Early Apparatus

Experimental apparatus with tine, tone bar, pickup and driver coil mount

#1 is the steel tine, #2 is the tone bar, the two together act as a tuning fork. The tine swings back and fourth in front of #3, a passive magnetic pickup. The hammer and felt damper are not part of this apparatus.

#4 is a plastic sewing machine bobbin on the end of a steel bolt. This has since been wound with approximately 600 turns of 30 AWG copper wire and serves as the magnetic actuator. Indeed, when supplied with DC, the coil generates a magnetic field and attracts ferromagnetic tine. Driven with a weak sine wave directly from a function generator, the tine vibrates slightly.

McPherson's Magnetic Resonator Piano uses a feedback loop in which magnetic actuators are driven by a single piezo pickup that senses the summed string vibrations on the soundboard. High-order bandpass filters precede each actuator in the signal path, with each filter tuned to the individual note. Each note is isolated at the source in the Rhodes piano with a dedicated pickup for each tine, but the close proximity of the magnetic actuator and magnetic pickup may be problematic.

In anticipation of a pickup/actuator proximity problem, a piezo film vibration sensor is taped to the end of the tone bar (#5) to test its effectiveness as a replacement for the magnetic pickup. It produced a very weak signal, and I suspect the tone bar vibrates at a different frequency as the tine.