Expression Pedal User Guide and Technical Reference

Quick Guide to Expression Pedals

This guide provides an overview of how expression pedals function, their internal circuitry, and how to troubleshoot common compatibility issues. Expression pedals are data entry devices used to control parameters in connected equipment, such as keyboards or guitar stomp boxes.

Volume vs. Expression Pedals

It is important to distinguish between these two types of pedals:

• Volume Pedal: Contains two jacks (in and out). Audio signals pass directly through the pedal. It is connected in-line using standard 2-conductor 1/4 inch instrument cables.
• Expression Pedal: Contains only one jack. Audio does not pass through it. It is connected using a 3-conductor 1/4 inch TRS cable. Its function is to control internal parameters of the connected equipment, which must be assigned via the equipment's user interface.

How Expression Pedals Work

An expression pedal generally contains a single electrical component: a potentiometer (a variable resistor). As the pedal is moved, the shaft rotates, changing the resistance. This resistance is then interpreted by the connected equipment to adjust a specific parameter.

TRS Wiring and Connections

The "tricky part" of using expression pedals is the wiring. There are three connections involved: Tip, Ring, and Sleeve (TRS). Different manufacturers wire their pedals differently. The most common scheme involves the potentiometer's wiper connected to the Tip of the TRS cable. Some pedals, such as the Yamaha FC-7, use a different scheme where the wiper is connected to the Ring. If you encounter compatibility issues, you may need an adapter cable to swap these connections.

Compatibility and Troubleshooting

Because there is no universal standard for expression pedal wiring or resistance, users must verify compatibility:

• Resistance: Most pedals use a 10k ohm potentiometer. If your pedal has a higher resistance (e.g., 20k), the parameter may hit its maximum value before the pedal reaches the end of its travel.
• Calibration: If your equipment supports it, use a calibration function to synchronize the pedal's physical range with the target parameter's range.
• Taper: Expression pedals typically use linear-taper potentiometers. Using an audio-taper potentiometer (common in volume pedals) may result in uneven control, where the parameter changes rapidly in one part of the pedal's travel and slowly in another.
• Wiring Mismatch: If a pedal is incompatible, it may not work at all or could potentially cause a short. Always check the manufacturer's specifications for the required wiring scheme.