[sdiy] Help with noisy VCA

[Tim Ressel]

1. Its a wave folder so I am not worried about distortion
2. I am at 100uA for Iabc
3. Interesting. I was wondering if a resistor to ground plus buffer 
would be a better configuration. What value resistor?
4. Right.

I've used this VCA circuit in another product and it worked fine. What 
else could be causing the noise? Did I get a batch of noisy 13700s?

--tr


On 9/1/2017 3:07 AM, Frédéric (Opensource) wrote:
> 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?
>>
>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com