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The Altec-Lansing M-20 Microphone System

Altec Model 21 Microphone
The Altec-Lansing Model 21 condenser microphone.

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Altec M-20 Microphone System
The Altec-Lansing M-20 Microphone System.

Operation
The “Lipstik” microphone system is designed to work with any existing microphone preamplifier or facility regardless of the input impedance. Aside from the outstanding advantages of this microphone, it differs from the conventional only in that the P-525A power supply must be connected between microphone and amplifier. The 165A base, containing the 5840 vacuum tube and 21 microphone, is terminated at the end of a ten-foot-long cable in a Cannon connector, which in turn mates with its companion fitting on the panel of the P-525A power supply. The output appears at the end of the two-conductor shielded cable extending from the power supply.

Altec Model P-525A Power Supply
The Altec-Lansing Model P-525A Power Supply.

21 Microphone
The condenser microphone depends for its operation upon variations in its electrical capacitance caused by microscopic deflections of the diaphragm to a fixed electrode when exposed to the pressure variations of a sound wave. The capacitance variation is converted to an electrical signal by supplying a polarized potential to the microphone through an extremely high resistance. In this case, the potential is 200 volts and the charging resistance is the insulation resistance of the associated vacuum tube and socket. Since the voltage e across the condenser is proportional to the charge Q divided by the capacitance c, it will decrease for a positive pressure applied to the diaphragm and increase for a negative applied pressure. Thus, the output is proportional to the diaphragm displacement for all frequencies above those at which the microphone reactance, 1⁄ωc, is commensurate with the leakage resistance. The capacitance of the condenser is approximately 6 mmf. Because of this high internal impedance, it is necessary to place the cathode follower tube, which serves as an impedance transformer, in close proximity to the microphone.

Polar and Frequency Response

Altec M-20 specifications

Text is from Altec-Lansing Corporation M-20 Microphone System Operating Instructions 12052.

Altec Model 21 Microphone

Altec Model 29A Microphone
The Altec-Lansing Model 29A condenser microphone.


The following ten images show the Altec Model 21D microphone and its components.

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone

Altec Model 21D Microphone


Altec Model 29A Microphone

M-20 instructions
Download the Operating Instructions for this mic.
 

 

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