The Phædrus audio PHILTER is based on an enhanced version of the EQ we developed for the London Console.
As you can see, the EQ is made up of capacitor and inductor circuits: there's not a gyrator, op-amp, or state-variable filter in sight!
Using the PHILTER is simple. I/O is via transformer-coupled, line-level XLR connectors on the rear, and the front-panel houses the tone-controls. The Bass and Treble controls are 11-position rotary-switches. Bass frequencies are controllable (boost/cut) by a total of 10dB, in 2dB steps. Treble frequencies are controllable (boost/cut) in the same way. However, treble boost curves are selected via the middle, multi-position rotary-switch. This selects boost centered upon the annotated frequencies: 2.5kHz; 3.5kHz; 5kHz; 8kHz; 10kHz; and 12kHz, or between a high and low shelf response. The response skein for the PHILTER (with the boost-curve selected to 5kHz) is given below. These responses were chosen to emulate famous EQ circuits of the past, and every one is magically "musical".
The only disadvantage of passive EQ is (being passive) signal is lost and the gain must be made up. In the PHILTER, this gain is provided by the internal PHILHARMONIC amplifier; a modular amplifier modelled after the "classiest" of all classical gear!
.....and using ONLY valves (tubes) for amplification
The PHILTER User Manual is available here
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