C-Bow — An e-bow for cymbals or any resonant surface (2020)

C-bow (cymbal electronic bow) is a feedback device that works exactly like a guitar e-bow but it is capable of exciting heavier resonant surfaces like metal percussion (cymbals, gongs, etc). It is made of a contact mic used as pick up, a transducer that re-projects the sound onto the same surface where the contact mic takes it, and a simple analog amplifier with a lowpass filter. The sound comes entirely from the object c-bow makes resonate.

As a contact mic I used a normal 35mm piezo. The transducer is a Dayton Audio DAEX19CT-4 (4 ohm / 5W). The first transducers I tried weren't even remotely powerful enough and Matthew Goodheart's platform about transducers spared me a lot of trial and error (The platform was called Evolving Door Music, but unfortunately it doesn't look online anymore). The circuit has a single op-amp (TL071) used as a unity gain buffer, a lowpass filter, and the LM386 used to drive the transducer. A tricky part was the isolation of the case: simply attaching a transducer and a contact mic to the case causes the case to feedback onto itself, and a the resonances of a little plastic box are far less pleasant than the delicate complexities of metal percussion. To isolate the contact mic, I tried wood, softer plastic, silicone layers, but the first easy solution was supermarket sponge. I cut a layer as high as the height of the transducer so that c-bow stays planted on the surface with two legs (see pic of the bottom part).

Features of c-bow:

  • A lowpass filter to cut the excessive high frequencies of the feedback. The 100k potentiometer controls the cutoff frequency.
  • A tactile momentary button that multiplies the gain by 10. It gives sort of feedback kicks that are useful to jump start the feedback loop or simply fun to play with. The sponge isolates the mic so the case doesn't feed back, but I left a high volume zone where pushing the button makes the object squeak without touching any surface.
  • A toggle switch bypasses the tactile button and keeps the gain to the maximum. To avoid very high piercing frequencies, the lowpass cutoff should stay low, thus the feedback loop requires more gain to pass the filter and sustain itself: this toggle helps with that.
  • An aux in with volume control allows you to add an external signal (a radio, an mp3 player, etc.) to the feedback loop or simply let you send onto the surface whatever input you want so that the surface works like a speaker (a pocket solution for David Tudor's Rainforest IV).
  • An aux out without volume control allows you to create networks of feedback between different c-bows on different surfaces.
  • Inputs are buffered so the output impedance of everything you send in won't affect the filter's behaviour.