[sdiy] op amp based filter

[JH.]

>Do you think that using op amps with very low GBW will make it (much)
>less noisy?
>
>Notice there are op amps with a GBW as low as 2.7 kHz as listed by
>Digikey, so they might not end up having any noise problems at all.

I haven't lookt into the respective data sheets, but I may well be that such 
a low GBW is the direct result of a low bias current.
I once bought a bag of alleged LM324 opamps for my PolyKorg clone, only to 
find that these had a slightly different prefix, and hat 1/10 of the power 
consumption, but also 1/10 of the bandwidth of the LM324. Normally, noise 
goes up also when current goes down.

>On the other hand, maybe an op amp with an external capacitor could
>work?

But then again, why not just use an "ordinary" opamp-based filter topology?
Using the compensation cap as an *additional* filter pole for an opamp that 
is already connected as a LPF may be interesing.
Eventide had a clever high-order LPF that only used one opamp. (A LM301 = 
LM101, actually.)
I have used that circuit in my SOST Flanger, if you want to take a look.


>So are you saying that every op amp is actually an OTA based LPF? (I
>know little about op amps)

Probably not all opamps, but certainly most opamps:
Input stage with differential pair is a gm stage (voltage controlled current 
source).
Then a voltage gain stage with compensation cap (forming an integrator)
Finally a unity gain buffer (output stage)

JH.