From Fonitronik:

This is the latest iteration of the PS3100 Resonators. The fonitronik mh21 Triple Vactrol Resonator prototype module has been tranformed from SMT to through hole, suitable especially for Eurorack DIY.

But what is it actually? Basically the module incorporates three voltage controlled band pass filters with fixed bandwidth (resonators), using the Dual-Vactrol-based core from the PS3100, and a simple triangle core LFO. By setting the BPFs to different center frequencies and modulating them, we get these nice resonating sounds. Depending on the source waveform and the modulation it might even resemble the human voice. The original purpose, however, was to make string patches sounding more vivid.

Fonik's Triple Vactrol Resonators (for Eurorack DIY) features:

• Three Band Pass Filters each with
• Single input
• CV input + attenuator potentiometer
• Initial frequency potentiometer
• LED
• Single output
• One summed output
• One common Audio input (normalled to single inputs)
• One LFO with
• Reset (sync) input
• Range switch (slow, fast)
• Rate potentiometer
• Output

The LFO is normalled to the 3 CV inputs, however, for the 3rd CV input the LFO signal can be inverted.