Mike Andrews: PZM Modifications

Radio Shack Pressure Zone Microphone (PZM) Modifications

After attempting to use this reportedly "pretty good" microphone in a variety of situations with some "pretty bad" results, I dissected the power supply and found the same audio transformer that's in their 180-in-one project kits! This DAT-Heads Digest article set me straight, from this, I culled a list of things I'd like to do to the microphone:
  1. Replacing the metal plate with a 6"x6" piece of plexiglas.
  2. Replacing the power supply with a 9V battery, 10uF metallized polyester film cap and 2.2K resistor.
  3. Replacing the cord coming out of the microphone with a Belden 8451 three wire balanced to XLR inline male.
For my application, the microphone would sit on a "jazz play-along station" in our music department most of the time, but I'd like to use it for live audio recording some of the time. That play-along station is assembled from mostly cheap consumer-grade equipment, so the microphone would need to interface to 1/4" unbalanced microphone input as well as plain old XLR balanced. My solution to wishes 2 and 3 follows in this schematic:

download PostScript version (pzm-mod.ps, ~9K)

The microphone element in this thing has a 2.2K ohm impedance, and needs a DC bias of anywhere from 1.5V to 10V. Increasing the bias voltage is generally regarded to be a Good Thing. Note the use of the large 10uF coupling capacitor - it really should be metallized polyester film or some equivalent type and it really should NOT be an electrolytic, if you're looking to improve the sound quality. I used a Cornell Dublier miniature metallized polyester film cap (Mouser #5989-100V10), expensive, but it sounds good - it's difficult to find film caps with that high of a capacitance.

Results were excellent. There's an expected "pop" when the switch is turned on, but the sound quality and noise level were on the level of "a whole new microphone." Highly recommended.

Update: 6/29/98 I originally built the thing in a dumb plastic case which really was subject to hum. I'm re-casing it. Results forthcoming.
Update: 8/28/2000 Sylvain in France proposed some modifications to the circuit. These may help you understand my baked diagram, and may improve hum problems! Thanks Sylvain!

Information updated on Tuesday, 4 May 2004.