NAD 7600 Stereo Receiver
As the only receiver in NAD's Monitor Series, the 7600 may be considered the paterfamilias of the entire line. It is strictly an audio model (though one input is designated for the audio from a video source), and its concentration on performance and value is what we have come to expect from NAD. The results are so exceptional that the 7600 remains an excellent value despite its fairly hefty price.
Among the unusual back-panel features that caught our eye are the main power switch (the front-panel switch offers standby, rather than off, so the receiver will respond to its remote) and a U.S.-standard threaded F-connector 75-ohm FM antenna input to accept coaxial antenna or cable inputs without an adapter of any sort. Also very unusual are the "lab" and "normal" input options for the power-amp section, which will be described later. Less startling, particularly in NAD products, are the bridging switch (doubling output power for mono use, depending on the load) and the load-impedance optimization switch.
You may be surprised by the front-panel tuning knob. Tuning is digital, but the manual control has the flywheel action of an "old-fashioned" capacitor front end. A delicate turn steps the tuning by quarter-channels (50 kHz) on FM, full channels (10 kHz) on AM; a swift twirl sends it rolling in either direction along the band. Rotation of the shaft is counted internally, driving the digital dividers that change the operating frequency. Mature users brought up on tuning knobs won't be the only ones delighted by this scheme.
Curiously, the balance control takes the opposite approach. A pair of buttons steps the balance in either direction from electrical parity. Just above them, an LED lights to let you know when electrical balance has been reattained. Admittedly, a knob is easier to use, but digital operation of both the balance and tuning controls is necessary for the remote. Besides, balance controls are seldom used.
The volume knob is at the bottom right, just outside the balance control. To the left is a group of buttons that control, respectively, "low level" (a roughly 20-dB attenuator, normally mislabeled "mute," which in this case is automatically engaged every time you turn on the receiver), mode (mono/stereo), an infra-sonic filter, and NAD's soft-clipping feature (designed to prevent the sort of tweeter-threatening distortion products characteristic of hard clipping in an overdriven amplifier). Farther left is a copy switch for dubbing in either direction between two connected tape decks. Similar three-position switches select the turnover frequency for each of the tone controls; they flank the bass and treble knobs, between which is a defeat button. At the far left are a bass EQ button, on/ off switches for each of the two speaker pairs (sturdy binding posts are supplied on the back panel), and a headphone jack.
Near the center of the front panel are the selectors: AM or FM, phono, CD, two tape decks, and video (or any other line-level audio) from external sources. Pressing either tape button simply changes the monitoring; the recording source remains the one chosen at the main selectors or by the copy switch. Next to the FM selector is a switch for NAD's FM noise reduction feature; beneath both are the presets, which can store the frequencies for eight stations on each band. The frequency readout also indicates when tuning is high, low, or spot-on for the nearest station; just below it is a signal-strength indicator.
The supplied wireless remote is NAD's unique design, which we find of average comfort when hand-held but well above average when used as a keypad resting on a table or chair arm. Powered by two AA cells, it offers power on/standby, all selectors (including tape), volume up and down (which rotate the front-panel control), balance, "low-level" on/off, the tuner presets, and an up/down tuner search that ferrets out the nearest receivable station.
Subjectively, FM receivability depends a lot on whether or not you're using the FM noise-reduction feature, which is extremely effective on very weak stations. It blends channels to cancel the stereo effect and, with it, the noise derived from the stereo subcarrier. As a result, 50 dB of S/N (signal-to-noise) ratio is maintained down to a signal strength of 27 dBf-astonishingly low for the stereo mode-but with so little channel separation (33/4 dB) that we decline to consider it stereo. However, these figures represent steady-state signals rather than the dynamics of actual broadcasting. With real stations, we were able to get very listenable results with a semblance of stereo on some stations that are relatively useless with conventional (if excellent) tuners-a feat that shouldn't surprise anyone familiar with past NAD tuners. With strong FM signals, we also were impressed by the more than 70-dB stereo S/N ratio, a figure that ranks among the top in our experience. NAD feeds the signal from the 75-ohm connector directly to dualgate MOS FETs (metal-oxide-semiconductor field-effect transistors), whereas typical receivers require an outboard balun transformer or, sometimes, employ an input transformer with primary taps for 75- and 300-ohm lines.
Frequency response of the FM section is exceptionally flat, and separation is unusually generous with the FM noise reduction defeated. Alternate-channel selectivity is about par, but that for the adjacent channel is well above average. The five elements of the signal-strength indicator trigger at thresholds from 12-1/2 to 71 dBf, an exceptionally broad range but one in which the steps are too far apart. Once the second element has triggered at 28 dBf, for instance, there is no further activity until the third lights at 48 dBf-that's 20 dB higher, skipping right over the most critical range for indicating borderline stations. Fringe-area users with rotating antennas thus will wish for closer spacing or (ideally) an analog meter. Otherwise, we are very pleased with the tuner section, particularly for fringe reception.
The phono circuitry is said to be identical to that in the company's top model, the NAD 1300 preamp. For fixed-coil pickups (moving-magnet or moving-iron designs), there are three input capacitance options, all switchable on the back panel (as is the MM/MC option). Nominal capacitances are 100, 200, and 300 picofarads; on Diversified Science Laboratories' test bench, they measured only nominally higher. Response with the moving-magnet option is extremely flat through most of the range, with only a very slight droop at the top end (down less than 1/2 dB at 20 kHz and a similar amount at 40 Hz, with a steeper rolloff below that). The moving-coil response is nearly as flat, with a very slight tilt toward the low end throughout the midrange. Even without the infrasonic filter, warp attenuation is better than 20 dB at 5 Hz with both options. All in all, we found the phono section distinctly above average on all counts for a receiver- and, as NAD implies, in the same league as separate preamps.
The infrasonic filter, with an attenuation of about 21 dB per octave below 15 Hz, can add radically to the warp suppression. Curiously, it also boosts response very slightly at the bottom of the audio band, flattening the negligible inherent droop in the 30-Hz region. The Bass EQ feature, intended to give extra punch below the resonance frequencies of typical speakers, provides a rolloff slope of about 20 dB per octave (below 30 Hz or so) but adds almost 7 dB of boost at its own resonance, just below 40 Hz. It thus should complement many speakers without overdriving them or contributing to any problem-causing infrasonics.
NAD describes the tone controls as amounting to a two-band semiparametric equalizer (offering no control over the bandwidth, or Q, of their action). Be that as it may, the controls are very well-behaved and quite useful. Bass offers about 12 dB of boost or cut at the extreme settings, with greatest effect very near the nominal center frequencies of 50, 120, and 250 Hz. The lowest of these has almost no effect above 1 kHz; the ±3-dB points of the highest are near 2 kHz. Behavior of the treble control is closely comparable over center frequencies of 3, 6, and 12 kHz.
So far, we have considered the 7600 in its normal amplifier mode, which includes deliberate bandwidth-limiting to prevent loading the amp (or the speakers) with infrasonics and ultrasonics that, although beyond hearing, can produce unwanted audible by-products through such mechanisms as intermodulation. This is the way NAD expects the receiver to be used. But, for those who believe in the wideband concept, the company offers an alternative: Shut off the receiver, remove the pre/main jumpers (which come plugged into the "normal" amp inputs), and place them between pre-out and the "lab" amp inputs, which bypass the bandpass filtration. The pros and cons of the two approaches have been argued since the dawn of high fidelity, and we see no end to the debate. Take your pick. We were too pleased with the receiver in the normal mode to work up much enthusiasm for the lab option, but we wonder whether our reaction would have been any different had NAD indicated that the lab mode were normal and band-limiting the option.
By receiver standards in particular, there's power to spare in the 7600. NAD is adamant that amplification should be approached in terms of the "power envelope"-in effect, graphing the output power against the time over which it can be sustained. Using the more succinct method of the IHF standard, we measure only the steady-state (FTC-rating, or RMS) power and transient headroom based on a 20-millisecond tone burst. Essentially, however, the DSL data confirm the design's excellent headroom.
However, interpreting the power figures is complicated by NAD's traditional approach to load optimization. A back-panel switch can be set for true 8-ohm-or-higher loads or (in the normal position) for lower impedances. The latter position is required if you have speaker pairs connected to both outputs. DSL made its 8-ohm measurements at the higher setting, which makes the most of such loads; lower-impedance loads were measured in the lower position. Thus, there isn't quite as much increase in measured power when impedance is lowered as there presumably would be if the load setting were left unchanged. Nevertheless, the increase is material. And it continues when (in the dynamic-power test) the load is further reduced to 2 ohms, which proves that the amp isn't running out of steam because of the high current drain. And with over half a kilowatt available for transients in each channel -even with a 4-ohm load-there's plenty of steam.
Invoking the soft-clipping feature reduces available power somewhat and increases distortion at high signal levels. This is deliberate. In effect, the feature alters the waveform to throw away the portion that threatens hard clipping. As a result, you get neither the full capability of the amplifier nor the danger (should you overdrive it). Overdriving the powerful 7600 is extremely unlikely but also exceptionally risky for the speakers.
In sum, we have no hesitation in calling the 7600 an outstanding receiver and, ultimately, a logical candidate for the status of a classic model. Consider one detail, for example: the automatic 20-dB attenuation when you turn the unit on. On first glance, it seems superfluous. But if you play music as loud as the 7600's excellence encourages you to do and then leave the volume at that level when you shut it down, the attenuator gives you a moment to lower the volume after turn-on to escape the full brunt of the sound. This detail could be conceived only through an exceptional ability to see with fresh eyes. Once again, NAD has demonstrated that it has few peers in that department.
