[sdiy] Roland SH-2000 key CV circuit detail

[Adam Inglis (sdiy)]

> On 24 Oct 2020, at 9:50 am, Ben Stuyts <ben at stuyts.nl> wrote:
> 
> On 24 Oct 2020, at 00:59, Adam Inglis (sdiy) via Synth-diy <synth-diy at synth-diy.org> wrote:
>> 
>> The lower third of this image shows the key CV processing circuit, including portamento…
>> 
>> 
>> 
>> 
>> 
>> I wonder if some clever person could explain to me the function of the twin FETs  Q504 and 505   ??
> 
> They form a high-impedance op-amp together with IC503. Q504’s gate is the + input, Q505’s gate is the - input and is connected to the output of IC503. Therefor IC503, Q504 and Q504 form a unity gain inverting amplifier. The reason they added both JFET’s is to increase the input impedance. This decreases the droop you would get on the sample/hold cap C504.

On 25 Oct 2020, at 12:33 am, Doug Forbes <dougforbes1 at gmail.com> wrote:
I am pretty sure they are there to provide a really high impedance for the .33 uF sample and hold cap. I am not sure if there were fet input op amps at the time of the SH2000. You need two so that the feedback loop sees the same characteristics as the input.



Many thanks Ben and Doug
The SH2000 was released in 1974.
I checked Walt Jung’s OpAmp Cookbook - the first of the reliable FET-input IC op amps didn’t arrive until about then (CA3130) or later (TL071) so this must’ve been the most economical way to do it!

cheers
Adam
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