recordinghacks

MXL V77S Tube

Cardioid Tube Condenser Microphone

The V77S is a large-diaphragm tube microphone manufactured by 797 Audio for MXL.

The name “V77” is engraved on the microphone body, but MXL’s website at the time this model was manufactured (circa 2000-2003) refers to it as V77S.

The capsule was described both as “27mm diaphragm” and “25mm diaphragm” by MXL. The capsule’s outside diameter is 34mm. The front half of the capsule appears to be a K67 type. The rear half of the capsule is a clear plastic plate with a non-K67 drilling pattern. This piece creates an acoustic delay network that is different from a typical K67.

A superficially similar capsule is seen on the Studio Projects B1 (although these capsules could vary in their tuning due to factors not obvious to visual inspection).

The capsule polarization voltage is applied to the capsule backplate from the B+ rail, through a voltage divider. The schematic we’ve found shows R3 = 51M and R4 = 20M, which would provide capsule voltage of 56VDC. The sample mic we inspected exhibited higher voltage, 67VDC, presumably due to the higher-value resistor (27M) at R4. We do not know whether this sample mic was an anomaly, or whether the found schematic is incorrect.

Audio signals from the capsule are coupled through a 1000pF capacitor to the tube grid. This high-impedance signal path is isolated from the PCB surface via insulated terminals.

The mic’s circuit is a single stage design based on a 6072 / 12AY7 vacuum tube. The second triode (pins 5-9) is grounded. B+ voltage is ~190V; this drops to 120V at the plate through a 100k resistor. Audio is taken from the plate, through a 1uF output coupling capacitor (electrolytic).

Signal injection reveals in-circuit EQ, resulting in a 2.5dB rolloff at 10kHz. Acoustic testing shows a remarkably flat frequency response from 1kHz to 12kHz, within 0.5dB of flat in the sample we tested.

The power supply uses a linear voltage regulator for the heater, with 6V output. The B+ supply is unregulated. The power transformer is shielded from the rectifier and filter circuits.

Special thanks to Orlando T. for the V77 loan and PSU photo.

The MXL V77S Tube is also known as: V77S, V-77S.

Specifications

Pickup Patterns	Pads & Filters
Cardioid (20 mV/Pa; 20 - 20,000 Hz)

Capsule Dimensions	Impedance	SPL/Noise
Diaphragm diameter: 34mm Diaphragm gauge: 3 microns	n/a

Weight	Length	Max Diameter	Interface(s)
n/a	195mm (7.68'')	55mm (2.17'')	3-pin XLR male (1)

Power Specifications
Includes tube power supply

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