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

[PaulSmith]

I would check the level at either end of the 1R on a good unit and then check on the problem unit with all controls the same. This could narrow it down to the output transformer.

Thanks!


Good unit 1.85ohm
Bad unit 1.10ohm

 

[jsteiger]

Thanks!


Good unit 1.85ohm
Bad unit 1.10ohm

Not DCR but audio level or ACV.

 

[PaulSmith]

Not DCR but audio level or ACV.

Apologies I’m not 100% on how to check that with probes! Either side of resistor or one probe elsewhere? Sorry I’m not very experienced with testing/diagnosing!

Either side of resistor reads the same measurement for both.

 

[jsteiger]

Apologies I’m not 100% on how to check that with probes! Either side of resistor or one probe elsewhere? Sorry I’m not very experienced with testing/diagnosing!

Either side of resistor reads the same measurement for both.

You need to run audio signal thru the preamp. Probe on the 1R with scope's ground clipped to a solid ground reference.

 

[PaulSmith]

You need to run audio signal thru the preamp. Probe on the 1R with scope's ground clipped to a solid ground reference.

Sorry for delay. The quiet unit reads around 5vac whereas the working unit is about 15vac. I used the same op amp for both but in the bad unit it gets very hot so obviously something up there!

 

[jsteiger]

Sorry for delay. The quiet unit reads around 5vac whereas the working unit is about 15vac. I used the same op amp for both but in the bad unit it gets very hot so obviously something up there!

I can't recall if you said what opamps are in the units and who built them? If they were built from kits have you swapped them around or tried the suspect preamp with a good known to be working opamp?

 

[PaulSmith]

I built the op amps and yes I have swapped in a good one to check. They are 0252. I also checked the opamp from the suspect unit in a known working one and it appears to be fine!

 

[jsteiger]

I built the op amps and yes I have swapped in a good one to check. They are 0252. I also checked the opamp from the suspect unit in a known working one and it appears to be fine!

At this point I would suggest desoldering all leads for the output transformer, make sure no leads are touching each other, check DCR for each winding (should be around 9 ohms) and make sure no windings are shorted together.

 

[PaulSmith]

At this point I would suggest desoldering all leads for the output transformer, make sure no leads are touching each other, check DCR for each winding (should be around 9 ohms) and make sure no windings are shorted together.

Apologies for the long delay I've been so swamped! I tried some different combinations and found some weren't 9ohms. Is this normal?

Red + black 9ohm
Pink + yellow 0.30ohm
Pink + grey/white 9ohm
Yellow + grey/white 9ohm
Orange + purple 9ohm
Grey + white 17ohm

 

[jsteiger]

Apologies for the long delay I've been so swamped! I tried some different combinations and found some weren't 9ohms. Is this normal?

Red + black 9ohm
Pink + yellow 0.30ohm
Pink + grey/white 9ohm
Yellow + grey/white 9ohm
Orange + purple 9ohm
Grey + white 17ohm

Pink and yellow must be internally shorted. Please send me an email with order # and I will get you a replacement.

 

[stolenbaby]

Hey there, I just built 2x 312's and the pre sounds fantastic. I love it! I feel like it has way more mojo somehow than my VP26's & VP28's to my ears although I love the like hell too. With my Lovechilds it's bliss. However, on one of the two 312's I built the phantom isn't working. I examined both pres and they are identical and were built side by side simultaneously. When I engage the phantom on the unit in question there is a loud pop & then a powering up noise that leads to dead silence and no signal. I have been using the unit with the phantom disengaged and getting great sounds with dynamics but the phantom is a no go right now.

The leads on the phantom switch are in the correct position. The solders on the PCB connected to the switch look good. Before I start de-soldering and re-soldering the whole thing, are there any dead giveaways/go to's I can try first?

Thanks,

C

I had a similar issue- the problem was that I did not install foam tape under the input transformer (not included and not mentioned in the 312 documentation- I found out when looking at the VP25 guide). If your input transformer is too close to the board, it seems the PCB pads for your signal winding can touch the case and short out, sucking up all your phantom.

 

[stolenbaby]

Hi this is my first build and I m not getting signal with LDC mics. Phantom power led is lit when turned on but there is only 4.8v at XLR pin 2 and pin 3 and with no load it measures 12.6mA. All my dynamic mics are working so I assume there is nothing damaged in the input transformer. My shure sm94 SDC has signal but crackles a bit. My rack is a brand new midas L6.

Replying to you as well, as this is the 2nd unanswered issue of this sort in the thread, and I was up a creek for weeks and reinstalled every component on that side of the circuit. I had the same issue, and the problem was that I did not install foam tape under the input transformer (not included and not mentioned in the 312 documentation- I found out when looking at the VP25 guide). If your input transformer is too close to the board, the PCB pads for your signal winding can touch the case and short out, sucking up all your phantom.

 

[Myhells]

Could be incorrect resistor placement, wrong facing diodes, incorrect transistor placement and/or turning them the wrong orientation. This is most likely all a problem with the opamp itself and not the preamp.

Rule of thumb, never test a fresh module build with a fresh opamp build.

Jeff,

The 2520 on the top is the known good one (BC-Q2) and the one on the bottom is the one that is heating up and turning the resister dark in color (V4.1). R12 near the O pin is getting dark. Is it possible that I have the yellow caps CD1 and 2 wrong?

Thanks

 

[Ricardus]

Jeff,

The 2520 on the top is the known good one (BC-Q2) and the one on the bottom is the one that is heating up and turning the resister dark in color (V4.1). R12 near the O pin is getting dark. Is it possible that I have the yellow caps CD1 and 2 wrong?

Thanks

Those caps aren't polarized so that isn't the answer. Unless you have a solder bridge and a short from when you soldered them.

 

[Myhells]

Those caps aren't polarized so that isn't the answer. Unless you have a solder bridge and a short from when you soldered them.

Good to know. I did put the bad DOA in the good preamp and the DOA was still getting hot and smelling bad. So I assume I did something wrong on the DOA. I just want to figure it out before I build another one.

 

[jsteiger]

Good to know. I did put the bad DOA in the good preamp and the DOA was still getting hot and smelling bad. So I assume I did something wrong on the DOA. I just want to figure it out before I build another one.

Look for a backwards or misplaced transistor(s).

 

[Myhells]

Look for a backwards or misplaced transistor(s).

I think I figured it out. I had BD 139-16 and BD 140-16 swapped. I didn’t realize they were different.

 

[Jacob Swogger]

Hi there. This is my second CAPI build, 4th pre-amp build. I've previously build a VP26 var gain. I'm now building a VP312 Stepped gain w/ Litz transformer (two actually). My board is Rev C. In the build documentation section, it's stated that Rev "C is identical to B with the square input option added." So I'm using Rev B documentation for my build. 1st question is what is the square input referred to here?

2nd question is on the Rev B BOM, there are parts with ~ and ~~ on them, specifically in the capacitor section. What does that signify?

3rd, R3 is optional, but I can't seem to figure out what my options are. In the 312 product description, R2 is mentioned as being optional depending on the transformer: "The input transformer you use will really dictate whether or not you use this Load R [R2] as it is optional and not required. With the EA2622, the circuit sounds great both with and without it. This Load R was typically not found on a stock API 312 card. The preamp will sound a bit more "open" without it." Is this what is being referred to? And if so, how would one describe my options if I'm using the provided CA2622?

thanks!

 

[Johan Olof Andersson]

Hello,

This might be a stupid question, but it's my first preamp build. I built the CAPI 511-VPR XLR Filtered Rack Bundle a year ago. Now it's time for the preamps.

I bought the following preamps and HiZ Plug-ins.

I can just swap op-amps between them, put the ret dots in my vp28 and vise versa? And the same with the Hiz-plug-ins?



2 x VP312DI-VPR-Litz Entire Bundle (Variable)
- Selected Discrete Op Amp: SL-2520 Red Dot
2 x HiZ Plug-In TX2 Pikatron Bundle US$64.00
2 x HiZ Plug-In FET-ZCON1 Full Kit US$19.00
1 x VPR Switch Mode PSU US$95.00
2 x VP28-Litz Entire Bundle
- Selected Discrete Op Amps: (2) CA-0252-DIY Single Kits

 

[geardude]

Hello,

I recently built a couple of these however one of them doesn't have any output signal. For reference I've tried swapping op amps ( currently using one of Capi's prebuilt 2520 clones) According to the guide c7 should be 120uf 50v nichicon however I received a 100uf 63v kingtronics gf2323 PET capacitor instead in the one that isn't working- besides that I've triple checked that all the resistor and capacitor orientations are the same/ correct.

I de-soldered the output transformer leads of the non working preamp and compared it to the others I have uninstalled and the non working card's output was in the expected ranges. For reference:

Orange & Yellow - 11.4 ohms
Green & Blue- 11.6 ohms
Violet & Gray- 11.2 ohms
Brown & red- 11.7 ohms


After ensuring both have the same gain settings (stepped input middle , max output attenuator) and the phantom is off-
without uninstalling the input transformers and comparing eachother unpowered, I did notice a discrepancy however

Good Installed input:
Pin 1 & 3 -12 ohms
Pin 2 & 4- 11.5 ohms
Pin 5 & 8- 1.89k

Non Working 312 installed input transformer:
Pin 1 & 3- 11.5 ohms
Pin 2 & 4- 11.5 ohms
Pin 5 & 8- 9.8 Ohms

Any further troubleshooting I can try here? Pretty stumped