TESTED THIS MONTH, by Hirsch-Houck Laboratories:
Sequerra Model 1 FM Tuner; Jensen Model 15 Serenata Speaker; Superex EP-5 Stereo Headphones; Technics RS-676US Cassette Deck
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Sequerra Model 1 Stereo FM Tuner
AUDIO-EQUIPMENT buffs have been aware for some time of the development of a new FM tuner by the same design team that created the classic Marantz 10B tuner some years ago. The Sequerra Model 1 has been in prototype form for a couple of years but is now in production, and we have had the opportunity of evaluating one of the early units.
In addition to a technically advanced multiplex decoding section and Dolby noise-reduction circuits, the Sequerra Model 1 has a unique multifunction oscilloscope display. Probably the principal purpose of the scope display is as a tuning and multipath-distortion indicator. (Multipath distortion is instantly visible on the screen and can be minimized by antenna rotation.) Unlike most such FM oscilloscope indicators, this one is accurately calibrated in signal level over a range of more than 100 dB. In other words, it is now possible to estimate the actual number of microvolts of signal reaching the tuner's antenna terminals from any tuned-in station. This could be a useful feature for anyone trying to evaluate the performance of different FM antenna systems in a difficult or remote receiving location.
The scope screen can also provide a vector audio display, showing the left and right audio signals from the received station. Mono or out-of-phase programs can be immediately identified. In the rear of the tuner are four input jacks intended for the audio outputs of a four-channel system. The Model 1's scope can then display the spatial distribution of a four channel program, with the level of each channel appearing as a deflection along one of four axes on the scope screen.
The last special (and optional) function of the oscilloscope display is truly unique to this tuner. There is a high-performance spectrum analyzer that scans a 2-megahertz portion of the FM band, centered at the frequency to which the Model 1 is tuned. All received signals are shown as "pips" whose horizontal positions and heights indicate the relative frequencies and strengths of the signals. Not only does this display show the presence of signals before they are tuned in, but one can easily identify from the pip shape whether the station is transmitting in stereo or even whether it carries SCA (Subsidiary Communications Authorization--e.g., Muzak) programming. Interference signals from other radio stations or from electrical devices can also be spotted readily and often identified from visible characteristics.
The tuner circuits are highly sophisticated and complex; unfortunately, space limitations do not permit a detailed description of the design. Basically, the Sequerra Model 1 has a differential (push-pull) r.f. amplifier, electrically tuned by varactor diodes to track the voltage-controlled local oscillator. There are no mechanical tuning capacitors, and the single tuning device is a ten-turn precision potentiometer. A four-digit frequency counter with bright red light-emitting-diode numerals displays the tuned frequency to the nearest 0.1 megahertz. Selectivity in the if section is provided by an eighteen-pole LC filter, followed by six limiter stages and the detector. The multiplex demodulator is designed for optimum phase characteristics, providing maximum separation across the entire audio-frequency range together with high rejection of ultrasonic subcarrier and SCA signals. There is provision for converting the FM de-emphasis from 75 to 25 microseconds when receiving Dolby broadcasts using the newly approved pre-emphasis characteristic for FM transmission.
The front panel of the Sequerra Model 1 is quite unconventional in appearance. The oscilloscope screen is its dominant feature, with the illuminated graticule (face-plate markings) changing to match the selected display function. The tuning control is the only knob on the tuner, and directly above it is the digital frequency display.
Along each side of the panel is a group of five pushbuttons. Next to each button is a large yellow illuminated identifier of its function, with one or two small red arrows alongside that light up in accor dance with the operation of the push buttons to show the selected functions.
For example, four of the buttons control the display, selecting PANORAMIC (spectrum analyzer), TUNING, TUNER VECTOR (audio), and EXTERNAL VECTOR (audio) modes. The SEPARATION button has MAXIMUM and HI BLEND positions (the blend mode sacrifices a bit of stereo separation at high frequencies to obtain a reduction of background noise). There are three MUTING buttons, labeled OFF, INTER-STATION, and INTER-STEREO (the last allows only stereo broadcasts to be heard). Other buttons control AUTOMATIC MONO/STEREO or full MONO operation, DOLBY operation, PANEL illumination level, and POWER.
At the rear of the tuner are separate fixed- and adjustable-level audio outputs, and inputs for external four-channel audio signals with a common level adjustment. Six screwdriver adjustments affect various aspects of the display section, and one sets the muting threshold level, from less than 3 microvolts (µV) to more than 30 µV. There is a detector output ahead of the de-emphasis, and a jack that converts the de-emphasis to the 25-microsecond characteristic when a shorting plug is inserted. Finally, an accessory socket will accommodate a re mote-control pushbutton tuning assembly (approximately $150) to be available some time this fall.
Though the Model 1 is not especially large (its dimensions are 16 3/4 inches wide, 14 1/4 inches deep, and 6 inches high), its weight of about 32 pounds makes it the heaviest FM tuner we can recall encountering. This can be attributed to its generally robust construction (each of its functional r. f. "blocks" is constructed in a separate shielded module), and to its very large, well-shielded power transformer. To many people, the most impressive specification of the Sequerra Model 1 tuner will be its price: $2,500, or $2,000 without the panoramic spectrum-analyzer section.
Laboratory Measurements. The de sign emphasis in the Sequerra Model 1 has been toward optimizing its audible characteristics-in respect to noise, distortion, and interfering signals of all kinds- rather than toward achieving more impressive but less useful "sensitivity" figures. In many respects, the performance of the Sequerra Model 1 approaches or exceeds the capabilities of the most advanced laboratory test equipment. Although we used the Sound Technology Model 1000A signal genera tor, acknowledged to be the finest commercial unit of its type, it was obvious that much of the time our readings reflected the limits of the generator's performance rather than those of the tuner.
The IHF sensitivity was 2 µV in mono and 3.5 µV in stereo. There is no automatic mono/stereo switching thresh old as such, so that stereo signals are received in stereo down to the minimum level that will lock the tuner's multiplex circuits (about 1.6 µV). The sensitivity for 50-dB quieting was 2.9 µV in mono and 33 µV in stereo. The ultimate distortion, at 1,000 µV, was about 0.1 percent in mono and 0.15 percent in stereo (we assume that both of these distortion levels reflect the limitations of our generator, rather than the performance of the tuner). The ultimate signal-to noise ratio was 69 dB in mono and 65.5 dB in stereo.
The frequency response of the Model 1 was obviously at least as good as that of our test instruments, measuring ±0.2 dB from 20 to 15,000 Hz. The channel separation was about 48 dB or better from 30 to 7,000 Hz. Accurate measurements in the upper audio-frequency range were hampered by a phase-shift problem in the signal generator, which yielded apparent separation figures exceeding 60 dB! With most good tuners, whose separation rarely exceeds 40 dB, this effect is not apparent, but in the case of the Model 1 we must content our selves with saying that its separation exceeds at least 42 dB, and is typically ...
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-- Among the features of the Model l's rear panel are controls for the oscilloscope (focus, intensity, position), means of selecting the amount of de-emphasis, and scope inputs for external signal sources.
-- In the graph of FM performance, the levels of both random noise and noise plus distortion are compared with the audio-output level as signal strength.
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... better than 48 dB, across the full audio-frequency range.
The capture ratio at 1,000µV was 1.1 dB, and it was still an excellent 1.4 dB at 10µV. The AM rejection measured 70 dB, which is apparently the limit of our test capability and surpasses that of any other tuner we have measured. Alternate-channel selectivity was a very good 86 dB. The image rejection of 62 dB was good, but not quite as outstanding as the tuner's other characteristics. The 19-kHz pilot-carrier suppression was a good 62 dB-a notable figure in view of the fact that the tuner's frequency response showed no signs of rolloff even at 15,000 Hz.
The fact that some of these numbers are not significantly different from those we have obtained from many tuners costing a fraction of the price of the Model 1 may seem rather strange. However, given the audible performance of the tuner (which we will get to in a moment), what this indicates is that tuner measurement standards are not as useful as they might be for delineating the characteristics of a tuner with such markedly superior qualities. For example, the IHF sensitivity measurement is based on the level of input signal (in microvolts) required to reduce the level of distortion plus noise (usually hiss) in the received signal to 3.2 percent (-30 dB). Practically, however, it matters little whether a tuner has an IHF sensitivity rating of 1.5 or 3 µV, because a signal-to-noise ratio of 30 dB is simply not suitable for serious listening. The 50-dB "quieting sensitivity" rating proposed for the new IHF FM standard approaches acceptable listening quality at the rated sensitivity, but just barely.
The overall operation of the Model 1 was flawless, with accurate tuning assured by the scope display (the low distortion of the tuner was maintained even with visible mistuning), and there was a drift of less than 10 kHz from a cold start. Compared with most solid-state tuners, the Sequerra Model 1 generates considerable heat, but its temperature compensation appeared to be excellent. The interstation muting was good, though at times we heard a slight noise burst when tuning on or off a station.
Comment. One may fairly ask how a $2,500 price tag can be justified for an FM tuner, no matter how good its performance. Certainly that question was in our minds when we evaluated the Sequerra Model 1. First of all, it must be conceded that the electrical performance of this tuner is, in every important respect, as advanced as the state of the art permits. Most good tuners we have seen cannot match its key performance specifications, and none can equal or even approach the total capability of the Model I. Furthermore, anyone familiar with high-grade laboratory instruments or military electronic equipment will appreciate the quality of the construction and the excellence of the parts used.
The Model 1 is quite unlike most consumer products, even the very best, and its price can be justified on the basis of the overall mechanical and electronic quality of the instrument.
But what about the Sequerra tuner's listening qualities? Does it sound better than other tuners, or offer other equally obvious audible advantages? Our answer must be somewhat equivocal. For casual listening to the vast bulk of ordinary FM programs, the Model 1 sounds no different from almost any good FM tuner costing a fraction of its price.
However, when listening critically with the best ancillary equipment-to the better-quality programs from "good-music" stations, and especially when comparing the sound of this tuner with that of another tuner in an A-B fashion, one can sometimes hear the slightly lower distortion and absence of obscuring "fuzz" in the sound of the Model 1.
The differences are extremely subtle, and we still would find it hard to justify the cost of this tuner if that were the only advantage it offered.
The most obvious difference in the sound delivered by the Model 1 (yes, there was a difference) was in the level of background hiss and noise. Whether the received signal was weak or strong, and no matter what other tuner it was com pared with (we tried several, all good), the background noise level from the Model 1 was noticeably lower. This is due partially to its superior limiting, and partially (according to the designer) to the lack of spurious noise products caused by intermodulation between the various signals entering the tuner. The fact that the sensitivity rating test done according to IHF standards does not reveal any particular superiority on the part of this tuner is merely an indication of the limitations of even the best avail able test equipment and procedures.
Another obvious superiority of the Model 1 is in reception of weak signals, particularly those afflicted with severe aircraft flutter, fading, or multipath distortion. Under conditions where other tuners suffer from background-noise modulation, distortion, or excessive hiss, the Model 1 generally delivers listenable and often full-fidelity sound. We convinced ourselves of this by an A-B com parison with a highly regarded FM tuner whose basic sensitivity and quieting measurements were almost identical to those of the Sequerra Model 1, and with both tuners operating from the same antenna system. In another location, ignition noise that severely marred reception on another fine tuner was completely suppressed in the Model 1. In short, the major feature of the Sequerra Model 1 is that its outstanding performance is realizable down to very low signal levels-signal levels that, with other tuners, show a substantial increase in noise or distortion, or both.
For fringe-area listeners, therefore, some of whom are unable to enjoy the benefits of FM with any previously available tuner, the Sequerra Model I may be a godsend, albeit an expensive one. If you can arrange to do so, it would probably be advisable to try it on a "money-back" basis. However, we are certain of one thing-if it won't do the job, nothing else will! The panoramic display is an intriguing and informative feature, both for the "DX-er" and the concerned listener.
One would probably not spend $500 for it as an accessory (even if it could be made to sell as one for such a low price, which it probably couldn't), but we feel that anyone willing to invest $2,000 in an FM tuner should go all the way and get the panoramic display.
Overall, the Sequerra Model I would seem to be the tuner that sets, current standards in the same way that the Marantz 10B did in its time. Those who have had their orders in and have been waiting for production to start will find, we think, that their patience has been well rewarded.
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Jensen Model 15 Serenata Speaker System
-------With speaker grille; ---- W/o speaker grille
HEADING the new line of Jensen loudspeakers is the Model 15, a large, floor-standing four-way ducted-port sys tem employing five drivers. The Model 15 Serenata is a handsome piece of furni ture, finished on all sides in oiled walnut, with a stain-proof top surface that has the appearance of black slate.
The acoustically transparent black grille snaps off to reveal a white speaker panel on which are mounted a 15-inch woofer, an 8-inch lower-mid-range cone driver operating from 300 to 1,500 Hz, and a 5-inch upper-mid-range that handles the 1,500- to 4,000-Hz frequency range. Two 1-inch dome radiators operate together above 4,000 Hz. There are separate, continuously variable level controls on the speaker panel to adjust the output of the mid-range and higher-frequency radiators (as a group) for the desired frequency balance. The spring-loaded binding posts are located under the wooden base, which raises the cabnet a few inches from the floor.
The Jensen Serenata is nominally an 8-ohm system, rated to handle up to 100 watts of power. It is relatively efficient, and can be driven by any amplifier rated at 10 watts or more per channel. The cabinet is 31 inches high (including the base), 23 inches wide, and 17 inches deep. It weighs approximately 75 pounds. Jensen's comprehensive five year warranty, which covers the entire system against manufacturing defects, includes parts, labor, and shipping costs both ways. Price: $426.
Laboratory Measurements. Preliminary listening tests indicated that the most uniform frequency response was obtained with the high-frequency control set to its maximum and the mid-range control set to the center of its range. As is our practice, we measured the integrated output of the speaker in a normally "live" room. A closely spaced micro phone was used for our bass response and distortion measurements below about 300 Hz. After correcting for the response of the room and the test micro phone, we combined the curves to obtain a composite frequency-response curve, which is roughly indicative of what can be expected from the speaker in a typical listening room.
The overall frequency response was smooth, varying only ±2.5 dB from 85 to 15,000 Hz. At lower frequencies, speaker placement and room dimensions can be expected to have a considerable effect; under our test conditions the over all frequency response was within ±5 dB from 42 to 17,000 Hz. Bass distortion was very low, typically about 0.5 percent above 50 Hz. It rose to 5 percent at 40 Hz and to 15 percent at 30 Hz. These measurements were made at the woofer cone only, and hence do not reflect the contribution of the port radiation, which is predominant below about 40 Hz. Therefore, the effective distortion at the very low frequencies is some what lower than our figures indicate. The distortion remained quite low at all frequencies whether we drove the speaker with a constant 1-watt input or to a constant 90-dB sound-pressure level (SPL).
The mid-range level could be adjusted over a range of about 8 dB, principally between 200 and 1,500 Hz. The high-frequency level control had its major effect above 1,000 Hz, with an increase of up to 7 dB from its center position to maximum. At its minimum setting, the tweeters were effectively shut off, rolling off rapidly above 1,000 Hz. The electrical impedance of the system measured-between 4 and 8 ohms from 20 to 20,000 Hz, except for the bass-resonance rise to.
about 15 ohms at 58 Hz. The speaker efficiency was high. With a 1-watt signal input in the octave centered at 1,000 Hz, a 97-dB SPL was measured at a distance of 1 meter. The SPL in the reverberant field was 87 dB at a 1-watt level. The tone-burst response within the operating range of each driver was good, with no spurious outputs, although we observed some moderate ringing from the 5-inch driver after a 3,500-Hz burst.
Comment. The Jensen Model 15 Serenata had a big, well-dispersed sound, with an excellent overall balance. In our simulated live-vs.-recorded tests, which operate from 200 Hz up, it proved to be an extremely accurate speaker at upper middle and high frequencies, with a slight added warmth in the lower mid range. The bass was solid and powerful, though it did not extend quite as far down in frequency as the best acoustic-suspension speakers do. (In our listening room the generous-size Model 15's had to be located several feet from the nearest wall, which inevitably had some effect on their low-bass performance.) The high efficiency of this Jensen system, typically 6 to 10 dB greater than that of most acoustic-suspension systems, makes it possible to achieve "live-performance" listening levels in average-size rooms without investing in a super-power amplifier. In fact, any good receiver with an output of 20 watts or so per channel can drive this speaker to the highest listening levels most people will ever require. By com parison, a high-quality acoustic-suspension system would need at least 200 watts per channel to achieve the same output levels. Furthermore, the Model 15 is rated for inputs as high as 100 watts per channel, which indicates that it should be able to play much louder than most home systems. Obviously, this goes a long way toward justifying the cost of the Model 15. In addition, its fully finished back makes it adaptable to installations where the rear of the cabinet must be exposed. All this makes the Jensen Model 15 Serenata attractive both aesthetically and sonically.
-------- Tone-burst response of the Model 15, shown here at (left
to right) 275, 3,500, and 7,000 Hz, was generally good despite some mild "ringing" at
3,500 Hz.
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Superex EP-5 Stereo Headphones
SUPEREX ELECTRONICS, whose broad line of stereo headphones includes both dynamic and electrostatic types, has announced a hybrid model, the EP-5 Electro-PRO, employing both electrostatic and dynamic driver elements.
Electrostatic phones are well known for their wide frequency range and smooth response, but they tend to be restricted, particularly at low frequencies, in the maximum volume levels they can pro duce without excessive distortion. Most of them also require an external coupler/power supply that usually must be plugged into the a.c. line. Dynamic phones, on the other hand, can deliver extremely high acoustic outputs approaching the threshold of pain, but of ten have a relatively irregular upper mid-frequency and high-frequency response.
In the Superex EP-5, many of the best features of both types are combined. A 2 3/4-inch Mylar-diaphragm dynamic "woofer" handles the low and mid frequencies. Mounted coaxially with it is a 2 1/4-inch permanently charged electret electrostatic diaphragm. The crossover, in the 3,000- to 4,000-Hz region, is accomplished acoustically through back-loading the tweeter and by tailoring the acoustic design of the woofer cavity.
Electrostatic elements, by their nature, require a high-voltage audio signal, and this is provided by a step-up trans former in the control "console" supplied with the EP-5 system. The console must be connected to the speaker outputs of a stereo amplifier (the usual headphone jacks cannot drive the transformer and phones), and the headset plugs into a five-pin socket on the console panel.
Terminals are provided in the rear of the console for the displaced speaker connections, and a rocker switch on the panel selects either speaker or head phone listening.
Unlike full-range electrostatic head phones (including those made by Superex), the EP-5 tweeters operate as single-ended (non-push-pull) drivers. Apparently the small diaphragm excursions needed for high-frequency operation also make it possible to use permanently polarized electret elements and eliminate the inconvenience and expense of a.c. line connections or polarizing supplies without incurring excessive distortion.
The left and right inputs are electrically separate, except for a 440-ohm resistance between their grounded sides. Most amplifiers share a common ground be tween channels and present no connection problems, but Superex suggests caution in connecting the EP-5 console to any amplifier which does not have one side of each speaker output grounded.
The headset of the EP-5 is similar in styling to other current Superex models, with relatively large, but light, square-shaped earpieces and a padded head band. The cushioned ear seals do not exert an uncomfortable pressure on the wearer's head, yet are quite effective in excluding ambient noise. The headset weighs 16 ounces, and it has a 15-foot coiled cord. The console measures 7 x 4 x 2 inches. Price: $80.
Laboratory Measurements. The frequency response of the Superex EP-5 phones was measured on a Koss-de signed test coupler, with the phones driven at a constant 3-volt input. The output of the dynamic woofer was un usually smooth down to the lower test limit of 20 Hz. There was a broad dip in the 1,000- to 3,000-Hz region that was about 8 dB below the 100-Hz level. The tweeter output above the crossover frequency was almost as great as the aver age woofer output, and it remained strong all the way to the upper test limit of 20,000 Hz. The impedance of the EP-5 phones reached a maximum of about 90 ohms in the 500- to 2,000-Hz range, falling off to 15 ohms at 20 Hz and to 8 ohms at 20,000 Hz.
Tone-burst measurements of headphones are difficult to interpret, since the dimensions of the coupler and the inside of the earpieces interact in a somewhat unpredictable manner to modify the burst response, particularly at the higher frequencies. Our measurements at 100, 1,000, and 5,000 Hz showed generally good response, with some evidence of ringing at the higher frequencies. A 3-volt test-signal input produced a sound-pressure level (SPL) of 105 to 107 dB at frequencies from 20 to 400 Hz, and be tween 100 and 105 dB at frequencies from 3,000 to 18,000 Hz. These are all fairly high listening levels (they would rarely be used in loudspeaker listening), and the EP-5 phones can produce them with any amplifier rated at 10 watts per channel or more. The EP-5 has internal overload protection circuits to prevent excessive drive signals from reaching the phones. We verified their effectiveness by driving the phones at high levels from a 200-watt-per-channel amplifier with no ill effects.
Comment. We judged the listening quality of the Superex EP-5 phones with a variety of amplifiers and receivers and in a side-by-side comparison with a rather expensive and highly regarded full-range electrostatic headphone. Superex states that the EP-5 was developed to provide high-quality listening for rock, modern, and synthesized music. A slight bass boost was incorporated in the phones to appeal to the taste of contemporary listeners and to compensate for possible air leaks between the ear cushions and the wearer's head. Our subjective reaction conformed in every detail to Superex's own appraisal of their phones. The EP-5 had a slightly "heavy" sound by comparison with the reference phones, although the high-frequency responses of the two phones seemed to be quite similar. For those users who prefer a more conventional sound balance, most amplifier tone controls are capable of flattening the bass response to match the mid-range level.
We could not hear any distortion or similar effects that might have resulted from the use of single-ended drivers.
This is not surprising in view of the high crossover frequency, since harmonics generated by the electrostatic tweeters would probably fall above the range of human hearing, and many intermodulation products would fall below the cross over frequency.
To summarize, we feel that the Superex EP-5 phones do provide the extend ed, smooth high-frequency response of an electrostatic phone together with the powerful bass of a dynamic phone, and at a price well below that of other electrostatic phones currently on the market.
For the listener who favors the low bass, the EP-5 phones are among the most potent that we have heard in the lowest octaves, yet when desired they can easily be equalized with amplifier tone controls to suit more conventional listening tastes.
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Technics RS-676US Stereo Cassette Deck
UNTIL recently, most cassette decks have followed a standard external for mat, with the tape-loading well, meters, and controls all mounted on the top surface. However, a recent trend in cassette-deck design, exemplified by the new Technics RS-676US, is the front-loading deck. External dimensions, styling, and general appearance of a front-loading deck generally match those of the amplifiers and tuners in the same manufacturer's product line, so that the tape deck can be stacked on, or placed next to, the other components--even be panel-mounted--without creating aesthetic problems. The tape is loaded (in a more or less horizontal plane) through a hinged front door, and is visible through a window in the door. In the case of the RS-676US, the cassette is tilted slightly forward in the lighted compartment and can be viewed either directly or via a built-in mirror. An EJECT button opens the door partially and, if the transport is disengaged, ejects the cassette.
The Technics RS-676US is a two-motor machine with a solenoid-activated transport mechanism controlled by light-touch pushbuttons in the center of the front panel. Symbols on the PLAY, REC, and PAUSE buttons light up when en gaged. The PLAY and STOP buttons are long bars, easily distinguished from the other smaller controls. Pressing the REC button turns on the recording circuits (for setting levels) and lights a red dot on the button, but does not start the tape.
Recording begins when the PLAY button is touched, but if one wishes to disengage the recording mode after setting the levels, it is only necessary to touch the STOP button. The PAUSE control latches when pressed, and is released by a second touch.
Above the transport controls are a three-digit index counter and its reset button, plus a button to engage the MEMORY PLAY, a feature exclusive to this machine. Several other deluxe recorders have a memory stop, which means that the counter can be set to zero at any part of a tape and the machine will rewind to that point and stop. The Technics system goes one step beyond this the tape stops at the preset point, then automatically goes into the play mode. If you wish, it can also be used as a memory stop by pressing both the PAUSE and REWIND buttons.
The Dolby-system switch includes a third position for a filter that removes the pilot carrier and other ultrasonic signals from stereo FM programs so that they do not affect the operation of the Dolby circuits. A second DOLBY FM switch connects the recorder's Dolby circuits to decode a Dolbyized FM broadcast for listening. The circuits are so arranged that the program may also be simultaneously recorded in encoded form. The recording can then be played back later through the recorder's Dolby system and heard with full noise reduction and proper frequency balance. Recessed screwdriver controls on the panel set the Dolby-FM levels with the aid of test tones transmitted by the FM stations.
A two-position TAPE SELECTOR optimizes recording and playback equalization and recording bias for normal ferric-oxide or chromium-dioxide (CrO2) tapes. The now-standard 70-micro second playback equalization is used for the CrO2 tape. Some brands of CrO2 cassettes are manufactured with a special notch next to the recording interlock tab, and when such a cassette is loaded into the RS-676US, the recorder automatically switches to the CrO2 operating mode for recording and playback.
A single large knob adjusts recording level for both channels, and a smaller BALANCE control provides gain adjustment between channels. Two small con centric knobs adjust the microphone-input gain, which is independent of the setting of the master recording-level con trol and can be mixed with the high-level inputs. The INPUT SELECTOR connects either the TUNER or the LINE inputs, or disconnects both for microphone record ing alone. The two quarter-inch micro phone jacks and a stereo headphone jack are located on the front panel.
The two illuminated VU meters read both recording and playback levels.
Normally, they have the ballistic characteristics of a standard VU meter, with a fast attack and decay and no significant overshoot. Pressing the PEAK CHECK button below the meters converts them to very fast-responding peak indicators, with a much slower decay time.
In the rear of the recorder are the in put and output jacks, left- and right-channel playback-level screwdriver adjustments, and a Dolby FM de-emphasis switch. Dolby FM programs are now being transmitted with a 25-microsecond (u-s) pre-emphasis, instead of the normal 75 us, for better compatibility with non-Dolby FM reception. When a 25-us transmission is received, the switch on the RS-676US can convert the output of a standard FM tuner to a 25-us characteristic for optimum frequency response and noise reduction. There is also a REMOTE CONTROL socket for use with an optional accessory ($34.95). The Technics RS-676US is 16 7/4 inches wide, 11 7/8 inches deep, and 5 1/2 inches high; it weighs 23 pounds. Price: $459.95.
FREQUENCY IN Hz (CYCLES PER SECOND)
Laboratory Measurements. The play back-frequency response, tested with the Nortronics AT-200 tape (standard 120-µs equalization), was ±0.5 dB from 125 to 10,000 Hz, rising to +4.5 dB at 31.5 Hz. The response with the Teac MT-116SP test tape (employing the 70-μs equalization used with CrO2) was within dB from 40 to 10,000 Hz.
We measured the overall record-play back frequency response with Maxell UD and TDK Krom-02 ( CrO2) tapes at a-20-dB recording level. The former had a slightly rising high end, and was within-1-4 dB from 30 to 16,000 Hz. The CrO2 response was somewhat flatter within ±3 dB from 27 to 16,800 Hz. We also checked the RS-676US with the new Sony Ferri-Chrome tape, using the NORMAL bias setting recommended for this tape. As expected, the high end was strongly accentuated, to +10 dB at 13,500 Hz. When we played the tape back with the 70-µs equalization used for CrO2, the overall response was ±4 dB from 30 to 16,300 Hz. Using the same tapes, we also checked record-playback response at 0 dB to determine the machine's freedom from the effects of magnetic saturation.
With the standard and CrO2 tapes, response at 10,000 Hz was 2 to 3 1/2 dB be low the mid-range level. With Ferri-Chrome tape, the 10,000-Hz level was actually 1 1/2 dB above the 1,000-Hz level.
With the Dolby system on, the overall record-playback response at a-25-dB level was accentuated slightly, by about 3 dB, above 1,000 Hz. This tended to dilute the effectiveness of the noise reduction, although the Dolby system still achieved a very good 7- to 8-dB improvement in signal-to-noise ratio (S/N).
The reference 3 percent distortion level was reached with a recording input of +4 dB (Maxell UD), +2 dB (TDK Krom-O2), and +3 dB (Sony Ferri-Chrome).
The differences in the S/N numbers among the three tapes were insignificant.
The unweighted S/N was about 50 dB, improving with IEC weighting to about 54 dB. Adding the Dolby system resulted in a 6 1 .5-dB S/N with all tapes.
When we used the Ferri-Chrome tape with normal recording bias and the 70-µs playback equalization, the S/N was about 1 to 2 dB better than with the other tapes.
The input sensitivity for a 0-dB re cording level was 62 millivolts (mV) for LINE, 85 mV for TUNER, and 0.24 mV for MIC. The mic input added about 11 dB of noise at maximum gain, but at any normal input-gain setting the noise in crease was small enough to be inaudible.
The playback output level from a 0-dB recording-level input was about 0.5 volt, varying somewhat with the tape used. A standard Dolby-level test tape gave a +6-dB meter reading, although the Dolby marks on the meter scale were at +3 dB.
The headphone volume was good with 8-ohm phones, but rather low with higher-impedance (200-ohm) phones.
The performance of the tape transport was excellent, with a speed error of less than 0.1 percent, and only 0.1 percent unweighted rms flutter. In fast-forward or rewind, a C-60 cassette was wound in 64 to 65 seconds. The meters had an exceptionally fast and well-damped response, even in their normal mode of operation. Tone bursts of a 0.3-second du ration gave exactly the same reading as a steady test signal, and the response of the meters was down only 3 dB with 0.1-second bursts. When we pushed the PEAK CHECK button, a 0.05-second burst gave a reading 3 dB below a steady signal.
Comment. While there are probably some installations that require a top-loading cassette deck, we found the de-sign of the Technics RS-676US exceptionally convenient to use, and we suspect that many others will have the same reaction. The positive, light-touch controls give it the feel of a fine open-reel deck, and add much to the enjoyment of the unit.
In respect to sound quality, the RS 676US is as good as any cassette recorder we have used (and that includes some very fine units). Its Dolby FM-decoding system worked perfectly, al though we could not evaluate the 25-µs FM de-emphasis feature since that characteristic was not yet in use by broad casters at the time of writing.
We encountered a potential problem with recorded Dolbyized FM broadcasts while listening to them in decoded form.
It should be noted, however, that this problem exists with all recorders having this feature, and is not peculiar to the RS-676US. With the DOLBY FM CAL controls set so that a 50 percent modulation level in the broadcast produced a Dolby calibration-level meter reading (+3 dB), high-level program passages regularly drove the meters to their limit (+6 dB) or beyond. This is apparently not enough to cause distortion in the recorder's electronic circuits, but is almost certain to cause tape saturation. One can switch to the normal TUNER INPUT mode, so that the proper recording level can be set, but this sacrifices the ability to listen to the decoded program while recording.
Having lived with the Technics RS 676US, we can say that anyone who has a chance to become accustomed to the conveniences of a high-quality solenoid-operated cassette deck such as this one will find it difficult to go back to an old fashioned "piano-key" design. The front-loading feature should make this machine a natural choice for rack or panel mounting or for shelf installations at or near eye level. Best of all, the Technics RS-676US sounds quite as good as it looks.
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Also see:
EDITORIALLY SPEAKING, WILLIAM ANDERSON