Multipath Filters

[Haible Juergen]

	>
> http://www.et.put.poznan.pl/~mbartkow/SYNTH-DIY/n-path.html
	>
	>Tell me what you think.
	>
	>regards,
	>
	>MB

Hi Maciej,

thank you very much for this article !
This does explain something. Whished I had it before I went to the
breadboard. Yes, I held back my "results", because they were so unpleasant,
but now I dare speak.

I built a circuit similar to the EN design (changes see below), and I was
very
disapointed with the results. Guess I was expecting miracles ...
There were some spectacular comb filter sounds indeed, but all was spoiled
by heavy clock feedthru and aliasing distortion.

I probably made a big error, using a LM13700-based SVF instead of the
manually
tuned opamp version of EN. I suspect the offset voltages and buffer currents
in the 
filter are open doors for the clock to come thru, and going high impedance 
(1nF caps to fit the OTAs) was not improoving things either. I even suspect
capacitor mismatch (didn't select the caps) plays some role, because the
clock
became even more dominant when I touched one of the capacitors with the
scope
probe.

I do not want to turn down this filter per se, because it's clear that my
version made
the problems described in the article much worse. (But even the 1:30 sweep
ratio
of an optimized filter -mentioned at the end of the article - does not look
very impressive.)
Let this just be a warning that you can run into quite some pitfalls (as I
did).

The filter (even my own crap version) was great for one thing, nevertheless:
Processing
white noise. Like a very strong phaser - and the aliasing is masked by
noise.

Thanks again for translating this article - very interesting stuff indeed !

JH.