Revox B710 MkII Cassette Deck Review

Like many other cassette decks, the Revox B710 MkII is a three-head model with both Dolby-B and Dolby-C noise-reduction systems. It is unlike other cassette decks in its appearance and operating features, however, and its performance is essentially at the highest levels that have yet been achieved in the cassette medium.

The B710 is a large, rugged, and relatively heavy machine. A rigid die-cast frame supports the tape-transport mechanism, which uses no fewer than four direct-drive motors. Its dual tape-drive capstans are driven by individual Hall-effect motors locked to a common quartz-crystal reference oscillator. The tape hubs are driven by individual d.c. motors with tachometer feedback control. The B710 has no belts or clutches, all tape motions being handled directly by the drive motors under control of a microprocessor. Even the head assembly (which contains a configuration of three Sendust heads that permits monitoring from the tape while recording) is moved into place by a solenoid, and the motion is smoothed by pneumatic damping.

The microprocessor also controls the B-710 MkII’s non-volatile control memories, tape counter, and digital clock. The built-in timing facilities of the deck not only allow unattended recording or playback, starting and stopping at preset times, but permit the tape playback to be stopped and started at any desired settings of the tape index counter, so that any part of the tape or one whole side may be repeated indefinitely. The index counter is a large four-digit, seven-segment LED numerical display. The same digits show the time in the clock mode, which can be selected at any time by a front-panel button and is available continuously while the machine is turned off.

The cassette snaps into an open well on the front panel and cannot be removed unless the tape is stopped. The cassette opening has a removable (and easily misplaced) plastic cover. To the left of the cassette opening is the time/tape-index display, below which are six square momentary-contact buttons. All internal switching is done by FET’s close to the affected circuits, so that the control buttons handle only low-level d.c. voltages.

The buttons are for pause, fast forward and rewind, play, stop, and record. A logic system permits the transport to be switched from any mode to any other without damage to tape or mechanism. To enter the record mode, both record and play buttons must be pressed simultaneously, but “flying start” recordings can be made while in play by pressing record and tapping the play button. The use of the record mode is shown by a red light on the right side of the digital display window. The pause control works as expected when recording, but it is inoperative during playback. The stop control can be used instead during playback, even for temporary interruptions, since the tape transport starts almost instantly and without an audible transient.

Similar buttons to the left of the numerical display are marked mode, runup, and zero. The mode button switches between clock and index displays, zero resets the index reading to 0000, and runup is used when setting index readings for repetitive playback or for changing the clock or timer settings. Toggle switches along the lower part of the panel control power, tape or source monitor, and Dolby noise reduction. One switch turns on the noise reduction, and another selects either the Dolby-B or Dolby-C system.

On the right side of the panel, opposite the time/index display, are the level “meters.” They consist of two parallel twenty-four-element rows of red LED’s corresponding to program levels from —30 to + 8 dB (0 dB being at the Dolby reference level of 200 nWb/m). The LED’s respond virtually instantaneously to level increases, but the response decays over a period of about 2 seconds. Their readings do not include the effect of recording equalization.

Two sets of concentric knobs below the meters (for left and right channels) adjust recording levels for the line and microphone inputs (which can be mixed). Standard 1/4-inch phone jacks below the knobs are used for the left and right microphone inputs (plugging a microphone into the left jack feeds its signal to both channels for mono recording) and for a pair of stereo headphones. The headphone output is designed for driving medium-impedance (200- to 600-ohm) phones, and a small knob next to the jack adjusts the headphone volume level.

A brushed-aluminum strip across the top of the panel hinges down to reveal a number of less-often-used controls. Four small buttons marked set, start, stop, and clear are used to set the index points for automatic start and stop operation of the machine and to enter the clock time. A three-position timer-mode slide switch sets the machine for unattended playback or recording under the control of its own clock. Four small buttons at the right of the group set up the equalization and bias for various types of tape. Three are marked for IEC Type I (standard ferric oxide), Type II (chromium dioxide or chrome equivalent), and Type IV (metal) tapes, respectively. The fourth button (auto) can be used with most recent cassettes having notches on the rear edge for automatic setting of the bias and equalization. Manual selection must be used with older cassettes that lack these notches. A slide switch inserts a 19-kHz filter in the signal path for recording from FM tuners with inadequate suppression of the pilot carrier signal (which could interfere with the operation of the Dolby circuits).

On the rear apron of the Revox B710 MkII are phono jacks for the line inputs and outputs, with screwdriver adjustments for setting the playback output levels (normally used at maximum settings). When the B710 is used with a Revox B739 or B780 receiver, the two can be connected via DIN sockets on the recorder, permitting remote switching of one by the other. The Revox B710 MkII is about 17-7/8 inches wide, 13-7/8 inches deep, and 6 inches high. It weighs 23 pounds. Price: $1,995.

Laboratory Measurements

We measured the record-playback frequency response of the Revox B710 MkII with a number of different types of tape (including samples of the IEC Type-I and Type-II reference standards) to determine if any were particularly suitable or unsuitable for use with this machine. For the most part, the response curves were so nearly alike that we would consider all the tapes equally compatible with the B710. The similarity of the curves also suggests to us that most current tapes conform so closely to IEC standards that user-adjustable tape bias or equalization, either manual or automatic, is a much less important feature for a top-of-the-line cassette deck now than it was only a couple of years ago.

We selected BASF Professional I Super as our Type I (ferric) tape, since it appeared to be closest to the IEC Type I in its response. For Type II (chrome) we chose TDK SA-X. The B710 is not designed to use Type III (ferrichrome) tape. For a Type IV (metal) tape we chose TDK MA, which we understand is essentially the standard IEC Type IV tape.

The playback response of the machine was measured (for both 70- and 120-microsecond characteristics) with the appropriate BASF IEC standard test tapes. Both tapes gave similar results, with a response flat within ± 1 dB from 80 to 12,000 Hz, rising to +3 or +4 dB at 18,000 Hz and to +2 dB at 31.5 Hz.

The 0-VU marking on the deck’s front panel corresponded to the Dolby reference level as claimed. The overall record-playback response was essentially the same with all of the selected test tapes at a — 20-dB recording level. It was typically flat within approximately ± 1 dB from 35 to 20,000 Hz, rolling off to about — 5 dB at 20 Hz. The “head bumps” in the low-frequency playback response were very small. At a 0-dB recording level, the playback output was — 6 dB at 11,800 Hz with BASF Professional I, 15,500 Hz with TDK SA-X (Type II), and 18,000 Hz with TDK MA tape.

The “tracking” of the Dolby encode and decode circuits was measured at recording levels from 0 to —40 dB (using Type II tape, which was marginally flatter in its response than the others). For the Dolby-B system, the change in overall response when the system was switched on did not exceed 1 dB at any level over the full frequency range except between 15,000 and 20,000 Hz, where there was a 1.5- to 2-dB change at —30 dB. With Dolby-C, the tracking error was about the same up to 10,000 Hz, with a change of 1.5 dB between 10,000 and 20,000 Hz at most levels. All these response changes are inaudible in normal use with music as program material.

However, a striking benefit of Dolby-C could be seen in the 0-dB record-playback response curve made with the noise reduction on. The high-frequency saturation, above 10,000 Hz, was virtually eliminated, resulting in the astonishing 0-dB response of + 1, — 1.5 dB from 32 to 20,000 Hz. This is a relatively unpublicized feature of Dolby-C, which employs a “spectral skewing” circuit to attenuate frequencies in the 10,000- to 20,000-Hz octave during recording and boost them in playback. The process greatly reduces high-frequency tape-saturation effects at the expense of about 12 dB of noise reduction in that octave. Since Dolby Labs has determined that the ear is relatively insensitive to low-level noise in the uppermost octave, the trade-off of noise reduction for headroom is highly beneficial.

The required input signal for a reference-level recording (at 400 Hz) was 62 millivolts (mV) for the ferric and chrome-equivalent tapes and 70 mV for metal tape. In each case, the maximum playback output from a 0-dB recording level was 0.81 volt. The third-harmonic playback distortion from the reference signal was 0.5 per cent for ferric and 1 per cent for chrome-equivalent and metal tapes. At 10 dB below reference level, the respective playback distortions were 0.13, 0.1, and 0.2 per cent. To obtain a playback distortion of 3 per cent, the recording level for the tapes was +5 dB for ferric, +4 dB for chrome-equivalent, and +5.5 dB for metal. The A-weighted signal-to-noise ratio (S/N) without noise reduction was approximately 55.5 dB for all three tapes. With Dolby-B and CCIR/ ARM weighting, the S/N readings improved to 64.8, 67, and 66.3 dB. With Dolby-C, they were 75.2, 74.5, and 74.8 dB, respectively.

The microphone input’s sensitivity was 0.3 mV, and it overloaded at 37 mV. The noise level through the microphone input increased by 17.5 dB over the line-input noise level at maximum microphone gain, but only by 3 dB with a center setting of the mike-gain control. Although we did not measure the headphone output, its adequacy can be inferred from the fact that the acoustic output of 600-ohm phones could be heard throughout the room even at partial volume settings.

The tape speed was 0.3 per cent fast. Flutter was ± 0.065 per cent weighted peak (DIN) and 0.043 per cent weighted rms (JIS). The predominant flutter rates were 8 and 35 Hz. In fast-forward and rewind modes, the B710 MkII handled a C-60 cassette in only 47 seconds. In spite of this exceptionally fast tape handling (or, more likely, because of it), the tape was stopped with unusual gentleness, slowing down perceptibly in the final seconds of the process to avoid undue stress on either tape or leader. The meters were accurate and as readable as their calibration intervals allowed.

Comment

The statement that we could find no significant flaw in the Revox B710 MkII should not be interpreted as meaning that it is perfect, but merely that it is so surpassingly well designed and constructed that even an occasional operating idiosyncrasy was easy to ignore. It should hardly be necessary to point out that this machine can record almost any program likely to be connected to its inputs with absolutely no audible degradation of signal quality. True, with interstation FM tuner hiss recorded at levels close to 0 dB, we could hear (barely) a minute change in the spectral balance when the playback was compared with the incoming signal. Not a very serious flaw, to be sure. Dubbing some wide-dynamic-range CX-decoded records we found (as expected) that Dolby-B noise reduction left an audible residue of hiss on the tape during quiet passages. However, the quiet background with Dolby-C was very nearly the equal of the CX residual noise level, and the tape hiss could be heard only on a critical source/tape comparison at a very high volume level.

The mechanical operation of this machine was a joy to experience. Instead of the usual “clunk” of solenoid operation, pressing any of the control buttons produced only a subdued and muffled click that never became obtrusive. The recorder is actually easy to use after one has carefully studied the manual (a well written, trilingual, spiral-bound book with numerous illustrations and a complete functional block diagram of the recorder). A little practice may be needed to master the techniques of setting the clock and using the programmed or timed-repeat modes, but for normal use the machine is as simple as any we have seen.

The Revox B710 MkII is a superlative machine by any standard that can be used to judge a cassette deck. The only nit we can pick about its design is the absence of a switched a.c. outlet. The accurate and convenient built-in clock/timer worked well, but it cannot switch on an associated tuner or receiver for unattended recording. As it is now designed only the fortunate owner of a Revox receiver can fully utilize this capability of the recorder.

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