[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

[onionseller]

This will be fine. Just caps charging. We are basically looking for a direct short which you do not have so carry on!

Ok, thanks. I assembled two of these a few years  ago and I don't remember this happening.  Also I think the solder pads were bigger back then.

 

[Blondino]

I just finished my 9th preamp and the first vp26 stepped gain. Everything seems works fine but I have a question.
Since I have no way to check the sound  with  fancy machines (of course I checked the testing point first), I just load the pre with 2 mics (matching pair), and do a little recording. Nothing weird  so far.
Right now during this quarantine I have available just another vp26 variable gain. After testing I noticed a little difference between the two.
The stepped one has a tiny bit more output gain at the same level (thing I 've already noticed between other Vp stepped gain models) but the strange thing is that always the stepped one has more proximity. the tone is exactly the same (at least at my ears). Is not even presence, is more like, when you move the mic far from the source, is more noticeable on the variable gain then the stepped gain. And getting a sound from a close source seems the stepped has more presences.
I swiped the mics and re did it, and the result still the same, so they are a real matching pair 

Now, those thing are not really bothering me since both of them sounds awesome, I'm just looking for tips and confirm  those difference if somebody else had the same experiences.

although I did apply a little modification on the build, maybe you can give me a tip also on this one. For some reason (I'm not going to explain here since I already wrote too much), I applied pins to the output transformer cable. I checked before installing the ohm of the cable with and without pin and saw that was the same, so I decided to go for it. Obviously the result is that the preamp works, not sure if the pins are making actually difference here (pic is attached)

My question is:  did anybody noticed those differences like me between those 2 preamp?

 

[jsteiger]

....The stepped one has a tiny bit more output gain at the same level....

I think you mean the stepped gain version appears louder when the knob is set to the same physical position as a variable gain preamp? If that's what you mean then likely yes as they are completely different tapers and were never intended to be the same.

 

[Blondino]

I think you mean the stepped gain version appears louder when the knob is set to the same physical position as a variable gain preamp? If that's what you mean then likely yes as they are completely different tapers and were never intended to be the same.

Yes exactly what I meant!
I noticed between the variable 26 and the stepped 25, 312 but I was not sure till now.
So for the rest I guess is all fine and the socket won’t affected the output transformer.
Thanks Jeff

 

[zonkola]

Aloha! For my first post I have an esoteric VP25 question:

I have two Cinemag CM-75101A input transformers and two Cinemag CMOQ-2s output transformers that have been sitting in a drawer for a decade, and I'm looking at the VP25 no-iron kits thinking, hmmm...

(I've built a CAPI 500 rack, two VP26, and have a BT50 on the workbench now, so I'm up to speed on the CAPI basics.)

I found the thread that discusses the CMOQ-2s and it looks like it's just a matter of wiring it up correctly due to the differences in the primary & secondary windings compared to a CA2503. But what about the CM-75101? Has anyone used one in a VP25 and lived to tell the tale?

 

[Blondino.82]

I have two Cinemag CM-75101A input transformers and two Cinemag CMOQ-2s output transformers that have been sitting in a drawer for a decade, and I'm looking at the VP25 no-iron kits thinking, hmmm...
[/quote]

try to buy the no iron kit first...

 

[zonkola]

try to buy the no iron kit first...

I know they're out of stock now, but I assume they'll be available eventually. Will the 75101As work in the VP25 circuit without major surgery, perhaps by replacing C2 and R4 with a 150K resistor?

 

[WFDE]

Hello I have A VP25 Stepped Gain kit. At position 8 & 9 on the stepped gain pot have a considerable amount of noise. which is defeated by engaging the phase switch. Measuring across the first and last pins of the stepped pot comes out at 25K. I have also gone back to reflow all of the solder joints. If someone could point me in the right direction to troubleshoot further, it'd be much appreciated, Thanks

 

[RohanHG]

Hey there,

Not sure if anyone is on this thread any more.

I've just built two VP25s and repaired two old VP26s. (The 26's had a bunch of resistors out of place and were really dark/rolled off)

The frequency response is now all fine since I have put the resistors in their correct place. I'm just noticing that the VP26's are way down on level compared to the VP25's. Like 20db+.

I can't find any other obvious miss placed components. And all the functionality of the switched/pots etc work as expected. Just really low level.

Any thoughts on where to start with trouble shooting?

Thanks!

 

[Whoops]

The frequency response is now all fine since I have put the resistors in their correct place. I'm just noticing that the VP26's are way down on level compared to the VP25's. Like 20db+.

I've built both the VP25 and VP26 and used them for years, I don't remember a noticeable diference in gain of one compared to the other.

The circuits are really really simple, did you try swapping the Opamps from one unit to the other?
That would be the first thing I will do.
If you have the same problem with both of the VP26 probably the same mistake was made in both units.

Print the schematic, use an highlighter pen, and check all components and connections/continuity one by one while highlighting on the schematic.

If I remember correctly I think in this project I soldered the components on top and bottom of the PCB not just on the bottom.

I'm pretty sure you have a wrong value component or connection on the OPAMP or PCB

 

[RohanHG]

I've built both the VP25 and VP26 and used them for years, I don't remember a noticeable diference in gain of one compared to the other.

The circuits are really really simple, did you try swapping the Opamps from one unit to the other?
That would be the first thing I will do.
If you have the same problem with both of the VP26 probably the same mistake was made in both units.

Print the schematic, use an highlighter pen, and check all components and connections/continuity one by one while highlighting on the schematic.

If I remember correctly I think in this project I soldered the components on top and bottom of the PCB not just on the bottom.

I'm pretty sure you have a wrong value component or connection on the OPAMP or PCB

Thanks! Yeah I ended up looking over the board again and it was a couple of miss placed resistors. All good now! Thanks for your response!

 

[stribor1]

Need some advice. I built 2 VP26 (built already more than 10 CAPI modules). With a dynamic mic everything is fine but if try to power a condenser mic with phantom power I get a very audible high pitch whining sound (at -19db FS See pic) the mic works and passes audio but it is mixed with the whining sound. The 48V led is lighting up and I can measure around 48V on the audio input rails (pin 8 and 10). it is the same with both pres. Any ideas?

the mic is an U87ai and works fine with a soundsculptor pre in the same lunchbox (fredenstein 6).
the whining doesnt apear if the mic is disconected and phantm power is on, so it seems to be some interaction with the mic circuitry which causes it…

 

[Ricardus]

Need some advice. I built 2 VP26 (built already more than 10 CAPI modules). With a dynamic mic everything is fine but if try to power a condenser mic with phantom power I get a very audible high pitch whining sound. the mic works and passes audio but it is mixed with the whining sound. The 48V led is lighting up and I can measure around 48V on the audio input rails (pin 8 and 10). it is the same with both pres. Any ideas?

Forgive the obvious question, but have you tested the mic with other preamps to see if it's whining with those preamps as well?

 

[stribor1]

Forgive the obvious question, but have you tested the mic with other preamps to see if it's whining with those preamps as well?

Yes, just edited the post, the mic works fine with Soudsculptor and AML 1073 pres in the same lunchbox. It‘s an U87 ai

 

[jujufuego]

Hello everyone! I have built two VP28s and while one of them is working perfectly, the other one has a strange error. I thought maybe someone here has an idea where to start in order to fix it.

When i power up my case the signal LED on the VP28 stays on regardless if there is signal going in or not. I checked the output - there is no signal coming out of the VP28. When I turn the Preamp gain switch up to the 12 o clock position, die signal LED turns off and functions as normal. But when I turn the preamp gain back to the "line level" mark, it stays always on again. All the other LEDs are working as intended.

I think I already narrowed the problem down to one gar1731 I assembled because if I use the another gar1731 for the preamp section, everything is working fine with the module.

So I think there is a problem somewhere on the gar1731.

Any ideas where to start looking for the error? And how?

Thanks for your time and I hope I can fix it Other than this the build has been fun and I am loving the sound of the VP28!

 

[Ricardus]

Yes, just edited the post, the mic works fine with Soudsculptor and AML 1073 pres in the same lunchbox. It‘s an U87 ai

So upload some hi res pics of the front and back of the board.

Have you tried different DOAs?

 

[stribor1]

So upload some hi res pics of the front and back of the board.

Have you tried different DOAs?

The DOAs are fine, I actually use two different ones in the two pres. The pres also work perfectly if phant0m power is not used. Will post pictures later today….

 

[TheseGoToEleven]

Need some advice. I built 2 VP26 (built already more than 10 CAPI modules). With a dynamic mic everything is fine but if try to power a condenser mic with phantom power I get a very audible high pitch whining sound (at -19db FS See pic) the mic works and passes audio but it is mixed with the whining sound. The 48V led is lighting up and I can measure around 48V on the audio input rails (pin 8 and 10). it is the same with both pres. Any ideas?

Since this mic works with other pres in the same lunchbox, the mic and the +48V rail produced by the lunchbox power supply should be good.

I would suggest pulling the VP26 out of the lunchbox to check the values of R7/R8 (6K8) and R9/R11 (750R) to make sure they are correct. If the resistors are placed correctly, measure the resistance from R6 (the 200R) to pin 8 and from R6 to pin 10. The result should be the same, otherwise there would be an impedance mismatch on the input pins.

 

[stribor1]

OK, so it seems that my two VP26 are OK, I tested it with my DIY 511 Alliance rack and the same Neumann U87 and everything works fine. Seems like the combination Fredenstein lunchbox + Neumann U87 + CAPI V26, has some issues with the Phantom power, this is even valid for the VP26 modules i finished years ago, same symptoms, didn’t notice it before since the I got the Neumann only few months back. Realy strange because with another condenser mic ( SE ) it works fine.
Anyhow, thanks for all the help….

 

[jsteiger]

OK, so it seems that my two VP26 are OK, I tested it with my DIY 511 Alliance rack and the same Neumann U87 and everything works fine. Seems like the combination Fredenstein lunchbox + Neumann U87 + CAPI V26, has some issues with the Phantom power, this is even valid for the VP26 modules i finished years ago, same symptoms, didn’t notice it before since the I got the Neumann only few months back. Realy strange because with another condenser mic ( SE ) it works fine.
Anyhow, thanks for all the help….

Check the series resistor before the pair of 6k8's. It should be 200R. The phantom part of the VP26 is extremely basic and textbook.