vcf5t update

urosuros urosuros at
Mon Mar 6 00:46:46 CET 2000

----- Original Message -----
From: tomg <efm3 at>
To: DIY <synth-diy at>
Cc: <efm at>
Sent: Sunday, March 05, 2000 12:25 PM
Subject: vcf5t update

> I did something really good guys! One of our members asked
> me to see if I could get this filter to work in his current project.
> Well...I wasn't successful doing exactly what he wanted ..but I
> did something even better.  I removed the opamp all together!
> Instead using a pair of inverting transistor amps. This cuts the
> parts count down to 3 trannys and the diodes of course.
> The coolest thing is it works soooo much better!! That kill
> the neighbors dog ,split the sky oscillation is gone. The
> cutoff response is very smooth. You can easily sneak up
> on the res advancing to full on without fear you are about
> to blow your speakers and when it does self oscillate it's
> even (more or less) across the entire res sweep.
> The update is here. This is very cool. you guys should build one
> today!
> Tom
Hi Tom and all others
I'm looking at your schematic and I must say that I'm puzzled by how VC is
first thing both ends of initial trimming pot are conected to Vcc ( btw what
is suply voltage ? )
Are you shure this is conected right ?
second thing both emitor and colector of transistor at which base is CV
conected are
conected to voltage suply across 10k resistors . Well this is definitly not
tranistor amplifier stage . It seems to me that transistor wwould be in
region of operation . Am I right whit this ? I rarely encoutered saturation
in linear
electronic ( and my impulse/digital electronic course is yet to come ) so Im
shure how to model it . But any way since E-Vcc and C-Vcc resistors are
conected across
same voltage ( and Vce is very small , if transistor works in sat. ) one
resistor will sink
curent and other will source curent , right ? Or wrong ? I would appreciate
if somebody explain
how this works .
Also , in old MS50 filter schematic from EFM we have 3046 array in diode
Are you shure Tom that 1N914 will work here ?
And two question not regarding this filter .
First if transistors in ladder filter are not matched that would decrese CV
but what else will be affected ? It seems to me that poles would spread a
little ( or a lot )
and that would decrese resonance and overall filter performanca , right ?
Could this be neglected by simply increasing feedback around ladder ?
And second : dif amp with active load has best performance in CV rejecting
discrete VCF and VCA configurations ) but I need three mached pairs ( for
in emiter , for active load , and for difpair ) Is there any simpler
( simpler = less parts and less matching ) with good Cv rejection ? The
would be one common colector transistor with controled curent source in
but it would have terrible Cv rejection and could only work in VCA ( has
anyone seen
VCF built around this ) ?
Thanks in advance
respect to you all

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