and black leads from one of the transformers.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: pitone on April 28, 2018, 11:42:54 AM
You can swap the red and black leads from one of the transformers.

Cool!easy fix... I will, thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: javibigcity on June 05, 2018, 10:30:50 AM
Hello everybody,
This is my first message after long years of lurking and assimilating valuable info from you guys. So first and foremost: a big thanks to all the contributors. Hopefully I can become one over time.

I have this pair of VP28 Litz preamps and man they sound great. To be honest I almost exclusively use them to amplify the output of my passive summing mixer, and so far so good (had them for one year now).
Yesterday night, however, one of them started to act funny, in these two ways:
-   The HPF switch, when in the -12dB position, entirely cuts the preamp output. In addition to that, when touching the switch with my fingers, I can hear a low but audible intermittent random ground (? ) noise. The noise stops when I touch ground anywhere else in my mixing rig.
-   I hardly use the switch, but more problematic (and maybe related somehow?) is the fact that this unit suddenly reproduces low freqs around 4-5 dB louder than the other unit. I realized this cause the mix yesterday was totally wrong, pan-wise. It was caused by the synth-bass although it was mono and centered in the DAW and in the summing mixer. So I used white noise to test the outputs of both the summing mixer and the CAPIs and apparently everything was ok. Then I tried with tones and it became clear that 80Hz and 100Hz were louder in one of the CAPIs (or quieter in the other). I did not go beyond those two freqs.

The -6dB position in the switch works ok, as so do the freq buttons.

Can a faulty HPF switch be causing all this trouble? Any hint would be very appreciated.

Thanks a lot,
Javi
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 05, 2018, 10:39:46 AM
Hello everybody,
This is my first message after long years of lurking and assimilating valuable info from you guys. So first and foremost: a big thanks to all the contributors. Hopefully I can become one over time.

I have this pair of VP28 Litz preamps and man they sound great. To be honest I almost exclusively use them to amplify the output of my passive summing mixer, and so far so good (had them for one year now).
Yesterday night, however, one of them started to act funny, in these two ways:
-   The HPF switch, when in the -12dB position, entirely cuts the preamp output. In addition to that, when touching the switch with my fingers, I can hear a low but audible intermittent random ground (? ) noise. The noise stops when I touch ground anywhere else in my mixing rig.
-   I hardly use the switch, but more problematic (and maybe related somehow?) is the fact that this unit suddenly reproduces low freqs around 4-5 dB louder than the other unit. I realized this cause the mix yesterday was totally wrong, pan-wise. It was caused by the synth-bass although it was mono and centered in the DAW and in the summing mixer. So I used white noise to test the outputs of both the summing mixer and the CAPIs and apparently everything was ok. Then I tried with tones and it became clear that 80Hz and 100Hz were louder in one of the CAPIs (or quieter in the other). I did not go beyond those two freqs.

The -6dB position in the switch works ok, as so do the freq buttons.

Can a faulty HPF switch be causing all this trouble? Any hint would be very appreciated.

Thanks a lot,
Javi
All are classic signs of a bad solder joint. I would touch things up specifically on the HPF PCB as well as the 5-pin header that connects the boards.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: javibigcity on June 05, 2018, 11:04:35 AM
Hopefully that solves both issues (esp. the second one).
I'll do that between tody and tomorrow and keep you posted.

Thanks so much!
Javi

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: javibigcity on June 07, 2018, 06:16:51 AM
Hopefully that solves both issues (esp. the second one).
I'll do that between tody and tomorrow and keep you posted.

Thanks so much!
Javi



It definitely solved the issue. There was a bad solder point in one of the pins of the HPF connector to the main board.
Thanks a lot : ]
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 07, 2018, 09:35:41 AM
It definitely solved the issue. There was a bad solder point in one of the pins of the HPF connector to the main board.
Thanks a lot : ]
Excellent! Glad you got it solved. Now back to making music... ;)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: PerspectiveSound on June 13, 2018, 12:27:14 AM
Hello, I had an issue before with a vp28 burning a resistor. Well, I built three more channels and ordered 2 pre-built opamps and traced the problem to a faulty opamp. All four channels pass audio however the third one I built now has an issue very similar to the one in this thread
 
Thanks Jeff,

Here are my measurements on the output transformer leads...

Black to Red, 15.7Ω
Pink to Gray, 32.8Ω
Violet to Orange 31.1Ω

I went ahead and measured other colors together after you mentioning there should be no measurable resistance between the other colors. I am getting resistance between these other leads..

Grey and Orange 33.3
Grey and Violet 63.7
Pink and Violet 31.1



 one channel is about 8 dB below unity gain. Checked possible bad solder joints, tested with working opamps and different rack slots, with 1.229 kV input the test points appear very close to the test guide except the final output is low on DMM, .296kV

I'm a bit unclear which of the transformers T2 or T3  to measure, I seem to be detecting resistance between these same leads on both of them, could they both be faulty? I should say the three working channels sound amazing and can pass a stereo image at unity gain and all the transformers were packaged together. Thanks for any help y'all could give me
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 13, 2018, 10:00:53 AM
I'm a bit unclear which of the transformers T2 or T3  to measure, I seem to be detecting resistance between these same leads on both of them, could they both be faulty? I should say the three working channels sound amazing and can pass a stereo image at unity gain and all the transformers were packaged together. Thanks for any help y'all could give me
Faulty output transformers are very rare and unlikely but sh!t does happen because of humans. If your TP voltages read good until the final output, I would look into T3. Make sure all T3 leads are desoldered from the PCB and none are touching each other. You should only getting resistance from end to end of the following windings.

Black to Red, appx 16Ω
Pink to Gray, appx 32Ω
Violet to Orange appx 32Ω

Check from black to all other colors besides red and make sure there is no DCR. Do this for ALL leads. Report your findings.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: PerspectiveSound on June 13, 2018, 08:28:46 PM
Thanks Jeff, I checked the resistances on T3 and got close to the proper values for black to red - 16.3 ohm, gray to pink - 33.1, violet to orange - 31.7, however I seemed to get very high resistances between most other combinations for example 6 MOhm between black - gray, 8.5 MOhm black -pink, 5.0 MOhm gray - violet. I feel like I could be making a mistake however the proper resistances consistently show up as well and the meter reads zero at rest.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 13, 2018, 08:39:24 PM
Thanks Jeff, I checked the resistances on T3 and got close to the proper values for black to red - 16.3 ohm, gray to pink - 33.1, violet to orange - 31.7, however I seemed to get very high resistances between most other combinations for example 6 MOhm between black - gray, 8.5 MOhm black -pink, 5.0 MOhm gray - violet. I feel like I could be making a mistake however the proper resistances consistently show up as well and the meter reads zero at rest.
OK then, this transformer is likely fine.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Homestudio on June 18, 2018, 04:18:37 AM
Hi all, I built a couple of gar 2550s and plugged them into my good working vp28. burned out PR1 and PR3 on the board. I'll replace all four resistors. Is there anything else I sould check on the vp28 before fireing back up? I'd like to test the op amp sockets before I install my tested gar 2550s but need the test procedure. Can a man get some help? Thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Homestudio on June 18, 2018, 04:59:30 AM
Pardon me guys I meant the g a r 2520 s
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 18, 2018, 10:44:59 AM
Hi all, I built a couple of gar 2550s and plugged them into my good working vp28. burned out PR1 and PR3 on the board. I'll replace all four resistors. Is there anything else I sould check on the vp28 before fireing back up? I'd like to test the op amp sockets before I install my tested gar 2550s but need the test procedure. Can a man get some help? Thanks
On the VP28 just replace the four 10Ω R's. The testing is pretty self explanatory if you have looked thru the VP2x build guide. Make sure you have the proper DC voltages at the V sockets and make sure the signal sockets are not directly shorted to ground.

Your problem will be on the gar2520 builds. You should visit that specific support thread. I will quickly say that the biggest problem is mixing up and/or turning the larger BD transistors. Wrong facing diodes is the 2nd most common issue.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Homestudio on June 18, 2018, 01:25:50 PM
Thanks Jeff for the quick response. The only things I see on the gar 2520 are some bridges on the pens to other points that I thought were ok. I'll clean those up and see what happens. Everything else looks good. Thanks, Patrick
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 18, 2018, 01:59:10 PM
Thanks Jeff for the quick response. The only things I see on the gar 2520 are some bridges on the pens to other points that I thought were ok. I'll clean those up and see what happens. Everything else looks good. Thanks, Patrick
Well, if the 10Ω damping R's smoked there is definitely something wrong on the opamp builds. Did you double check the larger BD transistors? Posting on the gar2520 build thread is best off from here.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Homestudio on June 18, 2018, 02:05:55 PM
Yeah the builds look good except for the bridges, everything's in the right position nothing's wrong there. The only resistors I could find local are 10 ohm .25watt  2% (instead of 5%). Will these be alright in the vp28? Thanks again,Patrick
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on June 18, 2018, 03:19:49 PM
...The only resistors I could find local are 10 ohm .25watt  2% (instead of 5%). Will these be alright in the vp28?
Yes, perfectly fine.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Homestudio on June 18, 2018, 04:13:27 PM
Thanks again Jeff. Awesome support!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on July 10, 2018, 02:32:40 AM
A fun thing I just found out about my VP28's is what happens when I really push it with program material. On a rock mix where the peaks are -3dBFS and RMS is around -12dBFS, I inserted my VP28's with Red Dots and BA512s and turned the input gain to just before two clicks to fully open and turned down the output fader to -16. The result is this almost finished mastered sound with everything moving forward in the sound field and increased perceived loudness. The peaks were now hitting around -6dB but the RMS was at -9dB, which would explain the volume feeling as it was greater than before. I could faintly hear the low end start to break up, but without any static sounding artifacts like the preamp was giving up. Maybe the kids would call what I'm hearing as wooly. Whatever it may be in the subjective audio dictionary, it seemed to immediately put me really close to what sounded like something ready to be released.

For mastering projects I incorporate two different sounding compressed mixes parallel to the original and then limit for overs, but I think for the next project I'm going have to incorporate this signal chain somewhere in the process.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: WillDuc on July 30, 2018, 12:04:29 PM
Hi guys,  just want to make sure my unit is working as it should. I noticed that in line mode (mic button depressed) the sound changes quite a bit. I would describe it as hi fi, almost to the point of harshness. After running a test tone i noticed that while in line mode, i don't get any harmonics even if i drive the input quite hard. In mic mode however, i get plenty of them (see attachments). Reading from the forum it is not something that this preamp is known for (pronounced highs). Is the unit working as it should ?


Regards,
William
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 30, 2018, 12:07:09 PM
Hi guys,  just want to make sure my unit is working as it should. I noticed that in line mode (mic button depressed) the sound changes quite a bit. I would describe it as hi fi, almost to the point of harshness. After running a test tone i noticed that while in line mode, i don't get any harmonics even if i drive the input quite hard. In mic mode however, i get plenty of them (see attachments). Reading from the forum it is not something that this preamp is known for (pronounced highs). Is the unit working as it should ?
There is nothing more than a u-pad before the input transformer for line mode. Basically only the level should change going in, nothing else.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: WillDuc on July 30, 2018, 12:31:51 PM
Ok so there's definitely something going on with the unit. Can you suggest a starting point ? Debugging without a schematic is tough.

Thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 30, 2018, 12:41:10 PM
Ok so there's definitely something going on with the unit. Can you suggest a starting point ? Debugging without a schematic is tough.

Thanks
We have a test points guide for this. Ruling out the opamps is always the first step.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: WillDuc on July 31, 2018, 12:58:35 AM
Thanks jeff for your quick reply. Here are the results i got.

TP1=101mV
TP2=698mV
TP3=1,241V
TP4=305mV
TP5=305mV
TP6=305mV
TP7=642mV
XLR output = 1,233V

Judging from the numbers i guess everything is working fine. As it's my first colored preamp, i guess it's the sound of a cranked preamp that i am not used to hear. That being said, i still find it weird that in line mode i don't see the harmonic content i see in mic mode when i feed a test tone.

Regards
Guillaume
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on July 31, 2018, 09:54:24 AM
...As it's my first colored preamp, i guess it's the sound of a cranked preamp that i am not used to hear. That being said, i still find it weird that in line mode i don't see the harmonic content i see in mic mode when i feed a test tone.
Not sure what type of levels you are sending it but hit the line input with appx 40dB more level than when in mic mode. We run 0dB in to measure line mode things and -40dB for mic mode. What level are you feeding it in mic mode?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: WillDuc on July 31, 2018, 10:40:12 AM
Not sure what scale you are referring to when you say 0dB. I read approx 1.23V on the input xlr using a DAW as a tone gen. Now using this level in line mode i can't get the preamp to distort and add harmonics like in mic mode (i obviously turn the level down when using the mic mode, can't tell how much in dB though) . In line mode, I push the signal with the channel fader and/or the preamp gain and when i start to hear the tone distorts it's when my converters start to clip. Maybe it's me and the line mode is cleaner than the mic mode? Keep in mind it's my first experience with your products.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: aka-pablo on August 02, 2018, 01:09:40 PM
Hello groupdiyers - I am new to this forum - I have just built 2 CAPI VP28 preamp mounting 2 capi gar2520 each. I built the 2 units at the same time using a "carbon copy" approach, if I can say that. When testing them I get 2 sort of issues:

1) one of the unit works and I get signal, while the other one does not get any signal (the leds light up though). Basically one of the gar2520 (the Fader Booster) warmed up to the point that i can smell the warming plastic and I had to remove the unit.

2) When I test the other unit, the one that works, I get a distorted signal as an output. I have double checked if there is any bridge between capacitors and parts but everything seems to be relatively clean.

Is there a way to isolate the problem? I have seen the document that shows how to test teh unit using the test points however at the moment I haven't got a signal generator. Do I necessarily need a signal generator to test the unit?

Thanks in advance
P
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 02, 2018, 02:58:07 PM
Testing a newly built preamp with newly built opamps is near impossible. You really need 2 known to be good working opamps. Short of that, we are just stabbing in the dark if there are problems at startup.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: csweet08 on August 07, 2018, 06:24:23 PM
Hello, I've noticed some slight browning of a single resistor on a couple of my VP28's and my FC526s. I'm told this resistor protects against ground faults or power problems but I'm not sure of this. I haven't noticed any extra noise on these units vs others without the browning. Anyone know the possible cause and whether its cause for concern? My best theory is it could be from a few hot swaps I accidentally did. I attached a couple  photo's here https://groupdiy.com/index.php?topic=69678.new;topicseen#new
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: WillDuc on August 14, 2018, 11:50:39 PM
Hey ya’ll,

Building a pair of VP28s and I’m to the output transformers. The leads on my EA2623’s don’t match, however, the colors on the PCB. Can anyone help me get these in the right place? My leads are blck/red/grey/pink/purple/orange...

Thanks! Sorry if this has already been addressed.

Ryan


There you go Ryan :

http://www.capi-gear.com/catalog/images/gallery/2623/2623-1-L-specs.pdf

Judging from the colors you mentionned I guess you have Litz transformers in your unit. You can find the conversion table on the lower left.

Cheers
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Studio Mollan on August 20, 2018, 10:17:13 AM
Hi,
I would like to make a line in patch point to the VP28. Planning on using the spare card edge connectors 9 can 11 for this. I already have this wired to an XLR in my Fredenstein 500 rack.  Would be easy if i got the schematics but i know and understand you won't make them public Jeff.  So if you could please pinot me in the direction on where to inject it on the PCB.

Best possible scenario would also be to make this input 48v free. Probably via a pair of capacitors. If i could get a value on these too would be great.
Thanks! Great kit this!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 20, 2018, 09:35:04 PM
Hi,
I would like to make a line in patch point to the VP28. Planning on using the spare card edge connectors 9 can 11 for this. I already have this wired to an XLR in my Fredenstein 500 rack.  Would be easy if i got the schematics but i know and understand you won't make them public Jeff.  So if you could please pinot me in the direction on where to inject it on the PCB.

Best possible scenario would also be to make this input 48v free. Probably via a pair of capacitors. If i could get a value on these too would be great.
Thanks! Great kit this!
Mic or Line shares the same input path which is the typical card edge pins 8 and 10. I would make this selectable somewhere outside of the rack. Also just make sure phantom on the pre is off. If you try to connect 2 sources without switching one of them out of the path, the 2 devices outputs would drive each other. I'm not sure I see the need or want to support something like this.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jkmang on August 22, 2018, 01:34:15 PM
Just checking something real quick. Organizing BOM before building 4 units. 

C3 - listed 33uf 25V on the BOM. I have all 4 33uf 63V.  I'm sure this is fine. Just checking.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 22, 2018, 07:42:44 PM
Just checking something real quick. Organizing BOM before building 4 units. 

C3 - listed 33uf 25V on the BOM. I have all 4 33uf 63V.  I'm sure this is fine. Just checking.
Yes this is correct. BC/Vishay discontinued the previous 25V part so the new 63V part is all that's available.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: brewhouse75 on August 29, 2018, 10:50:47 AM
Just finished my first build of (2) VP28s. Not sure why, but on one of the units the polarity switch must be engaged to get any decent amount of gain. When disengaged the signal is very weak and lacks any low end. No idea how to track down what component is to blame without a schematic. Second unit, which I built at the exact same time, works just fine.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on August 29, 2018, 11:34:27 AM
Just finished my first build of (2) VP28s. Not sure why, but on one of the units the polarity switch must be engaged to get any decent amount of gain. When disengaged the signal is very weak and lacks any low end. No idea how to track down what component is to blame without a schematic. Second unit, which I built at the exact same time, works just fine.
Check the DCR of the leads of the first output T2. Check for open windings and/or shorts. This has been posted about above, but not sure how far back.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: thisisntjohnv on September 05, 2018, 04:13:51 AM
In need of some help.

I loaded my newly built VP28 preamp into my 500 series rack today. It's passing signal but very low and only when the mic switch is engaged. The channel fader gain is working but the preamp gain is not affecting the signal.

I took the casing off and desoldered the pre gain resistors and check the opamp connections but the pre amp gain is still not working and the signal is low.

Can anyone point me in the right direction or help with this?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 05, 2018, 09:59:49 AM
In need of some help.

I loaded my newly built VP28 preamp into my 500 series rack today. It's passing signal but very low and only when the mic switch is engaged. The channel fader gain is working but the preamp gain is not affecting the signal.

I took the casing off and desoldered the pre gain resistors and check the opamp connections but the pre amp gain is still not working and the signal is low.

Can anyone point me in the right direction or help with this?
You never told us what opamps you are using?

It could be many things. With DIY the biggest trouble is typically bad solder joints or misplaced resistors.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jbmacdo3 on September 05, 2018, 02:05:43 PM
I took a look through this thread and could not find any guidance on this.   I very well could have missed it, so if I did I apologize in advance.

I tried to salvage a build for a buddy of mine and am having some issues getting the unit to output volume.  The unit has a tested GAR2520 in the preamp slot with a GAR1731 in the Fader slot, Litz transformers.

I hooked it up to my signal generator per the AC spec in the test point guide and am getting proper readings except for: 0.003VAC at TP6 and TP7 with 0.05VAC across the output.    Any help would be greatly appreciated to try to narrow down where I should be looking for the issue(s)

Thanks,

Jeff Macdonald
Is there some component or at least area of the PCB I should be focused on to try to hunt down the problem?   
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: thisisntjohnv on September 05, 2018, 02:12:57 PM
You never told us what opamps you are using?

It could be many things. With DIY the biggest trouble is typically bad solder joints or misplaced resistors.

I’m using two  pre built red dot opamps and Litz transformers.

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 05, 2018, 09:53:50 PM
I took a look through this thread and could not find any guidance on this.   I very well could have missed it, so if I did I apologize in advance.

I tried to salvage a build for a buddy of mine and am having some issues getting the unit to output volume.  The unit has a tested GAR2520 in the preamp slot with a GAR1731 in the Fader slot, Litz transformers.

I hooked it up to my signal generator per the AC spec in the test point guide and am getting proper readings except for: 0.003VAC at TP6 and TP7 with 0.05VAC across the output.    Any help would be greatly appreciated to try to narrow down where I should be looking for the issue(s)

Thanks,

Jeff Macdonald
Is there some component or at least area of the PCB I should be focused on to try to hunt down the problem?
Dbl check the wiring for the T3 output transformer. If that is correct, desolder the T3 leads completely and see if TP6 comes up to normal.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 05, 2018, 09:55:03 PM
I’m using two  pre built red dot opamps and Litz transformers.
You need to follow the steps on the Rev B Test Points guide and post the results here so we can narrow down your problem. Same as the dude just before you did.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: thisisntjohnv on September 05, 2018, 10:18:47 PM
You need to follow the steps on the Rev B Test Points guide and post the results here so we can narrow down your problem. Same as the dude just before you did.

If I can reach the test points when the unit is in my 500 series rack is it safe to carry out without the jig? Or should I order this for testing: http://capi-gear.com/catalog/product_info.php?products_id=406

JV
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 05, 2018, 10:30:09 PM
If I can reach the test points when the unit is in my 500 series rack is it safe to carry out without the jig? Or should I order this for testing: http://capi-gear.com/catalog/product_info.php?products_id=406

JV
No problem without a test jig if you can safely and accurately get to them.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: thisisntjohnv on September 06, 2018, 05:11:32 AM
No problem without a test jig if you can safely and accurately get to them.

I went to test my unit. I was only able to get a reading of 1.2V AC (connecting the red probe to pin 2 and black probe to pin 3) when the mic button was engaged. Even with the sine wave signal generator cranked in my DAW there was not enough signal when the mic button wasn't pressed in.

Should I still carry out the test on all the test points? Also, how do I get the reading for TP1, TP2 etc. I'm just a little confused of where to put the common and red when testing for each point. I'm using my 6 space 500 series chassis while testing with no other units so I'm able to reach the test points.

Since I hit a wall with testing I went back and cleaned the board, removed excess flux and reflowed parts of the board that looked like they could be causing the problem. Still no luck getting the Preamp gain switch to affect the signal. When I engage the mic button I can get about 24db of signal max. When I disengage the signal drops to where it barely hits my meters on my symphony mk2.


JV
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jbmacdo3 on September 06, 2018, 05:38:12 PM
Dbl check the wiring for the T3 output transformer. If that is correct, desolder the T3 leads completely and see if TP6 comes up to normal.

Hey Jeff,

Thanks for the quick reply!  I took a look at the wiring for T3 and it was correct so I desoldered the leads and TP6 is still not reading correctly, it is still showing 0.005VAC.

Thanks,

Jeff
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 06, 2018, 08:57:23 PM
Hey Jeff,

Thanks for the quick reply!  I took a look at the wiring for T3 and it was correct so I desoldered the leads and TP6 is still not reading correctly, it is still showing 0.005VAC.

Thanks,

Jeff
TP6 is post the fader booster amp so I would look at that.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: thisisntjohnv on September 07, 2018, 03:26:13 AM
Can anyone direct me or guide me through testing my VP28 test points? I set up my DAW to send a 400hz sine wave at +4db and I got my multimeter to read 1.2V AC when  connecting DMM red probe to pin 2 and black probe to pin 3, but Im not sure how to carry out the rest of the test. I can only get enough signal to  coming through to set up and carry out the test when the mic button is engaged.

I'm lost here. Could use some help on the proper way to test. I'm referring to the CAPI VP28 Rev B Test Points PDF.

JV
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 07, 2018, 09:45:31 AM
I can only get enough signal to  coming through to set up and carry out the test when the mic button is engaged.
If you follow the steps in the Test Point guide, all measurements shown are done in Line mode, not Mic mode. What is your reading at TP1 when following the proper procedure?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on September 08, 2018, 05:36:04 AM
Hi,
built several VP28 but it's the first time I am encountering this issues. On two VP28 the volume is higher than regular ones.
These are the measurements I have done with two Red Dots as opamps.

TP1: 112.5mV {1} 112.3mV {2}
TP2: 1.42V {1} 1.42V {2}
TP3: 2.60V {1} 2.50V {2}
TP4: 1.25V {1} 620mV {2}
TP5: 1.25V {1} 620mV {2}
TP6: 1.25V {1} 620mV {2}
TP7: 2.60V {1} 1.24V {2}
OUT:  2.60V {1} 1.28V {2}

Out has been measured with line in at unity gain and fader at 0.
Where could I begin troubleshooting?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 09, 2018, 12:34:08 PM
Hi,
built several VP28 but it's the first time I am encountering this issues. On two VP28 the volume is higher than regular ones.
These are the measurements I have done with two Red Dots as opamps.

TP1: 112.5mV {1} 112.3mV {2}
TP2: 1.42V {1} 1.42V {2}
TP3: 2.60V {1} 2.50V {2}
TP4: 1.25V {1} 620mV {2}
TP5: 1.25V {1} 620mV {2}
TP6: 1.25V {1} 620mV {2}
TP7: 2.60V {1} 1.24V {2}
OUT:  2.60V {1} 1.28V {2}

Out has been measured with line in at unity gain and fader at 0.
Where could I begin troubleshooting?
My guess would be you have misplaced resistors around the A1 opamp. Specifically R5-R8.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on September 09, 2018, 03:45:19 PM
My guess would be you have misplaced resistors around the A1 opamp. Specifically R5-R8.
Nope. R5 to R8 were good. I misplaced wrongly RF12 (it was an error of the small plastic bags specific of 24.9R values, probably one wrong resistor went inside the  bag and I didn't checked it).
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 09, 2018, 04:49:48 PM
Nope. R5 to R8 were good. I misplaced wrongly RF12 (it was an error of the small plastic bags specific of 24.9R values, probably one wrong resistor went inside the  bag and I didn't checked it).
We have a very specific way of sorting the 11 and 12 pc resistor bags so not sure how we could screw that up...especially the way your bulk orders were packed...but sh!t happens.  :o
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: frenkonio on September 10, 2018, 01:23:27 AM
We have a very specific way of sorting the 11 and 12 pc resistor bags so not sure how we could screw that up...especially the way your bulk orders were packed...but sh!t happens.  :o
Anything that comes to a good end... is good 😅 don't worry about that!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: radtunez on September 10, 2018, 08:17:36 AM
Hey,
I've got a problem with one of my Gar2520s in a VP28. The thing started giving off a wicked smell the first time I put it in my lunchbox, and it seemed to be coming from the op-amp. I tested that same op amp in my other VP28 that has been working perfectly, and same thing happened. This time around, however, I noticed the R9 and R13  56R resistors had been totally fried black :/ And may have singed the neighboring caps.  I double checked all of my resistor placement, transistor placement, solder bridges, and I'm not coming up with any reason.

Any ideas? Thanks in advance!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 10, 2018, 10:46:30 AM
Hey,
I've got a problem with one of my Gar2520s in a VP28. The thing started giving off a wicked smell the first time I put it in my lunchbox, and it seemed to be coming from the op-amp. I tested that same op amp in my other VP28 that has been working perfectly, and same thing happened. This time around, however, I noticed the R9 and R13  56R resistors had been totally fried black :/ And may have singed the neighboring caps.  I double checked all of my resistor placement, transistor placement, solder bridges, and I'm not coming up with any reason.

Any ideas? Thanks in advance!
Sounds like this should be posted on the gar2520 support thread.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: thisisntjohnv on September 11, 2018, 03:34:19 AM
If you follow the steps in the Test Point guide, all measurements shown are done in Line mode, not Mic mode. What is your reading at TP1 when following the proper procedure?

Just wanted to follow up on this. I upgraded my multimeter to get a more accurate reading. Here are my test results:

TP1: 110mv
TP2: 194mv
TP3: 194mv
TP4: 47mv
TP5: 47mv
TP6: 47mv
TP7: 47mv
Output: 35mv

Just so you guys don't have to read through the thread: my preamp gain switch isn't affecting the signal and I'm getting an extremely low level. I have litz transformers and I also have two pre built red dot amps. I went back and checked for cold solders and reflowed anything that looked like it might be an issue. Above are the current readings of my preamp.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 11, 2018, 09:47:26 AM
Just wanted to follow up on this. I upgraded my multimeter to get a more accurate reading. Here are my test results:

TP1: 110mv
TP2: 194mv
TP3: 194mv
TP4: 47mv
TP5: 47mv
TP6: 47mv
TP7: 47mv
Output: 35mv

Just so you guys don't have to read through the thread: my preamp gain switch isn't affecting the signal and I'm getting an extremely low level. I have litz transformers and I also have two pre built red dot amps. I went back and checked for cold solders and reflowed anything that looked like it might be an issue. Above are the current readings of my preamp.
Its very hard to tell in the pic but R8 does not look like a 20k?? Your problem starts with TP2 which indicates A1 or the support circuitry around it. That would be R5-R8 as well as R1, C1 and then the gain-switch R's.

Just to rule things out, I would swap the red dot positions and see if anything changes. I have never received a bad opamp from Scott though so I doubt this will change anything.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jbmacdo3 on September 12, 2018, 07:48:49 AM
TP6 is post the fader booster amp so I would look at that.

I just wanted to let you know I got the issue traced and sorted.  Thanks for the help!  Funny enough it was the 6/12db switch that was somehow bridged to ground. The output of the HPF board was getting continuity to ground and not passing audio.  Once I got rid of that the VAC readings went back to normal and I get to keep my hair and sanity  :)

Thanks for the great kits and amazing support. 

Cheers,

Jeff
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: CaseyI on September 12, 2018, 03:40:23 PM
I've just completed my VP28 and it has some issues.  The Signal LED is lit when my 500 rack is powered OFF, and it turns off when I power the rack on.  I can't do any audio tests as I've also managed to fry R14 and R15 on my Gar1731.  Any help is appreciated.

Thank you,
Casey
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on September 13, 2018, 12:04:59 PM
I've just completed my VP28 and it has some issues.  The Signal LED is lit when my 500 rack is powered OFF, and it turns off when I power the rack on.  I can't do any audio tests as I've also managed to fry R14 and R15 on my Gar1731.  Any help is appreciated.

Thank you,
Casey
I honestly have no idea how this can be happening. It makes zero sense to me.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: CaseyI on September 16, 2018, 08:38:21 PM
So I figured out the issue with my 1731 which was I transposed the Q7 and Q8 which led to frying R14 and R15.  Now that I've got that sorted out I've discovered that I've also fried PR1 and PR3 and the board itself.  I don't have spare resistors on hand but at least I have some direction on getting this thing up and running.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Homestudio on October 06, 2018, 07:12:05 PM
Hey Jeff, I've been tracking with the vp28 for a while now and it's been sounding great, I mean great. But last night I was tracking and halfway through the volume just cut completely almost to nothing. Fired it up this afternoon and again the same thing happened it sounded great and then completely cut out after about 10to 15 minutes. Any ideas?
Thanks
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: dmp on October 08, 2018, 10:23:55 AM
Hey Jeff, I've been tracking with the vp28 for a while now and it's been sounding great, I mean great. But last night I was tracking and halfway through the volume just cut completely almost to nothing. Fired it up this afternoon and again the same thing happened it sounded great and then completely cut out after about 10to 15 minutes. Any ideas?
Thanks
Sounds like something in heating up and failing. Check that the power supply is not the problem. Thermal fuses sometimes kick off.
If it is a problem on the vp28, after it cuts out examine it for any hot components.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mole-man on October 26, 2018, 06:43:28 PM
Hello gang,

Proud owner of a self built vp26 and vp28. The vp26 works as it should but the vp28 has issues with its phantom power. It works fine with dynamic mics but does not work with capacitor mics. It outputs no signal with P48 but when I engage the pad I get a hum and a highly distorted low level signal (the pad works fine with dynamic mics). I've gone over most of the joints and the two 6k8 resistors are in the correct place. I've measured the inputs pins 1 to 2 and 1 to 3 and I'm getting 47.5v on both.. Which is confusing for me. Is this a grounding issue? I'm very confused 😕.
Any light you guys could shed on this would be most appreciated.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on October 26, 2018, 06:57:40 PM
Pins 2 and 3 on the card are output. You should not be getting any phantom power on those. If you refer to any API 500 series pinout chart you will see that pin 8 and pin 10 are your inputs, which is where you should be seeing phantom power. This is also shown on the schematic for VP25, 26, and 312. Jeff doesn't have a schematic posted for the VP28, but the phantom power scheme should be the same or close to it. What you are describing sounds like there is little to no VDC being sent to the condenser mic or that the mic being tested is faulty. Double check for phantom on pins 8 and 10 and then go from there.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on October 26, 2018, 07:27:08 PM
Hey Jeff, I've been tracking with the vp28 for a while now and it's been sounding great, I mean great. But last night I was tracking and halfway through the volume just cut completely almost to nothing. Fired it up this afternoon and again the same thing happened it sounded great and then completely cut out after about 10to 15 minutes. Any ideas?
Thanks
Classic symptoms of a cold solder joint. The most common issue on DIY builds.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mole-man on October 27, 2018, 06:34:57 AM
Pins 2 and 3 on the card are output. You should not be getting any phantom power on those. If you refer to any API 500 series pinout chart you will see that pin 8 and pin 10 are your inputs, which is where you should be seeing phantom power. This is also shown on the schematic for VP25, 26, and 312. Jeff doesn't have a schematic posted for the VP28, but the phantom power scheme should be the same or close to it. What you are describing sounds like there is little to no VDC being sent to the condenser mic or that the mic being tested is faulty. Double check for phantom on pins 8 and 10 and then go from there.

Thanks!

Paul

Thank you for the reply.

 I meant pins 1&3 + 1&2 of the xlr when connected to the input! I can see how that was confusing..

If the mic is seeing enough juice I don't understand what's happening further down the circuit..
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on October 27, 2018, 11:48:12 AM
Thank you for the reply.

 I meant pins 1&3 + 1&2 of the xlr when connected to the input! I can see how that was confusing..

If the mic is seeing enough juice I don't understand what's happening further down the circuit..

Do you still have +48V on XLR Pin 2 and 3 in relation to pin 1 (card pins Pin 8 and 10) with the microphone connected? This check will require the test jig. If the preamp is working fine as you say with a dynamic mic, then the issue you are describing has to be either phantom power getting to the condenser or the mic itself. Switching on +48V doesn't change the signal path of the preamp, as you know, so I don't see how it can electronically can be anything else unless you have solder bridge somewhere that is bleeding 48VDC to the opamps affecting the voltage your DOAs are receiving. You'll have to trace down the path that phantom takes from the card to audio +/- input. If it's not there then it seems you are about to embark on the magically frustrating journey of quadruple checking components and reflowing any suspicious solder joints and then quadruple checking everything again until you find the problem which turns out to be something you were certain was not the problem.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mole-man on November 05, 2018, 01:22:44 PM
Thank you for your reply Paul.

I scrutinised the board many times over and compared to the picture on the capi website and all the components are in the right places. I noticed a tiny wisp of solder under C5 and since removing it the preamp is alot more stable with! I assume a tiny solder bridge...

The first couple of times I used it with capacitor mics it worked 85% of the time but would sometimes stop passing level again and only producing almost no output that's highly distorted.

I then discovered that engaging the pad would appear to pass audio normally when this problem arose.

The last 10+ times I've used it with capacitor mics there have been no problems. I'm concerned that it will act up again though.

I've tested both opamps in a vp26 and they seem to work fine.

The only place there could be a solder bridges is under the the switches. I'm assuming the problem is probably with the pad c&k switch but I don't know how to remove it without damaging the switch and probably the board as well.

Any thoughts?

Thanks in advance.

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on November 13, 2018, 08:19:41 PM
I've tested both opamps in a vp26 and they seem to work fine.

The only place there could be a solder bridges is under the the switches. I'm assuming the problem is probably with the pad c&k switch but I don't know how to remove it without damaging the switch and probably the board as well.
Extremely unlikely to have a solder bridge under one of the switches. Essentially impossible. If something is intermittent, its likely a bad solder joint somewhere on the build.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on December 19, 2018, 03:16:03 PM
So with a 400HZ test tone being injected at 1.228V, and all settings correct for the test I have the following results.

TP1 - 111.8
TP2 - .998
TP3 - 1.804
TP4 - .450
TP5 - .450
TP6 - .450
TP7 - .940
Output at pin 2 is 1.402

Im not sure why they are higher than the guide, but they look close. Are these acceptable values?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mylithra on December 22, 2018, 10:13:59 AM
Been forever since I posted here. Recently built a VP28 and a GAR2520 and a GAR1731. Took my time, examined every solder joint as I went and had zero trouble with it. Powered up and sounded gorgeous the first time. The Support Docs from the site coupled with the first revision build guide here, there were no surprises.  Took me about 8 hours beginning to end. (about 2.5/3 hours on the DOAs, and about 6 hours on the VP28s)

If I'm giving advice to anyone building these, take your time, double check everything prior to soldering and try to make every solder joint perfect.

Just as a note, this isnt my first CAPI kit, I built 4x 312s, 2x with DI, I think the 312 w/DI was harder to build than the VP28
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on December 23, 2018, 03:35:28 PM
So with a 400HZ test tone being injected at 1.228V, and all settings correct for the test I have the following results.

TP1 - 111.8
TP2 - .998
TP3 - 1.804
TP4 - .450
TP5 - .450
TP6 - .450
TP7 - .940
Output at pin 2 is 1.402

Im not sure why they are higher than the guide, but they look close. Are these acceptable values?
:bump:
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: JRoulat on December 30, 2018, 06:39:46 PM
Just chiming in here as a recent CAPI owner and Noob to the DIY world.

Built the 511 rack. All XLRS. It’s great.

Built a vp28 and watched it smoke and burn. That’s what happens when you mistakenly put PR 1-8 where R 1-8 are supposed to go.  Say nothing. I chastised myself enough.  Really stupid.  Likely due to hyper enthusiasm.

Just built my second VP28 and flawless startup.

Also bought the test jig.

My lessons learned:

Like my shop teacher said. Measure twice, cut once. Relax and enjoy the process. Very Zen to build one of these.

Know thine DMM.  I am getting better at mine and also organize your components. (See measure twice above).

Get a super small tip for your solder station to do the DOAs. A big tip is like trying to eat sushi with baseball bats.

Watch out for flying leads when clipping them off. One ended up in my sandwich. I kid you not.

Get a good soldering iron. It is your main tool. Seriously- temp controlled.

Yeah though I walk through the valley of the shadow of death I will have at my side a good solder sucker.

Anything else.....  not really, go slow learn and enjoy!

Jeff-BTW- sounds amazing!!!!  You will be seeing more orders. Rock On!!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mylithra on December 31, 2018, 08:22:59 AM

Yeah though I walk through the valley of the shadow of death I will have at my side a good solder sucker.
Anything else.....  not really, go slow learn and enjoy!


Get a Hakko 808 or FR-301. It will change your life.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: JRoulat on December 31, 2018, 12:18:31 PM
Many thanks for the recommendation.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 14, 2019, 11:50:34 AM
So with a 400HZ test tone being injected at 1.228V, and all settings correct for the test I have the following results.

TP1 - 111.8
TP2 - .998
TP3 - 1.804
TP4 - .450
TP5 - .450
TP6 - .450
TP7 - .940
Output at pin 2 is 1.402

Im not sure why they are higher than the guide, but they look close. Are these acceptable values?
:bump:
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 14, 2019, 02:39:04 PM
Have you confirmed that you have 1.228V AC when probing between pins 2 and 3 of your signal generator XLR? You cannot verify this level by referencing high or low to ground.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 15, 2019, 06:02:09 PM
Have you confirmed that you have 1.228V AC when probing between pins 2 and 3 of your signal generator XLR? You cannot verify this level by referencing high or low to ground.
Fluke 87 says 1.228 between pins 2 and 3
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 15, 2019, 09:51:16 PM
Fluke 87 says 1.228 between pins 2 and 3
OK. What opamps are you using? If they are built from kits, have you verified they are working properly in another know to be working properly module?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 15, 2019, 10:25:06 PM
OK. What opamps are you using? If they are built from kits, have you verified they are working properly in another know to be working properly module?
They are GAR 2520 OPAMPs built from kits. I did swap them out with a pair from another build and got the same results.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 19, 2019, 05:51:43 PM
They are GAR 2520 OPAMPs built from kits. I did swap them out with a pair from another build and got the same results.
Well, I replaced the opamps with a brand new pair of SL2520 and get the same results. Funny thing is, if I set the gain to 1 click below unity, the test numbers come up perfect. I’ve triple check component placement and all is correct. Maybe my signal coming from the daw isn’t exactly at 1,228v? Even though the meter says it is?
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 19, 2019, 10:11:16 PM
Well, I replaced the opamps with a brand new pair of SL2520 and get the same results. Funny thing is, if I set the gain to 1 click below unity, the test numbers come up perfect. I’ve triple check component placement and all is correct. Maybe my signal coming from the daw isn’t exactly at 1,228v? Even though the meter says it is?
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it

You should always meter what the signal is at the XLR going into the piece of gear being tested. Even though the software says it's sending out a certain level, that doesn't mean your converter doesn't have a little bit of loss before it leaves the output.

So everything meters correct when you make an adjustment to the gain? Does the gain keep tracking down/up when you make changes? If changing something at the switch corrects the issue, then that would tell me it's something at the switch. And you may have triple checked your component placement, but did you completely disassemble the module and have a long, loving gaze at your solder joints? What's on top looks neat and clean but it's usually what lies beneath that causes great irritation and distress. So go ahead do the thing that we all try to avoid and take it all apart and really, really look at those solder joints and check for bridges. You may have to use your meter and test for continuity between the switch pins (shorts) and/or follow traces between components and make sure they're actually connected (cold solder joint).

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 19, 2019, 10:25:26 PM
You should always meter what the signal is at the XLR going into the piece of gear being tested. Even though the software says it's sending out a certain level, that doesn't mean your converter doesn't have a little bit of loss before it leaves the output.

So everything meters correct when you make an adjustment to the gain? Does the gain keep tracking down/up when you make changes? If changing something at the switch corrects the issue, then that would tell me it's something at the switch. And you may have triple checked your component placement, but did you completely disassemble the module and have a long, loving gaze at your solder joints? What's on top looks neat and clean but it's usually what lies beneath that causes great irritation and distress. So go ahead do the thing that we all try to avoid and take it all apart and really, really look at those solder joints and check for bridges. You may have to use your meter and test for continuity between the switch pins (shorts) and/or follow traces between components and make sure they're actually connected (cold solder joint).

Thanks!

Paul
The bottom is also clean, Ill pull the module and post a photo. As stated in a previous post, the output was measured directly from the output of an Aurora 16 with a Fluke 87.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 19, 2019, 10:27:03 PM
The bottom is also clean, Ill pull the module and post a photo. As stated in a previous post, the output was measured directly from the output of an Aurora 16 with a Fluke 87.
I would also suggest that desoldering the entire PCB as a pragmatic excercise would make matters worse.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 19, 2019, 10:35:18 PM
Well, I replaced the opamps with a brand new pair of SL2520 and get the same results. Funny thing is, if I set the gain to 1 click below unity, the test numbers come up perfect. I’ve triple check component placement and all is correct. Maybe my signal coming from the daw isn’t exactly at 1,228v? Even though the meter says it is?
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it
I'm gonna go out on a limb here and ask if you are 100% sure you have the preamp gain knob properly aligned on the rotary switch? Lowest gain setting is knob to 7:00. Highest gain setting is knob at 6:00. The above quote tells me its not aligned correctly.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 19, 2019, 10:49:05 PM
I'm gonna go out on a limb here and ask if you are 100% sure you have the preamp gain knob properly aligned on the rotary switch? Lowest gain setting is knob to 7:00. Highest gain setting is knob at 6:00. The above quote tells me its not aligned correctly.

Beat me to it.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 19, 2019, 10:50:06 PM
I'm gonna go out on a limb here and ask if you are 100% sure you have the preamp gain knob properly aligned on the rotary switch? Lowest gain setting is knob to 7:00. Highest gain setting is knob at 6:00. The above quote tells me its not aligned correctly.
I will check to see if I have the knob is correctly aligned on the shaft tomorrow. Ill admit, this is something I hadnt considered. Although it has been my experience in the past that the simple things can  certainly slip past you. Ill let you know.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 19, 2019, 10:55:03 PM
The bottom is also clean, Ill pull the module and post a photo. As stated in a previous post, the output was measured directly from the output of an Aurora 16 with a Fluke 87.

My apologies, I did not that output of the converter was already verified. Also, I did not suggest desoldering the entire board. I would never wish such pain on a fellow DIY'er.

Let us know if Jeff's suggestions regarding knob alignment is correct.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 19, 2019, 11:03:24 PM
My apologies, I did not that output of the converter was already verified. Also, I did not suggest desoldering the entire board. I would never wish such pain on a fellow DIY'er.

Let us know if Jeff's suggestions regarding knob alignment is correct.

Thanks!

Paul

No worries , my friend. I appreciate your input.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Chrome Heart on January 20, 2019, 08:26:09 PM
No worries , my friend. I appreciate your input.
Yuuup, I get the "total noob move" award on this one. I had lowest gain setting keyed at 6:00. Dont know how I missed that one, but Im not surprised.
So now that I have a pair of GAR2520 and SL 2520, is there a way to pair one of each in a build that would make sense? At least I could save some money on another VP28 and have a matched pair.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on January 21, 2019, 12:44:07 AM
Yuuup, I get the "total noob move" award on this one. I had lowest gain setting keyed at 6:00. Dont know how I missed that one, but Im not surprised.
So now that I have a pair of GAR2520 and SL 2520, is there a way to pair one of each in a build that would make sense? At least I could save some money on another VP28 and have a matched pair.

Op amps are a preference thing in these types of builds. You kinda have to try them out see what you like. Some did an op amp comparison here on the forum a while back.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Mylithra on February 07, 2019, 06:40:52 PM
So now that I have a pair of GAR2520 and SL 2520, is there a way to pair one of each in a build that would make sense? At least I could save some money on another VP28 and have a matched pair.
Here is mine. I have SL-2520s in both sockets with a red dot in the pre and a blue dot in the fader.  I have the standard 2623 in the pre and the Litz in the fader.

I also tried the GAR 2520 w/ the GAR 1731 and like the sound of that as well.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: benplayskeys on February 13, 2019, 02:48:27 AM
Hey Guys,
I have an odd one.
I’ve built over 20 of these preamps and this is the first real issue I’ve had with one.

Here is the setup-
Pro Tools tone generator sending pink noise out into an RND R6 rack. VP28 with the issue is plugged into slot 1.

The main symptom is that the unit passes a very very small signal out (basically nothing at all) in line mode when first turned on. The green LED does not show signal and the pre has to be cranked as well as the output fader to hear any resemblance of sound. I discovered that if I engage the “Mic” switch and immediately disengage it, the Line level input is suddenly working and passing signal at a good level. The odd thing is that when I come back the next day to re-test the same issue is occurring. I have also left it on for somewhere in the ballpark of 15-20 minutes and it suddenly drops out. This could then be fixed by again, engaging and disengaging the “Mic” switch.

Here is the trouble shooting steps I’ve taken.
-re-flow all solder points
-replace the Toneluck switch in the Mic/Line path
-triple check resistors are in the correct position.
-I have placed known working Op Amps in both Fader and Pre positions (Gar as well as SL Red Dots)
-I have also used a known working unit in this chain and it has no issues

Thanks for your help in advance.
-Ben
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: brewhouse75 on March 05, 2019, 12:20:19 AM
One of my four VP28s has significantly quieter output than the others. Rechecked solder flows and everything looks good. Any suggestions as to where to go from here?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on March 05, 2019, 01:42:43 AM
Are these newly built or a new issue with currently working units? Are the knobs oriented correctly to the shaft position? Did you compare signal traces the signal between a working and a non-working one? Did you swap op amps? Are the transformers wired correctly? Component orientation/placement? Transformer winding DCR?

No to be too abrasive, but just saying you rechecked solder flows and not knowing where to go from there means you haven't read through the thread. There are multiple places where people have low output and it is one of the above mentioned issues, but it does require some reading and usage of the search feature at the top of the page. The guys who are putting out these awesome products are very helpful and fully support their designs and even are generous with their time to help sort out end-user issues, but this is still DIY and it does require you to read through the information that is readily available on each specific thread and throughout the forum.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on March 05, 2019, 02:11:12 AM
Hey Guys,
I have an odd one.
I’ve built over 20 of these preamps and this is the first real issue I’ve had with one.

Here is the setup-
Pro Tools tone generator sending pink noise out into an RND R6 rack. VP28 with the issue is plugged into slot 1.

The main symptom is that the unit passes a very very small signal out (basically nothing at all) in line mode when first turned on. The green LED does not show signal and the pre has to be cranked as well as the output fader to hear any resemblance of sound. I discovered that if I engage the “Mic” switch and immediately disengage it, the Line level input is suddenly working and passing signal at a good level. The odd thing is that when I come back the next day to re-test the same issue is occurring. I have also left it on for somewhere in the ballpark of 15-20 minutes and it suddenly drops out. This could then be fixed by again, engaging and disengaging the “Mic” switch.

Here is the trouble shooting steps I’ve taken.
-re-flow all solder points
-replace the Toneluck switch in the Mic/Line path
-triple check resistors are in the correct position.
-I have placed known working Op Amps in both Fader and Pre positions (Gar as well as SL Red Dots)
-I have also used a known working unit in this chain and it has no issues

Thanks for your help in advance.
-Ben

Have you swapped 500 series slots? Have you measured the VDC at the unit with this happens? If you had a second extender card you could run a signal trace between the two and see where it dies off and compare to the nonworking one. I would also check to make sure those caps behind the push button switch and fader are reading correctly. You can also pull the fader op amp, do the initial chassis power up (when you are having the issue), measure the signal at the IN+/- sockets, toggle the mic/line switch in and out, then measure the signal again at the fader op amp. If the signal is the same then the issue is somewhere downstream from there. It is also possible there is some sort of issue with Q1. From the description of the switching the mic/line setting and then it works sounds similar to latching issues with voltage regulators. Let us know.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on March 23, 2019, 07:53:11 PM
Built my VP28 today.  And it is not working.  :(

I believe it's probably my op amps (also built those).  Or one of them, anyway.  I've reached out to Scott for some red dots.  I've also ordered an extender from CAPI so I can take measurements when they get here.  The reason I believe it's my DOAs is because if they're inserted one way, I get a constant signal LED.  Switch them around and there's no signal led. 

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 23, 2019, 11:26:51 PM
Built my VP28 today.  And it is not working.  :(

I believe it's probably my op amps (also built those).  Or one of them, anyway.  I've reached out to Scott for some red dots.  I've also ordered an extender from CAPI so I can take measurements when they get here.  The reason I believe it's my DOAs is because if they're inserted one way, I get a constant signal LED.  Switch them around and there's no signal led. 

Brandon
Yeah, I would suggest using only known to be working opamps to test a new build.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on March 25, 2019, 09:28:42 AM
That makes sense.  And I've got two red dots that should be here Wednesday or so.  :)  I'll pop those in and see what happens.  I'm hopeful that fixes the issue and I can move forward.  I'll be back if not.  Haha!

Do you have a general troubleshooting guide, Jeff?  Specifically what voltages I should see where on the test points provided on the VP28?

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 25, 2019, 09:52:44 AM
That makes sense.  And I've got two red dots that should be here Wednesday or so.  :)  I'll pop those in and see what happens.  I'm hopeful that fixes the issue and I can move forward.  I'll be back if not.  Haha!

Do you have a general troubleshooting guide, Jeff?  Specifically what voltages I should see where on the test points provided on the VP28?

Brandon
There's a Test Points guide posted in the VP28 section of the Support Docs page at the store.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on March 25, 2019, 11:21:39 AM
Awesome.  I'll check it out.  Thanks, Jeff.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on March 25, 2019, 01:11:36 PM
Well, I'm quite embarrassed.  I have made some progress.  Decided to try a known good 990 op amp.  No dice.  Then I realized I didn't have the mic button pressed in.   :o ::) Completely forgot about that. 

So I now get clean, beautiful signal, but I have to crank the gain on input and output both to get the levels anywhere decent (which may explain why I didn't get ANYthing in line mode).

I'm a bit relieved I've not screwed things up as badly as I thought I had.  Still have something screwy with the output and the LEDs being really dim.  I'll hammer on those a bit more after I can check voltages.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: S-O on March 27, 2019, 08:11:18 PM
Hey there! Thanks for putting out a rad kit that is super easy to follow! I had very few troubles assembling! The only problems I had were my own fault and were pretty easy to fix!

I have made 2/4 VP28's I bought. Halfway there!

I have one odd issue that may never really be much of a problem in the long run but I would like to see if there is an easy solution. I searched a bit and didn't find anything that matched my problem exactly.

So When I have the VP28 in the CAPI 511-VPR Filtered rack with DB25 cables plugged into it and into my patch bay the green signal LED is on. As soon as patch something in the input or output all is well and it turns off! If I plug headphones in I can hear that something seems to be oscillating.

Maybe something is grounding out or solder bridged but I can't seem to find out where or how! Any input would be rad and appreciated!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 27, 2019, 08:43:29 PM
Hey there! Thanks for putting out a rad kit that is super easy to follow! I had very few troubles assembling! The only problems I had were my own fault and were pretty easy to fix!

I have made 2/4 VP28's I bought. Halfway there!

I have one odd issue that may never really be much of a problem in the long run but I would like to see if there is an easy solution. I searched a bit and didn't find anything that matched my problem exactly.

So When I have the VP28 in the CAPI 511-VPR Filtered rack with DB25 cables plugged into it and into my patch bay the green signal LED is on. As soon as patch something in the input or output all is well and it turns off! If I plug headphones in I can hear that something seems to be oscillating.

Maybe something is grounding out or solder bridged but I can't seem to find out where or how! Any input would be rad and appreciated!
What opamps are you using and who built them?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: S-O on March 28, 2019, 08:13:00 AM
What opamps are you using and who built them?

Thanks for the speedy reply!

Lil ol' me built them! Not my first DIY attempt, but one of the more complex projects I have done!

I am using your GAR2520 v4.1 opamps!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 28, 2019, 09:52:52 AM
Thanks for the speedy reply!

Lil ol' me built them! Not my first DIY attempt, but one of the more complex projects I have done!

I am using your GAR2520 v4.1 opamps!
That said, its quite possible that there is an error on one or more of the opamps so the green signal LED is doing its job due to maybe some noise or oscillations. I would test them with prebuilt or solid known to be working opamps.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: S-O on March 28, 2019, 10:20:15 AM
That said, its quite possible that there is an error on one or more of the opamps so the green signal LED is doing its job due to maybe some noise or oscillations. I would test them with prebuilt or solid known to be working opamps.

Will do and will report back when I do! Thanks!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on March 30, 2019, 12:35:05 PM
Ok, finally got everything put together and took some test measurements.

Here is what I have:

TP1:  110mv
TP2:  109mv
TP3:  192mv
TP4:  53mv
TP5:  53mv
TP6:  53mv
TP7:   54mv

Any thoughts on what I may have screwed up?  This is with Lieber red dot op amps, too, FWIW.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 31, 2019, 01:50:07 AM
Ok, finally got everything put together and took some test measurements.

Here is what I have:

TP1:  110mv
TP2:  109mv
TP3:  192mv
TP4:  53mv
TP5:  53mv
TP6:  53mv
TP7:   54mv

Any thoughts on what I may have screwed up?  This is with Lieber red dot op amps, too, FWIW.

Brandon
Try disconnecting the primary leads (PCB labels red and orange) from T2 and see if TP2 comes up to normal.

FWIW, to anyone who is troubleshooting the signal path...anything AFTER the first TP that does not measure right is not important until we get the FIRST place the signal is incorrect sorted.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on March 31, 2019, 02:16:10 AM
Thanks for the reply, Jeff.

This made TP1 379mv and TP2 374mv.  All others were 0.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 31, 2019, 01:08:27 PM
Thanks for the reply, Jeff.

This made TP1 379mv and TP2 374mv.  All others were 0.

Brandon
This should not have changed TP1 at all. Something is strange possibly with your input level?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 01, 2019, 12:08:49 AM
Well I tried again tonight.  Verified input was 1.22v and this time I got 111mv at TP1 and 109mv at TP2. 

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 01, 2019, 10:42:39 AM
Well I tried again tonight.  Verified input was 1.22v and this time I got 111mv at TP1 and 109mv at TP2. 

Brandon
OK so the issue is between TP1 and TP2.

This means you have a misplaced resistor from R5-R8 OR something is wrong with your gain switch resistor string. That is R1, C1 and then RG1-RG11.

Are you sure the preamp gain knob is orientated the proper way?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 01, 2019, 01:07:39 PM
Thank you for the reply, Jeff.

R1, C1, R5-R8 all look correct to me.  Some of them were soldered from the front and then again from the back (I used the wrong tip on the front during assembly, went back with a "chisel" tip to reflow the solder joints from the back after assembly), so they look not so great on top. 

I also went through RG1-RG11 and all of those are correct, too.  Which leaves me with the gain knob.  Is it possible to put the gain switch in backwards somehow? 

Here are pics of the mentioned sections:

https://www.dropbox.com/s/njob03rt28953hp/20190401_115508.jpg?dl=0

https://www.dropbox.com/s/wa2h9nj5r21vkym/20190401_115502.jpg?dl=0

https://www.dropbox.com/s/inxz6r1wfud2a9i/20190401_115458.jpg?dl=0

https://www.dropbox.com/s/az898f7ppdld69f/20190401_115454.jpg?dl=0

I'm at a bit of a loss. 

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 01, 2019, 02:08:15 PM
Thank you for the reply, Jeff.

R1, C1, R5-R8 all look correct to me.  Some of them were soldered from the front and then again from the back (I used the wrong tip on the front during assembly, went back with a "chisel" tip to reflow the solder joints from the back after assembly), so they look not so great on top. 

I also went through RG1-RG11 and all of those are correct, too.  Which leaves me with the gain knob.  Is it possible to put the gain switch in backwards somehow? 

Here are pics of the mentioned sections:

https://www.dropbox.com/s/njob03rt28953hp/20190401_115508.jpg?dl=0

https://www.dropbox.com/s/wa2h9nj5r21vkym/20190401_115502.jpg?dl=0

https://www.dropbox.com/s/inxz6r1wfud2a9i/20190401_115458.jpg?dl=0

https://www.dropbox.com/s/az898f7ppdld69f/20190401_115454.jpg?dl=0

I'm at a bit of a loss. 

Brandon
I have no idea how the bottom of the board looks but there are for sure some questionable solder joints on the top. All of those giant solder blobs must be cleaned up. I see at least 2 that look like they could be shorting to the top ground plane. Also, I would suggest cleaning up the residual flux. It can sometimes be conductive.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 02, 2019, 11:12:23 AM
Thanks, Jeff.  I'll go through them, clean them up, and try again.  :)

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 07, 2019, 03:54:53 PM
I have gone through and cleaned up all of the solder joints in the areas you pointed out are before TP2 and I am still getting 109mv at TP2.  I am at a bit of a loss here.  Any other ideas on things I can try?

https://www.dropbox.com/s/rfbauorn1grlqsw/20190407_151525.jpg?dl=0

https://www.dropbox.com/s/p12vewssq5hpqpj/20190407_151510.jpg?dl=0

Thank you for your assistance, Jeff.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 07, 2019, 08:46:05 PM
I have gone through and cleaned up all of the solder joints in the areas you pointed out are before TP2 and I am still getting 109mv at TP2.  I am at a bit of a loss here.  Any other ideas on things I can try?

https://www.dropbox.com/s/rfbauorn1grlqsw/20190407_151525.jpg?dl=0

https://www.dropbox.com/s/p12vewssq5hpqpj/20190407_151510.jpg?dl=0

Thank you for your assistance, Jeff.

Brandon
I can see in the pic that the R next to C5 is 200Ω and it should be 20k. That is your problem but also you need to find the missing 20k which I am guessing is in the spot of a 200Ω somewhere...
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 07, 2019, 09:22:56 PM
#facepalm  Thank you so much for taking a closer look.  I'll go back through everything again and see what I can find.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 07, 2019, 09:25:29 PM
#facepalm  Thank you so much for taking a closer look.  I'll go back through everything again and see what I can find.

Brandon
I did give you those part #'s to check previously.  ;)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 07, 2019, 10:44:55 PM
I did give you those part #'s to check previously.  ;)

 :-[ :-[ :-[ Indeed you did.   :) I really did check them, but clearly I didn't check that one close enough.  :) Fortunately I had transposed it with R1, so it was super quick finding where the 20k went.  I switched the resistors, put the transformer leads back in, and all is well!  :) Thank you so much for your help and for a great kit, Jeff.  If I still lived in Nashville, I'd buy you a beverage of your choice.  :)

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on April 12, 2019, 03:33:32 PM
The question probably has been asked before. Is the double-stick foam tap required for installing the input transformer? I don't see it in the kits. What is its purpose? I guess it helps holding the transform in place.

I have to say Chunger's tutorials are super helpful, without which I would never have the gut to take on the project. Thanks Chunger!

Here is what my progress so far after one evening's work (2-3 hours):
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 12, 2019, 04:49:30 PM
The question probably has been asked before. Is the double-stick foam tap required for installing the input transformer? I don't see it in the kits. What is its purpose? I guess it helps holding the transform in place....
The double sided foam tape is not nor has it ever been included. Its only a suggestion to elevate the bottom of the mu-metal can to keep if from shorting to the PCB. There is some more info here. http://capi-gear.com/catalog/Build_VP26-S_Rev_C.php#Sec5
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on April 12, 2019, 04:57:24 PM
Wow! Appreciate the fast response. I see the purpose now, and I am glad I asked. I will look at the VP26's build guild to assist finishing my 1st kit.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on April 12, 2019, 05:08:48 PM
One last question, is C3 33uF 60v or 25v? In the BOM it says 25v. The only 33uF capacity in my kit is 60v. Since 60v is greater than 25v so I think it should work. Not the other way round, right?

Thanks again,
Leon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Fuzz Face on April 12, 2019, 06:53:59 PM
One last question, is C3 33uF 60v or 25v? In the BOM it says 25v. The only 33uF capacity in my kit is 60v. Since 60v is greater than 25v so I think it should work. Not the other way round, right?

Thanks again,
Leon

No problem using a higher rated cap
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 12, 2019, 07:23:25 PM
One last question, is C3 33uF 60v or 25v? In the BOM it says 25v. The only 33uF capacity in my kit is 60v. Since 60v is greater than 25v so I think it should work. Not the other way round, right?
That is also covered in the VP25/VP26 Pictorial Build Guides. The old tan 25V parts were discontinued a few years ago by BC/Vishay.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on April 13, 2019, 09:57:29 AM
Thanks again for the help. I just finished the build yesterday evening and I gave it a test this morning. It worked and sounded amazing! This is almost too good to be true:) . The only mistake I made is applying one sticker to the wrong end of the knob. I had to peel it off and ended up with a not so nice looking sticker for the bigger knob.

Now it is time to build the second one.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on April 14, 2019, 08:17:26 PM
Just finished my second kit, also worked on first try. This time I made another silly small mistake near the end, I put the red button on the phase flip rather than phantom power. Is there an easy way to correct this? I deliberately used the grey buttons for HOF because I think it looks better that way.

Here is a picture of the newly built pair (next to a pair of BT50s someone else just built for me):
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 14, 2019, 09:32:14 PM
The caps can be removed unless they are super glued in place. You can use a tiny flat head screw driver to gently pry it off the switch. That's how I have done it in the past.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on April 14, 2019, 10:41:31 PM
Thanks Paul! Looks like I am not the only one making this mistake :). I tried your tips, but I am afraid that I was going to break it so I stopped. I installed the caps with the two openings slots facing other buttons which makes them hard to pry off. I guess I have to live with it now, until I can order some replacements.

Best,
Leon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 15, 2019, 03:02:32 AM
You would probably have to remove faceplate to do this. But if you don't have OCD then it won't matter.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: recarcar on April 15, 2019, 08:54:00 PM
Hello All,

I have been combing through the thread to see if anyone had similar issues but I can't seem to find anything.  My VP28 build does not seem to be passing any signal. The green LED does not light up and there seems to be nothing passing through. The other LEDs are working and everything seems to look fine but not sure where to start. Does anyone have any suggestions on where to look first? Thanks!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Potato Cakes on April 15, 2019, 11:12:35 PM
I have been combing through the thread to see if anyone had similar issues but I can't seem to find anything.  My VP28 build does not seem to be passing any signal.

I'm pretty sure that on this thread and most all the other build threads here on the GDIY forum there are several posts about builds not passing signal. Also, just saying it is not passing signal is not going to be useful in diagnosing the problem. Unfortunately you will have to go back through this thread a little more closely and read all the posts on this thread regarding op amps, solder joints, transformers, etc. There are also voltages posted for the test points on VP28 Support page on the CAPI website to aid in troubleshooting. I know it doesn't sound like fun, but trying to find errors in one's build rarely is.

Thanks!

Paul
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on April 16, 2019, 12:13:56 AM
Hello All,

I have been combing through the thread to see if anyone had similar issues but I can't seem to find anything.  My VP28 build does not seem to be passing any signal. The green LED does not light up and there seems to be nothing passing through. The other LEDs are working and everything seems to look fine but not sure where to start. Does anyone have any suggestions on where to look first? Thanks!
As always, please tell us first about the opamps. What are they and who built them?

Also, have you tried the Test Points doc? Where does your module stop showing the same results as what is posted in the TP doc?

As Paul mentioned, read back thru this thread and you will find a plethora of excellent info for you to at least start with.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: resilient on April 16, 2019, 09:13:24 AM
Did you also build your opamps?

You will want to get known working on amps to start with.

Brandon
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: noiro on April 19, 2019, 01:57:01 PM
Did I said that already CAPI freakin' ROCK 8) 8) 8) 8) 8) 8) 8) . Building new one today and yes having working op-amp to test with is a must, but also if you start building lots, it may be worthed to have the FiveFish test JIG or even better check this thread
https://groupdiy.com/index.php?topic=60558.0
Title: burnt PR1&PR3 after putting in a newly built DOA
Post by: Leon on May 29, 2019, 12:40:28 PM
My VP28 has been working great. Today I used it to test a newly built GAR2520. It did not passing signals, and I smelled something. Well looks like PR1 and PR3 got burnt (see pictures). Any idea what  could be wrong with the GAR2520? I did check the critical points mentioned in the guide botg virtually and using multi meter. There is no short.  Any help will be greatly appreciated!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on May 29, 2019, 12:42:09 PM
Here is a photo of the DOA
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on May 29, 2019, 12:43:23 PM
bottom side of the DOA:
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on May 29, 2019, 01:14:10 PM
Just saw it in the GAR2520 thread, I switched BD138, BD140, thinking they are the same. Making mistakes at the very end has become a habit to me . Too bad    :(
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: TillM on May 29, 2019, 01:36:21 PM
No worries,
This can happen.
Just change the burned resistors and swap the BD139 with the BD140 and everything will work.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Leon on May 29, 2019, 07:10:29 PM
No worries,
This can happen.
Just change the burned resistors and swap the BD139 with the BD140 and everything will work.
Yeah, just did that and now it works again. Thanks!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: lalalaetc on January 17, 2020, 09:01:58 AM
Hello,
I've had a few VP28's that have been working excellently for a couple of years now. I recently swapped some modules around in my rack, and now one receives, but won't pass signal.

Here are my TP measurements:

Tp1- 105mV
Tp2- 668mV
Tp3- 20mV (that's right, .020V)
Tp4- 0V
Tp5- 0V
Tp6- 0V
Tp7- 0V

These measurements seem to clearly demonstrate the area of the problem, but I have been unable to find a published schematic to effectively trace the circuit. Any thoughts?

Thank you in advance!
Wyatt

Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 17, 2020, 10:29:27 AM
Wyatt

If things were working fine and then they stopped...this is likely a cold solder joint. I have heard of this exact situation more times than I can count.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: lalalaetc on January 17, 2020, 11:00:05 AM
Thanks for getting back so quickly, Jeff.
I'll definitely have a close inspection on the pcb, and re-flow any questionable spots.
From the values reflected in my reading of the test points, does it seem logical that I could isolate this to a particular area of the circuit?

Thank you again for your support

Best,
Wyatt
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: lalalaetc on January 17, 2020, 04:10:43 PM
Welp... Traced between tp2 and tp3, touched up a few questionable joints, and I'm back up and running. Thanks again, and sorry to bog down the thread with such simplistic problem solving measures.

Best,
Wyatt
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Spiritworks on January 19, 2020, 05:27:11 AM
What's the purpose of the small washer placed between the transformers and the PCB? I forgot to install it on two builds. Do I need to take them apart to place the washer?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 20, 2020, 01:25:17 PM
What's the purpose of the small washer placed between the transformers and the PCB? I forgot to install it on two builds. Do I need to take them apart to place the washer?
The transformer lams are connected to the chassis ground. The top copper layer is 0V audio common. The washers just help to make sure chassis ground is not directly shorted to 0V.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: Spiritworks on January 21, 2020, 04:45:13 AM
The transformer lams are connected to the chassis ground. The top copper layer is 0V audio common. The washers just help to make sure chassis ground is not directly shorted to 0V.

Thanks Jeff.  So I better install them.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on January 21, 2020, 10:27:01 AM
Thanks Jeff.  So I better install them.
I would, just to be sure.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: ramtazz on February 23, 2020, 07:59:09 PM
Hello Everyone,

While I am not a noob to DIY I am to building these CAPI V28's.  I purchased 2 and put one together with 2 2520 op amps (kits) and then the fun began.... I have signal going in through the mic (both reg and condenser w/ 47v), signal Led lights up... and upon listening to the output... its a constant 60 cycle hum ( the knobs do not reduce or make it louder)....  (although if I crank the gains you can slightly hear a distorted signal way in the background).  I've tried swapping the 2520's, I've tried it without them... and swapped cards in different ports and working cards in those ports so I know the ports are good. I get the same hum.

I am awaiting more banana plugs to do further testing (published test points), but this is what I've tried so far....
Resoldered both amps and main PCB board
Traced back the output to the last of the transformers.

Without a schematic I cannot do anything more. I'm assuming since I have the hum with and without the op amps, that they are not an issue.  I don't know if its a bad trace, bad part or what.

Any guidance would be very much appreciated. 

Thank you in advance!!

Bill
 
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 23, 2020, 08:47:04 PM
Bill, I think we emailed about this. Unless you have 2 solid, known to be working opamps, trying to troubleshoot a new preamp build with unknown opamps is almost pointless. You will have no way of knowing what if anything is working or not working. Could be a problem with the preamp, one or both opamps or all of the above.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: ramtazz on February 27, 2020, 11:00:05 PM
Thank Jsteiger for the nudge in the right direction.  I borrowed 2 J Hardy Ops from another project and then verified the main board was sound. Then by solder sucking and resoldering a few connections on both of the 2520 op amps one by one I got both to work. Great sounding preamp.  Bill
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on February 27, 2020, 11:54:30 PM
Thank Jsteiger for the nudge in the right direction.  I borrowed 2 J Hardy Ops from another project and then verified the main board was sound. Then by solder sucking and resoldering a few connections on both of the 2520 op amps one by one I got both to work. Great sounding preamp.  Bill
That's great Bill. Congrats!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: FivePointsRecording on March 28, 2020, 11:26:34 PM
Hey guys, I am having some strange high pitched whistle sounds come from the VP28 I built, as well as no signal. Unfortunately, I mismatched the Q7 and Q8 transistors (#139 & #140) which caused a little smoke and burnt the PR2 & PR4 resistors. I’ve replaced those resistors and actually built a whole new Gar1731 but there is noise from the pre, as well as no audio from the microphone.  There also seems to be some small sparking happening near the connection point (UNDER J3-12) when I insert the pre into the rack, whereas none of my other pres do so.

The make things short, I’ve checked:

*Made new Gar 1731, tested on Capi Vp312 and VP25 pre and worked fine
*Gar 2520 opamp also works fine
*went thru and reconnected all solder points for gray hills and resistors
*Filter PCB no cold solder joints
*All LEDs are working correcting

As for testing points:
—I was able to run a sine wave thru the preamp (WITH THE MIC BUTTON ENGAGED?) successfully BUT the preamp stepped gain switch seems to be stuck in a specific location, as it has no change when adjusting. The fader knob works fine, but the preamp knob might be in a more gain-forward state, as you can hear the harmonics very clearly.
—Even though the sine wave is coming thru with MIC engaged, the preamp still passes no microphone signal when connected to a mic.
“With nothing connected to the output of your generator, assuming a balance device with an XLR, connect your red probe to pin 2 and black probe to pin 3. Your DMM should read 1.228V AC. “
—Read 0.000000 v  so I didn’t continue on this path yet as the instructions are a bit difficult for me to follow.

I’ve included a couple of pictures, any help would be greatly appreciated! Excuse my amateur soldering!

(https://scontent-atl3-1.xx.fbcdn.net/v/t1.0-9/90797871_10222898063539998_6042415589811552256_o.jpg?_nc_cat=107&_nc_sid=8024bb&_nc_ohc=BewG9sEOeTcAX9tgHNH&_nc_ht=scontent-atl3-1.xx&oh=959a7674285ba119424c81b6b5e6650e&oe=5EA4DB3A)

(https://scontent-atl3-1.xx.fbcdn.net/v/t1.0-9/91649671_10222898063860006_3912342644012875776_o.jpg?_nc_cat=102&_nc_sid=8024bb&_nc_ohc=bomITnEV5L8AX9n63ji&_nc_ht=scontent-atl3-1.xx&oh=88a6b59edc6b097b29f0e9bf15d05360&oe=5EA76414)
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on March 29, 2020, 03:25:53 PM
@FivePointsRecording

I'm not trying to be mean but in looking at the bottom of that board, your problem could be almost anywhere and in more than 1 spot. I see multiple solder joints that look cold. Also, many spots where there is way to much solder than could potentially be shorting out to the ground plane.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: iturnknobs on March 29, 2020, 04:36:39 PM
I believe in the past, Jeff had said he solders around 800F.  I took his advice and haven't had any issues since(minus dirty solder tips). Using chore boy combined with a wet sponge in between each use has helped me out also. Reflow all your points as recommended by Jeff. You should see the solder "flash" shiny silver and it shoud simultaneously adhere to the component and pcb trace/hole. As soon as that occurs, remove the iron. Also, I'm not sure what "common" practice is, but I always turn off power when inserting modules to the rack/chassis.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: FivePointsRecording on March 30, 2020, 04:35:45 PM
Well... it works now :D  I basically went through @ 800 and sucked out any excess solder and retouched every point. Thanks guys!!
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: AttamanuBe on March 31, 2020, 06:53:32 AM
Hi there  8)
Someone saved the  pictures of Chunger's build guide ?
They are no longer displayed on the thread  :-\
Thanks,
Stay safe !
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: deep_transmissions on April 13, 2020, 01:01:00 PM
Took me a 6months to get approved for this board ... yes 6 months! Dude.

Anyways, so I'm just now able to add some thoughts here after building 2 CAPI VP28's back in 2019. My previous experience was pretty limited, I built a DIYRE summing mixer, and that's about it. A few wees ago I just finished a Hairball 1176 Rev D for my 500 rack. Totally worth it!

My VP28's worked without any troubleshooting, so if you're worried about being able to do this I'll say that if you take your time and don't rush anything, go slowly, follow the intructions in this thread carefully, make notes to yourself of any conflicts in parts, or potential confusions so you can track possible errors, I think you'll do just fine!

In fact, I bought a Hakko FX-888D knowing I was going to build these pre-amps, since the passive summing mixer I built requires amplification. These 2 togetehr are a pretty killer way of getting analog console mixing sound in your home studio. I cannot put into words how good they sound. Smokin'. And running my analog gear through the CAPI's into the 1176, forget it. So good.

So the Hakko blew a capacitor right when I started installing the first resistors. The unit had to get shipped to the company for a repair which they did fo free under warrantee and you know what? I used my sh*tty Radio Shack wand soldering iron for the entire of both the VP28's! It was probably not the best decision, but just shows if you work carefully you don't necessarily need pro grea to get this stuff done.

My Advice:

- Read through this whole thread. I did that while waiting for my VP28 kits, it went pretty fast, you'll have a good idea of what you're going to be doing and you'll also see where other people had problems, and you can then work sensitively around those trouble areas.

- Definitely read through the whole build thread at the beginning. You want to knwo wahts' coming before you get there.

- Test every component you can, I didn't have any problematic parts but if you do, finding them at the beginning and re-ordering instead of going through your entire build is a huge time saver.

- Do the LEDs very carefully.

- Wire the transformers very carefully. Get a good wire splicer that won't cut extra wire, if you cut those too short you're going to have a hell of a time fixing that problem.

- Be sure you have the right diameter wrench for the knobs before you start. You'll be bummed if you can't finish your build with the knobs on when you rack em.

That's it! This was such a good project. I feel like doing these 2 was equal to years of therapy. I learned to be confident in myself, to slow down and pace and breathe and just focus on quality and not speed. And it then led me to have the confidence to build that 1176 compressor which is just the absolute cats meow on drums.  My next project is a Syncussion drum machine, this one has a lot of SMD parts so it's going to be a toughy.

Good luck and you'll be blown away by the thump these things bring your mix.
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: fancytimestudio on May 23, 2020, 09:58:11 PM
Hi all,
I have two VP28 preamps on my bench here. One is quite a bit louder at line level (unity gain).

I swapped opamps and the issue followed the PCB. I reflowed solder and still have the same issue. So I started probing around and found the following:

I have a signal generator suppling 1.212v at 400Hz. At the +/- points on the first opamp socket I'm getting different readings on the two preamps. Preamp 1 has .706v and Preamp 2 has .110v.  I'm unsure if either of these are correct but I know they aren't the same! Any ideas?`
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 23, 2020, 11:21:15 PM
Hi all,
I have two VP28 preamps on my bench here. One is quite a bit louder at line level (unity gain).

I swapped opamps and the issue followed the PCB. I reflowed solder and still have the same issue. So I started probing around and found the following:

I have a signal generator suppling 1.212v at 400Hz. At the +/- points on the first opamp socket I'm getting different readings on the two preamps. Preamp 1 has .706v and Preamp 2 has .110v.  I'm unsure if either of these are correct but I know they aren't the same! Any ideas?`
Misplaced resistors....?
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: fancytimestudio on May 24, 2020, 01:19:22 PM
Misplaced resistors....?

Hi Jeff, Thanks for the quick response. I went through and confirmed all resistors were in their correct places this morning. Thought I had a 70k at R34 so I removed it, tested and it tested at 20k as it should (the red looked very purple, my daughter confirmed). So I re-soldered it in place.  I should mention that when I have the "mic" button depressed, they function identically. It is only when in line mode that they differ that one is quite a bit louder than the other. Not sure if that helps
Title: Re: [BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
Post by: jsteiger on May 24, 2020, 02:35:00 PM
Hi Jeff, Thanks for the quick response. I went through and confirmed all resistors were in their correct places this morning. Thought I had a 70k at R34 so I removed it, tested and it tested at 20k as it should (the red looked very purple, my daughter confirmed). So I re-soldered it in place.  I should mention that when I have the "mic" button depressed, they function identically. It is only when in line mode that they differ that one is quite a bit louder than the other. Not sure if that helps
Post a pic of the resistors that are just behind the switches and below the T2 output transformer.