<html><head><style> body {height: 100%; color:#000000; font-size:12pt; font-family:times new roman,new york,times,serif;}</style></head><body><div>100K certainly predates Ray Wilson.</div><div><br></div><div>In the early modular synths, there was often a series resistor on the output of modules (op-amps) to prevent failure due to a shorted</div><div>cable or bad connection, or back-feeding a voltage. Often 1K was used to limit current to 1.5mA.</div><div><br></div><div>100K is a 1% load on that. Feeding two module inputs yields 50K, four = 25K.</div><div><br></div><div>The error of 1% might already be too much for a pitch CV...</div><div><br></div><div>So 100K is a good rule of thumb. Couple of orders of magnitude larger than the output impedance can probably be ignored.</div><div><br></div><div>Also, in a modular someone might parallel two outputs (I never do) and the 1K output allows a passive mix without damage.</div><div><br></div><div>H^) harry</div><div><br></div><div>PS. Nice to see Bernie hanging out in the list. Welcome.</div><div><br></div><div>----- Original Message -----<br>From: Mattias Rickardsson <mr@analogue.org><br>To: Ian Fritz <ijfritz@comcast.net><br>Cc: Synth-Diy mailing list <synth-diy@synth-diy.org><br>Sent: Fri, 13 Nov 2020 15:56:35 -0500 (EST)<br>Subject: Re: [sdiy] Voltage Feedback Resistors and Circuit Stability<br></div><div><br></div><div dir="auto"><div><div dir="auto">Out of curiosity, what is your reasoning behind 100k being a good compromise? </div><div dir="auto">I thought that the widespread use of 100k was an unfortunate consequence of having 100k input impedance as standard in eurorack. :-)</div><div dir="auto"><br></div>A bog-standard TL07x draws 1.4 mA per op-amp and has a noise around 18 nV/rtHz.<div dir="auto"><br></div><div dir="auto">- Any resistor above 22k is noisier, so let's not go higher than 10k.</div><div dir="auto"><br></div><div dir="auto">- Assuming a 5 V signal through 10k, the current is only 0.5 mA. Quite small compared to what the op-amp eats constantly.</div><div dir="auto"><br></div><div dir="auto">I'd say anything between 4k7 and 10k is a good compromise between noise and power consumption for TL07x circuits. And perhaps 10k-22k for TL06x. For low-noise op-amps 2k2 is often a good choice, since higher values increase noise levels.</div><div dir="auto"><br></div>/mr<br><div><br></div><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Den fre 13 nov. 2020 20:13Ian Fritz <<a href="mailto:ijfritz@comcast.net" rel="noreferrer noreferrer" target="_blank" data-mce-href="mailto:ijfritz@comcast.net">ijfritz@comcast.net</a>> skrev:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex" data-mce-style="margin: 0 0 0 .8ex; border-left: 1px #ccc solid; padding-left: 1ex;"><div dir="auto"><div dir="ltr">R values around 100k or so give a good compromise between noise and power consumption.</div><div dir="ltr"><br></div><div dir="ltr">Ian</div><div dir="ltr"><br><blockquote>On Nov 13, 2020, at 9:59 AM, tpuefke via Synth-diy <<a href="mailto:synth-diy@synth-diy.org" rel="noreferrer noreferrer noreferrer" target="_blank" data-mce-href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a>> wrote:<br><div><br></div></blockquote></div><blockquote><div dir="ltr"><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">I was hoping someone more experienced could help shine some light on this issue. This is still quite puzzling to me as a non-engineer.<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">Looking through my collection of schematics from various places like MFOS (bless his soul), I see a lot of the standard 100k resistor in negative feedback op-amp configuration, for inverters, mixers, attenuators... For a long time i have been adapting these as my go-to values without giving it much thought and usually it works pretty well. The odd schematic here and there uses lower values.<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><div style="width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="width: 100ch; word-wrap: break-word; word-break: break-word;">Considering that high feedback resistance implies a higher gain potential, shouldn't lower values (e.g. 10k, 1k even) usually be a better choice in terms of circuit stability? <br></div><div style="width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="width: 100ch; word-wrap: break-word; word-break: break-word;">Or is this a non-issue for fractional gain setups? <br></div><div><br></div></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">I've been wondering about this for a while now and recently stumbled across this post discussing Rf values and op-amp input capacitance:<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><div style="width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="width: 100ch; word-wrap: break-word; word-break: break-word;"><div style="width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="width: 100ch; word-wrap: break-word; word-break: break-word;"><a href="https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html" style="color:rgb(45,113,184)" rel="noreferrer noreferrer noreferrer" target="_blank" data-mce-href="https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html" data-mce-style="color: #2d71b8;">https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html</a><br></div><div><br></div><div><br></div><div>What are the pros and cons in your experience for using high resistances like these, in attenuator / unity inverter setups especially? <br></div></div></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">Higher resistances simply to reduce current consumption in bigger circuits? What about resistor noise?<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">Just trying to wrap my head around this to be able to make more informed decisions. <br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">Any feedback is HIGHly appreciated. :)<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">thanks,<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;">Tom<br></div><div style="color:rgb(60,60,60);font-family:monospace;font-size:15px;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;width:100ch;word-wrap:break-word;word-break:break-word" data-mce-style="color: #3c3c3c; font-family: monospace; font-size: 15px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; width: 100ch; word-wrap: break-word; word-break: break-word;"><br></div><div><br></div><span>_______________________________________________</span><br><span>Synth-diy mailing list</span><br><span><a href="mailto:Synth-diy@synth-diy.org" rel="noreferrer noreferrer noreferrer" target="_blank" data-mce-href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a></span><br><span><a href="http://synth-diy.org/mailman/listinfo/synth-diy" rel="noreferrer noreferrer noreferrer" target="_blank" data-mce-href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a></span><br><span>Selling or trading? 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