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

[jsteiger]

I would firstly swap opamps around. Also make sure they are seated properly http://capi-gear.com/catalog/DOA_Install.php

Next rule out any outside possibilities like rack, cables, convertor settings yada yada

Bingo!  The DOAs all seemed to be seated properly but when I swapped them my gain came back.  Thanks for the advice.

This is a tell tale symptom of a bad solder joint.......somewhere on the build. The jarring action of removing/inserting the opamps probably "made it good".....for a while.

 

[northamrec]

I've probed between the +V and C DOA sockets and the -V and C DOA sockets and got about 15.5V for both. Are there any other tests that I could do to make sure everything is good to go before installing a new functional op amp? I found the VP28 top overlay with test points highlighted (see attached), but I'm not entirely sure how to do it. Is anyone willing to explain how to use my DMM with these test points? Any help would be very much appreciated. Thanks!

 

[jsteiger]

You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

 

[cbhris]

If I neglected to install the piece of foam tape beneath the 2622 transformer would that potentially cause problems shorting something?

The issues I'm trying to track down are very low, noisy output (gain and fader both fully clockwise to get usable signal level) and my polarity switch stops all sound from passing when engaged.

When I send a 1khz sine wave to the lunchbox at ~0db and I measure the hot and cold pins with my dmm, i get 4.5 at the input cable and 2.6 at the output.  The pad and hpf are not engaged, and the signal gets lower when they are engaged.

I bought pre-assembled DOA's. 

Also, thanks to northamrec for posting that test points image.  i had a hard time finding it.  can anyone hold my hand a little bit regarding how to test those points?  I'm very much a newb at this... i connect my vp28 to power, with the DOA's installed, sending a low signal sine wave to the input.  one probe goes on the test point, where does the other go?

Thanks very much for your help

 

[jsteiger]

Black probe goes to a 0V reference like the pad directly adjacent to gold finger #5 or #13.

These symptoms (like polarity cutting out signal) sound like bad/cold solder joints. The polarity flip is nothing more than the switch swapping the 2 primary wires of the first 2623-1. Bad solder joints or shorted pads on the switch would be a likely cause.

The foam tape is just for a spacer to keep the bottom of the can from shorting out to pads under it. Chances are, if it is shorted, you would have no signal passing thru the 2622.

 

[Golgoth]

Hi,

I'm thinking about replacing the EA2623's for Litz's into my VP28 (more uniform highs and deeper bass? That's all I need on my mix buss!  ;D) but something got into my mind, what if I put Carnhill's or Cinemag's instead?

Did you try when designing the 28, Jeff? Or did somebody tried something crazy here?

 

[jsteiger]

Hi,

I'm thinking about replacing the EA2623's for Litz's into my VP28 (more uniform highs and deeper bass? That's all I need on my mix buss!  ;D) but something got into my mind, what if I put Carnhill's or Cinemag's instead?

Did you try when designing the 28, Jeff? Or did somebody tried something crazy here?

No, I went with grandma's recipe when I originally came up with the VP28. I personally would not stray farther than the Litz 2623-1 but by all means, try out some other stuff!

 

[Golgoth]

Thanks Jeff

 

[cbhris]

I desoldered my 2622 to make sure it wasn't shorting...and i guess i wasn't careful enough as now two of the legs are loose.  they slide about 1 mm in and out of the transformer body.  is it shot?  is there a way to test with my voltmeter?

thanks

 

[jsteiger]

Yes. Measure DCR between pins 5-->8, 1-->3, and 2-->4.
http://www.capi-gear.com/catalog/images/gallery/2622/2622-specs.pdf

 

[cbhris]

Yes. Measure DCR between pins 5-->8, 1-->3, and 2-->4.
http://www.capi-gear.com/catalog/images/gallery/2622/2622-specs.pdf

thanks for the quick reply.

5-->8= no resistance
1-->3=55.4 ohms
2-->4=60 ohms

 

[jsteiger]

5-->8= no resistance

That is an open secondary so sadly this one is no good anymore.  :'(

 

[cbhris]

5-->8= no resistance

That is an open secondary so sadly this one is no good anymore.  :'(

Okay.. i will order a replacement.. 
i put an order for another vp28 kit in yesterday... if i order this replacement 2622 tonight do you think you could ship them together?

 

[jsteiger]

5-->8= no resistance

That is an open secondary so sadly this one is no good anymore.  :'(

Okay.. i will order a replacement.. 
i put an order for another vp28 kit in yesterday... if i order this replacement 2622 tonight do you think you could ship them together?

Yes they will ship together. Today is a federal holiday so no mail. All current open orders will ship out tomorrow.

 

[johnlukeyoung]

Hello,

I have two VP28's, one of which is ~10dB quieter than the other. I swapped the op amps and checked that all my polarity was correct, and still nothing changed. I then purchased the extender rig and I am attempting to measure at the test points with a sine wave at 800hz and 0dB, but the measurements are super inconsistent. If I measure the same point multiple times, the readings vary drastically.
Though I am not sure what use they will be, here are the readings I took. Any advice would be very helpful!

Unit 1 (quieter):
#1=0.355 (0.480) (0.380) (0.551)
#2=1.114 (1.464) (1.370)
#3=0.677
#4=0.705
#5=0.475
#6=0.526

Unit 2:
#1=0.409
#2=1.000
#3=1.738
#4=0.604
#5=0.599
#6=1.112

 

[northamrec]

You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

I set up a signal generator (-40db sine) in my DAW going out of my Lynx Aurora and into the lunchbox with my CAPI VP28 connected to an extender cable. I'm actually getting 0 V on all test points except #2, which initially measured .24 V. Clearly something is wrong! I'm still getting ~15.5 V at the DOA sockets.

 

[jsteiger]

.....If I measure the same point multiple times, the readings vary drastically.....

Readings that vary can mean a poor connection for your probes or more likely bad solder joints.

If you are injecting a 0dB signal I assume you are in line mode? If that is the case, are you sure the preamp gain is set to the Unity position (and knob is properly located) and the output fader is at 0 (and knob is properly located)? With 0dBu going into line mode, you shook have appx 0.39V AC at test point #6 which is just before the 1:2 output transformer which is providing 6dB of "free" gain.

 

[jsteiger]

You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

I set up a signal generator (-40db sine) in my DAW going out of my Lynx Aurora and into the lunchbox with my CAPI VP28 connected to an extender cable. I'm actually getting 0 V on all test points except #2, which initially measured .24 V. Clearly something is wrong! I'm still getting ~15.5 V at the DOA sockets.

Are you measuring ACV or DCV? Sounds like you are measuring DCV. Audio is ACV.

 

[northamrec]

You can inject a low level -40dB-ish sine wave into the preamp and monitor the test points shown in the doc. Follow in order which follows signal flow.

I set up a signal generator (-40db sine) in my DAW going out of my Lynx Aurora and into the lunchbox with my CAPI VP28 connected to an extender cable. I'm actually getting 0 V on all test points except #2, which initially measured .24 V. Clearly something is wrong! I'm still getting ~15.5 V at the DOA sockets.

Are you measuring ACV or DCV? Sounds like you are measuring DCV. Audio is ACV.

You're right. All the test points are between .7 and 1.0 mV AC, although the first one may be higher, i.e., ~ 1.5 mV AC.

 

[northamrec]

I just installed my new pre-assembled op amps and everything works! I'm very, very happy with how it sounds. Thanks for the help! Looking forward to building the 3 VP 28s that I just got in the mail...