Altec M11 System

The Altec M11 microphone system, produced around 1950, was nicknamed the "coke bottle" for the distinctinve shape of the 150A amplifier base (microphone body). The M11 system was intended for professional broadcasting, live sound, and studio recording.

The system was compatible with 21-series capsules, including the 21A, 21B, 21C, and 21D models. Each revision introduced minor changes to the capsule housing, particularly in how sound entered the transducer, while maintaining similar performance and omnidirectional polar patterns.

The Altec M11 is a modular tube condenser microphone system introduced around the early 1950s. It comprises the P518A power supply, the 150A amplifier base (microphone body), and interchangeable screw-on 21-series condenser capsules. The power supply was available in two formats: the free-standing P518A and the rackmount P519A, which are electrically identical but differed in form factor.

The 150A base houses a 6AU6 vacuum tube configured as a cathode follower, providing a low-impedance output of approximately 1000 Ω. The base accepts the 21A, 21B, 21C, and 21D capsules, which are condenser transducers with similar designs but variations in capsule porting and mechanical structure.

Output impedance can be internally configured to 30 Ω, 250 Ω, or 500 Ω. Audio is fed via a cable to the external power supply, which provides heater, plate, and screen voltages. The power supply includes selectable low-frequency attenuation settings at 20 Hz, 40 Hz, or 120 Hz.

The microphone's nominal frequency response is 20 Hz to 15 kHz, with an open-circuit sensitivity of 48 dB re 1 mW per 10 dynes/cm² (~94 dB SPL). The system was capable of operating effectively with cable runs up to 400 feet, though increased heater wire gauge was recommended for longer distances.

Altec M11 system technical specification, instructions and promotional material.

An article about the M11 system at the Modesto Radio Museum