After turning the world on its damaged ears with a pair of volatile distortion boxes, the Agonizer and the Butt Probe (see for the reviews),
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The Metasonix TM-6 is a tube-based filter that can be used with synths, drum machines, and even electric guitar (if you boost the instrument''s signal enough).

After turning the world on its damaged ears with a pair of volatile distortion boxes, the Agonizer and the Butt Probe (see for the reviews), Metasonix has returned to a more traditional synth paradigm by releasing a vacuum tube — based multimode filter, the TM-6 ($399). The company's previous filter, the TM-2 (an Editors' Choice Award winner in 2003), had a unique sound and included a VCA circuit. Besides being the only other commercially available tube filter, the TM-6 has a distinctive sonic character that combines a vintage vibe with an aggressively modern attitude.


The TM-6 contains a pair of military-grade 6AK5 pentode tubes, illuminated by a small LED and visible through a Plexiglas portal. A green light on the front panel indicates when the unit is receiving power: although there is no power switch, there is a bypass switch. The TM-6 requires a 12 VAC wall wart for power; don't accidentally use a DC supply, or you'll damage the unit.

Like the other TM-series processors, the TM-6 can be used as a stompbox, as a tabletop effect, or mounted in a 19-inch rack or a modular synth using the optional mounting kits. The TM-6 works well with any line-level signal, from drum machines and synths to prerecorded mono audio tracks. When used with an expression pedal, the TM-6 works exceptionally well as a wah-wah on electric guitar, as long as you boost the guitar's signal using a preamp or effects processor (see Web Clip 1).

Double Your Fun

The TM-6's filter circuit is based on the classic Twin-T notch design using two parallel bandpass filters, with ranges from 300 Hz to 1 kHz (lower filter) and 800 Hz to 1.9 kHz (higher filter). The Bandpass Range switch sets the distance between the two filters, from roughly two octaves (Low setting) to three octaves (High setting). By comparison, the TM-2 had only a 2-octave spread between the filters. The TM-6's wider range is a welcome feature that substantially increases the timbral palette of the device.

The Bandpass Range switch also determines the resonating behavior of the filter circuit when it goes into oscillation. The circuit is more peaky, and consequently more prone to self-oscillate, in the Low setting.

The Filter Type knob lets you set the overall behavior of the filter from bandpass (fully clockwise) to 4-pole lowpass (12 o'clock setting) to Bass Only (fully counterclockwise). This last setting passes audio through within a frequency range of 80 to 200 Hz. As soon as you move the knob from its fully counterclockwise setting, the bandpass filter's signal returns to the output.

The Filter Tune knob moves the center frequencies of the filter pair simultaneously. The Tune CV input allows you to sweep this control using a control voltage (a 1-octave sweep requires 0 to 4V) or expression pedal.

Filter resonance is also given a knob and a CV input on the TM-6, and it's these two controls that can make the filter scream. The TM-6 goes into oscillation easily, especially when full-range control voltages are sent into the Resonance CV input. If you're using CVs from analog modules to control the filter frequency or resonance, you'll want to attenuate the CV because the TM-6 is very sensitive. (Keep your master fader down while you're tweaking your patch to protect your hearing and tweeters.)

Turning up the module's Input Level control curbs the self-oscillation and adds extra beefiness to the sound. To hear the full range of the TM-6, the input control should be set near the 12 o'clock position. However, the module sounds great when the input gain is fully maxed out. But don't expect the TM-6 to give you the extreme distortion of the TX-1 or TX-2: this circuit pushes the tubes just enough to add a chubby richness to the signal rather than completely obliterating it.

The Envelope, Please

Because the resonance is so volatile, it's easy to modify drum tracks and sequences by setting the filter at the edge of oscillation, so that the various peaks in the signal give you sweeping pitches or xylophone-like pings. One of my favorite patches involved sending a drum machine through an envelope follower that controlled the TM-6's Filter Tune while simultaneously processing the drum sequence. I set the resonance to the near breaking point to add subtle downward pitch sweeps (see Web Clip 2).

Bandpass filters are great for adding “glorpiness” to synth patches, and the TM-6 works well in this regard. It's a little wild and rambunctious in its behavior, but once you get to know it, the TM-6 offers great subtlety and a range of sounds unavailable elsewhere.

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