[sdiy] Roland SH-2000 key CV circuit detail

[Ben Stuyts]

> 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…
> 
> 
> <10.gif>
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> 
> 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.

Ben