[sdiy] Help with noisy VCA

Jay Schwichtenberg jschwich53 at comcast.net
Fri Sep 1 19:58:16 CEST 2017


With 100uA Iabc I think you're doing you gain in the op-amp. I'd boost Iabc to get it closer to the 1mA limit and drop the gain of the op-amp buffer.

What is the signal input (Vpp) and what do you expect at the output?

Could be a noisy op-amp also.

Jay S.

> On September 1, 2017 at 9:20 AM Tim Ressel <timr at circuitabbey.com> wrote:
> 
> 
> 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
> 
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