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

Hello and welcome to the forum.
Looks about right to me.When cranking the input gain with a hot signal from condenser mics you'll definetely overdrive the input stage.When a signal overdrives it starts to flatten out at its' peaks (these look like square waves then),after that (god beware) it even turns to dc.
It is unlikely that both preamps have been built wrong,it's more a matter of level matching.
E.g. an U87Ai will be good for speech and moderate singing at a gain of say 35 dB.
The input gain of these guys goes to +60dB,that's a lot and too much for a such a signal.
Do a short test and listen to it while increasing the gain,the distortion amount should follow your gain meaning starting clean.then start to gently overdrive up to heavy distortion..
Bringing down the output level won't help because-as said- the distortion has already taken place at in the input stage(s).

Best regards,

Udo.

kante1603 said:

Hello and welcome to the forum.
Looks about right to me.When cranking the input gain with a hot signal from condenser mics you'll definetely overdrive the input stage.When a signal overdrives it starts to flatten out at its' peaks (these look like square waves then),after that (god beware) it even turns to dc.
It is unlikely that both preamps have been built wrong,it's more a matter of level matching.
E.g. an U87Ai will be good for speech and moderate singing at a gain of say 35 dB.
The input gain of these guys goes to +60dB,that's a lot and too much for a such a signal.
Do a short test and listen to it while increasing the gain,the distortion amount should follow your gain meaning starting clean.then start to gently overdrive up to heavy distortion..
Bringing down the output level won't help because-as said- the distortion has already taken place at in the input stage(s).

Best regards,

Udo.

Click to expand...

Thanks, Udo!  Unfortunately I didn't get any time to do more testing tonight because I ran into a snag with my interface.  While I'm not intimately experienced with API-style preamps, I feel like I understand the concept of analog saturation/clipping well enough to feel worried.  The issue I'm having isn't that I've got a single, sinusoidal wave-form that's been shaved/flattened/clipped at the top; what's happening is that I'll have a loud transient (say, a clap), and about 50 ms or so (very rough guess) AFTER the initial transient, the waveform looks like it's going into a feedback loop before it spikes up in a very unusual/digital-looking nasty triangle-wave-meets-square-wave-meets-random-wave spike.  The sound that accompanies it is a very strange tweeting/chirping noise... almost like a high-pitch blip or beep.  Would something weird like this be expected from a properly built unit?  I had in mind that things would start to distort gradually and just become a more granulated version of the original signal, but this is something altogether different.

As soon as I get the time, I'll try some more testing.  All I can do is hope that the builds were successful and that if they weren't I can get these babies working properly ASAP!

Thanks again for your help.  I'll report back.

O.k., a very simple thought comes up......are you sure it is really coming from the preamps?
I mean could it be that your microphone has an issue?

Just my 2ct,

Udo.

Looks like the issue on the one VP28 that wasn't working was some solder that had flowed on the underside of a capacitor and bridged it. Fixed and both units are working great now.

Just checking back in.  It appears that I had some really weird stuff going on in my session and the only thing that I can figure was the session was corrupt.  I did some line-level tests and the analog saturation appeared to be behaving exactly as expected.  I also took the pair in to a local pro studio I've got an affiliation with and did a comparison with an API 512c for some simple voiceovers, and let's just say I was very pleased with the results (as were the owner and engineer at the studio).  I'll check things out with the original mics I was using in a new session, but for now I think I'm good to go.  Thanks for giving it your consideration, sorry if it was just a red herring. 

No problem.glad it seems to work o.k.!

Have fun,

Udo.

I need to get the front panel off my VP28. Does anyone have a trick for removing the 1/8" to 1/4" knob adaptor? Seems to be pretty well on there and I don't want to damage the Grayhill.

EDIT Got it off by using pliers to carefully twist it against the end of the Grayhill's rotation. Made me hesitant but the switch seems to be fine.

An IC puller works a treat for this.

Thanks for a great build Jeff-quiet and clean and a fun construct!
Can this preamp be dropped into a 51-X box or does it need to be modified in some way?
If so, what modifications would be required?

Thanks
Mac

maq3396 said:

Thanks for a great build Jeff-quiet and clean and a fun construct!
Can this preamp be dropped into a 51-X box or does it need to be modified in some way?
If so, what modifications would be required?

Thanks
Mac

Click to expand...

No modifications needed.
edit:  will work fine in a 51x box.  it will run as standard VPR spec within the 51x box.

Thanks Gunpoint!

Cheers
Mac

Built up 3 of 'em in a day   2 for me, one for a buddy.  Only headache was that one unit was sounding all glitchy, so I took it apart and reflowed all the solder joints.  I put it back together and tested again only to find out that it was my interface that was spitting out glitchy audio, not the preamp.  Restarted the comp, tested again, boom, done!  easy breezy! 

Also, jeff's card extender test rig made it very easy to test 'em out without having to load them into the racks :-D 

Hey chunger

Thanks for the great build threads. May I just check please do you tin the legs of the resistors and caps and diodes etc?

Or no need now a days ?

Thanks
Mkr

Just Finished two here. Worked out the block. Great guide by Chunger.
Sounds great and a wonderful project. 
The only thing that could improve it for me being in the UK is if I could have paid a little extra for the small Allen key as they are a bugger to get here. The only place I could find it was in one of the trade delivery shops which have a twenty pound delivery minimum.  :-\
I ground down a precision screwdriver in the end as I got impatient.
Thanks to Jeff for this project and I will be definitely trying out some of the other CAPI stuff when they come in.

Mark C

+1 for the hex key (had the same trouble here) & congrats for your builts.

Hello everybody ! I'm french and I'm the happy owner of 10 vp 28! I already built 4 of them, but my first one is still not working properly.
I have a very low level, without the low frequencies, as If the hpf was engaged. I can use the mic and line modes, i can use the pad on the signal, and use the hpf, even If the original sound Is already filtered. All this with a very low level. The switches are working properly too.
I have to tell that i tried this first preamp with my first gar2520, that was not good, because i switched the bd139 and 140. So, the oamp heat a lot, and i wonder If there was no damage because of this try.
I checked the voltage on the oamps pins, and compared the r values with the other vp28 i built. All the values seems to be the same between the bad one and the good ones.
If someone can help me, it would be great!
I have to say that this amp is impressive, and I'm very happy that we can built a such beautiful preamp!
Thanks a lot.

I'd like to add that the green led is not flashing (because of the low level I think). I tried with several oamps, discrète and ic.

Last thing: If it can helps, on the link below, a PDF file i did, that help me to check and classify the resistors for the vp28.
https://db.tt/9z8T69oi

OOstudio said:

I have a very low level, without the low frequencies, as If the hpf was engaged. I can use the mic and line modes, i can use the pad on the signal, and use the hpf, even If the original sound Is already filtered. All this with a very low level. The switches are working properly too.

Click to expand...

Hello and welcome to the forum.
A thin or lacking low frequency signal at a low level indicates a balancing issue most of times.
I'd check transformer wirings and functionality first,seems like a "hot" or "cold" signal part is missing.

Best,

Udo.

Jeff and guys,
I finished my first 2 28's i couldn't be happier, it's an amazing kit with an amazing sound.
Just a question, I am measuring with a 1khz tone thru both units and I find a return signal with a difference of .3 db, this is really tiny but it's possible to improve it? .1 or .2 or something like that.

Cheers from Argentina!