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It's a 2 pole all-pass filter, or a phase shifter. It's intended use is to allow low frequencies to "catch up" with the phase of the mids and highs in an attempt to correct for low end phase shift that can occur when running audio through several stages of gear. If your gear is phase linear like all of mine (each piece has a 2 hz to 200k hz bandwidth to eliminate phase shift in the audio band) you will never need it. If you run through gear with roll-offs at 20 hz or so, it may help. The same circuit with a dual reverse log pot to tune the frequency is what the variable phase alignment tools are made from. The Aphex is set at 360 degrees, making the two tuning resistors variable would allow a 0 to 360 degree adjustment.
Jim Williams
Audio Upgrades
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