Electro-Voice Model 950 Cardex Microphone

A crystal microphone whose published polar plots made it a teaching example for a generation of tape recordists — and whose identity has since been swallowed by a same-numbered field strength calibrator from a different company.

Overview

Here is a piece of vintage audio trivia that trips up collectors constantly: search for "Model 950" and you will mostly find a Measurements Corporation field strength calibrator — a 13-pound, $360 rack instrument from Boonton, New Jersey, with a 54–250 MHz carrier range and a piston attenuator. That is not this microphone. The Electro-Voice Model 950 is a crystal microphone, the "Cardex," and the two products share a number and nothing else. Anyone chasing an EV 950 on the used market needs to know the difference before money changes hands.

The Cardex shows up in the literature in a way most microphones of its day did not: as a worked example. A 1956 Audio article (1956, p.44) reproduced the Model 950's directivity pattern at several frequencies as part of a discussion of how directional characteristics affect a microphone's frequency response — the point being that the polar behavior of a mic is not a constant but shifts with frequency, and the Cardex's plots illustrated it. For a 1950s crystal mic to be chosen as the illustrative case in a recording-technique text says something about how widely it was known among tape hobbyists, even if the article was teaching a general principle rather than reviewing the mic itself.

What the surviving documentation does not give is the mundane stuff a buyer wants: the Cardex's frequency response, its output level, its impedance, its finish, its original price, its production years. None of that appears in the sources available here, and it will not be invented to fill the page. What can be said is that it is a crystal (piezoelectric) design — the "Cardex" name and the Audio reference both establish that — and that crystal elements of the period carried the usual caveats: they were high-impedance, they were sensitive to heat and humidity, and they aged. A working example today is a small miracle of storage conditions.

What the 1956 Polar Data Tells You

The Audio piece is worth dwelling on because it is the one substantive period document tied to this microphone. The article's argument is that a directional pattern measured at one frequency is misleading; you need the pattern across the band to understand how a mic will actually behave on a session. The Model 950's plots were presented as the case in point. For a collector, that is a useful reminder that this is a mid-century crystal mic with all the directivity quirks that implies — not a modern cardioid with a flat off-axis response.

The same 1956 issue of Audio (p.43) frames the whole discussion around the tape recordist choosing microphones the way a photographer chooses lenses — a general-purpose tool plus specialists for particular jobs. The Cardex appears in that context as one of the tools under discussion, which places it squarely in the amateur and semi-pro tape-recording world of the mid-1950s rather than the broadcast studio. That is the honest read of the evidence: a known, widely referenced crystal microphone for home and project recordists, documented mainly because it made a good teaching example.

The Identity Trap: Two Different 950s

Collectors note that the number collision is severe enough to poison search results. The Measurements Model 950 field strength calibrator is a completely unrelated instrument — a calibration source for field strength meters, traceable to the National Bureau of Standards, priced at $360 f.o.b. Boonton, N.J., measuring 7" high by 12¾" wide by 7½" deep and weighing approximately 13 lbs, with a 75-ohm source impedance and output maintained at 100,000 microvolts (+40 dBmV) across that load, attenuable down to 0.3 microvolts. Every one of those specs belongs to the Measurements instrument, not to the Electro-Voice microphone, and any listing that mixes them is confused. If a seller describes a "Model 950" with a carrier frequency range or a piston attenuator, it is the calibrator. If it has a crystal element and a 1950s microphone body, it is the Cardex.

The same caution applies to the Electro-Voice microphone listings that turn up in period dealer catalogs. A Burstein-Applebee catalog page from the era lists an "Electro-Voice Model 950" among a run of EV microphones — the 695, the 880 Cardax, the 726, the 688 — but the catalog text for the 950 entry is fragmentary and does not yield usable specifications. It confirms the model existed in the EV line alongside those siblings, which is worth something, but it does not tell a buyer what the mic will do. Treat catalog appearances as evidence of existence, not as a spec sheet.

Owning and Maintaining One

Because the sources here give no impedance or output figures for the 950, matching it to a modern preamp is a matter of experiment rather than specification — crystal mics of the period were typically high-impedance and wanted a high-impedance input, and a modern low-Z mic preamp will usually need an adapter or a buffer to get sensible level. That is general period practice, not a documented spec for this model, and should be treated as a starting point for testing rather than a guarantee.

Collectibility

The Cardex occupies an odd spot in the collector market. It is historically interesting — a microphone that earned a place in a 1956 recording text as the illustrative example of directivity — but it is not a celebrated studio classic, and the documentation that survives is thin on the things that drive prices. There is no reliable current-market figure to quote from the sources available, and the identity confusion with the Measurements calibrator means listings are frequently mislabeled, which distorts any casual price survey. Buyers should confirm which 950 they are looking at, ask whether the crystal element produces output, and price accordingly. A working Cardex is a charming period piece with genuine documentation behind it; a dead one is a shelf ornament.

For anyone assembling a mid-1950s tape-recording kit, the 950 is a legitimate period-correct choice, and the Audio article gives it a small place in the literature that most crystal mics of its generation never earned. That is the honest case for it: not a hidden gem, not a legend, but a documented working microphone from the era when home tape recording was becoming a hobby — with all the fragility and uncertainty that implies.