Neumann M 49 (1952–)
A tube condenser that let an engineer change a microphone's polar pattern from the control room - and changed how orchestras were recorded.
Overview
Before the M 49, a condenser microphone's directional character was fixed at the capsule. If you wanted a different pattern mid-session, you swapped microphones or swapped capsules. The M 49 was the first condenser microphone with remote pattern control, and that one feature is why it ended up hanging above orchestras across the world.
The patent belongs to Dr. Grosskopf. Neumann's own documentation lays out the reasoning: exaggerated reverberation could be diminished, recording distance increased without a vague timbre, and the business of exchanging microphones with different polar patterns avoided. The engineering problem was that a switchable-pattern mic of the day - the U 47 - jumped about 5 dB in output when you moved it from omni to cardioid, because the rear half of the capsule was simply disconnected electrically. The M 49 solved that by keeping both capsule halves connected to the impedance converter for signal while insulating them from each other for DC bias. The front diaphragm sits at a constant 60 Vdc. The rear diaphragm's bias is adjustable from 0 Vdc to 120 Vdc. Sweep that control and the pattern moves continuously from omni through cardioid to figure-8 while sensitivity stays practically constant.
The pattern control lives on the power supply, not the microphone. The M 49's power supplies carried a potentiometer with a pointer knob and a matching scale, and the manual gives the numbers behind the dial: 0 volts at the omni position, 58 volts at cardioid, 116 volts at figure-8. The documentation notes the M 49 was adopted as a main microphone above large orchestras because of this, and also found work as a spot mic for wind and string instruments, on piano, and as an announcer's microphone.
The tube story runs in two acts. Early versions used a directly heated triode, the Hiller MSC 2, which was later succeeded by the Telefunken AC 701 (k). That AC 701 k is selected for low noise, and Neumann's manual is emphatic that replacements must come through the company or its representatives. The connecting leads are pre-curled at the factory for quick installation. The manual warns that changing the tube takes real care: grip the helical tube leads with long-nose pliers and apply minimal heat from a miniature soldering iron, so the spiral and the pliers together act as heat conductors and keep the glass envelope from cracking.
Mechanically, the M 49 is built to keep structure-borne noise out of the signal. The amplifier sits on a solid rubber disk that isolates it from the housing, and the capsule is decoupled by a swing-metal shock mount - a method Neumann's documentation notes was incorporated into most of the succeeding Neumann microphones. The amplifier can be pulled free of the base with a slight upward tug, the rubber ring leaving its groove, and re-seated with a screwdriver. The microphone uses an 8-pole connector, a consequence of the several supply voltages it needs.
The documented response is 40 to 15,000 cps, with total harmonic distortion under 0.6% from 40 to 15,000 cps up to a sound pressure level of 114 dB. Sensitivity is roughly 0.7 mV/dyne/cm² at 1000 ohms. One detail buried in the service instructions: the M 49 b's response drops below 40 cycles by design. If a flat response down to 20 cycles is wanted, a resistor - number 3 on the schematic - has to be removed. That's not a fault. It's a factory choice, and it's reversible.
Impedance is a strapping matter, not a fixed figure. The microphones were usually supplied strapped for a 1000-ohm load impedance, with 250-ohm versions available on request. Those 250-ohm units are identified by a red dot on the type number tag. Per the manual, source impedance is set to one-fifth the load impedance - 50 ohms source for a 250-ohm load, 200 ohms source for a 1000-ohm load - which keeps the microphone's own preamplifier from overloading. Re-strapping between the two is done after removing the wire-mesh cover, following the diagram on the terminal board. One more strap sits under the plexiglass cover. Opening it drops the equivalent loudness level by 4 dB but permanently locks the microphone to cardioid, making the pattern control on the power supply inert. If a used M 49 seems to ignore its pattern knob, check that strap first.
That plexiglass cover hides a terminal board with a strap, and opening it produces two results together - the 4 dB reduction and the permanent cardioid lock. It made sense for a broadcaster who only ever wanted cardioid and wanted a hotter signal. It's fully documented, so it can still be diagnosed decades later.
Power came from the NN 48 b supply, which delivers 4 volts DC heater, 116 volts DC plate, and 0 to 116 volts control potential, drawing 100 mA heater and 0.8 mA plate current. Line voltage is strapped for 110 or 220 volts, visible through a small window on the side of the cover, with fuse values of 80 mA for 110 volts and 50 mA for 220 volts. The supply carries a pilot lamp - a 6-volt, 180 mA Rafi L. 2821 - and Neumann notes it's harmless to run the supply for long periods with no microphone connected. Where AC wasn't available, a self-contained battery supply, type BB 9, could stand in. For permanent installations, the manual recommends wall outlets (T 3033/S) feeding concealed lines back to a rack, where an N 52 a plug-in supply could be mounted in a suitable frame.
Cabling is bayonet-fitting throughout, and the manual's first piece of advice is to make sure those fittings are locked. The seven-conductor shielded cable came normally at 33 feet, with lengths up to 100 feet admissible between microphone and supply. Past 70 feet, Neumann advises bridging the heater voltage series resistor (part R 8) inside the supply. The C 28 s stand mount incorporates a swivel joint and came with a ½-inch right-hand thread, or a ⅜-27 thread for the USA. For boom work, the MZ 49 suspension permits rotation through almost 360 degrees. The floor stand M72 and table stand M70 both accept the C 26 extension cable at their bases.
Neumann's own advertising, decades later, grouped the M 49 with the U 47, U 67, and KM 53 as legendary microphones coveted by engineers and producers worldwide — a claim from a company with an obvious interest in its own back catalogue, but one that the M 149 Tube's design decisions quietly corroborate. When Neumann set out to build a new tube microphone in the 1990s, it borrowed the K 49 capsule and headgrill from the classic M 49 and M 249, as Mix (February 1996, p.36) reported in the launch advertising. You don't reach back forty-odd years for a capsule unless the capsule earned it.
Servicing a surviving example is not casual work. The manual is blunt that servicing should go to the factory or an authorized service center or the guarantee is void, that repairs should only be attempted by a skilled technician with dry hands, and that under no circumstances should anyone blow into the capsule. A dusty diaphragm after long service can be cleaned with an extremely soft camel hair brush. The capsule and its plexiglass mounting plate come off the amplifier by unscrewing three set screws, with the connecting wires carefully unsoldered. Neumann recommends checking the power supply's operating voltages every six months, and asks that spare-parts orders include the serial number so the correct parts for that particular version can be supplied. Between the Hiller and AC 701 tube eras, the impedance strapping options, and the internal strap variations, "an M 49" is not one thing.
The M 49 was introduced to German radio broadcasters in 1952, and the documentation notes it conquered international recording studios rather quickly. It's a piece where the historical importance and the practical utility are the same fact: remote pattern control wasn't a gimmick, it was the reason the microphone got hung over orchestras, and everything else — the shock mounting, the selected tube, the constant-sensitivity bias scheme — exists to make that one feature usable in a real session.
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