[BUILD] CAPI VP312DI~500/51x Series~Preamp + Direct Inject~Official Support Thread

[Claudio]

Thanks, Jeff and Mike, for your replies. So everything measured fine and I inserted the VP312DI in my rack with an extension jig. I have a Trident DecaDent, but for my initial tests I wanted to use my Fredenstein Bento. I thought if something went wrong, I’d rather blow the cheaper of the two racks. I fired up the preamp, everything worked as expected, and it sounded amazing! The DI on my bass alone was killer. The only issue I had was the high squeaking noise when I engaged phantom power. However, I know that this is an issue with my Fredenstein, I’ve had that before with one of Volker’s preamps. (Actually that’s why I also bought a Trident chassis).

I then tried a different slot that is further away from the Fredenstein’s PSU to see if that changes the squeaking. Same extension jig, same setup. A few seconds after I engaged phantom power, I saw some smoke rising and R10 burnt. Bugger. I highly suspect that this is due to the Fredenstein’s weird 48V rail. But is there anything else I should doublecheck when I replace R10 and fire the VP312DI up next time? Or do I maybe need a resistor with .5 W, not just .25 W?

If this should matter, I have the version with a stepped gain switch, the gar2520 and the FET-ZCON1 plug-in.

Thanks a lot,
Claudio

 

[mrmike186]

I was able test the DI and it works with post 2622 engaged and not engaged. The gain and output attenuators both work so I assume  this rules out input and output transformers. I touched up all solder joints and checked for bridges but mics still don't work.

 

[jsteiger]

Thanks, Jeff and Mike, for your replies. So everything measured fine and I inserted the VP312DI in my rack with an extension jig. I have a Trident DecaDent, but for my initial tests I wanted to use my Fredenstein Bento. I thought if something went wrong, I’d rather blow the cheaper of the two racks. I fired up the preamp, everything worked as expected, and it sounded amazing! The DI on my bass alone was killer. The only issue I had was the high squeaking noise when I engaged phantom power. However, I know that this is an issue with my Fredenstein, I’ve had that before with one of Volker’s preamps. (Actually that’s why I also bought a Trident chassis).

I then tried a different slot that is further away from the Fredenstein’s PSU to see if that changes the squeaking. Same extension jig, same setup. A few seconds after I engaged phantom power, I saw some smoke rising and R10 burnt. Bugger. I highly suspect that this is due to the Fredenstein’s weird 48V rail. But is there anything else I should doublecheck when I replace R10 and fire the VP312DI up next time? Or do I maybe need a resistor with .5 W, not just .25 W?

If this should matter, I have the version with a stepped gain switch, the gar2520 and the FET-ZCON1 plug-in.

Thanks a lot,
Claudio

R10 the 200Ω resistor? This will have nothing to do with phantom power. My first thing would be to look at the gar2520 (if it was built from a kit). I have honestly never heard of smoking that R in any of my pre's so not sure what would have caused that except for an opamp problem.

 

[jsteiger]

I was able test the DI and it works with post 2622 engaged and not engaged. The gain and output attenuators both work so I assume  this rules out input and output transformers. I touched up all solder joints and checked for bridges but mics still don't work.

The pre should switch from Mic to DI when a 1/4" plug is inserted. FYI, the DI jack is auto-switching so you can only use Mic mode with NO 1/4" plug inserted.

Otherwise, I would look at the plug in relay board for the Mic/DI switching. Its likely stuck in DI mode which means an error on that PCB.

 

[Claudio]

R10 the 200Ω resistor? This will have nothing to do with phantom power. My first thing would be to look at the gar2520 (if it was built from a kit). I have honestly never heard of smoking that R in any of my pre's so not sure what would have caused that except for an opamp problem.

Hey Jeff,

I did some more tests and strangely enough now the preamp works without any problems (except for the Bento  noise). Not sure what went wrong when I saw the smoke. Possibly I had the extension jig inserted incorrectly and was causing a short? Anyway, would you recommend I still replace R10 or can I risk running the preamp as it is now?

Best regards,
Claudio

 

[jsteiger]

Hey Jeff,

I did some more tests and strangely enough now the preamp works without any problems (except for the Bento  noise). Not sure what went wrong when I saw the smoke. Possibly I had the extension jig inserted incorrectly and was causing a short? Anyway, would you recommend I still replace R10 or can I risk running the preamp as it is now?

Best regards,
Claudio

Its likely OK but if its darkened, I think I would replace it since the rest of the preamp is fresh and new.

 

[mrmike186]

The pre should switch from Mic to DI when a 1/4" plug is inserted. FYI, the DI jack is auto-switching so you can only use Mic mode with NO 1/4" plug inserted.

Otherwise, I would look at the plug in relay board for the Mic/DI switching. Its likely stuck in DI mode which means an error on that PCB.

Should I be looking at the relay PCB behind the input transformer? I assume the one behind the 1/4 jack would not cause this.

This may be a silly question but when the relay swithces and makes a click sound,  does it come from the actual unit or though the speakers/headphones? 

Sorry for all the questions but lastly is there any way to bypass the relay to test the rest of the preamp? 

Thanks for all your help.

 

[jsteiger]

Should I be looking at the relay PCB behind the input transformer? I assume the one behind the 1/4 jack would not cause this.

This may be a silly question but when the relay swithces and makes a click sound,  does it come from the actual unit or though the speakers/headphones? 

Sorry for all the questions but lastly is there any way to bypass the relay to test the rest of the preamp? 

Thanks for all your help.

The clicking is mechanical from the relay itself, not audible in the output. Check the schematic to see which one is for Mic/DI. I can't recall and would have to look.

 

[Ricardus]

I have two of these on the horizon for a next possible build.

 

[mrmike186]

I ended up replacing both relay PCB's  and one  of the toggle switches I damaged while troubleshooting  so now I have a functioning  312DI......well almost anyway.  The DI doesn't work while in post 2622 mode but it does while not engaged and the mic input works and sounds great.  I hear the click on the relay when I insert a 1/4 inch plug and when I toggle the 2622 switch so I'll look at the FET ZCON1 Hi-Z plugin next.  It's getting close!

 

[jsteiger]

I ended up replacing both relay PCB's  and one  of the toggle switches I damaged while troubleshooting  so now I have a functioning  312DI......well almost anyway.  The DI doesn't work while in post 2622 mode but it does while not engaged and the mic input works and sounds great.  I hear the click on the relay when I insert a 1/4 inch plug and when I toggle the 2622 switch so I'll look at the FET ZCON1 Hi-Z plugin next.  It's getting close!

Also make sure the 1/4" plug is mono. A stereo plug WILL NOT WORK with the relay switching of the VP312DI.

 

[JOHN5]

Hi -

I'm troubleshooting a build and have gotten stuck.  Here's my components:

-GAR 2520 from a kit
-HiZ Plug-In TX2 Pikatron Bundle
-Stepped gain version

This is what I've found:

-Resistance Readings on DOA sockets:

+V to -V  <200K
-V to C      <200K
+V to C      10K
0 to C        <200K
+V to 0      <200K
-V to 0        <200K

Here's what's occurred:

-Plugged in unit to API lunchbox.  Lunchbox has LEDs for +16V, -16V and 48V.  Left unit powered on for approx 1 minute with no observable issues
-Turned on phantom power switch.  LED on pre lit and went dark after approx 1 sec.  Observed smoke from an unknown location in pre.  Lunchbox tripped and its LEDs went dark.
-Removed pre from lunchbox

After reviewing assembly guide and this thread, so far, I noticed the following screw ups:

-No 10K resistor installed w/LED
-JP1-1 not installed
-Solder bridge between R13 and -V pin on DOA

I fixed the issues and re-installed in the lunchbox.  About 30sec after power up, noticed smoke from DOA.  Lunchbox was not tripped. Powered down.  Removed pre from lunchbox.  R12 and R13 on DOA are charred. 

Removed DOA from board.  Re-checked resistance values on board's pins.  Values are same as above. 

Re-installed in lunchbox and powered up.  Waited approx 5 mins.  No smoke or trips.  Checked DC voltage on DOA pins.  Read +14 V and - 14 V, respectively to pin C. 

From reading this thread, I understand that the common issue is with stuffing the relays, specifically the diodes.  Should I check the relays by unsoldering the headers to get a closer look at the diode values or is there something else I should be looking at based on what's occurred?  To my eye, everything seems to be pointing to the DOA but figured I would ask before blowing more stuff up. 

Thanks!

Pics to follow.

John

 

[Mylithra]

If you built the DOA, closely examine it with something that has high magnification. Make sure there isnt a soldering error there first.  There are a few places that the pads are really close together and bridge easy.

 

[JOHN5]

If you built the DOA, closely examine it with something that has high magnification. Make sure there isnt a soldering error there first.  There are a few places that the pads are really close together and bridge easy.

Thanks, will do

 

[Mylithra]

If you built the DOA, closely examine it with something that has high magnification. Make sure there isnt a soldering error there first.  There are a few places that the pads are really close together and bridge easy.

Something that happened to me early on. I had made the switch to no-clean core solder and had stopped cleaning up the residue.  I completed a DOA and had it promptly burn up. When I took a closer look, I didn't make any bridges but I did have a small fleck of solder that splattered and caused a short that burned it up.  Had I cleaned up like I do now, I wouldnt have fried it. Now I still use no-clean core solder but I use a cloth with isopropyl alcohol to clean the boards after soldering. If you do this, be careful of parts sensitive to solvents (polystyrene caps for example) so you dont melt them.

 

[JOHN5]

Something that happened to me early on. I had made the switch to no-clean core solder and had stopped cleaning up the residue.  I completed a DOA and had it promptly burn up. When I took a closer look, I didn't make any bridges but I did have a small fleck of solder that splattered and caused a short that burned it up.  Had I cleaned up like I do now, I wouldnt have fried it. Now I still use no-clean core solder but I use a cloth with isopropyl alcohol to clean the boards after soldering. If you do this, be careful of parts sensitive to solvents (polystyrene caps for example) so you dont melt them.

Cool, good tip.  A couple additional questions for you, Mylithra (and anyone else): 

Does my issue seem exclusively related to the DOA?  I know it may be difficult to answer.  The reason I ask is that I'm going to purchase a pre-assembled DOA to eliminate that variable.  Naturally, I'd rather not fry that one too. 

In other words, are the resistance and voltage values I have (first post) sufficient proof that my issue is limited to errors in the DOA build? 

I appreciate the assistance!

John

 

[jsteiger]

Cool, good tip.  A couple additional questions for you, Mylithra (and anyone else): 

Does my issue seem exclusively related to the DOA?  I know it may be difficult to answer.  The reason I ask is that I'm going to purchase a pre-assembled DOA to eliminate that variable.  Naturally, I'd rather not fry that one too. 

In other words, are the resistance and voltage values I have (first post) sufficient proof that my issue is limited to errors in the DOA build? 

I appreciate the assistance!

John

The resistance measurements shown in the old VP2x guide do not apply to the VP312DI. Yours posted should be fine. Not sure what rack you are using but +/-14V is pretty low.

 

[JOHN5]

The resistance measurements shown in the old VP2x guide do not apply to the VP312DI. Yours posted should be fine. Not sure what rack you are using but +/-14V is pretty low.

Thanks, Jeff.  It's an API 8-slot lunchbox:  https://apiaudio.com/product.php?id=137

Is 14 V something I should be concerned with?  I have two other pres installed in it which seem to work fine (Hairball Elements Bronze).  With the pre removed, I checked the pins on the free slots in the rack: 

4th pin from bottom on slot to ground:  15.8 V
2nd pin from bottom to ground:  -15.9 V
bottom pin to ground:  47.2 V

I re-checked w/ the 312DI installed and am reading 15.0 V and -15.3 V now at the DOA pins.  Maybe I screwed up the measurement earlier?  Maybe I've got some periodic voltage drops in my lunchbox?  Perhaps my meter sucks more than I thought? 

It sounds like I should try w/ a pre-assembled DOA and go from there.  Unless I hear differently, I will give that a shot and let you all know. 

Thanks!

John

 

[JOHN5]

Hi again,

I bought a pre-assembled DOA, a Rogue Five (https://www.diyrecordingequipment.com/collections/assembled/products/rogue-five-discrete-opamp).  Here's where I'm at: 

-lunchbox powers up without issue. Reading +15.3 and -15.3 V on the top side of the DOA. 

-I get no signal, regardless of Preamp Gain knob position, using a dynamic mic following step 12.3.a of the VP assembly guide (which I know is not specific to this Pre). 

-LED does not illuminate when engaging 48 V switch.  Reading 48 V to ground from red wire terminal on switch and 48V on orange wire when switch is engaged.

--------------------------

It seems like I should move on to checking the assembly of the relays, specifically the diodes. 

Any other thoughts at this point? 

Thanks,

John

 

[JOHN5]

UPDATE -

False alarm - - I don't think my coffee had kicked in.  I had a DAW "operator issue" (forgot to turn on my ADAT which works as the master clock). 

Tested everything out:

-Dynamic Mics - - good
-Condenser Mics - - good 
-Mute, Phase and Pad function normally

-Bass on DI circuit - - good (as well as all switches on DI circuit)

The only problem I have is with the LED.  It does not illuminate.  My guess is that I fried it because I originally forgot the 10K resistor upon initial startup.  The LED lit up for a moment and then went dark. 

I will try replacing that and circle back if that doesn't resolve the issue. 

Thanks!

John