wasp filter clone works

Haible Juergen Juergen.Haible at nbgm.siemens.de
Mon Jan 26 15:36:51 CET 1998


Hi,

I am back again (had to stay in bed one week - high temperature, no
work,
no electronics, but at least I re-read the whole Stephen King Dark Tower
series (;->) ).

On the weekend I have finally tested the wasp filter clone, and made
some 
modifications. Results so far:

It sounds considerably different than the ordinary SEM-type state
variable filter.
The maximum Q is lower on the Wasp version. And there is an additional
distortion
coming from the CMOS inverter nonlinearities. This distortion is
gradually increasing
with input level, and you can slightly hear it way before the circuit
actually clips.
The CMOS inverters seem to be the dominant source of distortion; the
CA3080
input dividers are rather on the save side. (100k / 1k ; but remember
the absolute
maximum voltage swing is limited to 5V anyway.) I have replaced the 100k
resistors
with 51k without noticeable increase of distortion.

Ok, the Wasp Filter sounds different than other state variables. But
what is it good?
IMO, every filter's overdrive characteristic  has its own special
applications. 
The SSM2040 in LP configuration, for example, is unbeatable when you
want to
process a full chord of buzzy "Jump"-type saw voices.
The Wasp Filter works best on Farfisa-type *organ* sounds. I tried
various sounds
from my OB-8, and really, the best results came with bright organ
sounds.
My favorite patch is the filter in LP mode with medium cutoff, and then
an envelope
with slow attack opening the filter (resonance quite low). This certain
"edge" that is
added by the filter's distortion is hard to describe, but very pleasant.

I had this circuit at the breadboard first, and I was so pleased with
its sound that
I build it again on a tiny veroboard. The whole filter consists of 3/6
CD4069 and
two 3080's, counting the active components. I built a notch filter fom
another 1/6
CD4069 (add HP and LP), a heavy distortion section (1/6 4069 with 1meg
feedback
resistor, and 100nF input capacitor without series resistor), and a
fixed 2pole, 5kHz 
LPF from the remaining 1/6 4069 as a speaker simulator for the
distortion section.
Now use the SVF to preshape the frequency response of some input signal,
and then go into the overdrive / speaker simulator. LPF, BPF and even
HPF settings
sound pleasant thru the overdrive, with or without resonance. For some
reason
the notch filter doesn't work well in this configuration. (But it makes
a great "one
notch phaser" without the overdrive, and controlled by an LFO.)

I was really astonished what you can get out of one single CMOS chip
(the OTAs
just act as variable resistors, so they don't get much credit here.)

I have drawn a little schematics diagram of the whole circuit. I'll scan
it in the next days,
and if you're interested we can put it at some web page.

I've also made some other experiments with the 4069. You can build a
little saw
VCO just with one 4069 and a npn pair: One inverter as integrator, two
more for
the hysteretic switch, another one as opamp that regulates the current
in the
exponential converter. Use another one for scaling / level shift and
drive a pnp
transistor with equal collector and emitter resistors, and you have a
poor man's
version of the VCS-3 variable triangle/saw VCO.
Or overdrive another inverter with a triangle wave, and add a DC offset,
and you have
a poor man's VCS-3 - "sine with waveform control".
Ok, these things have limitations. You *always* get a glitch when the
integrator
in the VCO changes direction. And the waveshapers work far from ideal,
i.e.
bend a positive slope more than a negative one, and I have not even
tested the
accuracy of the expo converter. But for one little CMOS chip plus a few
transistors
this makes a hell of a utility and modulation VCO.

Ok, these were the results from the weekend. And no, I have not touched
the
frequency shifter since one week. But I received some nice photos from a
friend
who took some photographs recently, and there is one with the FS in
progress
and me at the solder iron, so maybe I can scan this one as well.

JH.


PS.: Thanks again for all the PC/CD-R recommendations. I have not bought
anything
        yet, but I was overwhelmed by the amount and knowledge of the
response I got!

 



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