one more on filters ...

Joachim Verghese jocke at
Wed Apr 17 17:29:52 CEST 1996

On Tue, 16 Apr 1996, Haible_Juergen#Tel2743 wrote:

> 3080 to (-) input of op-amp integrator, feedback from integrator to positive 
> (!) input of 3080 would also make a Lag like [2]. Should work even with 
> SSM2024's (which only have (+)-inputs and require a summing node at the 
> output). But I haven't tested it.

It does work! I have been waiting for an opportunity to build a
filter with 2024 gain cells, and last night I finally did it.
(Inspired by you folks!).

I used TL072 op-amps and 100pF caps for the integrators, and it
works just great. As the 2024 control inputs are effectively
a diode to ground, I used a sinking exponential converter via
a PNP mirror.

I've only tried the "safe" gain structure so far, i.e. 100k/220 ohm
resistors for integrator feedback and inter stage coupling. The
sound is very. very clean and almost identical to the ARP 4075,
which is not really surprising since the topology is similar.

One possible difference is that the 4075's maximum usable Q (before
self-osc) seems to decrease with increasing cut-off frequency.
The 4075 I used for reference did not have the cut-off range fix.
While the 2024 based filter allows for higher Q settings, it is
fairly difficult to control the Q near oscillation. You raise the
Q control a little, and suddenly you have oscillation. This is not
the case with the 4075, which is much more controllable in that sense.
It doesn't *jump* into oscillation.

Compared to a Moog ladder filter (using 3086 arrays), the 2024 seemed
to have considerably lower noise, and a warmer sound. The ladder
sounded more aggressive, but that might have been because of slight
input overdrive.

Going home now to do some more tests...


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