[sdiy] Voltage Feedback Resistors and Circuit Stability

[Michael E Caloroso]

100K is a "one-size-fits-all" solution.  Low power or specialty opamps
have weak output stages would not like a 1.5mA draw with a 1K load.  I
have encountered a circuit in a vintage Minimoog where a modern opamp
would not work in place of the original 741.

MC

On 11/15/20, Harry <hbissell at wowway.com> wrote:
> 100K certainly predates Ray Wilson.
> In the early modular synths, there was often a series resistor on the output
> of modules (op-amps) to prevent failure due to a shortedcable or bad
> connection, or back-feeding a voltage. Often 1K was used to limit current to
> 1.5mA.
> 100K is a 1% load on that.  Feeding two module inputs yields 50K, four =
> 25K.
> The error of 1% might already be too much for a pitch CV...
> So 100K is a good rule of thumb.  Couple of orders of magnitude larger than
> the output impedance can probably be ignored.
> Also, in a modular someone might parallel two outputs (I never do) and the
> 1K output allows a passive mix without damage.
> H^) harry
> PS.  Nice to see Bernie hanging out in the list.  Welcome.
> ----- Original Message -----
> From: Mattias Rickardsson <mr at analogue.org>
> To: Ian Fritz <ijfritz at comcast.net>
> Cc: Synth-Diy mailing list <synth-diy at synth-diy.org>
> Sent: Fri, 13 Nov 2020 15:56:35 -0500 (EST)
> Subject: Re: [sdiy] Voltage Feedback Resistors and Circuit Stability
>
> Out of curiosity, what is your reasoning behind 100k being a good
> compromise? I thought that the widespread use of 100k was an unfortunate
> consequence of having 100k input impedance as standard in eurorack. :-)
> A bog-standard TL07x draws 1.4 mA per op-amp and has a noise around 18
> nV/rtHz.
> - Any resistor above 22k is noisier, so let's not go higher than 10k.
> - Assuming a 5 V signal through 10k, the current is only 0.5 mA. Quite small
> compared to what the op-amp eats constantly.
> 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.
> /mr
>
> Den fre 13 nov. 2020 20:13Ian Fritz <ijfritz at comcast.net> skrev:
> R values around 100k or so give a good compromise between noise and power
> consumption.
> Ian
> On Nov 13, 2020, at 9:59 AM, tpuefke via Synth-diy <synth-diy at synth-diy.org>
> wrote:
>
> 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.
>
> 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.
>
> 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?
> Or is this a non-issue for fractional gain setups?
>
> I've been wondering about this for a while now and recently stumbled across
> this post discussing Rf values and op-amp input capacitance:
>
> https://www.analog.com/en/analog-dialogue/raqs/raq-issue-122.html
>
>
> What are the pros and cons in your experience for using high resistances
> like these, in attenuator / unity inverter setups especially?
>
> Higher resistances simply to reduce current consumption in bigger circuits?
> What about resistor noise?
>
> Just trying to wrap my head around this to be able to make more informed
> decisions.
> Any feedback is HIGHly appreciated. :)
>
>
> thanks,
> Tom
>
>
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