Pro-One Modifications: Oscillator A and Filter


Grab the schematic and follow along.

The Pro-One has triangle waves on both oscillators, but panel space restrictions kept them from having a switch to select the triangle wave on oscillator A. I'm a big fan of triangle waves so I did something about it. U7b amplifies the triangle wave to make it the same magnitude as the sawtooth wave. The sawtooth and triangle waves are then fed to opposite ends of pot VR4 for manually selecting triangle, sawtooth, or a mix of the two. This circuitry needs to be inserted between the point where U102 pin 8 reaches switch S106. Break the trace so that the sawtooth wave does not reach S106 then insert this circuit.

Filter resonance can be selected as a modulation destination simply by attaching one resistor to a switch that selects the signal from either the Direct or Wheel modulation buses. The middle position of the switch disconnects both modulation buses from the resonance control input of U111. The connection to the filter resonance input is best done by attaching R39 right to the Pro-One motherboard near U111 pin 9.

The filter's envelope generator signal is inverted by U7a. The inverted and non-inverted envelopes are selected with switches SW5 and SW6 to modulate filter frequency and the Direct and Wheel modulation buses. This circuit needs to be inserted between the output of U116 pin 2 and the tops of pots R101 and R1190. Break the two traces leading to these two pots then insert this circuit.

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