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

Tom Wiltshire tom at electricdruid.net
Wed Nov 25 19:50:46 CET 2015


I think the key phrase you used here is "not awfully much" ;)

The 30K/100p gives a Fc at 53KHz, which is a negligible effect on the audio range - a ½ dB you say - no big deal. You could use 220p and you'll still not notice. Likewise, unless you connect a ton load of 2164s together, you could use 47p without any problems. The SSM2164 is pretty forgiving in my experience.

You ask "Should I worry?". I think the answer is "No", unless you've got some good reason to believe it's causing a problem. But you say you've reduced the cap's value on a pair of 2164s with no problems. I've used TL07x op-amps and forgot to put any cap at all, and never saw any ill effects from it. If there was any ultrasonic oscillation, it either wasn't of large amplitude, or it was filtered by subsequent stages before it caused serious trouble. I suspect the 100p value is fairly conservative (larger than necessary), and it can afford to be, when the cutoff is set a full octave off the top of the audio range.

HTH,
Tom

On 25 Nov 2015, at 16:43, Mattias Rickardsson <mr at analogue.org> wrote:

> 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
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy



More information about the Synth-diy mailing list