Neumann M50
A pressure microphone that behaves like a directional one up top. The sphere does the work, and every classical engineer in Europe knew it.
Overview
Neumann mounted a pressure capsule flush in a Plexiglas sphere, diaphragm sitting right on the surface, and let the geometry of that ball shape everything above a kilohertz. The low end is as linear as any pressure transducer. The top end rises gradually to +6 dB between 8000 and 16000 Hz and gets more directional as frequency climbs. Pressure-mic bass with pressure-gradient-like treble directivity is the whole trick. It's why the M50 became the default spot microphone for woodwinds, brass, and strings in classical rooms across Europe.
Mechanically it's built like its sibling the M49. The amplifier sits elastically mounted on a solid rubber plate, and the capsule rides in its Plexiglas sphere on the plastic amplifier cover via an elastic suspension. The active element is a Telefunken triode, type AC 701 (k). Early capsules used a highly stretched aluminum diaphragm. Later versions moved to gold-sputtered polyester foil. To get the sensitivity and low equivalent noise the design needed, Neumann set the diaphragm-to-electrode gap at an extremely narrow 10 µm, roughly half the spacing typical of other condenser capsules of the period. That tight gap is the source of the M50's reputation for both quietness and fragility.
The M50b variant, documented alongside the M49b, is a pressure-type microphone with non-directional characteristic, the diaphragm mounted in a lucite ball. Neumann's own manual notes the response rises slightly and becomes increasingly directional at higher frequencies when exposed to direct sound, but stays essentially flat in a diffused field. The M50b carries a white jewel for identification, while the M49b uses a red one. That's the fastest way to tell them apart on a shelf.
The usable range is listed as 40 to 15,000 cps, sensitivity approximately 1.0 mv/dyne/cm² at 1000 ohms, source impedance 200 or 50 ohms, total harmonic distortion under 0.6% from 40 to 15,000 cps up to a sound pressure level of 114 dB. The tube complement is a single selected Telefunken AC 701 k. The mic stays linear where pressure mics normally roll off, and gets directional where most omnis go soft.
The M50 was accepted quickly as *the* high-quality microphone for classical recording work. It served as a spot mic for woodwinds and brass, and was equally at home on strings. One documented session from a Berlin studio placed two M50s as spot microphones on a violoncello at 2.5 meters, with the optimum sound achieved by turning both microphones off-axis by roughly 15 degrees. Here the directivity itself became part of the voicing.
The M50b runs on the NN 48 b power supply, which delivers 4 volts DC heater, 116 volts DC plate, and 0-116 volts control potential, drawing 100 ma heater and 0.8 ma plate. The supply is strapped for 110 or 220 volts at 50-60 cps and fused at 80 ma for 110 volts, 50 ma for 220. A pilot lamp (Rafi L. 2821, 6 volts, 180 ma) indicates operating condition. Neumann notes it's not harmful to run the supply for long periods without a microphone connected. A BB 9 self-contained battery supply could substitute where AC wasn't available.
Connections are bayonet-fitted throughout: microphone, cable, power supply. The M50b uses an 8-pin plug into the C 26 extension cable or C 28 (C 28 s) interconnecting cable with stand mount. The C 28 s stand mount includes a swivel joint and comes with a ½-inch right-hand thread (or ⅜ - 27 for US use). The standard seven-conductor shielded cable is 33 feet. Runs up to 100 feet are permissible between microphone and power supply, though Neumann advises bridging the heater voltage series resistor (R 8, accessible under the supply cover) if the microphone sits more than 70 feet out. The microphone is designed to work into a load impedance of 250 or 1000 ohms or higher, with source impedance set at one-fifth the load - 50 ohms for a 250-ohm load, 200 ohms for 1000. Units ship strapped for 1000 ohms unless ordered otherwise. The 250-ohm versions carry a red dot on the type number tag.
For suspension work, the MZ 49 mounts allow near-360-degree rotation for film and television boom use. Permanent installations typically used T 3033/S microphone outlets with concealed lines running back to an N 52 a plug-in power supply in a rack frame.
Maintenance is not casual. Neumann's own documentation warns that servicing should go through the factory or authorized service centers or the guarantee is void, and that any work is undertaken at the owner's risk. The wire-mesh cover on the M50b comes off after advancing three set screws at the base of the housing. A dusty diaphragm can be cleaned with an extremely soft camel hair brush. Nothing else touches it, and blowing into the capsule is explicitly forbidden. Capsule and Plexiglas mounting plate separate from the amplifier by unscrewing three set screws, then unsoldering the connecting wires. Tube replacement requires gripping the helically spiralled tube leads with long-nose pliers and applying minimal heat from a miniature soldering iron. The spiral and the pliers both act as heat sinks to keep the glass envelope from cracking. Replacement tubes must come from Neumann or their representatives. The AC 701 k is specially selected for low noise, and the connecting leads are pre-curled for quick factory installation. Neumann recommends checking power supply operating voltages every six months.
The M50 arrived together with the M49, the M49 as the first remotely controlled microphone and the M50 as the omni counterpart. The M49 offered switchable patterns via remote control from the power supply. The M50 committed to a single non-directional characteristic with that high-frequency rise baked into the sphere.
Recording Engineer (December 1985, p.74) described the design approach in a survey of stereo microphones — the technique of embedding a tightly stretched, gold-impregnated 12 mm polystyrene diaphragm on the surface of a 40 mm plastic sphere — noting that in combination these properties yielded a microphone with an omnidirectional pattern and the distinctive high-frequency behavior that made it a reference tool. The same publication's subjective evaluation series (December 1985, p.79) placed the M50-era Neumann designs in context with the Schoeps CMC541U and Bruel & Kjaer 4000 series, judging the classic Neumann valve microphones as having a very positive appeal among the listeners surveyed.
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