The Radio Shack PZM microphone (SKU 33-1080, 33-1090) contains a generic electret microphone capsule which consists of an electret microphone element connected to a field-effect transistor. This circuitry is housed within the microphone element itself and is not generally accessible unless you are predisposed to micro-surgery.
The stock microphone connects to a small box housing a battery and a small transformer. Even as supplied, the microphone has a balanced output, which can be accessed by simply cutting off the supplied plug and replacing it with an XLR connector. Others have replaced the battery with smoke alarm batteries, which helps out in the output headroom department.
This circuit replaces everything but the microphone capsule. In return, the microphone is 48V phantom powered, and we fondly bid farewell to the output transformer. I have not tested the circuit ant anything but 48V because I don't have any equipment with other than 48V phantom powering.
Q4 is a field-effect transistor connected as a current-source load for the microphone capsule, which improves headroom and linearity. Matched dual transistors, Q2A and Q2B are connected as a differential amplifier, with R5 and R11 providing a substantial amount of emitter degeration. The diff-amp performs the single-ended to push-pull (balanced) conversion needed to drive the balanced microphone input. Emitter coupling resistor, R2, sets the overall gain at about 6dB. Current source Q1 sets the overall collector current for Q2 at about 8mA total.
The balanced microphone line connects directly to the collectors of the diff-amp. R8 and R9 set the output impedance as well as providing a means separating the phantom powering from the audio. Since the resistors are driven by a push-pull signal, their junction is essentially free of any audio signal. Since the preamp is direct coupled to the microhone preamp, a matched transistor is necessary to minimize any offset voltages between the collectors, which might cause problems with a transformer coupled microphone input.
The voltage divider consisting of R4 and R7 divides the collector voltage of the diff-amp (24V) down to about 12V. This voltage biases the diff-amp via R3 and R10. The current in R4 and R7 also biases the reference diodes for current source Q1.
Q5 operates as a voltage regulator, using the R4/R7 voltage divider as its reference to derive approximately 12V for operating the PZM's electret element.
The PZM element is connected so that its connecting cable's shield is normally above (voltage wise) its output. From a schematic standpoint, this would be the FET's drain connection. Whatever, it is not at ground potential, except for AC. The output of the microphone element corresponds to the FET's source connection (as the FET is used as a source follower), and this connects to the Q4 current source.
C1, C2, and C4 are all bypass capacitors.