[Build] CAPI VP312~500/51x Series~Preamp Kit~Official Support Thread

[jsteiger]

***ATTENTION!! The stepped VP312 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!!***

As the title states, this is the official build/support thread for the VP312, 500 and/or 51x series mic preamp build that can be found at our capi-gear.com website. A decent amount of PCB’s have been sold so I felt it proper to start this support thread.

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September 21st, 2015 Update:
All support docs for this project can be found on the recently added Support Docs page at www.capi-gear.com
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This is about as easy as a preamp build can get. Keep in mind, if you are using the 51x version, you must be very careful with the voltage selected and the loaded DOA. I definitely don’t want anyone to accidentally smoke a vintage API 2520 and be mad at me!

Also, if you are using the preamp in the bipolar 24V position (with an appropriate DOA), you should check the DC offset for the particular DOA you have chosen. If you have more than a few mA of DC present at the output of the DOA, you will want to use a good quality cap to block this DC from the output transformer. This cap can be positioned in place of R11, the 1 ohm R.

FWIW, PPA’s 2055E running on 24V sounds nothing short of amazing in this circuit! Kudos to Pier! 

So, if you have any questions feel free to let them fly. 

Best regards, Jeff

 

[[silent:arts]]

running this on with the “VPR Mod” is childish  ;D ;D ;D

 

[brichie]

Hey Jeff!  I'm running into a slight problem with my VP312 build - using it with either a gar1731 or a stock api2520 results in low output that sounds overdriven and thin.  I just read through the VP2X thread where someone had a similar sounding issue that was eventually traced back to a cold solder joint on one of the switches.  I went back over all of my solder points and they all seem to check out.  One thing I did notice was when checking the resistance between the DOA sockets, between C and O I get 0 resistance.  All the other sockets check out with high resistance -- should this be a concern? 

I should also mention a slight f*up on my part, I followed the VP2X manual a little too closely when cutting the leads for the output transformer and thus had to extend them a little -- just the gray and yellow wires. 

BTW I have a newer API 6B lunchbox, and I did the VPR Mod as described.

Enjoyed the build immensely, I look forward to getting it up and running!
-br

 

[jsteiger]

brichie,

You should measure around 9 ohms resistance between the Com and Output DOA sockets. The Yellow transformers lead is important, I would dbl check that. It's part of the output transformer's secondary series connection. Also dbl check the t-pad wiring and connections. I have seen that be an issue for a couple of guys in the past. As you did with the switches, closely look at and even touch-up any component's solder joints to ground. These solder pads sometimes don't heat as nicely as say one only on a track. The heat dissipates out into the ground plane and can sometimes lead to a cold solder joint.

Keep us posted.

Best, Jeff

 

[jsteiger]

If the above doesn't get you there, you could also monitor the output directly from the opamp. It will be unbalanced but will bypass the t-pad, polarity switch, mute switch and the output transformer. Take this signal from the opamp side of the Zero ohm resistor. This may help you to isolate the problem.

Best, Jeff

 

[brichie]

Jeff,

Unfortunately after checking the solder points I still have 0 resistance between the COM and Output DOA sockets.

The t-pad looks pretty solid, all the leads are soldered sufficiently and they go to their respective CBA locations on the board.

Check this one out, not only had I beefed the yellow and grey wires, but it appeared I somehow confused GRY and GRE (it was about 2am when I was putting it together, guess I got a little too excited to finish) and I'm slightly concerned that maybe the initial power up in the rack may have damaged the output transformer.

I did a little search on how to pass signal and monitor the op amp but it yielded no results as far as instruction is concerned, would you mind pointing me towards a resource on how to execute?  I'm seeing a XLR connection with some alligator clips in my future. 

Thanks for the assistance Jeff, I'm a total beginner when it comes to troubleshooting electronics - hand me a bass guitar and I'll hold it down all day
-br

 

[jsteiger]

Unfortunately after checking the solder points I still have 0 resistance between the COM and Output DOA sockets.

This is entirely possible depending upon the range of your DMM. It is actually a good thing. It would be a problem if it was a few ohms.

...Check this one out, not only had I beefed the yellow and grey wires, but it appeared I somehow confused GRY and GRE...

I'm thinking this is the source of your problems. If you look at the schematic and the secondary windings, well they were pretty jazzed up that way.

I'm slightly concerned that maybe the initial power up in the rack may have damaged the output transformer.

That shouldn't have happened.

I would say to fire it up and see what you've got happening now. Then report back.

Cheers, Jeff

 

[brichie]

Good news Jeff,

Fired it up and all is well! Thanks for the pointers!
-br

 

[jsteiger]

Good news Jeff,

Fired it up and all is well! Thanks for the pointers!
-br

10-4, that is good news. I will have to remember about the GRE/GRY potential switch-a-roony. 

Have fun with it.
Jeff

 

[letterbeacon]

How close is does the VP312 sound to the API 3124+?

I have an API 3124+ at work and it sounds good, but not 'warm' enough for me.  Is the VP312 based on older API technology, or will it sound similar to the current API 3124+ that's in production at the moment?

Thanks a lot!

Matt

 

[jsteiger]

Matt,

My pre's do sound similar, yes. They sound like they are all related. Mine are all a little less "hard" sounding than the current production company's version. I believe this is mostly because of Ed's 2622 compared to the Jensen that they use for the input. Next important component contributing difference, the DOA's from Scott and Gary just plain sound better than a current 2520. They both sound more like their '70's counterpart.

That is just my opinion. Do a search on gs for "3124 VP26" and you will find some opinions of other peeps. Some hang here to so maybe they will have some input as well.

I have noticed on the 3124 skiz that they have a coupling cap after the opamp with a small parallel bypass cap. These components were not on the original 312 cards. I don't have them on the VP312 card either.

Best, Jeff

 

[letterbeacon]

Thanks Jeff, that's exactly what I wanted to know.  I know what you mean about the 3124+ sounding 'hard' - I'm glad to hear the VP312DI doesn't have that character to it.  

One more question - I should be able to fit two VP312DI pre amps into a 1U rack enclosure, right?

 

[jsteiger]

...One more question - I should be able to fit two VP312DI pre amps into a 1U rack enclosure, right?

Yes for sure. All remaining PCB's for the VP312DI project should be here tomorrow. I have a few outstanding parts, but 98% of the components are here already.

If you plan to mount the VP312DI in a 1RU, you will need to be very careful and meticulous with your metal work, especially where the 3-sub mini toggles are at. The back of the faceplate will need a milling in it so the switches and the Bo DI board seat properly. I have to get some up close and detailed pics of how all of that goes together. If one is not careful, nothing will fit to the faceplate or line up properly. It is not impossible, it just won't be a super easy task.

Best, Jeff

 

[letterbeacon]

I'm planning on using switches and pots that I have lying around here so will probably just use flying leads to connect to the PCB.  That should make things a bit easier when designing the front panel too, I hope.

Thanks for your help!

 

[Seditionary]

when building our vp312's... i remember there being an optional resistor?... what is the tone difference with and without this resistor?  we included it. 

thanks

 

[Knuckleduster]

Hi Jeff, I built my vp312 a few weeks ago. After finishing building it I did a quick test and everything seemed to work with no issues. About a week later I took it on location for a session. I couldn't get much of a signal out of it. When I got home I tried it again and everything seemed to work fine. I took it to another session yesterday. Everything worked fine for the first few hours and then all of the sudden in between takes I stopped getting a good signal again. I fiddled around with some of the switches while tracking and once I flipped the phantom power off and back on again then everything worked. So I guess something isn't quite working with the phantom power. The phantom power light was still on by the way when the issue occurred (it does turns off when phantom power is in the off position though).

Any ideas on where to start looking?

Thanks

Georges

 

[jsteiger]

Georges

Below is some "boilerplate" help text that I send out to folks. This solves the majority of issues people have.

First, what DOA is it? A pre-built or a DIY? If it's a kit, swap the opamps and see if the problem follows. Also, have you seen the DOA install page http://classicapi.com/catalog/DOA_Install.php I highly recommend never testing a new preamp build with a new opamp build. There are too many variables and you may never know what the issue is. If it's a pre-built opamp from me, chances are, it is not bad. Gary and Scott both run them in for many hours before sending them to me.

Have you completed the pre-start tests at the end of the old VP2x Assembly Guide? If you are building a VP312, the C to O DCR value will be about 8.5 ohms. Make sure all voltage readings are correct.

Double check all component values and make sure caps are facing the right way. Resistors cannot be measured once both ends are soldered to the PCB. You will get many odd readings. Either use the color codes to verify or one end must be desoldered to get an accurate DCR reading.

Double check the lead colors/pads for the output transformer. Take your time and be sure. Countless times I have heard back from folks who checked and checked and then a few days later realized that some of the leads were placed incorrectly.

Next, reflow the solder on the 3 C&K pushbutton switches.

If it's the Grayhill stepped gain option, do you have 25k when measuring between the to outside pins of the switch? If not, check with a magnifying glass as there probably is a short between some solder pads. These can be checked end to end by probing between adjacent pins. You should get the resistance of each of the R's on the pdf as you work from one end to the other.

Scan the board and check all solder pads with a magnifying glass of some kind. The biggest issue, besides what I mentioned above are cold solder joints. Sometimes they can look fine but not be good.

 

[Timh]

I just got my kit in the mail, and I'm so excited to start my first preamp build, but I feel like I'm taking crazy pills. There's a 10ohm resistor in here that I don't see on the BOM. Also two 10k resistors when only one is needed for R3. I just want to be sure I'm not mixing them up before I start soldering stuff.

 

[jsteiger]

Tim, you must follow the BOM that I emailed out after I shipped. The name of the BOM file will match the Rev and title on the PCB. This support docs email often ends up in spam so check there for a date about 2 days after the "shipped" order update was sent out. Of course, that may also be in your spam. 

 

[ruairioflaherty]

I just got my kit in the mail, and I'm so excited to start my first preamp build, but I feel like I'm taking crazy pills. There's a 10ohm resistor in here that I don't see on the BOM. Also two 10k resistors when only one is needed for R3. I just want to be sure I'm not mixing them up before I start soldering stuff.

One very very important trick when building.  Do not go by visuals alone - measure every component on the DMM before you stuff it, this has saved my ass in the past.  Use the parts called for in the build instructions and schematic, if there are extras you can safely ignore them.

Edit - Jeff beat me to it....