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Switch Selectable Range for the 2720-3B Filter

Improve The Versatility And Variety Of Sounds Available


In the 2720-3B manual, it is indicated that changing the values of C3, C4 and C5 will change the range of the filter. Halving the values will double the upper and lower limits of the range, while doubling the values will cut the limits in half.

Original 2720-3B Circuit


If the three capacitors are replaced by the following switching network, the filter will have three switch selectable ranges. When the switch is in position #1, the two capacitors in that branch are in parallel, which gives an equivalent capacitance of twice the original value. This corresponds to halving the frequency limits.

When the switch is in position #2, only one capacitor is switched in, thus duplicating the original circuit.

When the switch is in position #3, there are two capacitors in series to give an equivalent,capacitance of half the original value. This position will double the filter's frequency limits.

Switches S1a, S1b, and S1c are a three pole, three position rotary switch (similar to the range switch on the 2720-5 Control Oscillator). Note that all capacitor values are equal to the original values.

Modified 2720-3B Circuit



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Polyphony - Copyright: Polyphony Publishing Company

 

Polyphony - Apr 1976

Donated & scanned by: Retro Synth Ads

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Feature by Robert Matarazzo

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