REVOX B710 MKII CASSETTE DECK
to 20 kHz with chrome or metal tape.
Studer Revox makes a limited number of products for the audiophile market, bul they are all of very high quality. It is always ol interest, therefore, when they introduce a new model, such as the B710 MKII stereo cassette deck. It has microprocessor control (including a built-in timer), three heads mounted on a high-stability common base, four-motor direct drive, and other goodies which I will cover later. The cassette holder and drive assembly dominate the front panel with their size and rugged looks. The dual capstans draw altention because they are of much larger diameter than those on other decks. Also unusual and noteworthy is the light and sensor assembly to detect clear leader for automatic cueing right at the start of the actual oxide.
The transport control buttons are the standard ones, and the logic allows direct changing between most modes, including "punch-in" flying-start recording (shifting to "Rec" while in "Play") by pressing both "Rec" and "Play" at once. There is no pause function in "Play," but there is one in "Rec." where it's needed. The counter display above shows either four bright LED-type digits for tape Iocation or, with a push of the "Mode" switch, local time. Counter, memory, time and timer settings are controlled with the adjacent buttons for "Zero" and "Run-Up" (which set and find Ihe zero positions on the tape), while the timer's "Set," "Start," "Stop" and "Clear" buttons are behind a swing-down panel at the top. There are also slide switches for timer mode ("Play/Off'Rec") and the multiplex filter, as well as four interlocked pushbutton switches for tape selection (IEC Types I, II, and IV or auto-selection from sensing holes in the cassette shell).
When the B710 MKII is placed in record mode, a red indicator appears at the very right of the "Counter/Time" display, just above the "Rec" button. Below the transport buttons are the large-handled toggle switches for "Power." "Monitor," and "Dolby-NR" ("On/Oil") and ("C-Type/B-Type"). If the power switch is in the "Standby" position. The clock continues to function, and the time can be read out by pushing "Mode." When the Dolby-mode switch is on, a Dolby double-D symbol is illuminated al the right end of the horizontal LED-type "Peak-Reading Meter," If the multiplex filter is on, "MPX" appears just above the Dolby symbol, and if the tape seleclor is set to "Auto." there is an "IEC" just below it.
The "Peak-Reading Meter" covers a wide range, from " - 30" to " + 8" for each channel. Meter resolution is very good, with 24 bars for each channel and with separate thresholds for each bar. The dual-concentric level pots for both mike and line inputs allow for complete mike/line mixing. There is a fair amount of friction between sections, but the large, well-knurled knobs make interchannel balancing quite easy. There are phone jacks for left and right mikes and (or headphones. (A single mike in the left jack sends a mono signal to both channels.) The separate headphone volume control does not affect the outgoing line level—a considerable convenience at times. The large, white lettering on the gray background makes lor excellent legibility under any normal lighling.
The connections on the back panel are under a fairly deep overhang of the chassis, so they're a bit hard to see from the front. There are the expected line in/out phono jacks, plus sockets for a remote control and for power-on connections to a Revox receiver, especially useful with the built-in timer. The screw-type fuseholder and a line-voltage selector are potentially helpful if you move overseas.
Access to the interior was gained by removing the metal top cover. The attention that Revox gives to detail was indicated by the cover's smooth, gentle curve which ensures firm sealing at all points when installed. The soldering on the several p.c. boards was excellent, with no flux residue Most of the inter-card connections were made with multipin cables. All of Ihe ICs were in sockets, for easier service replacement. None of the parts or adjustments were labelled, however, so internal work would best be left to those with the necessary servicing information. The four-motor transport assembly caught my attention with its rugged and rigid construction, large flywheels, lack of pulleys and belts, and damped head-gate solenoid. The drive system was very quiet in all operating modes
The playback responses were very good for 120-uS, but showed some high-end roll-off (- 3 dB) lor 70-uS EQ. Playback level indications were exact within the resolution of the bar-graph meters. Play speed was about 0.25% fast, very satisfactory. This Revox deck was supplied with TDK AD. SA-X and MA cassettes, and they were used for Ihe normal test program. Other tapes that were well-matched to the B710's internal sensitivity and bias sellings were BASF Professional I Super and Metal IV. JVC ME-P. Maxell UD XL I and XL II-S. Memorex HBII and TDK MA-R. Swept-frequency responses were obtained tor the three supplied tapes at Dolby level and 20 dB below that both with and without Dolby C NR As Figs. 1 to 3 and Table I show, all of the responses were quite flal and well extended. The bene-fits of Dolby C at higher levels were especially noteworthy with SA-X and MA tapes. yielding response that would do many decks proud at levels 20 dB lower. Dolby tracking was excellent with SA-X, and good with the other tapes. In running Ihe swept responses, some leak-through from the inputs to the outputs at the highest frequencies was noted. A check with pink noise at maximum meter level proved that the coupling was a testing oddity and definitely not delectable with music-type signals.
Playback of a recorded 10-kHz tlone had about 30° of phase error between tracks and a total jitter of 10°, better than most decks overall. The multiplex filter was 1 dB down at 16.0 kHz and a good 32.8 dB at 19.00 kHz. Bias in the output during record was very low in level. Separation be-tween tracks at 1 kHz was a very good 47 dB, and crosstalk at the same frequency was an outstanding -87 dB. Erasure of metal tape at 100 Hz was a good 62 dB.
The third-harmonic distortion was a very linear function (in dB) from -10 dB to the HDL3 = 3% limits (Fig. 4) for all three tapes. Notice the very low figures at - 10 dB for both SA-X and AD tapes The SA-X tape was also used for tests of HDL3 versus frequency from 50 Hz to 7 kHz, with Dolby C NR, at 10 dB below Dolby level. The results from 50 Hz lo 2 kHz were excellent (Fig. 5). but the sharp rise above 2 kHz was a litlle unexpected.
The signal-to-noise ratios (Table II) were measured at Dolby level and at the 3% limit points, both with and without Dolby C NR and with both IEC A and CCIR/ARM weightings. The perceptive reader may notice that the difference between a Dolby-level ratio and a 3%-point ratio is less than the level difference shown in Fig. 4. The discrepancy is the result of compression in the recording: Fig. 4 shows the difference in incoming record levels, but Table II reflects the compression revealed in playback. (Please note that this is a characteristic of all recorders, even though rarely reported.) A further point is tlhat the 3% points are higher with Dolby NR than without—another plus for noise reduction. The results are all excellent, and there is no doubt about the considerable advantage Dolby C provides over Dolby B in lowering noise.
The input sensitivities were 0.64 mV for mike and 64 mV for line, both higher than the specification and by exactly the same ratio (64:35). In some cases, it would be advantageous to have the specified sensitivities. The overload points were 40 dB higher, as specified: Mike at 65 mV and line at 64 V. The output clipped at a level equivalent to + 15.1 dB relative to meter zero. The line input impedance was close to 200 kiloohms over much of the band, falling to about 25 kiloohms at 20 kHz—still plenty high enough. The line input pot sections tracked within a dB from maximum down 60 dB, which is outstanding. The mike pot sections tracked to the same limit for 45 dB, quite acceptable. The line outputs were right on the specification of 775 mV for meter zero, falling to about 747 mV with a 10-kilohm load. The maximum headphone output was a very high 2.61 V, even with an 8-Ohm load. That was far more than enough for all of the phones tried, and it was necessary to turn the headphone volume control down for normal use. The line output impedance was close to 400 ohms (low and good) across the band, but increased somewhat when the output level-set pots were turned down to their maximum attenuation of 28 dB. The output polarity was reversed in both "Source" and "Tape" modes.
Tests with a 5-kHz tone burst demonstrated that the B710's meters were truly peak reading, meeting the require-
Table 1— Record/playback responses ( - 3 dB limits).
Table II—Signal/noise ratios with IEC A and CCIR/ARM weightings.
ments of IEC Standard 268-10 both for response and decay time. A full zero response was reached with a 20-mS burst. The frequency response was 3 dB down at 6.6 Hz and 25.9 kHz. Calibration of the scales was excellent over the entire range from " - 30" to " + 8." It is a rarity to find 24 separate thresholds so accurate, and the deck's meters were definitely superior in that regard. The high brightness of the segments made for easy reading over a range of lighting levels
Playback of a 3-kHz tone recorded with 120-V line power had no detectable change in speed over a range from 110 to 130 V, one indicator of good transport design. Over a considerable period of time, any speed variations were less than the minimum readable, 0.02%. Throughout the length of a C-90 cassette, maximum flutter was 0.038% wld. rms and 0.07% wtd. peak, both excellent figures Wind times were just 68 seconds for a C-90 cassette, a shorter time than many decks need for C-60s This last wind, however, was very smooth and quiet (constant-speed, according to Revox), and the stops were soil and shock-free. The times for run-out to stop, changing winding directions and going from wind to play were all less than a second.
Use and Listening Tests
The instruction manual has 11 pages each of English, French and German. There are many excellent illustrations, well integrated into fhe fairly lucid text, although I would have preferred more detail in the discussion of memory functions. The block schematic is very good and a most welcome inclusion. The design of the cassette drive, which projects out from the front panel, made for very easy loading/unloading and provided excellent access for all maintenance tasks. All of the controls and switches were completely reliable throughout the testing I found il a little frustrating That I could not set "Start" (the timer's control behind the swing-down panel which zeroes the counter) while in record or play mode, because this caused the deck to stop I would also like to have been able to rewind past "Start," but pushing the rewind button again caused the B710 to go into play. Having the clock function built in was nice, and the timer scheme worked fine.
The listening tests utilized pink noise for the Dolby tracking tests, as usual, and the discs included Mobile Fidelity's Romantic Russia with Georg Solti and the London Symphony Orchestra and Chorus. The biggest effect noted in any of the switching was the great improvement in the high end at higher levels with SA-X tape when Dolby C was switched in; there were changes with the other tapes, but less obvious. At times, there were small variations in the playback compared to the original, but they were fleeting, and the sound quality was always excellent Dolby C NR was used for these tests, and its low-noise character was apparent in a number ol very quiet passages.
The Revox B710 MKII does not have a sophisticated array of microprocessor-controlled functions such as music-selection programming, intro-search. etc.. but it does have a solid list ol good things for its premium price. These include excellent response, low distortion, very low noise, full mike/line mixing, excellent peak-responding meters and an out standing transport drive system. All in all, the B710 MKII impresses me as one deck that will deliver excellent performance for many years to come.
Howard A Robefson