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

[AudioHak]

Hmm, OK this is the first issue I have heard of with the signal present on the new PCB. If both are acting the same, it is likely the same build error on both or possibly a rack situation? What rack are you using?

Agreed it does suggest the same error. I am using a Radial PowerStrip 500 Series 3-Slot Power Rack. I do have 2 other lunchboxes I can try. An aphex usb 500 and a bento 6. But I have never had any other problems with the radial. Of course that doesn't mean it's not causing the problem.

I guess the question would be what feeds the ic when the 48v button is pressed vs not pressed? As this seems to be related somehow.

I followed the guides by both you and chunger in this thread. Used the BOM provided on the site and carefully separated each part and tested individually with a volt meter taping each part to a piece of paper similar to the way chunger does. Never had any discrepancies. All parts matched up to their respective pcb names. Many builds under my belt. First time I've had a problem.

That said I am a human. Weird that I would repeat the mistake unless there is something I repeated in the guides?? Maybe I am missing something. Thanks for your help.

 

[AudioHak]

I did notice that the original guides are Rev A. Is there a difference somewhere in the guides between A and B that could lead to this problem?

 

[jsteiger]

Did you follow the Rev B BOM?

Is there possibly low level noise where the signal light is actually correct?

 

[AudioHak]

Did you follow the Rev B BOM?

Is there possibly low level noise where the signal light is actually correct?

I did follow the Rev B BOM. As far as noise, I have not noticed any. Seems very quiet. But I have only tracked electric guitars through it so far. If there is any noise what would that suggest?

 

[AudioHak]

I think I mentioned this already. But the preamp works and sounds great regardless of the Phantom power button position. The only issue is the green signal led staying lit when 48v is not pressed in.

 

[jsteiger]

I think I mentioned this already. But the preamp works and sounds great regardless of the Phantom power button position. The only issue is the green signal led staying lit when 48v is not pressed in.

Yeah this is not right. I have not witnessed this issue myself so not sure what to say. The 48V switch has nothing at all to do with the sig present LED. The phantom switch simply unshorts its respective LED and also allows 48V to flow to the 200Ω R which feeds the 6k8's. I am wondering if you can measure or see anything at the output when the sig LED is on before you engage the 48V LED?

To add, the sig present pickoff point is after the preamp opamp not before it. If the sig light is on, I would measure the module's output to see if there is something there.

 

[AudioHak]

Yeah this is not right. I have not witnessed this issue myself so not sure what to say. The 48V switch has nothing at all to do with the sig present LED. The phantom switch simply unshorts its respective LED and also allows 48V to flow to the 200Ω R which feeds the 6k8's. I am wondering if you can measure or see anything at the output when the sig LED is on before you engage the 48V LED?

To add, the sig present pickoff point is after the preamp opamp not before it. If the sig light is on, I would measure the module's output to see if there is something there.

That is indeed strange then. To add to the strangeness, the first VP28 I built is no longer doing this. It now functions as normal. I did not change anything.

I did some testing and verified that there is no low level noise present when the 48v button is not pressed. But the second VP28 still has this problem. I will change the preamp opamp tonight and see what, if anything, happens.

 

[AudioHak]

Switched opamps out with 4 different amps, same result. This is pretty strange. I did find that the VP28 with the problem has slightly lower input volume. When the 48v button is pressed in and the led is functioning "normally", the preamp gain takes one more click to light the signal led in the same way as the other VP28 I have. But I don't think this would be causing the signal led to stay lit constantly when the phantom power is not on.

Should I post some pictures? Any recommendations?  Frustrated... :-\

 

[jsteiger]

Well, due to part tolerances the level required to fire the sig present will vary about 1-2dB but should not be more than that.

In line mode (set to unity), it takes an approximate -16dBu signal applied to the input to illuminate the sig present LED. I have built many of these and seen it vary from -15 to -17. Those are worst case. Its usually closer to +/- 1 db around -16.

Do the pre's match in level or do the sig present LED's light at slightly different levels? Its two separate things actually.

The fact that the first one is working normally now makes me think a faulty solder joint somewhere. There is nothing here that needs to "break in" or anything.

 

[Commander Fluffypants]

With the Mic button in and all others out, you should have 3k6Ω from 5 to 8 and 5 to 10. Between 8 and 10 will be around 29Ω.

Hey there. Still looking for the next step.

From a previous post:

This is what I have (almost.)  3.61k for 5 to 8 and 5 to 10 and 27.5 for 8 to 10.

What do I check for now?

Thanks again.

 

[AudioHak]

Well, due to part tolerances the level required to fire the sig present will vary about 1-2dB but should not be more than that.

In line mode (set to unity), it takes an approximate -16dBu signal applied to the input to illuminate the sig present LED. I have built many of these and seen it vary from -15 to -17. Those are worst case. Its usually closer to +/- 1 db around -16.

Do the pre's match in level or do the sig present LED's light at slightly different levels? Its two separate things actually.

The fact that the first one is working normally now makes me think a faulty solder joint somewhere. There is nothing here that needs to "break in" or anything.

Thanks Jeff. I sent a test tone through both preamps into pro tools and using the same settings they measured  -10. So it seems to just be the signal led that is having the problem. I also recorded the same source and found no discernible difference between the two preamps.

I repeated the same tests after changing and swapping the op amps (using 8 different op amps, 2 from ADK). No difference. I then tried a different chassis. Again, no change at all. Exact same problem. So it seems the only problems I am having is that the signal led stays lit unless I press the 48v power button and when pressed, the signal led required more gain on the preamp (2-3 stepped notches higher) than the other to activate the led in the same manner. But again the audio levels are the same at matched settings when recording.

I do realize that nothing needs to "break in" and have tried reheating many connections following the signal path, but to no avail. My solder connections are very clean. I have been soldering for over 15 years. Not trying to say that i am infallible, I know its most likely human error, but I have checked multiple times now and all the parts are correct according to the documentation I have. I have been quite meticulous and possibly even anal... about my connections. 

Does this get us anywhere? Any ideas on where to focus my efforts in troubleshooting?

I really appreciate your help,

Jason

 

[AudioHak]

Well Jeff, I am happy to report everything is working. But the fix still puzzles me. I was running a session and decided to the leave the 48v power on while tracking for 2 hours or so. I shut everything down for the night and came in the next day, powered everything up, switched off phantom and the led worked as normal. I have had one instance where the signal led came back on, but I did the same thing, left the phantom power on for about 30 mins, and everything was working again after that. Since then, everything has been working without any issues.

The preamps both sound incredible, compared to my 2 VP26's and my 2 VP312's ( and those sound fantastic). I hope this post helps anyone who might have this same problem, as it really was not a major issue, just a very minor annoyance.

Have a happy holiday everyone!!

 

[scratchyTheHobo]

Evening, Gents! (morning?)

I've got a VP28 Rev B with blue dots and litz, and everything works minus the signal LED. Everything passes signal, the test points read out, but the signal LED won't light up. The LED is in the right direction, and I read no output voltage at the leads or the 4558. I also can't find VCC+ at the chip, so that's probably the issue.

I've tried a different LED, but not a new chip. The chip's in the right direction and always has been.

Any thoughts? I've been chasing around this thing for hours now  ???

 

[byoung]

I just finished 2 VP28's Plugged the first one in without opamps and checked voltage, everything seemed good +-16v(+16.07 and -16.04) at the opamps pins. So I powered it down and put in a gar1730 and gar2520. when I powered it back up I saw the green signal LED flicker and PR3 smoked. I saw a small glow(arc) on the gar1730 around the general area of R14/R11 before I got the power off, but that could have been PR3 going up in smoke possibly? I assume PR3 is a protection resistor and am hopeful it did it's job at preventing any further damage. I've checked the gar1730 for solder bridges visually and didn't see any or any smoked parts, but that doesn't mean something isn't dead. Does anybody have an idea what I should be looking for based on what happened? 

I'm cautious to plug in my second VP28 until I have this resolved since I populated them at the same time, if I screwed something up then it very well is on the second preamp/set of opamps too.

Also is there a breadboard circuit I can put together to test the opamps on their own?  to prevent any further hardware damage?

 

[jsteiger]

When a PR smokes it indicates trouble on the respective discrete opamp. The most common errors are swapping and/or turning the larger BD transistors. The next thing is wrong facing diodes.

 

[byoung]

Am I assuming correctly that PR3 would be for the fader booster opamp? I want to make sure I'm focusing on the proper one.

 

[byoung]

Argh :-[ I swapped the BD139 and 140 on the 2520's,  seriously don't know how I missed that. Would this cause PR3 to blow? Also do you think they need to be replaced now, or just moved to the right position? I don't see any external damage to them, but not sure how robust they are.

 

[jsteiger]

Argh :-[ I swapped the BD139 and 140 on the 2520's,  seriously don't know how I missed that. Would this cause PR3 to blow?

Yes indeed it would. You can just try putting them where they belong and see if it works ok.

FYI, PR1 and PR3 are for the preamp opamp. PR2 and PR4 are for the fader booster amp.

 

[byoung]

Well I swapped the 140 and 139 in my 2520's and and put my second VP28 in the rack. Works great so far, going to let it burn in for a bit. I've got to get another 10r before I get #1 up running because I don't have any on hand, have hundreds of other values but no 10R.

 

[hecjrivera]

I recently bought the vp28 Rev B.1 and noticed its different from this build. Does anyone know where I can find the right guide?