<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=""><div><blockquote type="cite" class=""><div class="">On 5 Oct 2020, at 05:28, ColinMuirDorward <<a href="mailto:colindorward@gmail.com" class="">colindorward@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class=""><a href="http://www225.pair.com/audio/waltzingbear/Schematics/Urei/545.JPG" class="">Here</a> is the schematic for the Urei 545 PEQ. I understand that it's a highpass filter followed by four BP SVFs summed into the dry signal, then a lowpass filter.</div><div class="">I'm stuck on the in/out switch, though. What's going on here? I know what it's supposed to do, but I don't understand how it works.</div></div></div></blockquote><div><br class=""></div>The switch connects the output of IC1b directly to the output stage IC7. The output of IC1b is very low impedance and is connected directly to the + input of IC7. Therefor, the processed signal (coming in via R52) is completely attenuated (short circuited you might say) by IC1b’s output.<div class=""><br class=""></div><div class="">There might be some bleed but it is attenuated by R52 (10k) over the output impedance of IC1b (extremely low).</div><div class=""><br class=""></div><div class="">Ben</div><div class=""><br class=""></div></div></body></html>