Reissmann MR51

From the data sheet (translated from German).

The highly sensitive MR 51 condenser microphone is an elegantly designed instrument of the highest precision, intended for the transmission of musical and speech performances. The diaphragms, the heart of the microphone, are edge-mounted gold foils of exceptionally thinness. The specially shaped, fixed backplate ensures high sensitivity and maximum frequency fidelity.

The device includes a single-stage preamplifier and is suitable for driving any standard power amplifier or radio. The required anode voltage of approximately 55 V = 2 mA should be taken from the end of a generously sized filter network. If using AC heating, this network must be balanced.

The MR 51 condenser microphone is a highly sensitive precision instrument and requires careful maintenance to remain fully functional. It must be protected from strong mechanical shocks and exposure to moisture.

Technical specification taken from an original Reissmann data sheet (translated from german)

• Construction: Gold leaf foils, edge-tightened in a galvanized steel housing; built-in preamplifier.
• Polar pattern: approx. cardioid
• Tube complement: EF 86
• Capacitance of the microphone capsule: approx. 100 pF
• Frequency range: 60 Hz - 14000 Hz
• Output impedance: > 100 kOhms
• Connection: 3-core shielded special cable without intermediate transformer. One 5 m connection cable is included.
• Supply voltages: Anode voltage approx. 55 V = 2 mA
• Heat voltage 6.3 V 200 mA:
• Power supply from the power amplifier or mains connection unit NR 55

• Connector connections:
  • Power supply from the power amplifier or mains connection unit NR 55
  • 1 = Heater; 2 = Heater; 3 = Sound + Anode; Ground = Earth

• Dimensions: Overall length 145 mm; Diameter 4.2 mm; Weight: 235 g

MoMics technical note:

As far as we can tell, neither the MR50 nor the MR51 was supplied with a separate stand alone power supply. Instead, they were powered from the DIN socket of a Reissmann power amplifier (pictured) which provided 120V DC and 6V AC power for the heater. The microphones do not have internal output transformers - the signal is instead piggy-backed on top of the high voltage DC supply, to the next amplification stage. This would likely be a noisy arrangement in a modern studio, but was undoubtedly good enough for public address duties at the time.

View some traced-out schematics for our MR50 and MR51.

A fantastic resource documenting the history of Reißmann and their products. This link takes you directly to their page on the MR51 microphoine