the legendary ReVox A77...

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Brian Reeves

The Authorised  ReVox Service Centre for the UK

STEREO REVIEW

EQUIPMENT TEST REPORTS

By Hirsch-Houck Laboratories


The ReVox A77 Tape Recorder.


It is a pleasure to report that the widely acclaimed, but no longer available, Revox G-36 Mk III tape recorder has actually been surpassed in performance by Revox's new Model A77.


The A77 has fully solid-state electronics, a bias-oscillator frequency of 120 kHz (as opposed to 70 kHz for the G-36), and a new electronic motor-speed control. The A77 model we tested is a three-motor, four-track, two-speed recorder; however, it is substantially lighter and smaller than its predecessor.


The Revox A77 has its operating controls grouped into separate recording and playback areas. On the playback side there are two rotary switches with concentric knobs. One switch establishes the playback mode - stereo, either channel through both outputs, or both channels combined for mono. It's concentric knob controls playback level. The other switch connects either the signal input or the signal output to the playback amplifiers (before/after tape monitoring) routed to the output jacks on the rear of the deck. Two playback equalization characteristics are provided: NAB or IEC (for European tape recordings). The recording equalization is to the NAB standards. The knob concentric with this switch is a playback channel balance control.


On the right side of the recorder panel are two VU meters with real VU-meter characteristics. Adjacent to each is a red button of the push-on, push-off type. Depressing either channel's button alone records both inputs on that channel. If both buttons are depressed, a stereo recording is made. These supplement a record-interlock button, providing a double safety against accidental tape erasure. Recording levels may be set up before the tape is put into motion. When the recorder is in operation in the recording mode, the selected channel's VU meter (or meters) is illuminated. Under each meter is a recording input-selector switch with a concentric recording-level control. There are inputs for high and low-impedance microphones (with front-panel jacks in parallel with rear phono connectors), radio (via a rear DIN connector), and auxiliary inputs with connectors in the rear. In addition, each switch has a position for recording the output of that channel combined with any additional source on to the other channel (multi tracking).


The transport mechanism is operated by a row of five pushbuttons, activating relays to control fast speeds, stop, play and recording. A connector in the rear permits the use of an accessory remote control unit for these functions. The tape speeds (7 and 3 ips) are selected by a switch that also controls a.c. power to the recorder. Each speed setting has two switch positions that set the tape back-tension to optimum values for either 10 inch or smaller reels.


The servo-controlled drive system of the Revox A77 is unique and effective. An eddy-current motor that delivers a high torque, free of the pulsations that are inevitable with any motor having a pole structure, powers the tape-drive capstan. The speed of this motor can be adjusted by varying a d.c. control voltage, with relatively little torque variation. The motor has a built-in tone generator that produces an a.c. signal whose frequency is proportional to motor speed. This signal is amplified, limited, and applied to a discriminator, whose d.c. output is proportional to speed. This is further amplified and used to correct the motor speed. The change between 7 and 3 ips is accomplished electronically by shifting the resonant frequency of the discriminator circuit. The chief advantages of this technique are independence from power-line voltage and frequency variations, as well as reduced flutter. Flutter of the A77 motor is inherently so low that the capstan can be driven directly from the motor shaft instead of through a separate belt-driven flywheel. According to the manufacturer, line voltage fluctuations of +/-20% cause a speed change of only +/-0.05%, and a change in the a.c. line frequency of 50 to 60 Hz also causes a speed change of less than 0.05%. Thus, the Revox A77 is a truly universal machine, capable of operating from 110 volts to 240 volts, simply by adjusting a switch at the rear of the recorder regardless of either a 50 or 60 Hz supply.


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