[sdiy] op amp based filter

[cheater cheater]

On Sat, Jul 24, 2010 at 20:19, JH. <jhaible at debitel.net> wrote:
>
>> 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 know very little about low-bandwidth op amps (and not so much about
op amps in general) but I think they might just be trading bandwidth
for precision?

> 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.

Good to know though. I'll be wary of that.

>> 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.

Thanks. Found it. They've directly taken it from the N01 app note for N = 1..3.

I've also noticed that if you look at the 1/2/301 schematic (page 15
of the app note), then the part between the OUT terminal (pin 6) and
the BAL/COMP1 terminal (pin 1) looks dauntingly like a moog ladder
filter, but that might just be my imagination. Check it out though:
the first pair seems to be Q17/Q9, the second pair Q14/Q13, the third
Q16/Q12. But that's just vague guesses, I have no idea what that
circuit actually does do. It just looks similar ;-) Of course, the
tell-tale capacitors separating the differential pairs are missing,
but there's a resistor between Q16/Q12 and OUT is indirectly a tap to
that, so if you put a capacitor there, it gets interesting, right? I
wonder what Tim S would say to this.

>> 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.

Good to know. But as I understand simply every op amp has a bandwidth
of some sort that deteriorates with the gain, so this approach of
abusing that deficiency may work even with different types of
topology.

Roy,
On Sat, Jul 24, 2010 at 20:29, Roy J. Tellason, Sr.
<rtellason at verizon.net> wrote:
> Are you looking for multiple letter prefixes?  LM301, uA301,  etc.?  I can't recall offhand what else it might be under,  but that's a pretty common and generic part...

No, not necessarily anything special. Yes, it did seem to be common so
I was surprised why it didn't show up. Oren seems to have found
something cool though! Thanks Oren.