WHEN THE Revox A77 tape recorder was introduced to the American market some
years ago, it received generally enthusiastic reviews from the hi-fi press.
Since then it has undergone several updatings and refinements to the basic
design. These included a special A77 incorporating Dolby B noise-reduction
and the HS77, a "high-speed" (7 1/2 or 15 ips) unit with half-track
stereo heads. You may recall I reported on the high-speed type as part of a
lightweight, portable recording system.
The Revox A77 has kept pace with its competition and still answers the needs of many audiophiles. However, in line with current trends, it was obvious there was a need for a more sophisticated recorder incorporating the technological advances of recent years. Willi Studer, manufacturer of Revox recorders, was aware of this and of the fact that his "encore" would have to be something quite special. I think it is safe to say that his new Revox A700 tape recorder is special indeed. This A700 has been shown at past AES conventions and I have given you a brief description of the unit. Revox furnished me with an A700 recorder with half-track stereo heads, and now that I've lived with it for some time, herewith are my observations and impressions of this important new recorder.
The A700 is bigger than the A77, a bit more than 19 inches wide by 18 inches in height by 7 inches in depth.
The weight has increased too, and portable use will require more muscle. The increase in size and weight is understandable in light of the new features and functions built into this machine.
Let's take a look at the tape transport section. The A700 is a three-motor unit operating at 3 3/4, 7 1/2, and 15 ips. There is a servo-control system for all 3 a.c. motors, which is integrated with the tape-motion full-logic system. The capstan motor is a high torque, asynchronous unit. Some 120 teeth are milled into the circumference of the rotor and are inductively scanned by a ring-coil around the motor. This constant 360-degree scanning is an improvement on the single-point scanning found in the Revox A77. There is less translation error for one thing, but the big advantage is in the quietness of the motor. When I say quiet, I mean you have to put your ear right up to the capstan shaft before you hear any whir. For any recording situation where the tape machine must be in the same room as the microphones, the A700 takes top honors. The signal produced by the inductive tacho-generator is fed into the servo-control system. This is a new Studer development, using special Large Scale Integrated (LSI) circuits which contain a quartz crystal-stabilized oscillator of 1.638400 MHz, along with the comparators, frequency dividers, and other functions of the servo circuit.
The capstan motor is thus phase-locked to the quartz clock frequency. However, there is also provision to override the reference frequency with a variation control (soon to be available) which will give plus or minus seven half-tones of any selected speed. An external oscillator vari able from 1 kHz to 10 kHz will enable continuously variable tape speeds from 2 1/2 to 21 1/2 ips. The three tape speeds are selected by pushbuttons on the right side of the tape transport. A nice touch is that these buttons are of the illuminated type, but will not light up until the selected speed is in full synchronous mode. As you might expect, it takes a tiny bit longer to "lock-in" the slower tape speeds.
The A700 has a tape motion sensor in its right idler roller, which feeds signals into the logic circuits controlling power to the reel motors, the pinch roller, and the tape-lift solenoids. On each side of the idler rollers is an idler wheel which is on a progressively-sprung tension arm. This is part of the tape tension control system of the A700, where the tension is constantly measured and maintained at optimum value by two closed-loop servos, each consisting of both the tape tension sensors, motor control circuit, motor control amplifier, and the reel motors themselves. All this adds up to fail-safe tape handling logic and very smooth spooling. The system is so good that on fast forward and re wind the tape pack is as free of "ridging" as normal play winding.
Located with the speed change but tons on the right side of the unit is a tape timer of the type found on the professional Studers. The readout is in minutes and seconds, with real-time indication at 7 1/2 ips. Accuracy is stated to be within 0.5%, and when I checked it against my stop watch, it was only 14 seconds fast in a half-hour program. A central grouping of five illuminated pushbuttons comprises the normal tape motion controls. On the left of the machine are illuminated pushbuttons for special operation controls. The pause button remains in effect as long as it is depressed and operates on all main transport modes. After release, the control logic switches automatically to the previously selected mode. By pushing the repeat button, the transport switches for rewind, releasing it selects the play mode. Using tape with translucent leaders at both ends and pressing the auto button enables the tape to be played, then rewound and played again, endlessly.
How good is the Revox A700 tape transport? The specs state a weighted peak flutter of less than plus or minus 0.06 percent, but a friend with a quality flutter bridge measured it consistently around .02- .03 percent. All in all, the tape transport of the A700 is one of the most advanced yet offered to the public and I found that it operated flawlessly.
As with most of the tape transport controls, the record and play electronics, and all special function (such as stereo multi-play and echo effects) electronics are on PC boards. The bias oscillator frequency is a high 150 kHz.
Studer has kept to the use of metal-laminate heads, and with the servo tension system gets a good tape head wrap for smoother low frequency response and extended high end response. It is interesting to note two other things about the heads. One is that Revox states there is room in the head/electronic complex for "extra electronics as necessary in later models." The other is that the die-cast head support is quickly interchangeable. One can but speculate that all this is in anticipation of a four-channel model.
The A700 input and output controls consist of both rotary switches and slider-type mixing pots. The en tire mixing/electronics chassis is re movable for servicing. There are four balanced 50-600 ohm microphone in puts with high- and low-level switching. A phono input preamp for magnetic cartridges with RIAA equalization is provided as well as two high level line inputs. The four slider faders are under the control of a stereo master slide fader. Also unusual for this kind of recorder is the provision of bass and treble controls. Main reason for them is that there is an out put controlled by a slide fader for connection to a fixed gain amplifier, such as Revox's A722 model. Line out puts A and B on the A700 have a fixed output level at plus 6 VU corresponding to 1.55 volts.
The VU meters are horizontally op posed, and unlike previous Revox machines, the meters read the play back output. In addition to the regular ASA VU indication, there are two red LED overload monitors, set to peak at plus 6 VU.1 hooked up an external meter, and fed some signal to the recorder and sure enough, the LED's blinked on at exactly plus 6 VU! As to performance, the Revox A700 is a winner. We have already covered the wow and flutter figures. The frequency response from tape is specified as 30 to 22,000 Hz, plus 2, minus 3 dB at 15 ips; 30 to 20,000 Hz plus 2, minus 3 dB at 7 1/2 ips; 30 to 1600 Hz, plus 2, minus 3 dB at 3 3/4 ips. I copied some wide-range master tapes on the A700, and the copy was liter ally a mirror image of the master. I checked the playback frequency response with my Magnetic Reference Library alignment tape, and the A700 was flat within one and a half dB from 30 to 20,000 Hz. Distortion is stated as less than 2 percent at plus 6 VU. I made some live recordings and deliberately activated the LED monitors without hearing anything approaching distortion. I used Ampex 406 tape in recording, which is close enough to the 3M 206, for which the unit was biased. With a claimed minus 65 dB S/N (ASA, A weighting), the tape copies I made had the slightest extra whisper of hiss compared to the very quiet master. Well, noise has to increase 3 dB, but I think the S/N claims are reasonable. All the controls acted smoothly, and it is a joy to see such precision tape handling. The recordings I made were beautifully clean and there is no question in my mind that the Revox A700 is a very strong competitor for leader in its price range.
(Source: Audio magazine, Jan. 1975; Bert Whyte)
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