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previous: Dave B. Hey,Is there any way to make a ... -- 11/2/1998 9:08 PM View Thread

Re: Alternate Phasing?

11/3/1998 3:11 PM
Mark Hammer
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Re: Alternate Phasing?
Here's a nutty, kooky, wacky, kinda idea...variable resistors!!  
 
An old issue of POLYPHONY (1978) had an article on design of phasers, and has an illustration of a two-stage phaser using a pot as a variable resistor for each stage instead of a FET (indeed, the FET is supposed to sub for the resistor, when you think about it).  
 
If you have a spare footpedal like a volume pedal, mount a dual-ganged pot in there instead of a single pot, and wire up the pot lugs between the cap input on the phase-shift stage and ground. Voila! If you're careful and lucky, you may even be able to get decent matching and a steep notch. The circuit would emulate a MXR Phase 45, albeit foot-controlled rather than with an LFO-based sweep.  
 
Feelin' even wackier? Then step on the gas and go the distance by wiring up 4 stages of phase-shift, and have the centre lug of each of the pots connected to ground. The #1 lug of each pot would connect to stages 1 & 2, and the #3 lug would connect to stages 3 & 4. Sweeping the footpedal would move the notch created by 2 stages up, and by the other two stages downward, at the same time. Pick your cap values right, stick some fine-tuning resistors in series with the pot, and you could have notches that pass each other, or notches that come together and move apart. Naturally, a linear-taper pot will likely provide the most predictable element for design purposes and matching.  
 
One more possibility for foot-controlled FET-less phasing is to have a buffer stage, split the signal into three, with one route going to a mixer/output stage, and the other two going to bandpass stages that can each be tuned by a single variable resistor going to ground (I forget the name of the circuit type, but it may be Sallen-Key). Each of the resistor elements would be part of a dual ganged pot, of course. Mix the outputs of the bandpass filters with the straight signal such that the filter outputs are out of phase, and you have an overall output with two tunable notches (an ersatz Phase 90). The nifty thing about this design is that you can easily tweak the resonance of each bandpass filter, as well as how much each notch contributes to the overall sound. You can also easily stick in a fine-tuning pot in series and adjust the spacing of the notches.  
 
Just to clarify, all of these use op-amps and are foot-operated.

 
Replies:
Jehan Sappideen I ripped open my colorsound phaser ... -- 11/26/1998 10:12 AM