[sdiy] 2164 -> TL074, 100 pF in feedback needed?

[David G Dixon]

I believe a value as low as 22pF works fine.

 

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] 
> On Behalf Of Mattias Rickardsson
> Sent: Wednesday, November 25, 2015 8:44 AM
> To: synthdiy diy
> Subject: [sdiy] 2164 -> TL074, 100 pF in feedback needed?
> 
> Hi fellow 2164 addicts,  ;-)
> 
> the typical application circuit in the SSM2164 datasheet 
> shows a 100 pF capacitor in parallel with the 30k feedback 
> resistor on the op-amp, saying
> 
> "The 100 pF capacitors ensure stability and reduce high 
> frequency noise.
> They can be increased to reduce the low pass cutoff frequency 
> for further noise reduction."
> 
> [...]
> 
> "If additional SSM2164s are added, the 100 pF capacitor may 
> need to be increased to ensure stability of the output amplifier.
> Most op amps are sensitive to capacitance on their inverting 
> inputs. The capacitance forms a pole with the feedback 
> resistor, which reduces the high frequency phase margin. As 
> more SSM2164’s are added to the mixer circuit, their output 
> capacitance and the parasitic trace capacitance add, 
> increasing the overall input capacitance. Increasing the 
> feedback capacitor will maintain the stability of the output 
> amplifier."
> 
> The problem is that they already lowpass filter the sound a little bit
> - more than ½ dB rolloff at 20 kHz - and sometimes you might 
> need a bigger resistor than 30k in order to optimize other 
> aspects of the circuit, resulting in even more HF rolloff. 
> Not awfully much, but you might have several of these stages 
> in an audio path adding up to some decibels.
> 
> But is 100 pF really needed?
> Is TL07x sensitive to capacitance on their inverting inputs?
> How about other common op-amps?
> 
> Using the bread-and-butter op-amp TL074, has anybody tried 
> out any variations on the 100 pF theme and run into problems?
> 
> I've used as low value as 33 pF & 47k after *two* summed 
> 2164s, withouth known problems yet. The datasheet pole 
> recommendation would be 127 pF for this scenario. Should I worry? :-)
> 
> (I'm sorry if this has been discussed already)
> 
> /mr
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