Mod Squad: KOMA Elektronik Poltergeist

Analog quadraphonic audio mixer, panner, and VCA
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Analog quadraphonic audio mixer, panner, and VCA
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While quad panning is very exciting, it requires a module-intensive setup. For this reason, I’ve always been attracted to the integrated 4-channel spatialization capabilities of the Buchla System Interface Model 227e. However, that module is expensive and has other, unrelated features (mic preamp, EQ, etc.).

KOMA Elektronik has come to the rescue with the Poltergeist ($849), a 4-channel analog mixer that offers both stereo and quad panning capabilities in a single, 28HP Eurorack module. Each of the four channels has its own audio input with gain control, click-free solo and mute buttons, CV inputs for panning, and VCA (with an attenuverter and attenuator, respectively), and a master knob to set the panning range between the speakers. A pair of Aux inputs, without input gain controls, is also onboard.

All of the audio inputs are controlled by the Master Gain knob and associated CV input, which ducks the output level as the incoming voltage increases. The Poltergeist’s CV inputs accept 0-8V signals.

It has four 3.5mm master-output jacks: In stereo mode, the two jacks on the left are assigned to the left channel, while the others go to the right; in quad mode, the four outputs can be assigned to individual speakers.

The length of time a signal remains in its panned position is set using the global Slope control. Turn it fully counter-clockwise to get a peaked shape, so the sound ducks as it moves between speakers. Turn the knob clockwise and the signals stay longer in each position, creating a smoother transition between speakers. You also have voltage control over the slope’s shape.

The global Origin control is used to tilt the panning of the channels away from their individual paths. Origin has a master CV input with an attenuator, and each channel has a bipolar attenuverter to set the amount of positive or inverse tilt. This allowed me to alternate the panning of two input signals in different directions using a joystick patched to the Origin’s CV input and setting the individual channels’ Origin knobs at opposing levels.

Each channel also has a Field knob, which introduces a phase-inverted version of an audio input to the main mix outs (post VCA). When panning with Field turned up, the target speaker is silent due to phase cancellation, and the input sound is heard from the other speakers: The manual refers to this as the Field Ghosting Effect.

The Poltergeist’s full panning range is traversed as a control voltage goes from 0 to +8V. It starts at the lower left output at 0, reaches the lower right output by 6V, then jumps back to the lower left at 8V. This allows you to pan in circles using an upward ramping sawtooth and the Pan’s attenuverter to set the direction. (If you hear a click when the sawtooth goes back to zero, adjust the Slope control to smooth it out.) Careful adjustment of the pan’s attenuverter will help smooth out the circular rhythm, to avoid hearing the lower left output twice (at the beginning and at the end of the cycle).

But that’s a minor issue once you discover the joy of moving four discrete sounds around the room using the Poltergeist. Moreover, the interface is easy to grok, making it suitable for live performance: Even in the heat of a gig, it’s easy to remember what everything does. And although the Poltergeist isn’t cheap, you definitely get what you pay for.