From The Hague, Rob Hordijk has been creating musically sophisticated electronic instruments for years, such as the Blippoo Box and the Benjolin. Some of his designs have been licensed for mainstream distribution: For example, Epoch Modular (epochmodular.net) offers a Eurorack version of the Benjolin (read our review at emusician.com/benjolin).
Epoch’s latest release is the Hordijk-designed TwinPeak ($335), an ingenious filter design with a variable response/curve. Taking its name from the Blippoo Box’s TwinPeak Resonator, the Epoch design is a realization of the inverse-parallel principle, where two lowpass filters of equal gain but opposing phase are combined to create a bandpass response with peaks at the corners of the pass band, as the filter with the lower cutoff frequency is subtracted from the one with the higher cutoff. The result is a variable width bandpass filter where the cutoff frequencies of the two filters can be crossed and, regardless of which is tuned higher or lower, the resonator output will be a bandpass response. Furthermore, you can set the passband to “near zero” bandwidth with both filters tuned in unison, regardless of cutoff frequency. The Epoch TwinPeak has a 4-pole architecture and 12dB slope, as well as a wavefolding circuit that lets you morph the filter response from lowpass to variable-width bandpass.
The module has two audio inputs and a level-input/fade control. When the knob is at 12 o’clock, you hear both signals equally. When the Curve control is fully counterclockwise you get a basic, resonant lowpass response, which you can modify using the 1V/octave input, Peak A knob, CV Mod A input, and the Peak A Mod attenuverter. Add resonance with the Res/Ping control and CV Res input.
Turn Curve counterclockwise to hear a dual-filter response; each can be individually tuned and modulated using its associated CV inputs and knobs. The Dual Mod CV input affects both cutoff frequencies simultaneously and has a greater range than the A and B modulation inputs.
Like many filters, the TwinPeak provides percussive timbres when pulses and gates are patched into the audio inputs. Epoch’s design yields a longer sustain than other filters when pinged, and its builtin “all harmonic distortion” gives you a wider range of sonorities than expected—from metallic to woody. Audio-rate modulation takes the timbres even further.
Turn Res/Ping clockwise to lengthen the tone and use the Peak A and B controls to set the peak frequencies. With Res/Ping fully clockwise and one or both tones tuned low, your note will sustain for several seconds. Back off Res/Ping to about 2 o’clock to shorten the sustain and create bell-and marimba-like sounds.
You can dynamically alter the length of the tone by patching a voltage into CV Res, then use the A, B, and D CV inputs to change the pitches in real time. I sent a triangle wave into Input 1 and a pulse into Input 2—each separately sequenced—then patched a voltage into CV Res to create a vibraphone-like sound that randomly alternated between sustained and muted.
The TwinPeak excels as a sweet-sounding resonant filter, and it can be made aggressive or provide vocal-style formant filtering. And if you’re a fan of lowpass gates, you’ll find that the TwinPeak has a similar feel when fed pulses, but offers a wider range of timbres than any other thanks to its variety of modulation inputs.