1000pF caps on api 312 inputs??

[rascalseven]

What is the purpose of these?

I'm converting an API 325 card to the 312 circuit using a Cinemag CM-75101A for the input transformer. Do I need to add these 1000pF caps between primary leads and ground as on the 312 schematic? I believe that schem was drawn for the 2622 input, and I'm just wondering if there is a need to keep them when using the Cinemag.

Thanks for any and all info.

JC

 

[louder]

alo jc
i read somewhere at the lab that you don`t need those caps.they were used to prevent some oscilation.i builded 4 api`s with those cinemags and didn`t used them,and they sound fabioulous.
best regards
pedro

 

[Whoops]

In an API 312 type circuit using ED Anderson 2622 replica transformers, the 1000pf caps from the primaries to ground are there to prevent RF noise? used as decoupling caps? or to tame down the 2622 frequency response?

Should those caps be soldered at the XLR input pins, or should they be on the pcb close to the 2622 primaries?

Thank you

 

[gar381]

alo jc
i read somewhere at the lab that you don`t need those caps.they were used to prevent some oscilation.i builded 4 api`s with those cinemags and didn`t used them,and they sound fabioulous.
best regards
pedro

I second pedro's response on the CM-75101A.  I didn't use the caps either on my
312 builds and they do sound fabulous !!

I know that Jeff uses these caps on the EA-2622 in the VP26 build so I would use them
with that trafo.

GARY

 

[ln76d]

In an API 312 type circuit using ED Anderson 2622 replica transformers, the 1000pf caps from the primaries to ground are there to prevent RF noise? used as decoupling caps? or to tame down the 2622 frequency response?

Should those caps be soldered at the XLR input pins, or should they be on the pcb close to the 2622 primaries?

Thank you

1000pF forms low pass filter, preventing RF. You can find some opinions that's the part of sound of API312. Is it audible?
I don't know
Best way is to compare. You can use DPDT switch and make A/B comparison.
If you will decide to use it, i think better way is to mount them on PCB at primary windings.

 

[Andy Peters]

1000pF forms low pass filter, preventing RF. You can find some opinions that's the part of sound of API312. Is it audible?
I don't know

I'm pretty sure that the RFI is audible, as it rectifies down into the audio band, or perhaps makes the input amplifier oscillate at supersonic frequencies which eats into headroom..

-a

 

[Whoops]

Thanks for your replies,
why is it recommended to use these caps the 2622 transformer and not with the Cinemag CM-75101A?

thanks