[BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread

[onss13]

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.

 

[pitone]

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

 

[jsteiger]

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.

 

[pitone]

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...

 

[jsteiger]

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.

 

[pitone]

You can swap the red and black leads from one of the transformers.

Cool!easy fix... I will, thanks

 

[javibigcity]

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

 

[jsteiger]

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.

 

[javibigcity]

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

 

[javibigcity]

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 : ]

 

[jsteiger]

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...

 

[PerspectiveSound]

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

 

[jsteiger]

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.

 

[PerspectiveSound]

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.

 

[jsteiger]

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.

 

[Homestudio]

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

 

[Homestudio]

Pardon me guys I meant the g a r 2520 s

 

[jsteiger]

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.

 

[Homestudio]

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

 

[jsteiger]

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.