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Your phaser is an anti-feedback notch filter

8/27/1999 1:29 AM
Mark HammerYour phaser is an anti-feedback notch filter
Reading the current newsstand issue of Guitar Player today (heavy focus on electroacoustics), I was reminded that not every electroacoustic player can shell out big bucks for a deluxe amp optimized for acoustic playing, and that your average phaser design can serve very nicely as a practical and cheap antifeedback device. Here's how.  
 
 
 
First, electronic antifeedback devices. They can work in a wide variety of ways (e.g., some shift the pitch a wee bit so that what comes out of the speakers ain't what you're playing), but a tried and true traditional method is to use a notch filter (opposite of a bandpass filter, in which everything BUT a given range of freqencies passes untainted), with a variable notch frequency so that the user can dial in exactly the freqency that is feeding back and knock it out. You COULD use the traditional bass and treble controls but they tend to throw the audio baby out with the bathwater. Something far more selective is always preferred.  
 
 
 
Where do phasers fit into this?  
 
 
 
Remember that phasers produce their sound by introducing phase shift changes in a copy of the signal which, when recombined with the original straight signal, results in cancellation (notches) at those frequencies where the most phase shift was introduced. The "better" phasers have more stages, which produce more notches (one notch per two stages of phase shift). The spacing of these notches generally remains fixed as the sweep generator (LFO) moves through its range, driving the elements that determine where the phase shift is for each stage in perfect sync.  
 
 
 
In many circuits, these "elements" are either FET's or photoresistors, whose purpose is to behave like a varying resistor. In theory, you COULD have a foot-operated 6-stage phaser pedal that used a 6-way ganged pot, but lord knows where you'd find one, lord knows where you'd find a replacement if it ever bust, and only the devil knows how awkward the installation and wiring would be or how precisely the 6 pots would track each other. Far easier, obviously, to use something that WORKS like a variable resistor and can be more easily matched.  
 
 
 
Do all these resistive control elements have to be controlled in unison? Nah. If you had a dual-ganged pot (VERY easy to get) for every two stages of phase shift, you could control each notch individually by using the pot, instead of FET's or photoresistors driven by an LFO. Since the audible depth of the notch is a function of the mixing of phase shifted and straight signal, the amount of notch-filtering can be varied by what would normally serve as a phaser "intensity" control.  
 
 
 
A good design would allow the user to dial in pretty specifically (i.e., each two successive phase shift stages would be optimized to cover a specific range so that pot rotation could let you select precisely), and defeat individual notches (being able to select between 0, 1, 2, or 3 notches seems sufficient).  
 
 
 
Okay, there's the idea. I haven't seen a decent DIY project in Guitar Player in years. Ladies and gents, start your engines. Incidentally, I've seen more than one schematic of a tunable notch filter that looks exactly like this.