[sdiy] MORE Russian synth schematic...

[Jaroslaw Ziembicki]

----- Original Message -----
From: "harrybissell" <harrybissell at prodigy.net>
To: "mikas" <mikas at turbodsl.lt>; <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, September 07, 2004 5:59 AM
Subject: Re: [sdiy] MORE Russian synth schematic...


> If the transmission gate IS a 4016... the polarity of thsi signal is
> wrong IMHO.
> The oscillator gives negative going pulses, which would make the switch
> closed almost all the time.

That's right, it looks like a mistake in the schematics. DD4 pin 10 instead
of pin 11 should drive the pins 13 and 12 of DD9.

> Any experiences with this type of switched filter ??? I would have to
> believe that dropping the modulation into the audio range would be heard.

I built such a VCF some twenty years ago. It was based on a schematics from
the soviet "Radio" magazine, either... A "classical" variable-state filter
with three
op-amps (741 type); the integrator resistors were switched by analog
switches
(1/2 of 4016), both driven from a VCO (approx. 10kHz - 1MHz) with a
constant-width pulse output (1µs).
Unfortunately, I cannot recall if there was a crosstalk of the switching
frequency
on the output... As far as I recall - there was NONE. I think the switching
frequency is being filtered out in the integrators. An alternative could be
a
constant-frequency clock (eg. 1MHz) with pulse-width modulation.
Note: there are some other musical applications of "switched resistors", for
example VCA's, voltage-/frequency-controlled mixers, phasers etc. They are
all very simple! The only difficulty is to generate a high frequency clock
with fine
and precisely controlled duty cycle. A task for an FPGA?

Regards
Jarek