Pro-One Modifications: LFO and MIDI


Grab the schematic and follow along.

The Pro-One's LFO signal is inverted by U6a. The inverted LFO, the non-inverted LFO, and the Sample & Hold signal are fed to switch SW3 which sets the signal that becomes the LFO modulation signal to the Direct and Wheel modulation buses. The circuitry built around U6a must be inserted between the output of the Pro-One's LFO summing amp (U101 pin 8) and the top of the LFO MOD AMOUNT pot (R103). You'll have to break the trace that runs between U101 and R103 and insert this circuit.

The LFO pulse width can be set by applying a 0V to +5V signal to U106 pin 5. The Pro-One has this value fixed at 2.5V with R1107 and R1108. The circuit built around U6b allows changing the pulse width from 0% to 100%. You'll have to break the trace that leads from U106 pin 5 to the junction of R1107 and R1108 so that both of these resistors are no longer connected to U106. Then connect the output of U6b to U106 pin 5.

The Kenton MIDI interface provides a voltage signal that they recommend be connected to the filter frequency control input. The source of this signal can be MIDI key down velocity, MIDI aftertouch, or one of two user selectable MIDI controllers. Rather than hard-wire it to filter frequency, I chose to connect it to the Direct modulation bus (via attenutation pot VR3) so that I can apply MIDI controllers to modulate oscillator frequency, oscillator pulse width, filter frequency, or filter resonance. The connection to the Direct modulation bus is best done by attaching R36 right to the Pro-One motherboard near the summing node of U101.

Back to Pro-One Modifications