About VCF's

[Joachim Verghese]

On Wed, 10 Apr 1996 gstopp at fibermux.com wrote:

>      Oh yeah I forgot about the ARP filter based on the LM3900!!!
>      How about some opinions about the sound? To be honest I don't 
>      remember.

Well, the 4075 is a very clean filter for a mid-70s design. Low noise
and distortion, that is. There's an extra differential PNP pair at
the input which kind of works as a limiter. Thanks to this limiter,
the filter behaves very well at high resonance settings, and will
even pass the input signal while self-oscillating.

Some people complain about the 4075 being too clean or "CEM-like",
and last time I did listening tests I found that it doesn't quite
pick out the harmonics the way some other designs do.

For DIY'ers the 4075 design is pretty straightforward, provided
your are willing to Vbe-match and thermally couple the five differential
PNP pairs. Or you could use PNP array chips such as Elantec's EP2015.
The LM3900 is easy to find, and there's very little offset build-up
in the circuit since the four 3900 amps are identical and the output
of each filter stage feeds the inverting input of the next stage (and
of course you have to have well-matched PNP pairs, too).

The schematics will be available through my upcoming web page. If you
can't wait, send me your fax number. or maybe I could covert it to a
uuencoded gif to transmit via email.

-joachim