Crown PZM-30GPG
A gold-finished pressure zone microphone that asked recording engineers to put the mic flat on the floor, not on a stand.
Overview
The PZM-30GPG is the gold-finished version of Crown's Pressure Zone Microphone. It is a boundary mic, and the whole design premise is that the capsule sits about a millimeter above a backing plate instead of hanging in free air. Crown built it around a pressure-calibrated electret condenser element. The polar pattern is hemispherical, so the mic listens to half the room, not a cone of it. The gold finish is cosmetic. Under it, the housing is a conductive plastic shell filled with flexible potting compound, and the electret cartridge and its miniature amplifier are held in place upside down near the plate.
The rated frequency response runs 50 Hz to 15 kHz. Electrical impedance is 150 ohms balanced when paired with one of Crown's two power supplies, the PA-18B (active) or the PX-18B (transformer). Crown recommended the PX-18B for feeding consumer recorders with unbalanced inputs, which tells you who the company expected to buy it. Either supply runs from two internal 9-volt batteries (MN1604) or from external phantom power between 15 and 48 V d.c. Total harmonic distortion is specified at 3% at 150 dB SPL, the maximum input level. The noise floor is quoted at less than 26 dB equivalent input SPL. Sensitivity is given as an open-circuit voltage of -76 dB re 1 volt per microbar, or -56 dB re 1 V/Pa.
Audio (March 1983, p.50) laid out the same manufacturer's specifications and then measured a review sample. Self-noise with the PX-18B supply came in at 19 dB A-weighted, or 28 dB across a flat 15-kHz bandwidth. That was low enough that the reviewer had to correct his readings for the noise of his own measurement amplifier between 5 and 15 kHz. He noted that some recorder preamps would likely be noisier than the microphone itself. So the weak link in a modest recording chain will usually be something downstream of the mic.
The physical package is modest. The microphone measures 5 × 6 × ⅜ inches and the power supply 5 × 1⅜ × 1⅜ inches, with a 5½-foot cable joining the two. Crown shipped it with a windscreen and a plastic storage case. The list price was $349.00, the same for the gold PZM-30GPG and the black PZM-30GPB. Audio (March 1983, p.51) noted that the electret cartridges, sourced from a hearing-aid microphone manufacturer, are made in volumes high enough to be cheap. Crown's careful testing and selection to meet the PZM's tolerances is what pushed the selling price up.
The baffle is the whole trick, and it is also the catch. The PZM's low-frequency behavior depends on the surface it sits on. Audio (March 1983, p.52) explained the formula for a square baffle's cutoff frequency - f = 283/D, with D the length of one side in feet - and noted that below the cutoff the pressure-doubling effect vanishes and output drops by 6 dB. A 2 × 2 foot baffle puts that cutoff at 142 Hz. A 4 × 4 foot baffle puts it at 71 Hz. The reviewer's blunt assessment was that a PZM performs rather poorly on baffles smaller than 2 feet on a side.
The same reviewer put the PZM-30GPG through a comparison. Using a 2-inch omnidirectional source in his listening room, he recorded the PZM's output against three conventional microphones - a Nakamichi CM-700 with omni and unidirectional capsules and an RCA 44-BX in figure-8 - and found that none of the conventional mics produced as smooth a curve. The comb-filter effect showed up as a series of holes between 200 and 1,000 Hz in the conventional responses. Even an omni placed head-down a millimeter off the floor did not eliminate it. His conclusion, paraphrased from Audio (March 1983, p.53), was that there was no way to equal PZM performance with conventional commercial microphones. He was careful to note he had not tried laboratory condensers, whose noise he considered too high for practical voice or music work.
The listening tests were more equivocal. At a Crown press event in a mobile home wired to the Grand Opera House, three pairs of PZMs - two on the stage floor, two on a disc overhead, two on a false ceiling - were compared against two pairs of flown conventional condensers while the Delaware Symphony played. The floor PZMs won on a piece with a guitar solo. The ceiling pair won on the Brahms Fourth. The conventional mics were never judged superior. The disc-mounted pair generally beat the conventional mics but trailed the other PZMs. The reviewer flagged two flaws in the demo. The press could not sit in the house to hear the live sound, and the conventional mics were clustered in one spot, so the claim that this was their best placement rested on Crown's word rather than an independent test.
The Audio review closed on convenience, saying that amateur recordists should discover PZMs as the best route to maximum audio quality with minimum effort. It is not a mic for shaping a sound with placement and proximity. It is a mic for a natural, low-hiss capture of a room or a stage without a forest of stands. The hemispherical pattern and the boundary mounting mean fewer decisions, and the low self-noise means fewer regrets.
The microphone is only half the system. The PZM-30GPG needs a PA-18B or PX-18B supply to run, and those supplies are the parts most likely to go missing or to have sat with leaking batteries. The Audio review noted that battery-powered supplies should be switched to "Phantom" when packed up to prevent drain. The transformer-based PX-18B, per the same review's impedance measurements, has output impedance that runs well below its 150-ohm rating at 1 kHz but climbs above 150 ohms by 20 kHz. That suggests leakage inductance in the transformer and a susceptibility to r.f.i. with a long, inductively terminated line. The reviewer's suggested workaround was a Shure A97A transformer yielding a 200-ohm resistive termination.
Collectors and studio people who seek out the PZM-30GPG tend to want it for the same reason the reviewers did: it does something conventional mics cannot, and it does it quietly. The $349 list price in early-1980s dollars puts it in the serious-amateur and small-studio bracket. It is not rare the way a limited-run tube amp is rare, but complete, working examples with the correct supply and the original case are the ones worth having. Put it on a boundary too small and it gives up 6 dB in the low end.
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