[sdiy] VTL5C2 Phase Shifter (was "Vactrols (oops)")

[Scott Stites]

Peter Grenader wrote:
> Sounds like the Serge Phasor,,  also vactrol driven.


I seriously doubt it has any of the elegance of a Serge circuit, but so far, it
works for me!  

It's about as simple as a phase shifter can get.  It's the classic op amp all
pass filter configuration, with the VTL5C2 LED's all driven from a common expo
sink, which in turn was 'borrowed' straight from Rene Schmitz's VCO4069 (thanks,
Rene!).  The output is mixed back in with the input for the resonance.  Right now
I've got a switch to mix the feedback either in-phase or anti-phase, which seems
to change the tone color with high levels of resonance.  The dry input and the
output of the all pass stages are mixed in another simple op amp mixer on the
output.

Currently it's just a four stage phaser, though I plan to keep adding stages to
see how things pan out.  One TL084, four VTL5C2's, four caps, the usual number of
resistors make up a four stage block, so it'll be easy to do.  What the circuit
could use is a limiter in the resonance loop (a la Jorgen Bergfors VCP), or,
better yet, a compansion scheme like the ARP Quadra or a JH variation of it.  All
of my experimentation with it (and everything else) is about to come to a
screeching halt, as I'll be out of town in the UK for four weeks pretty quickly
now.

As I mentioned before, the VTL5C2 response is pretty 'lazy' (especially on the
decay), but this does lend some interesting qualities in the response of the
phaser to hard edged LFO waveforems or faster LFO frequencies.  One thing I like
about it is that the Vactrols can't 'keep up' with the LFO as it approaches audio
frequencies, and the depth of modulation reduces as the LFO is increased.  It's
actually quite a pleasant effect - where my Small Stone clone tears up a signal
at those modulation frequencies, this phaser has a more subtle, pleasant effect. 
Shifting the LFO from very low to very high frequencies and vice versa is pretty
interesting.

Cheers,
Scott

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