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

[blacet at blacet.com]

I've been using 22pF and 33K for several products for years with the TL074
and SSM2164.

No problems but your results may vary.



> 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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