<<The problem in your set-up is probably that the piezo is a high-
impedance element (other microphones have a much lower output
impedance). This means that the piezo foil shield and the wire
connected to it (XLR pin 3) can act as an antenna, which obviously
picks up the LED noise.>>
A piezo element is both high impedance as well as unbalanced. They
benefit from being balanced through a transformer or opamp before
being sent to a mic preamp. The sort I use have an inner disk & an
outer ring, to which I solder the hot & ground wires respectively on a
short single conductor lead, terminated with a 1/4" plug. Either a
transformer DI or active DI then balances & converts the signal to low
impedance, suitable for a long run to the mic preamp. I've made many
of these & even going through a 200 to 300 foot audio snake in large
PA installations, it provides a very robust signal. There's no
inherent hot nor earth in a piezo element, & it seems to me that
Patrick's wiring implementation on his transducer is suspect. I
believe you want to develop the voltage differential between pins 1 &
2 (or between 1 & 3, although I think the issue of polarity is moot
here.)
Scott Fraser
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