Electro Harmonix produced the SYNTH9-U Synthesizer Machine as a polyphonic effects pedal designed to transform the signal of a guitar or bass into synthesizer tones. It carried nine presets tuned to emulate vintage synthesizer sounds, spanning lead synth, synth pad, and synth bass characters. The unit required no modifications to the instrument, no special pickups, and no MIDI implementation, and it functioned with both guitar and bass without additional configuration.

The SYNTH9-U provided four controls — DRY volume, SYNTH volume, CTRL1, and CTRL2 — allowing adjustment of each preset's parameters. Its usable tracking range extended to approximately the 23rd fret on the high-E string of a standard guitar and down to the open A-string on a bass guitar. The pedal was designed for use alongside other units in the EHX 9-series line, including the B9 and C9 Organ Machines, the KEY9 Electric Piano Machine, and the MEL9 Tape Replay Machine. A 9.6VDC-200mA power supply was included with the unit.

Owners seeking setup guidance, pedal chain optimisation, or assistance identifying compatible power solutions should consult a qualified guitar technician.