Phaser LFO idea

[WeAreAs1 at aol.com]

In a message dated 1/21/01 8:47:00 AM, Linc at christeld.freeserve.co.uk writes:

<< 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. >>

Hello Lincoln,

Yes, it's pretty easy to control the high and low peaks of an LFO wave as 
well as the overall depth of amplitude.  It just requires two knobs:  One 
that acts as a standard depth attenuator, and another to add or subtract DC 
offset to the LFO wave.  I realize that this may not be exactly what you had 
in mind, but the end result would be almost the same.  (the wave itself would 
still be symmetrical, though.  I think you are imaginging some kind of 
asymmetrical wave, right?)

I'm running out the door right now to get to the NAMM convention, so I don't 
have time to explain any further right now.  Maybe someone else can fill in 
the details, or come up with a better idea.  

For the asymmetrical wave idea, it would indeed be cool if you had two knobs: 
 One for the positive peak depth and one for the negative peak depth.  This 
would allow you to very easily create some unusual and useful LFO waveshapes. 
 I'm not quite sure how one would achieve this with circuitry - it might be 
tricky to get the zero-crossing part of the wave to be smooth.  Also, you 
would still probably want a third attenuator for controlling the overall LFO 
amplitude without affecting the shape of the wave that you created.

Michael Bacich