Ok. So i just went through the schematic.
All caps on the main board seem to be the rigth ones. (dont know about the 3 small extra boards yet - the PK1 and PK2 and then i have one more board on it, i think its the 2520 blue dot).

I found a really weird problem though.
Whenever i engage the phase it turns the phase AND makes the signal a lot louder?!? What the heck?
Looking at the schematic i dont really see how that could be done?
Any suggestions?
Mute/Pad works.
I still have very poor bass repsonse on both DI and MIC input. There is some serious low cut going on.
Since it is happening on both DI and mic, my logic tells me that the low cut problem is some where after PK2, right?
Are there some measuring points later in the circuit where i could probe with a headphone amp and see where the problem arises?

So in a nutshell:
-Why the added volume when phase reverse?
-Between what points in the signal path would you say the low cut error is happening?
-CR6 and 5 were reversed by mistake and the unit powered up. What needs to be replaced?

Schematic link:http://www.capi-gear.com/catalog/images/gallery/VP312DI/RevC/VP312DI-Rev-C.1-schematic.pdf


« Last Edit: December 26, 2015, 04:16:29 AM by the19thbear »

So another post:

I go out of my 500 rack with crocodile clips connected to the xlr out pins on the 500 rack into my soundcard via line in
By mistake i hooked up the clips on xlr output pins 2 and 3 (hot and cold) and not 1 and 3 (ground and cold).
But the weird thing is that when hooking it up via pins 2 and 3 (hot and cold) it works perfectly! If i plug in the clips correctly i still get low cut and the big volume jump with phase pressed in. But connected to pin 2 and 3 the low freq seem perfect and the phase switch only switches the phase.

What could be wrong here?

Its his 500 rack as well (lindell audio).. Could it be damaged?
Thanks again!
« Last Edit: December 26, 2015, 04:51:43 AM by the19thbear »

Hmm.. seems it could all be narrowed down to bad cabling (for now).
 I found my proper XLR to jack converter cables, and once everything was plugged in to/from the soundcard/500 rack all seemed to work fine!
I have to test it with balanced/xlr connections later though:)


February 2nd, 2016 Update:
Rev D boards are now shipping for the VP312DI. To easily identify, the Rev D boards are green. The circuit is 100% identical although a few part labels and designations have changed. Like with all projects, it is crucially imperative that you follow the BOM that matches the revision of the PCB that you are building. I still have a good amount of the Sub DI boards left in black. Once those are gone, the reorder will be green and have the CAPIĀ® affixed  :D
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Ok gents, time to get over my fear of troubleshooting....

I checked for resistance without op amp installed. All seemed to check out.

Powered up the module without the op amp and I started to get smoke....which looked like it was coming from C12, which appears to be installed correctly...

Any suggestions as to next steps greatly and humbly appreciated!



C12 is the storage cap on the 48V rail. That would most likely not be the cause of smoke. If it were in backwards it would probably have exploded, quickly. I don't think it would be from C12. Typically, letting the smoke out would be an opamp thing.....
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I didn't have the opamp in the socket though...

Any suggestion as to next steps?


I would dbl and triple check all parts and orientation. If there is smoke, you have something wrong somewhere. Tracing it down can be hard without know what/where to look. There is always the option of letting whatever the problem is go up in smoke so you know for sure. I am not exactly recommending that though. Sometimes, there is only so much I can do from here.....
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I finished my first VP321DI yesterday. On Mic and DI input I have a distorted signal. The distortion stays the same on all gain settings and all switch positions. Except for the mute switch ;)
I read the thread but can't find a similar problem. I recorded a short example; maybe someone had a similar problem.
I checked all orientations and diodes; everything looks good to my eyes.
What voltages should I check first?
Recorded through the VP321DI:


The first piece of data that should be disclosed when posting a fresh build problem is, what opamp is installed and who built it?
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The first piece of data that should be disclosed when posting a fresh build problem is, what opamp is installed and who built it?

Sorry  I forgot to wrote that. It's a SL-2520 Red Dot, Assembled. The build is VPR 16V.
500 Rack is a Fredenstein Bento 10

ok a few issues with my CAPI VP312DI.  DOA prebuild red dot SL-2520, Hi-Z Pikatron UP3096M kit, VPR Floor Box PSU (tested and working)
  • No mic input
  • DI pre 2622 doesn't really work.  lots of hum and noise and no signal.  strangely when i have my bass plugged into the DI and touch the strings with one hand and the other on the rack, i lost some of the noise and get some bass signal.  this is freaking me out a bit!!!
  • DI post 2622 does seem to work.  no noise and i get signal.  -10 doesn't work and neither does LPF

Sounds like its a bit of a mess.  one thing to note, i had the PK1 soldered in the wrong way when i first powered on.  i realised today and thought this must solve my issues but everything is still the same when i changed to the correct orientation. 
I've checked my diodes on the relays and all seems good there.  CR1 is different to CR2 and CR3. 
reflowed solder to both relays, DI jack and transformer wires. 
any idea where i should start to troubleshoot this?


The biggest error folks make on these are on the relay PCB's. Its usually swapped around or wrong facing diodes. I have also seen people have the relay rotated the wrong way.
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Yeah i've read through the whole thread so i'm aware of that so i did really try and check all that stuff before i posted.  i've attached a photo.  maybe you can quickly check and see if there is anything obvious.

more photos


i fixed it.  all working perfectly now.  i hadn't realised that the connections between the Sub-DI.1 PCB board and the main board were actually part of the circuit and i hadn't solder in one.  i just added that and now everything works. 

Hi Jeff (and everyone else who is smarter than me).

I'm wrapping up a build on 2 VP312DI's (VPR Rev C.1 Versions), and am getting nothing when plugging into my 11 space rack that has had no issues for years.  I chose against the DC Block / Cap coupled output (just from a purist standpoint).  This is my 3rd CAPI build (2x VP26, 2x VP28), and the only one that I have had any issues with.  Phantom power lights are turning on, but I'm getting no signal with an sm58 plugged in. 

I'm at my 3rd "walk away" moment, where I just need to take a break for clarity.  I have read this thread front to back at least 3x over the last four days, and I know that there are readings I can take that will help assessing the possible issue (which I will do after the weekend). 

Before I do so though, here's the question:  I forgot to buy a HiZ plugin kit when purchasing the preamp.  If this is not plugged into the circuit, will the microphone side of the preamp work?  I don't seem to be getting any output at all.  Even when I plug and unplug cables from my patchbay, I am seeing no small jumps in Pro Tools when I move cables before the preamp, but am clearly seeing meter jumps when I pull the cable on the output side.


-Mike   :)


Hey Mike

With no HiZ Plug-In installed, the VP312DI will work fine in mic mode. It will only work for DI when in "Post 2622" mode. The most common error with these builds are wrong locations/orientation for the diodes on the relay PCB.
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Thanks Jeff!  I'll be diving back into this later in the week.  I'm almost positive that the diode orientations are correct, but will look over the PCB's closely.  I'll go back through this thread and take the same readings that others have taken and report back my results.  Hopefully that will lead me somewhere.


I just finished the Rev D VPR version of the VP312Di with stepped gain. While it wasn't necessarily a difficult build, thanks to Chunger and Jeff's build guides, there were a few things that I got hung up on at the end. R16 is now where you put the 10k vertical resistor and the LED for the phantom power (not R17).  I got hung up on trying to figure out how to mount two components in one position on the PCB. Jeff's build does show this but I couldn't tell what I was looking at so I just wanted to point it out that the long leg of the LED is connected to the resistor that is mounted vertically. The short leg goes into what is labeled LED on the board. Then those two are soldered together to make one component in a sense. Hopefully this pic will help.
« Last Edit: December 22, 2016, 09:33:58 AM by bartbusch »

The other thing that neither build guide made mention of is that there HAS TO BE a jumper or switch installed in SW1 and SW2 in order for this thing to work. I had no switches and didn't know if it was somehow being connected inside the PCB.  Long story short, save yourself a little extra wire and install jumpers where it indicates "for VPR". 


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