Variable phase subwoofer

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palesha said:
argo,
Can u give details of the circuits u have mentioned at the two sites? More information about the Exponential curve tracking potentiometer is required.
MP

Isaac has already shown a typical allpass filter - also called delay filter i.e. a filter that passes all frequencies and only delays the signal to certain degree and consequently alters the phase. The gain of the filter is 1. The value of the Rf is kept equal and unchanged while by changing the value of resistor R you can delay the signal - here is where you need potentiometer. But changing the resistor R value doesn’t alter phase by linear relationship but logarithmically. For an example: turning your 10k linear potentiometer from 0 position to 50% of its turn will not change phase from 0 deg to - 90 deg as you would have wanted, 75% turn won’t change the phase to 135 deg, and so on. That’s why you may want to use exponential (inverse logarithmic) potentiometer in order to maintain linear scale when turning your potentiometer’s knob.
Now if you use one of such filter bloc, you can change the phase from 0 to - 180 deg, if you use two of those identical blocs (hence the dual pot need) in series it will be possible to change the phase of the signal from 0 to -360 deg.
Look also at http://www.linkwitzlab.com/filters.htm 4 - Delay correction
allpass.gif


and http://www.rane.com/pdf/ac22sch.pdf find detail 2: Rane's typical delay circuit.

Argo
 
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