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

[Mylithra]

Yeah though I walk through the valley of the shadow of death I will have at my side a good solder sucker.
Anything else.....  not really, go slow learn and enjoy!

Get a Hakko 808 or FR-301. It will change your life.

 

[JRoulat]

Many thanks for the recommendation.

 

[Chrome Heart]

So with a 400HZ test tone being injected at 1.228V, and all settings correct for the test I have the following results.

TP1 - 111.8
TP2 - .998
TP3 - 1.804
TP4 - .450
TP5 - .450
TP6 - .450
TP7 - .940
Output at pin 2 is 1.402

Im not sure why they are higher than the guide, but they look close. Are these acceptable values?

:bump:

 

[jsteiger]

Have you confirmed that you have 1.228V AC when probing between pins 2 and 3 of your signal generator XLR? You cannot verify this level by referencing high or low to ground.

 

[Chrome Heart]

Have you confirmed that you have 1.228V AC when probing between pins 2 and 3 of your signal generator XLR? You cannot verify this level by referencing high or low to ground.

Fluke 87 says 1.228 between pins 2 and 3

 

[jsteiger]

Fluke 87 says 1.228 between pins 2 and 3

OK. What opamps are you using? If they are built from kits, have you verified they are working properly in another know to be working properly module?

 

[Chrome Heart]

OK. What opamps are you using? If they are built from kits, have you verified they are working properly in another know to be working properly module?

They are GAR 2520 OPAMPs built from kits. I did swap them out with a pair from another build and got the same results.

 

[Chrome Heart]

They are GAR 2520 OPAMPs built from kits. I did swap them out with a pair from another build and got the same results.

Well, I replaced the opamps with a brand new pair of SL2520 and get the same results. Funny thing is, if I set the gain to 1 click below unity, the test numbers come up perfect. I’ve triple check component placement and all is correct. Maybe my signal coming from the daw isn’t exactly at 1,228v? Even though the meter says it is?
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it

 

[Potato Cakes]

Well, I replaced the opamps with a brand new pair of SL2520 and get the same results. Funny thing is, if I set the gain to 1 click below unity, the test numbers come up perfect. I’ve triple check component placement and all is correct. Maybe my signal coming from the daw isn’t exactly at 1,228v? Even though the meter says it is?
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it

You should always meter what the signal is at the XLR going into the piece of gear being tested. Even though the software says it's sending out a certain level, that doesn't mean your converter doesn't have a little bit of loss before it leaves the output.

So everything meters correct when you make an adjustment to the gain? Does the gain keep tracking down/up when you make changes? If changing something at the switch corrects the issue, then that would tell me it's something at the switch. And you may have triple checked your component placement, but did you completely disassemble the module and have a long, loving gaze at your solder joints? What's on top looks neat and clean but it's usually what lies beneath that causes great irritation and distress. So go ahead do the thing that we all try to avoid and take it all apart and really, really look at those solder joints and check for bridges. You may have to use your meter and test for continuity between the switch pins (shorts) and/or follow traces between components and make sure they're actually connected (cold solder joint).

Thanks!

Paul

 

[Chrome Heart]

You should always meter what the signal is at the XLR going into the piece of gear being tested. Even though the software says it's sending out a certain level, that doesn't mean your converter doesn't have a little bit of loss before it leaves the output.

So everything meters correct when you make an adjustment to the gain? Does the gain keep tracking down/up when you make changes? If changing something at the switch corrects the issue, then that would tell me it's something at the switch. And you may have triple checked your component placement, but did you completely disassemble the module and have a long, loving gaze at your solder joints? What's on top looks neat and clean but it's usually what lies beneath that causes great irritation and distress. So go ahead do the thing that we all try to avoid and take it all apart and really, really look at those solder joints and check for bridges. You may have to use your meter and test for continuity between the switch pins (shorts) and/or follow traces between components and make sure they're actually connected (cold solder joint).

Thanks!

Paul

The bottom is also clean, Ill pull the module and post a photo. As stated in a previous post, the output was measured directly from the output of an Aurora 16 with a Fluke 87.

 

[Chrome Heart]

The bottom is also clean, Ill pull the module and post a photo. As stated in a previous post, the output was measured directly from the output of an Aurora 16 with a Fluke 87.

I would also suggest that desoldering the entire PCB as a pragmatic excercise would make matters worse.

 

[jsteiger]

Well, I replaced the opamps with a brand new pair of SL2520 and get the same results. Funny thing is, if I set the gain to 1 click below unity, the test numbers come up perfect. I’ve triple check component placement and all is correct. Maybe my signal coming from the daw isn’t exactly at 1,228v? Even though the meter says it is?
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it

I'm gonna go out on a limb here and ask if you are 100% sure you have the preamp gain knob properly aligned on the rotary switch? Lowest gain setting is knob to 7:00. Highest gain setting is knob at 6:00. The above quote tells me its not aligned correctly.

 

[Potato Cakes]

I'm gonna go out on a limb here and ask if you are 100% sure you have the preamp gain knob properly aligned on the rotary switch? Lowest gain setting is knob to 7:00. Highest gain setting is knob at 6:00. The above quote tells me its not aligned correctly.

Beat me to it.

 

[Chrome Heart]

I'm gonna go out on a limb here and ask if you are 100% sure you have the preamp gain knob properly aligned on the rotary switch? Lowest gain setting is knob to 7:00. Highest gain setting is knob at 6:00. The above quote tells me its not aligned correctly.

I will check to see if I have the knob is correctly aligned on the shaft tomorrow. Ill admit, this is something I hadnt considered. Although it has been my experience in the past that the simple things can  certainly slip past you. Ill let you know.

 

[Potato Cakes]

The bottom is also clean, Ill pull the module and post a photo. As stated in a previous post, the output was measured directly from the output of an Aurora 16 with a Fluke 87.

My apologies, I did not that output of the converter was already verified. Also, I did not suggest desoldering the entire board. I would never wish such pain on a fellow DIY'er.

Let us know if Jeff's suggestions regarding knob alignment is correct.

Thanks!

Paul

 

[Chrome Heart]

My apologies, I did not that output of the converter was already verified. Also, I did not suggest desoldering the entire board. I would never wish such pain on a fellow DIY'er.

Let us know if Jeff's suggestions regarding knob alignment is correct.

Thanks!

Paul

No worries , my friend. I appreciate your input.

 

[Chrome Heart]

No worries , my friend. I appreciate your input.

Yuuup, I get the "total noob move" award on this one. I had lowest gain setting keyed at 6:00. Dont know how I missed that one, but Im not surprised.
So now that I have a pair of GAR2520 and SL 2520, is there a way to pair one of each in a build that would make sense? At least I could save some money on another VP28 and have a matched pair.

 

[Potato Cakes]

Yuuup, I get the "total noob move" award on this one. I had lowest gain setting keyed at 6:00. Dont know how I missed that one, but Im not surprised.
So now that I have a pair of GAR2520 and SL 2520, is there a way to pair one of each in a build that would make sense? At least I could save some money on another VP28 and have a matched pair.

Op amps are a preference thing in these types of builds. You kinda have to try them out see what you like. Some did an op amp comparison here on the forum a while back.

Thanks!

Paul

 

[Mylithra]

So now that I have a pair of GAR2520 and SL 2520, is there a way to pair one of each in a build that would make sense? At least I could save some money on another VP28 and have a matched pair.

Here is mine. I have SL-2520s in both sockets with a red dot in the pre and a blue dot in the fader.  I have the standard 2623 in the pre and the Litz in the fader.

I also tried the GAR 2520 w/ the GAR 1731 and like the sound of that as well.

 

[benplayskeys]

Hey Guys,
I have an odd one.
I’ve built over 20 of these preamps and this is the first real issue I’ve had with one.

Here is the setup-
Pro Tools tone generator sending pink noise out into an RND R6 rack. VP28 with the issue is plugged into slot 1.

The main symptom is that the unit passes a very very small signal out (basically nothing at all) in line mode when first turned on. The green LED does not show signal and the pre has to be cranked as well as the output fader to hear any resemblance of sound. I discovered that if I engage the “Mic” switch and immediately disengage it, the Line level input is suddenly working and passing signal at a good level. The odd thing is that when I come back the next day to re-test the same issue is occurring. I have also left it on for somewhere in the ballpark of 15-20 minutes and it suddenly drops out. This could then be fixed by again, engaging and disengaging the “Mic” switch.

Here is the trouble shooting steps I’ve taken.
-re-flow all solder points
-replace the Toneluck switch in the Mic/Line path
-triple check resistors are in the correct position.
-I have placed known working Op Amps in both Fader and Pre positions (Gar as well as SL Red Dots)
-I have also used a known working unit in this chain and it has no issues

Thanks for your help in advance.
-Ben