[sdiy] Voltage Feedback Resistors and Circuit Stability

[Harry]

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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