Technics RS-B905 Cassette Deck Review

Dolby HX Pro headroom expansion, dbx and Dolby noise-reduction systems, separate record and playback heads, and a closed-loop, dualcapstan drive system are among the many attractions of the new Technics RS-B905 cassette deck. As the flagship deck of the Technics line, the RS-B905 also incorporates phase-compensation circuitry to improve stereo imaging, a user-adjustable bias control, and separate tape-sensitivity controls for each channel.

The take-up and supply-side capstans of the RS-B905 are belt-coupled and are driven by a DC servomotor. A small difference in the capstans’ diameters gives them slightly different rotational speeds, which produces the tape tension that holds the tape against the heads (The cassette’s pressure pad exerts additional tension, of course, but from an engineering viewpoint the pad is superfluous in a closed-loop transport.) A second DC motor drives the two reel hubs.

Although they share a common casing, the record and playback heads of the RS-B905 are electrically separate units, which allows the user to make instant quality comparisons between the source signal and its just-recorded counterpart. Such a design also allows the use of a relatively wide head gap for the record element, yielding a better signal-to-noise ratio, while using a narrow head gap for playback, yielding improved high-frequency response.

The Dolby HX Pro headroom-expansion system also extends high-frequency response at high signal levels. The HX Pro circuitry continuously monitors the signal being fed to the record head, and when it detects treble levels that would result in tape saturation (a serious problem at the 1-7/8-ips cassette tape speed), it instantly lowers the record bias slightly. The lowered bias enables the tape to hold more treble energy at just those moments when it is needed. The bias is restored to its normal level when the treble demand declines so that overall signal distortion will not increase.

The RS-B905 has a conventional cassette well with a nicely damped door mechanism, but the lack of rear illumination and a narrow viewing window render label and tape-remaining visibility poor. In contrast, the fluorescent peak-level indicators, with eighteen segments per channel calibrated from — 40 to + 18 dB, are highly readable. The three-digit fluorescent tape counter registers reel-hub revolutions only, with no indication of elapsed or remaining time.

The main adjustments for bias and equalization for different tape types (ferric, high-bias, and metal) are made automatically, but a user-adjustable bias control is provided to compensate for the bias requirements of different brands of ferric or chrome-type tapes. Since no tone-generating circuits are included, the user must make such refinements by ear while switching back and forth between tape and source positions. (A good test signal is FM interstation hiss, if it is recorded at a level well below 0 dB.) With most decks, slight differences in tape sensitivity, manifested as differences in loudness between tape and source switch settings, make such noninstrumented bias adjustments extraordinarily difficult. The RS-B905’s tape-sensitivity controls for each channel both ease this problem and increase the accuracy of the Dolby noise-reduction system.

The front-panel headphone jack has its own volume control, but there is no playback level control for the main output jacks on the rear. A conventional switch for external-timer record and playback activation is provided, as is rewind/repeat-play operation. The FM-multiplex filter is defeatable so that high-frequency response of CD and LP dubs is not limited.

The Technics RS-B905 measures 17 inches wide, 4-3/8 inches high, and 11-1/4 inches deep, and it weighs approximately 11 pounds. Price: $600.

Lab Tests

Measured with our IEC-standard calibrated BASF test tapes, the playback frequency response of the RS-B905 was extraordinarily flat for both ferric and chrome or metal cassettes. On an overall record-play-back basis, the low end rolled off very smoothly to — 3 dB at 30 Hz, and there was no sign of the usual low-frequency undulations (“head bumps”). At the normal — 20-dB measurement level, the response of our “center-line” ferric tape (TDK-AD) dropped to — 3 dB at 18,000 Hz, as specified, and our chrome-equivalent tape (TDK-SA) made it all the way to our 20,000-Hz upper measurement limit with a deviation of only +0.5, —2.5 dB.

With our centerline metal tape (TDK-MA), the response between 10,000 and 20,000 Hz rose to +3 dB, indicating a slight underbias in the factory setting, which also exaggerated the metal-tape response at a 0-dB level. The user bias adjustment does not affect the deck’s setting for metal tape, so we could not try to flatten this response, but it was (just) within specification in any case.

The signal-to-noise ratios of the RS-B905, as shown in the accompanying box, were extremely good, though the raw numbers perhaps need some additional explanation. The 0-dB indication on the unit’s display was factory set at what is, for today’s tapes, an unrealistically low level. Specifically, inputs at the IEC standard reference level, which is often used to set the 0-dB point for today’s high-quality decks, read + 6 dB on the left channel and +4 dB on the right. Also, we measured 3 percent third-harmonic distortion, both with no noise reduction and with the two Dolby noise-reduction systems, at a staggering indication of +12 dB. In my opinion, encouraging recordists to record at such an indicated level for signal peaks is absurd, yet if they do not do so residual background hiss will become an almost inescapable audible problem.

Measuring the signal-to-noise ratio (S/N) with the DBX noise-reduction system posed an additional challenge, though of a pleasanter sort. The conventional 3-percent point for third-harmonic distortion could not be reached by any input level, for the input stages of the RS-B905 began to clip at a +22-dB input, 4 dB above the maximum of the deck’s own indicators! We measured the DBX S/N, therefore, at a + 20-dB input, though I do not advise readers to run their indicators offscale when recording with DBX. With this noise-reduction system it is more important to use some of the “excess” signal-to-noise capacity to reduce—drastically— the so-called “normal” distortion on the tape.

Our wow-and-flutter measurements showed good but not exceptional performance, and overall speed accuracy was 0.7 percent high. Dolby tracking was extremely good, as was that of the DBX system. Both input sensitivity and output level were entirely normal, and fast winding was slightly better than average.

Comments

We made most of our listening tests in advance of measuring the RS-B905, and some of the residual background hiss we detected (especially with Dolby C, with which we expected to hear next to none) dropped into insignificance when we later learned how severely miscalibrated the deck’s 0-dB indicators were. The sonic imagery and ability to pinpoint sources across the sound stage were absolutely superb, among the best I have heard. How much of this is attributable to the deck’s special “phase correction” circuitry is not certain since the feature can’t be defeated temporarily for comparison, but the clarity of the record-playback reproduction was outstanding.

The smoothness of the frequency response was also audibly evident, and while our measurements later showed that perhaps we should have been able to detect some slight wow-and-flutter, the fact is that we did not. The DBX system itself showed very little audible “pumping,” though when we were listening for it specifically some very slight noise shifts could be heard on solo piano notes in a recording of the Schubert Trout Quintet. On prerecorded materials the playback was as clean as any we have heard. That so much top sound quality can be provided in a deck at the relatively modest price of the RS-B905 is a tribute to the engineering acumen of the Technics designers.

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