[sdiy] Help with noisy VCA

[Tim Ressel]

Hi,

its +/- 5 volts in and out. with the divider that works out to about +/- 
100mV at the input of the 13700.

I'll make the gain change and see.

--tr


On 9/1/2017 10:58 AM, Jay Schwichtenberg wrote:
> 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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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com