Electro-Harmonix

Oct 1, 2004 12:00 PM, By Doug Eisengrein

Electro-Harmonix's new Bi-Filter ($990) is an all-analog, dual-envelope, and LFO-controlled filter processor that was inspired by such classics as the original Mutron, Beigel Sound Lab's envelope controlled filter, and EH's Q-Tron. The 2U device is a single-channel processor with dual VCF filters that operate in either series or parallel. The unit was designed by Mike Beigel of Beigel Sound Lab fame, and according to Electro-Harmonix, “exquisite tone was the bottom line” in designing this unit. Bi-Filter sports an attractive retro-modern look, with a brushed aluminum faceplate, deep blue and teal section panels, and '70s-style graphics.

Command Central

The Bi-Filter's front-panel layout is feature-rich yet straightforward, with four main areas. The Drivers section has controls for sweeping the filters. There are slider controls for Envelope Drive, Sweep Oscillator Rate, and Sweep Oscillator Shape. A red Over LED indicates envelope saturation, and a Log/Lin switch gives you alternate envelope tracking. Other switches select Audio Boost and LFO Shape, which has Triangle, Square, and Off positions.

The next two sections house the controls for Filters 1 and 2, which are identical but separately adjustable. Envelope Response sliders control Attack and Decay, accompanied by Envelope Amount, Oscillator Amount, and Filter Frequency sliders. Range switches apply to the frequency sweep of the filters, and Filter mode switches select lowpass, bandpass, or highpass. Finally, there is a Q control for resonance and handy LEDs that track frequency Over Range and Under Range.

The Output Mixer section has separate main mix sliders for Filters 1 and 2, along with Dry, which mixes in the unfiltered (post — Audio Boost selector) control signal. Below each Filter Output slider is a phase Invert switch, and below the Dry slider is a switch that selects between parallel and series for the signal path through the filters. To the right of the Output Mixer are switches for bypassing the filters and FX loop. The front panel also includes the power switch and its accompanying blue LED.

The rear of the Bi-Filter contains an ample selection of ¼-inch jacks. In addition to the Audio Input and Main Output jacks, the rear panel houses separate outputs for Filters 1 and 2. Prefilter Effects Send and Return jacks are included, and a Bypass Footswitch jack takes a TRS connector, where tip carries the filters and ring carries the FX Loop. The icing on the cake is the control voltage jacks: each filter has separate CV inputs, and each envelope has separate CV outputs. The CV inputs will accept a TRS-type expression pedal for controlling the sweep of the filters. If you plug only into CV Input 1, the pedal will control both filters.

All Analog, All the Time

Electro-Harmonix suggests that the Bi-Filter is great for all manner of soft synths, so I loaded up a bleepy 16-bar techno loop created entirely in a Reaktor drum machine. I thought this would be a good first test and contrast with the analog filters, because all the sounds in this percussive snippet were created entirely with software-based FM tone generators. The sounds cover the audio spectrum, from thick kick drum to sticks, bleeps, and square-wave tones. I patched the main output and both filter outputs of the Bi-Filter to separate channels on my console so that I could analyze them individually, but first I checked out the Bi-Filter's own output mixer. All sliders are sturdy, and the unit was sonically very smooth.

My initial impression of the sound was analog with a capital A. The Bi-Filter's output was fluid, organic sounding, and punchy, producing floorboard-rumbling lows and screeching highs. The unit was typically warm, yet easily driven into distortion. While listening to different mixes of the filtered signals with the dry signal, some sounds exhibited phase cancellation, but that was easily remedied with the invert switches. I tried all manner of oscillator, frequency, invert, and parallel/series settings and got interesting results. The Bi-Filter was fun, and I got sucked in. Several audio examples of Bi-Filter processing are online at emusician.com (see Web Clips 1-20).

I continued my tests by feeding solo acoustic-guitar recordings, spoken word, and finally an old analog synth (an Oberheim Matrix 6) through the Bi-Filter, with delightful results. A few of the more satisfying features were the Audio Boost switch, which amplifies the signal according to the Envelope Drive; the Q slider, which really does a number on the resonance; and the sweepable LFO Shape control, which has a subtle, silky crossfade.

All About Tone

The tone of the Bi-Filter lives up to Electro-Harmonix's goal of exquisiteness, and its many features, including CV control possibilities, are sure to please analog purists. I found the Bi-Filter to accentuate higher frequencies better than lower ones, but it may just take time working the unit to find the sweet spots. I would have liked a switch to swap the order of the filters in the signal chain, but that would be a cherry on top. The Bi-Filter sounds fat, and with this much control at this price, I'm not complaining.

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Overall EM Rating (1 through 5): 4.5
Electro-Harmonix
tel.: (718) 937-8300
email: info@ehx.com
Web: www.ehx.com