[sdiy] Dual Voltage Controlled Delay

[Scott Stites]

Hi All,

As synth-diy goes, I’m still pretty much a rank
amateur.  It was actually some 15 years ago when, as a
guitarist, my interest in electronic effects led me to
a great passion for the synthesizer.  Started DIY way
back then with a hand-full of Curtis chips and big
plans, until life got in the way.  Only recently has
the bug bitten me in a really big way and I’ve gotten
back into it.  

Anyway, my favorite guitar effect was my old DOD analog
delay (sorry, Henry), followed closely by my old ADA
flanger.  As I have started breadboarding (sorry again,
Henry) a lot of the great designs you all have so
generously made available, I got sucked into a tangent
experimenting with the PT-2399 digital delay chip.  I
came up with an application that I’m not sure has been
done before or not.  If it has been done, I guess I’ve
been doing it in unknowing parallel.

It has to do with two separate voltage controllable
delay stages designed into one module.  The kinky thing
about my design is that I have provided a path between
the two delay stages for what I’ll call
‘cross-regeneration’ for want or ignorance of a better
term.  In essence, each delay stage has the usual
controls – delay time and regeneration.  The extra
control for each stage is a regeneration tap from the
opposite stage.  The module includes the two stages,
plus a final mixer where wet and dry balance for both
stages can be mixed.  There are actually more controls
for input level, and CV level for inverting and
non-inverting CV inputs.  I’ve also thrown in a
selectable inverter on the output of each PT2399 for
the even/odd harmonic thing.

As mentioned before, each delay stage has an extra
control that allows the wet signal from the opposite
stage to be mixed in to the other stage’s delay loop. 
The second stage signal input is normalized to the
first stage input with a switching jack so that the
delays can be easily run in cascade/parallel, or it can
be used as two separate delay devices.  With all of the
regeneration, care must be taken to balance the four
regeneration mix controls to prevent runaway
regeneration, but it’s worth it.  Running in cascade,
with complementary control voltages controlling each
stage’s delay (pushing and pulling time so to speak)
can provide some very unique effects/timbres.  In
addition to that, I’ve found that some really neat
rhythmic effects can be created using the setup.

Last night I was experimenting with Rene Scmitz’s VCO
3.  I was FM modulating it at 200 Hz and .3 Hz, and the
envelope was controlling the cutoff of a 12 dB low pass
filter and the delay time of both stages in a
complementary fashion. The AR envelope generator was
being gated with an LFO.  Out came this very strange
tone that sounded like someone talking to me on a
sideband radio in an unknown language.  I turned down
the delay mix, and the effect went away.  I thought
that this is just too cool, so I thought I’d throw it
on the table for comment or input from all of you very
knowledgable and gifted minds.  I haven’t presented a
schematic for two reasons:

1. Being a rank amateur, it is a very bloated and crude
circuit involving way too many op amps, and, frankly,
I’d be embarrassed to display it.

2. I don’t have a web page to upload it to.

Instead, if you would want to try it out, I would point
to Scott Bernardi’s excellent PT2399 design.  Just
build two and add a mixer or two.  I based my design on
the same ap-note that he did, and used a FET to control
the delay time, whereas Scott is using an opto-coupler,
which I’m pretty sure will give much better control and
isolation of the delay CV.  Plus, he provides for
voltage control of the regeneration depth.  And
finally, he knows what he’s doing.

Or, it could be done using a couple of other delays and
some mixers (the Blacet Time Machine springs to mind
*drool*).

For a larf, I do have a very crude schematic of my
ridiculous design with no component values (not hiding
anything, just got lazy) that I could email if anyone
is interested in just how out of control I really am.

Sorry about the long letter.

Scott Stites

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