[BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread
- Thread starter jsteiger
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Help Support GroupDIY Audio Forum:
Chrome Heart
Well-known member
Chrome Heart said:
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
Well-known member
Fluke 87 says 1.228 between pins 2 and 3jsteiger said:
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 said:
Chrome Heart
Well-known member
They are GAR 2520 OPAMPs built from kits. I did swap them out with a pair from another build and got the same results.jsteiger said:
Chrome Heart
Well-known member
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?Chrome Heart said:
Anyway, there it is.
If anyone sees anything obvious, I’d be happy to hear it
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Potato Cakes
Well-known member
Chrome Heart said:
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
Well-known member
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.Potato Cakes said:
Chrome Heart
Well-known member
I would also suggest that desoldering the entire PCB as a pragmatic excercise would make matters worse.Chrome Heart said:
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.Chrome Heart said:
Potato Cakes
Well-known member
jsteiger said:
Beat me to it.
Chrome Heart
Well-known member
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.jsteiger said:
Potato Cakes
Well-known member
Chrome Heart said:
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
Well-known member
No worries , my friend. I appreciate your input.Potato Cakes said:
Chrome Heart
Well-known member
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.Chrome Heart said:
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
Well-known member
Chrome Heart said:
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
benplayskeys
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- Joined
- Jul 6, 2018
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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
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
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