Phaser LFO idea

[Scott Bernardi]

We have to distinguish here between controls which are part of the
phaser circuitry, and part of the LFO circuitry. A phaser is basically a
set of staggered voltage controlled all pass filters that result in
notches in the response. The reactive voltage controlled elements are
OTA's, LDR's (LED/photoresistor pairs like Vactrols) or FET's. The first
two are actually controlled by currents, so you need voltage to current
converters (such as an exponential converter, or a simple linear
converter - opamp with a transistor in the feedback loop).
Feeding a DC voltage in to the voltage to current converter will control
the center frequency of the allpass filters. This could be done with a
pot labeled "CENTER". Feeding an ac LFO signal in to the v-c converter
gives you the sweep. If you put a pot on the output of the LFO to
control the amplitude, this gives you the "DEPTH", and of course you
would have a "SPEED" control to control the frequency of the LFO. By
choosing the CENTER frequency and controlling the DEPTH (amplitude) of
the LFO, you can precisely control the top and bottom of the sweeps as
you desire.
This is the approach taken by the classic Univibe (guitar phaser
stompbox made famous by Jimi Hendrix and Stevie Ray Vaughan) circuits.
Roger Mayer's improved version, the Voodoo vibe, uses an ICL8038 for the
LFO. Here are schematics:
http://www.green-fuz.freeserve.co.uk/vvibe.html



Lincoln Fong wrote:
> 
> Forgive me if this has been done but I've always wondered if it would be
> possible to build a phaser with an LFO that, rather than the usual GAIN and
> DEPTH controls or sometimes just DEPTH, had 2 pots marked TOP OF SWEEP and
> BOTTOM OF SWEEP. The advantage being that instruments in different registers
> need a different sweep to get a nice even swirl and avoid the part where
> nothing much happens.
> Put in another way can you make an LFO where you specify the exact height of
> output swing, top and bottom using analogue stuff? I'm sure it can be done
> but can't picture it. Another thing that springs to mind (that could be
> used) is the idea of a slider pot with 2 wipers (homemade?) to avoid
> problems of crossing over ranges.
> 
> Lincoln

-- 
Scott Bernardi
sbernardi at home.net