[sdiy] Help with noisy VCA

[Frédéric (Opensource)]

Hi Tim,

as Jay suggested:

1) No more than 100mVpp on LM13700 input for acceptable distortion
2) 1 mA to 1.2mA in IABC for an open VCA
3) I suggest you add a resistor between LM13700.12 and OPA1678.6
     because without it you forbid any voltage swing of the OTA output 
driver
     Although it works, my experience is, it is way less noisier when 
you have
     an output amplitude of 5Vpkpk at pin 12 as an example.
4) Then adjust R43

You can get really good results with the LM13700.
In my lastest design I managed to get below -80dB (compared to 0dB =
10Vpkpk) of noise floor with this VCA structure.

All the best from Germany.
Frédéric


Le 31/08/2017 à 23:09, Tim Ressel a écrit :
> Hi,
>
> I am still having trouble with this VCA design:
>
> https://drive.google.com/open?id=0B2yaoNYlV1nMWFhSRWUwdlU3TzQ
>
> It is noisy, and it gets worse as you turn up the gain. I tried 
> switching to a 2164 design but its no longer available in SMT.
>
> Any ides?
>