<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"><meta http-equiv="content-type" content="text/html; charset=utf-8"><div dir="ltr">This a convention that goes way back to the early days. Of course, having a high enough input impedance to avoid excessive loading has always been a consideration. With 1k output resistances, two outputs can usually be mixed directly, which is quite handy sometimes. So if you want no more than 1% loading, 100k input impedance is reasonable. But if noise from 100k resistors bothers you, then you should definitely go lower. Myself, I don’t notice it, and would prefer having the extra power savings.</div><div dir="ltr"><br><blockquote type="cite">On Nov 13, 2020, at 1:56 PM, Mattias Rickardsson <mr@analogue.org> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><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></div></div></blockquote><div><br></div>Sure, if noise is your only criterion.<div><br><blockquote type="cite"><div dir="ltr"><div dir="auto"><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></div></div></blockquote><div><br></div>0.5 here, 0.5 there can add up, though.</div><div><br><blockquote type="cite"><div dir="ltr"><div dir="auto"><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><br><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">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"><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 type="cite">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">synth-diy@synth-diy.org</a>> wrote:<br><br></blockquote></div><blockquote type="cite"><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">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"><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">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"><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"><div 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">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">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"><div style="width:100ch;word-wrap:break-word;word-break:break-word"><div 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"><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">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"><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">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"><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">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">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"><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"><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">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">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"><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">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">http://synth-diy.org/mailman/listinfo/synth-diy</a></span><br><span>Selling or trading? 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