The Altec-Lansing M-30 Microphone System
The “Lipstik” microphone system is designed to work with any existing microphone preamplifier 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 175A base, containing the 5840 vacuum tube and 29A Microphone, is terminated at the end of a fifteen-foot cable in a Cannon connector, which mates with a fitting on the 525A. A fifteen-foot, two-conductor shielded output cable is attached to the power supply. The power supply must be energized from the 117-volt, 50-60 cps supply.
The condenser microphone produces voltages in response to sound pressure through the variation of capacity between a moving diaphragm and a stationary electrode connected to a source of polarizing voltage through an extremely high resistance. For the 29A, the potential is 60 volts and the charging resistance is the input resistance of the associated vacuum tube. The output is proportional to the diaphragm displacement independent of frequency for all frequencies above those at which the microphone reactance is commensurate with the charging resistance. The capacitance of the condenser is approximately 50 mmf. Because of this high internal impedance, it is necessary to place the cathode follower tube, which serves as an impedance transmitter, in close proximity to the microphone. To achieve a uniform cardioid characteristic the sound pressure acting on the back side of the diaphragm passes through a multiple acoustic phase-shifting network. The network allows optimum phase-shift adjustments over the whole frequency range. To provide sensitivity the diaphragm is made of .00025 Mylar, gold plated and stretched to a resonance frequency of approximately 1500 cps. The movement of the diaphragm is resistance-controlled by the thin air film between diaphragm and back plate. Due to the low resonance frequency, the applied polarizing voltage may not exceed 60 volts. Higher voltages may cause the diaphragm to collapse on the back plate and the microphone to become inoperative, although undamaged.
(See the cautionary note at the bottom of this web page.)
CAUTION: The Altec 29A Microphone requires a 60-volt polarizing voltage, as compared with 200 volts for the Altec 21D Microphone. The Altec M-20 System’s P-525A Power Supply may be used with the 29A microphone if appropriate modifications are made to the P-525A unit. Specific instructions for these modifications are provided on pages 2 and 3 of the older Altec M-30 Microphone System Operating Instructions booklet, which is available above for downloading.