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CardioidAlesis AM51

Cardioid Condenser Microphone

The AM51 was one of the first microphones released by GT Electronics, the analog product division created when Alesis acquired Groove Tubes. It is a fixed Cardioid FET condenser with switchable pad and filter and transformer output, and was billed as an all-purpose studio condenser.

The capsule is the same as was found in all the Groove Tubes large-diaphragm models, a ~33mm K67 derivative with a resonator disk attached to the active diaphragm to smooth out the high-frequency response.

AM51 PCBThe circuit is a Class A JFET design with an output transformer. It has high gain (32mV/Pa) and relatively high self-noise (22dBA); these characteristics were common to all Groove Tubes and Alesis/GT condensers.

Switches on the mic independently enable a pad (-10dB) and high-pass filter (12dB/octave at 75Hz).

The AM51 circuit is extremely similar to the GT55, ST55, and M-Audio Nova circuits that we’ve seen.

Permalink: Alesis AM51

The mic was released in 1999.


Pickup Patterns Pads & Filters
Cardioid (32 mV/Pa; 20 - 18,000 Hz)
  • Pad: -10dB (Via Switch)
  • Filter: HPF: -12dB/octave @ 75Hz (Via Switch)
Capsule Dimensions Impedance SPL/Noise
Diaphragm diameter: 25mm
Capsule diameter: 32mm
Diaphragm gauge: 3 microns
200 Ohms (Low) Self-noise: 22.0 dB(A)
Weight Length Max Diameter Interface(s)
n/a n/a n/a
  • 3-pin XLR male (1)
Power Specifications
  • Requires phantom power
  • Phantom voltage: 48v

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