Australian newcomer Sebatron offers up the Copernicus ($349), a true stereo multi-mode analog filter. The Copernicus has lowpass, highpass, and bandpass modes, with external CV control of the cutoff frequency. The unit is intended for tabletop use or for DJ coffins and lacks the large bypass footswitch of its guitar-oriented brethren.
The true stereo design lets you manipulate stereo signals from a single set of knobs or by using control voltages. That simplifies the sweetening and mangling of stereophonic material.
The Copernicus has two unbalanced ¼-inch inputs, two unbalanced ¼-inch outputs, and a TRS ¼-inch CV input. Power is supplied by a 12 VAC wall wart, which is available separately. The Copernicus seems sturdy and its knobs turn smoothly.
The unit has a Bypass/Active switch for introducing the filter effects. The Effect Dry/Mix knob, as you would expect, balances the dry input with the processed signal. Setting the Effect Dry/Mix control to its midpoint mixes the filtered and unfiltered signals together, giving you the interesting phase anomalies you would anticipate.
The Input and Output gain controls on the Copernicus allow you to add overdrive to the signal. The Stereo Offset control, as the name implies, offsets the response of the left and right filters. That lets you change the stereo spread in interesting ways. Incoming CVs can be offset as well.
The Low- and Highpass modes sound warm and clear. Bandpass mode acts like a band of voltage-controlled, boost-only parametric EQ. I especially enjoyed the character of the Highpass mode, with the resonance set just before self-oscillation begins. It adds a hollow, whispery detail which, when offset and mixed with the dry signal, consistently produced interesting results. To create a deeper response slope in any mode, the left output (filter 1) can be plugged in to the right input (filter 2). However, that setup lets you process mono signals only.
The Copernicus I tested offers only manual control over filter cutoff, which has a slightly skewed response; there is little change up to the center position and most of the audible spectrum is swept in the final quarter turn of the pot. With Cutoff set to or near minimum, manipulating the Stereo Offset pot produces an audible scratching sound. In addition, I detected a low-level hum at those settings. The manufacturer states, however, that these problems occur only in the early production run and have been fixed in the newer models.
Plug It In
The Copernicus may be controlled from a modular analog synthesizer or other CV source. The TRS CV jack sends 0 to 5 VDC over the ring connection, in proportion to the front-panel Cutoff control, with 5V output at the minimum setting. External voltage is received over the tip connection. This allows you to control another Copernicus or an external device that accepts CVs, with the caveat that the voltage range is inverted from what is typical in the world of synthesis. Signals from 0 to 5 VDC will change the filter cutoff frequency, but 0 VDC opens the filter completely and 5 VDC closes it completely. That means that a small voltage will sweep the filters at the top of their range, while larger voltage excursions lower the cutoff frequency.
The Copernicus has a few other quirks. For example, it doesn't respond according to either the volt-per-octave or hertz-per-volt standard. Additionally, quick voltage changes from a keyboard or analog sequencer produce an audible tick. The Copernicus responds to control voltage with the same exaggerated slope as its Cutoff control. I would enjoy seeing the unit altered to exhibit the standard volt-per-octave response, to simplify interfacing with a modular synthesizer setup.
A device such as the Big Briar Moogerfooger CP-251 (reviewed in EM's March 2001 issue) would be an ideal companion to the Copernicus because it provides the offset that is necessary for doing small sweeps at low cutoff frequencies. A CV expression pedal can also be used with the Copernicus, but again, you will need a device such as the CP-251 to manage its behavior.
Input and Output
The Copernicus is a clean, clear-sounding filter that will add character to any audio source. The convenience with which it can be added to a stereo setup is enticing. However, I wouldn't recommend the Copernicus to audiophiles or users who have complex CV requirements.