<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On 24 Oct 2020, at 9:50 am, Ben Stuyts <<a href="mailto:ben@stuyts.nl" class="">ben@stuyts.nl</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">On 24 Oct 2020, at 00:59, Adam Inglis (sdiy) via Synth-diy <<a href="mailto:synth-diy@synth-diy.org" class="">synth-diy@synth-diy.org</a>> wrote:<br class=""><blockquote type="cite" class=""><br class="">The lower third of this image shows the key CV processing circuit, including portamento…<br class=""><br class=""><br class=""><br class=""><br class=""><br class="">I wonder if some clever person could explain to me the function of the twin FETs Q504 and 505 ??<br class=""></blockquote><br class="">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.<br class=""></div></blockquote><div><br class=""></div><div class="">On 25 Oct 2020, at 12:33 am, Doug Forbes <<a href="mailto:dougforbes1@gmail.com" class="">dougforbes1@gmail.com</a>> wrote:</div><div class=""><span style="caret-color: rgb(0, 0, 0); font-family: "Times New Roman", serif; font-size: 12pt;" class="">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.</span></div><div class=""><div class=""><br class="webkit-block-placeholder"></div></div><br class=""></div><div><br class=""></div><div>Many thanks Ben and Doug</div><div>The SH2000 was released in 1974.</div><div>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!</div><div><br class=""></div><div>cheers</div><div>Adam</div></body></html>