[sdiy] Voltage Feedback Resistors and Circuit Stability

[Oren Leavitt]

A lot of the early Moog modules used 100k input impedance.

I am *still* using a big batch of 100k 1/4 watt resistors I bought 
almost 30 years ago!

On 11/13/20 4:06 PM, tpuefke via Synth-diy wrote:
> >100k shows up a lot in eurorack circuits since it's the defacto input 
> impedance.
>
> Could it be that a lot of (more recent) Eurorack circuits are heavily 
> inspired by Ray Wilson? ;-)
>
> As to the context and design intention behind my question, in this 
> case it's less specific, general curiosity. I'm just trying to weigh 
> the options, for example for small circuits, or like Mattias mentioned 
> in low-noise signal paths.
>
> I was suspecting that input impedance plays into this, but for op-amp 
> stages other than inputs there would be no obvious reason to use 
> values as high as 100k unless to conserve power.
>
>
> thanks,
> Tom
>
>
>
>
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Friday, 13. November 2020 22:48, ben gebhardt via Synth-diy 
> <synth-diy at synth-diy.org> wrote:
>
>>
>> (first time poster!)
>>
>> Hi Ian,
>>
>> Usually in my experience one would scale down resistors around an 
>> opamp to generally lower the johnson noise contribution.  
>> Additionally there is something to be said for high impedance nodes 
>> and coupling in radiated noise from external sources but usually 
>> that's only problematic over 1M.
>>
>> As to why it's commonplace, I'm not sure which schematics you are 
>> referring to, but as mentioned, 100k shows up a lot in eurorack 
>> circuits since it's the defacto input impedance.
>>
>> There are a ton of other factors in the decision of this resistor, 
>> it's hard to generalize rationale w/o the context of the circuit and 
>> design intentions.  Happy to comment on something more specific.
>>
>> Ben.
>>
>> On Friday, November 13, 2020, 04:16:17 PM EST, Mattias Rickardsson 
>> <mr at analogue.org> wrote:
>>
>>
>> Mattias Rickardsson <mr at analogue.org <mailto:mr at analogue.org>> skrev:
>>
>>
>>     And perhaps 10k-22k for TL06x.
>>
>>
>> I should have written 22k-47k.
>>
>> /mr
>>
>>
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