ARP Axxe: Patch Points

I added a lot of patch points to allow better control from the outside. Just above where the keyboard used to be is a row of 1/8" jacks that allow inputs from the outside to be run into the faders above. A toggle switch above each 1/8" jack allows you to select which signal goes into the fader. If the switch is in the UP position, then the original signal (what went in before I started drilling holes) goes into the pot.

I tried using "short-circuit" type jacks for this, which make a normal connection when there is no cable plugged in. I had a lot of trouble with these. Many times they either connected NEITHER signal to the input or connected BOTH signals to the input. So I gave up on those and resorted to the much more reliable toggle switches.

Inverters

Both the VCA and the Pulse Width control for the oscillator expect negative control voltages. I wanted to be able to control these with positive voltages from the outside, so I built a little board with two basic unity-gain inverting amplifiers to reverse the polarity. These use the two halves of an LF412 op-amp.

VCA Control

Here is a schematic of the inverter used to drive the VCA:

Pulse Width Control

Here is a schematic of the VCO after the inverter has been added to allow pulse width control:

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