[sdiy] LM13700 Noise

[Tom Wiltshire]

Isn't the problem there the "ridiculous amount of gain" rather than the unsurprising amount of noise? I think you may be looking in the wrong place ;)

Would there be a way to reduce the thresholds for the wave folder circuit so you can get a similar amount of folding without so much gain required? Or pass the signal through two stages of folding or something instead to get a similarly complex effect?

I don't think there's a sweet spot on the 13700 that will suddenly make everything ok. Sure, you can abuse it, but as long as you're using it right, it is what it. Fine-tuning isn't going to change the fundamentals of the device at all. More control current gives more control feedthrough. That might be ok if your control signal doesn't change fast, but it'll thump worse if you're feeding it envelopes (for example).

HTH,
Tom

On 6 Jul 2017, at 19:05, Tim Ressel <timr at circuitabbey.com> wrote:

> Thanks for all the info, but I am concerned about broadband noise. I'm already using the lin diodes.
> 
> I am making a wave folder that has a ridiculous amount of gain on the back end, and the VCAs are making too much noise for my customer. I'm trying to minimize the noise through optimizing the LM13700 circuit if possible. The alternative is to switch to a linearized 2164 VCA, but I am trying to avoid the redesign.
> 
> I'd like to know if there is a 'sweet spot' on the 13700. Less input, more control current, or vice-versa. that kind of thing.
> 
> --timbo
> 
> 
> On 7/6/2017 5:33 AM, Tom Wiltshire wrote:
>> There's a great paper on minimising *distortion* in the LM13700:
>> 
>> http://www.openmusiclabs.com/2015/10/my-summer-vacation-by-guest/trackback/index.html
>> 
>> It covers noise in passing, but you might learn something useful.
>> 
>> In short, I'd agree with Richie. Maximise the signal level, accept some distortion (just call it "warm analog character") and be sure to use a decent op-amp I-to-V convertor on the output not just a resistor to ground.
>> 
>> HTH,
>> Tom
>> 
>> ==================
>>        Electric Druid
>> Synth & Stompbox DIY
>> ==================
>> 
>> 
>> 
>> 
>> On 6 Jul 2017, at 12:51, Tim Ressel <timr at circuitabbey.com> wrote:
>> 
>>> Hi,
>>> 
>>> Does anyone know how to minimize noise in an LM13700 design? I've been playing with the balance between input level, control current, and output I/V conversion gain trying to minimize noise but with limited success. The datasheet doesn't seem to cover this.
>>> 
>>> Thanks!
>>> 
>>> -- 
>>> --Tim Ressel
>>> Circuit Abbey
>>> timr at circuitabbey.com
>>> 
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at synth-diy.org
>>> http://synth-diy.org/mailman/listinfo/synth-diy
>> 
> 
> -- 
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy