Electro-Harmonix Bi-Filter – Vintage Analog Dual Filter

One of the rarest and most interesting analog filters ever built by Electro-Harmonix.

Designed by Mike Beigel (1947–2026), inventor of the legendary Mu-Tron III. RIP Mike.

Beigel later brought his distinctive analog filter expertise to Electro-Harmonix, where he and Mike Matthews designed the Q-Tron, Q-Tron Plus, Stereo Polyphase and Bi-Filter. The Bi-Filter has roots deeply connected to Beigel's earlier filter designs and represents a particularly sophisticated development of his approach to analog filtering.

The Bi-Filter essentially puts two analog filters into one unit, with extensive routing and control possibilities. It can be used as a stereo effect, with both filters operating independently, or as a serial dual-filter, allowing much more complex filtering and resonance effects.

It can produce everything from classic warm analog filtering and resonant sweeps to extremely expressive vocal- and vowel-like sounds. A fantastic creative tool for synthesizers, drum machines, bass, guitar, samples and studio processing.

A very unique piece of vintage Electro-Harmonix gear and a particularly interesting part of Mike Beigel's legacy. His Mu-Tron III became one of the defining envelope filters of all time, and the Bi-Filter shows his continued exploration of sophisticated analog filter design.

Condition:
The unit is in excellent overall condition and looks almost new. It has been very well cared for.

The only cosmetic issue is that the top-right rack ear is slightly bent, as shown in the photos. This is purely cosmetic and does not affect the functionality of the unit.

A rare opportunity to own one of Electro-Harmonix's most unusual vintage filters, designed by the inventor of the legendary Mu-Tron III.

I made a demo featuring a Hohner Clavinet D6. Check out the video link.