Crown PX-18B
A transformer power supply that exists almost entirely to solve one problem: getting a Crown pressure-zone mic into an unbalanced recorder input without dragging along a hum.
Overview
The PX-18B is not a microphone, and it isn't a preamp in any meaningful sense. It's a power supply and impedance-matching box, the passive-transformer sibling of Crown's active PA-18B. Both units sit between a PZM capsule and whatever comes next. Both run off either two internal 9-volt batteries (MN1604) or external phantom power. Crown's own documentation is blunt about why the PX-18B exists: it is the supply the company recommends when the destination is a consumer recorder with an unbalanced input. The PA-18B is the active version. The PX-18B swaps that active circuitry for a transformer, and the swap has consequences that show up the moment you put it on a bench.
The transformer is the whole story, and it comes with an asterisk. In a March 1983 equipment profile, Audio (March 1983, p.51) ran impedance curves on the PZM-30GPG through the PX-18B. The output impedance sat well below its 150-ohm rating at 1 kHz, then climbed above 150 ohms by 20 kHz. The reviewer read that rise as high leakage inductance in the transformer and warned that the inductive termination of the mic line could invite radio-frequency interference. With the PZM disconnected, the same supply measured like a 420-ohm resistor. Cheap and simple, and not perfectly behaved at the top of the band.
The grounding situation is worse. Audio (March 1983, p.50) reported that three of six PZM plates in its test sample were not grounded to pin 1. It also found that the PX-18B's standard 3-pin input connector left a 9-volt potential between the PZM plate and the supply chassis, and that a future short across those two points would burn out a battery and produce a memorably noisy recording. The reviewer recommended that Crown move to 4-pin connectors and 3-wire shielded cable.
Noise is where the PX-18B does its job cleanly. Driving a PZM-30GPG, the supply's self-noise measured 19 dB A-weighted, or 28 dB across a flat 15-kHz bandwidth, according to Audio (March 1983, p.51). The reviewer noted that the hiss was low enough that his readings had to be corrected for amplifier noise between 5 and 15 kHz, and that many recorder preamps will be noisier than the microphone itself.
One practical note from the same review: many Japanese recorders of the period were designed around 600-ohm microphones, and a 150-to-600-ohm line matching transformer was suggested to get optimum signal-to-noise from those inputs. The PX-18B's transformer gets you to an unbalanced consumer input, but it doesn't match every load.
Treat the PX-18B as an accessory, not a collectible in its own right. It turns up attached to PZM-30GP and PZM-30GPB microphones, sometimes in the plastic storage case with the windscreen and the 5½-foot cable. It also gets separated from its microphone and sold in a box of odds and ends. The batteries are the usual failure point. Per Crown's own instruction as quoted in the review, the supplies should be switched to "Phantom" when packed up to prevent drain. Inspect the transformer and the connector wiring too.
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