By Hirsch-Houck Laboratories


It is a pleasure to report that the widely acclaimed, but no longer available, Revox G36 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 are two rotary switches with concentric knobs. One switch establishes the playback mode stereo, either channel through both outputs, on both channels combined for mono. Playback level is controlled by the concentric knob. The other switch connects either the signal input or the output of the playback amplifiers to the output jacks in the rear. Two playback-equalisation characteristics are provided; NAB or IEC (for European tape recordings). The recording equalisation 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-an, 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.
The Transport mechanism is operated by a row of five pushbuttons, activating solenoids 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 (71/2 and 33/4 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 tension to optimum values for 10 1/2 inch or smaller reels.
The servo-controlled drive system of the Revox A77 is unique and effective. The tape-drive capstan is powered by an eddy-current motor that delivers a high torque, free of the pulsations that are inevitable with any motor having a pole structure. 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 71/2 and 33/4 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 per cent cause a speed change of only 0.05 per cent, and a change in the a.c.-Iine frequency of 50 to 60 Hz causes a speed change of less than 0.05 per cent. Thus, the Revox A77 is a truly universal machine, capable of operating from 110 volts to 240 volts, 50 to 60 Hz, by adjustment of a switch in the rear of the recorder.
When the full-width head cover is swung down, two more pushbuttons are revealed. One cuts off the signal to external speakers, and the other switches off the power to the reel motors. This is for convenience in editing. When the reel motors are turned off, and the recorder placed in a fast-speed mode, the reels may be turned by hand with the tape in contact with the playback head. At the desired point, the tape may be lifted from the heads and placed in the tape splicing guide which is moulded into the fixed portion of the head cover. The only problem with this arrangement is the possibility that one may spill tape by forgetting to turn on the reel motors before placing the machine back into normal operation.
We stated that the A77 surpassed the older G36 in performance. This is best illustrated by its phenomenally flat record/playback frequency response, measured with Scotch 203 tape, for which the machine's bias was adjusted. At 71/2 ips, the response was within + 0.5, - 2.0 dB from 20 to 20,000 Hz. This has never been equalled by any other recorder we have tested. Perhaps even more impressive is the response at 33/4 ips, which was + 2.5 - 3.5 dB from 20 to 20,000 Hz. The high end falls off smoothly and is perfectly usable all the way to 20,000 Hz. The NAB playback response, with the Ampex 31321-04 test tape, was + 1.5, - 0.5 dB from 50 to 15,000 Hz.
The signal-to-noise ratio was very good, 51 dB at 71/2 ips and 48.5 dB at 33/4 ips, referred to a 0-VU recording level. Noting that the distortion at 0 VU was a mere 0.65 per cent, we increased the recording level until the distortion reached approximately 3 per cent, which occurred at + 10 VU for the higher tape speed and+.9 VU for the lower speed. At these levels,- the signal-to-noise ratio was 59 dB at 71/2 ips and 54.5 dB at 33/4 ips, figures that closely approach true professional performance.
The Transport worked smoothly and with complete silence. Except for the turning of the reels, one could not tell the machine was operating from a distance greater than about 12 inches. Wow was 0.01 per cent (actually the residual inherent in our instruments) and flutter was 0.09 per cent at 33/4 ips and 0.07 per cent at 71/2 ips. In fast speeds, 1,800 feet of tape was handled in about 90 seconds, and the machine could be brought to a stop in about 2 seconds.
The Revox A77 is housed in a teak cabinet with a fold-away carrying handle. It is one of the handsomest, as well as best-performing, tape recorders we have seen. We have never been a recorder that could match the performance of the Revox A77 in all respects, and very few that even come close. It sounds as good as it tests, which speaks for itself. The Revox A77 is offered in a variety of configurations. It is available with either half or quarter-track heads, in either the teak cabinet or a portable carrying case. The price of the deck in a wood base is $569; the deck with built-in power amplifiers is $599-

Revox Corporation, 155 Michael Drive
Syosset New York 11791 New York USA
Tri-tel Associates Ltd
Toronto Canada
C E Hammond & Company Limited
Lamb House Church Street Chiswick London W4

(Remark: Historical information, original published approx. end of 1969. Please note that the adresses and phone numbers may not be correct any longer.)

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