recordinghacks

Groove Tubes GT57

Multi-Pattern Condenser Microphone

The GT57 is a multipattern FET condenser, designed by Aspen Pittman.

Three switches around the base of the grille allowed selection of:

• -10dB pad (which raised the max SPL to 144dB @ 1% THD)
• a high-pass filter, -12dB/octave @ 75Hz
• Polar pattern: Cardioid, Omni, Figure-8

The mic used Pittman’s disk resonator capsule. This was a 32mm, center-terminated capsule believed to be based on the Neumann K67 backplate design. A small metal disk is attached to the center termination screw to increase the capsule’s sensitivity to high-frequency sounds.

The mic’s impedance conversion circuit was built with through-hole components. It used a custom-wound output transformer with a nickel core.

The published self-noise figure is relatively high for a FET mic, at 22dBA. The GT57 also had very high sensitivity of 32mV/Pa.

The mic’s body as all brass, nickel-plated and painted black. The grille is made of stainless steel.

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Specifications

Pickup Patterns	Pads & Filters
CHART Omnidirectional (32 mV/Pa; 20 - 20,000 Hz) CHART Cardioid (32 mV/Pa; 20 - 20,000 Hz) CHART Bidirectional (32 mV/Pa; 20 - 20,000 Hz)	Pad: -10dB (Via Switch) Filter: HPF: -12dB/octave @ 75Hz (Via Switch)

Capsule Dimensions	Impedance	SPL/Noise
Diaphragm diameter: 26mm Capsule diameter: 32mm Diaphragm gauge: 3 microns	200 Ohms (Low)	Max SPL: 144 dB Self-noise: 22.0 dB(A)

Weight	Length	Max Diameter	Interface(s)
n/a	19mm (0.75'')	64mm (2.52'')	3-pin XLR male (1)

Power Specifications
Requires phantom power

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