With some basic soldering skills, building a custom controller that works with the expression inputs on Neunaber Stereo Pedals is fairly easy. This can be accomplished using a traditional treadle or simply a small box with a knob and optionally a knob cover.
The expression inputs accept a 3.5mm TRS (tip-ring-sleeve) plug, where:
- Sleeve (shield) is common (ground)
- Ring is a 3.3V output (top of pot)
- Tip is control voltage input (wiper on pot)
Most expression pedals have a minimum-value pot (P2) connected between the bottom of the expression pot (P1) and sleeve (GND) connection, as shown in this EV-5 schematic:
Since the maximum value is controlled by the knob on the pedal, you will have full control over the minimum and maximum range of the expression controller.
Single Knob Preset Switch
Here is a schematic for a preset switch that switches between the knob on the pedal and the knob on the external controller:
R1 controls the brightness of the LED. Values less than 1k ohm are not recommended, as this may limit the maximum value of the expression control. For best results, use a high-efficiency LED.
Dual Knob Preset Switch
Here is a schematic for a preset switch that switches both knobs:
Note that both knobs are a ratio of the knob setting on the pedal. In other words, you can switch between a high-Mix, high-Depth setting and a low-Mix, low-Depth setting.
External Bypass Switch
If you want to bypass the pedal using an external controller plugged into the Mix expression input, simply
- connect the switch between the tip and sleeve contacts
- connect the tip contact through a 20k resistor to the ring contact
The following schematic shows these connections:
Single Knob Mix Preset with Bypass
You can combine an external bypass and preset switch, but note that the external bypass only works with the Mix control: