[sdiy] Any noise avantage to parallel 2164 VCAs?

[rsdio at audiobanshee.com]

Right. I forgot that these are current output, which can be summed directly.

Stacking op-amps doesn’t work because they’re voltage output.

A (additional) mixer circuit isn’t needed for a current output device.

Brian


On Jun 17, 2018, at 1:32 PM, electronicpresskit <electronicpresskit at gmail.com> wrote:
> The datasheet shows an example of 4x 2164 cells being summed into one current to voltage converter, so no additional circuitry is needed. I think it’s more accurate to think of the 2164 gain cells as a voltage controlled resistor rather than an opamp stage.
> 
> epk
> 
> On Jun 17, 2018, at 10:24 PM, rsdio at audiobanshee.com wrote:
>> Stacking doesn’t work because you’ve tied two outputs together. You need at least a few Ohms of output resistance for this to work. A mixer solves this issue.
>> 
>> The theoretical maximum improvement is 3 dB S/N, but the added mixer circuit will reduce this. How many dB of noise does a resistor add? It depends upon the resistance.
>> 
>> Brian
>> 
>> On Jun 17, 2018, at 1:16 PM, Schwarz Raphael wrote:
>>> Wouldn’t this cause impedance issues all over the place ? Like the idea of physically stacking op-amps is a good intuition but yields bad results (never tried it myself but it seems to be a recurrent topic in guitar pedal DIY )
>>> 
>>> Le 17 juin 2018 à 17:34, Ian Fritz a écrit :
>>>> Circuitry? Can’t you just parallel them pin for pin?
>>>> 
>>>> On Jun 17, 2018, at 6:05 AM, Schwarz Raphael wrote:
>>>>> Hi Tom, 
>>>>> Don’t know if the effort of using them in parallel is worth the circuitry , I reckon noise is already low enough in the 2164 for synth use, however you usually use a pair per vca to linearize the vca response (or a mixed response) as 2164 has a log response that is generally unwanted (log envelope into a linear vca produces a natural envelope , log envelope into a log vca will produce loud clicks) but maybe you are pairing this with a digital linear envelope ? That would make sense !
>>>>> 
>>>>> Cheers !
>>>>> Raph
>>>>> 
>>>>> Le 17 juin 2018 à 13:49, Tom Wiltshire a écrit :
>>>>>> A quick query, if I may: Is there any noise advantage to using two 2164 VCAs in parallel?
>>>>>> 
>>>>>> I’ve got a circuit that uses two VCAs, so there are two others spare on the chip. And then it occurred to me that there might be a minor gain by using them in parallel pairs. It’s a case of “well, they’re there so we might as well use them” if there’s no reason not to.
>>>>>> Or would it be better to simply disable the unused sections?
>>>>>> 
>>>>>> Thanks,
>>>>>> Tom