Phaser LFO idea

[Colin Fraser]

> -----Original Message-----
> From: owner-synth-diy at node12b53.a2000.nl
> [mailto:owner-synth-diy at node12b53.a2000.nl]On Behalf Of KA4HJH
> Sent: Sunday, January 21, 2001 7:18 PM
> To: Linc at christeld.freeserve.co.uk
> Cc: synth-diy at node12b53.a2000.nl
> Subject: Re: Phaser LFO idea
>
> This is a good time to repeat my request for a Constant Slew
> Rate LFO. The
> Speed and Depth controls counteract so that a constant slew rate is
> maintained. Would be great for phasors, flangers, and
> anything that does
> pitch-bending. I'm sure if we all had one it would be like "why didn't
> somebody do this before?!" Everytime I get something
> interesting happening
> with one of these processors I get annoyed having to constantly switch
> between the knobs to keep the slew rate from getting too fast
> or too slow.
>
> I can see the block diagram in my head but have gotten anywhere near a
> bench in quite a while...

Thinking out loud...
Your rising/falling output will be generated by an op-amp integrator (as
usual).
You just need to choose a current (ie a voltage thru a resistor) into the
input of the integrator to set your rate, and flip the sign of this
current/voltage when your upper or lower limit is reached - usually done
with a comparator on the output of your integrator, the square wave output
of which is divided by a rate control then fed back to the integrator input.

Now, if you take the output of your comparator/schmitt trigger and use it in
combination with at least a 2 way, 2 pole analogue switch (4052 will do) you
can change the threshold voltage of your comparator to select a different
positive and negative limit, and also switch between a different rising and
falling rate.

You should end up with an LFO with 4 controls - rise rate, rise limit, fall
rate, fall limit.

Need to get some stripboard out...


Colin f