NAD 7400 Receiver (Equip. Profile, Aug. 1989)

Manufacturer's Specifications:

FM Tuner Section

Usable Sensitivity: Mono, 10.3 dBf.

50-dB Quieting Sensitivity: Mono, 13 dBf; stereo, 35 dBf (25 dBf with NR on).

S/N: Mono, 80 dB; stereo, 75 dB at 65 dBf.

THD: Mono, 0.08% at 1 kHz, 0.2% at 100 Hz and 6 kHz; stereo, 0.08% at 1 kHz, 0.3% at 100 Hz and 6 kHz.

Frequency Response: 30 Hz to 15 kHz, ±0.5 dB.

Capture Ratio: 1.5 dB.

Alternate-Channel Selectivity: Wide, 75 dB; narrow, 80 dB.

Adjacent-Channel Selectivity: Wide, 7 dB; narrow, 20 dB.

AM Rejection: 65 dB.

Image Rejection: Greater than 90 dB.

l.f. Rejection: Greater than 100 dB.

SCA Rejection: 70 dB.

Subcarrier Suppression: 60 dB.

Separation: 50 dB at 1 kHz, 40 dB from 30 Hz to 10 kHz.

AM Tuner Section

Usable Sensitivity: 300 µV/m.

Selectivity: 35 dB.

Image Rejection: 50 dB.

I.f. Rejection: 50 dB.

S/N: 45 dB.

THD: 0.5%.

Amplifier Section

Power Output: 100 watts per channel, 8-ohm loads, 20 Hz to 20 kHz; bridged mode, 300 watts, 8 ohms, 20 Hz to 20 kHz.

THD: 0.03%. Clipping Power: 130 watts per channel, 8 ohms.

IHF Dynamic Headroom: +5.7 dB.

Slew Factor: Greater than 50.

Slew Rate: Greater than 30 V/µS.

SMPTE IM: 0.03%.

CCIF IM: 0.03%.

Input Sensitivity: MM phono, 0.28 mV; MC phono, 0.02 mV; high level, 15 mV.

Phono Overload at 1 kHz: MM, 180 mV; MC, 13 mV.

S/N: MM and MC, 76 dB; high level, 96 dB.

Frequency Response: Phono, RIAA, ±0.5 dB; high level, 20 Hz to 20 kHz, ±0.3 dB.

Tone-Control Range: Bass, ± 10 dB at 50, 120, or 250 Hz; treble, ± 10 dB at 3, 6, or 12 kHz.

Bass EQ Action: +3 dB at 60 Hz, +6 dB at 36 Hz.

Infrasonic Filter: -3 dB at 12 Hz, 12 dB per octave.

Audio Muting: -20 dB.

General Specifications:

Power Requirements: 110, 120, 220, or 240 V a.c., 50/60 Hz; 390 VA.

Dimensions: 17 1/8 in. W x 4 3/4 in. H x 15 3/4 in. D (43.5 cm x 12.1 cm x 40.1 cm).

Weight: 26 lbs. (11.8 kg).

Price: $999.

Company Address: 575 University Ave., Norwood, Mass. 02062. USA.

It is always a pleasure to come across an audio component where the manufacturer takes the trouble to tell us exactly how the product measures up, using approved standards of measurement. It's an even greater pleasure to encounter a product which meets or exceeds virtually all of those published specifications. The powerful NAD 7400 receiver is just such a product. If you've glanced at NAD's published specs, above, you may feel that my calling this receiver "powerful" may be a bit of an overstatement. Believe me, it is not, for although the continuous power rating is 100 watts per channel, the "power envelope" circuitry, for which NAD is noted, can deliver short-term power peaks of 300 to 500 watts per channel, depending upon your speaker's impedance. It is also possible to operate this receiver in the bridged or mono mode, for a continuous power output of about 300 watts into an 8-ohm load. I doubt very much if most users of this product would want to do that, however, for it would be necessary to add a second amplifier of the same power-handling capability to drive the right-channel speaker. Seems to me most people opt for a receiver in order to minimize the number of components needed in the system. Still, I guess it does no harm--and doesn't add much to the cost--to include this extra feature.

NAD has always had the knack of providing all the useful features most people want in an audio component, without going overboard on needless frills. Instead of those seldom-used features that some manufacturers insist upon giving us, NAD concentrates on providing a product in which performance levels are well balanced. In the Model 7400, we have superb FM tuner performance coupled with high-level and phono preamp audio stages that deliver excellent sound reproduction.

The FM tuner section's 75-ohm coaxial antenna input is directly connected to the first stage of r.f. amplification, avoiding the use of--and the losses generated by--a 300-to-75-ohm balun transformer. The input circuit employs a dual-gate MOS-FET. A buffer stage precedes the i.f. circuitry, and a balanced quadrature detector is used for demodulation of the composite audio signal. That signal is then phase-compensated to make sure that the phase-locked-loop, multiplex-decoder circuit maintains high levels of stereo separation. There's a carefully designed "narrow" i.f. mode which increases adjacent-channel rejection to 20 dB--although I actually measured 21.5 dB! NAD seems to have a habit of calling certain features by names which are not used by anyone else. For example, the partial FM-stereo blend circuit, useful when stereo signals are weak and noisy, is dubbed "FM NR" by NAD. By whatever name it's called, this circuit's net effect is to reduce noise at the expense of stereo separation. NAD makes sure the trade-off is worthwhile, however, by carefully controlling the amount of blend. The 7400 offers two "banks" of seven presets, and you can program any combination of AM and FM station frequencies into the resulting 14 memory preset locations. Though tuning is digital, as is the frequency display, the tuning knob offers the intuitive feel of an analog system, providing a sense of extreme accuracy and eliminating those manual up and down tuning buttons or rocker switches which are often awkward to use and yet are found on so many of today's tuners and receivers. With the NAD 7400. you can give the knob a quick spin to scan rapidly across the FM or AM band or turn the knob slowly for fine tuning.

I have discussed NAD's "power envelope" circuitry in previous test reports of their receivers, but a quick review might be useful to those not familiar with this circuit. Essentially, two supply voltages are available for the amplifier's output stages. At moderate signal levels, the normal (lower) voltage supply provides all current. When the audio signal level rises above the rated power level, a controller turns on a "gate" transistor, so maximum current can flow from the alternate (higher) voltage supply. While the IHF Standard for dynamic headroom calls for measuring the ability of an amp to deliver levels in excess of its rated power for 20 mS, the NAD "power envelope" circuit provides this extra power for several hundred milliseconds. Of course, if constant, high power-output currents are called for--as, for example, during my bench tests--a second controller gradually shuts off the flow from the high-voltage supply to prevent overheating and possible damage to the amplifier. The Model 7400, according to NAD, can supply peak output current of up to 40 amperes.

The volume control of the 7400 is part of a feedback loop which varies preamplifier gain. As a result, when volume is turned down from maximum, residual circuit noise is also reduced proportionately. When the volume up and down buttons on the supplied remote control are used, a miniature motor inside the receiver actually turns the volume control, avoiding the noise and distortion which sometimes occur with all-electronic level control circuits.

With three distinct crossover frequencies for both the bass and treble controls, the tone-control flexibility of this receiver approaches that of a parametric equalizer. Rather than provide a loudness control, NAD chose to incorporate a "Bass EQ" circuit. Instead of a loudness circuit's volume-dependent boost of the low and upper bass, NAD's circuit simply boosts the lowest bass frequencies by a fixed amount. At the same time, a sharp infrasonic filter is switched in, to avoid boosting or amplifying signals below the audible range. The 7400 has input/output circuits for two tape decks and allows dubbing in either direction. In short, this receiver offers most of the essential conveniences found in separate power amplifiers, preamplifiers, and tuners-all on a single, well-designed chassis.

Fig. 1--Frequency response, FM tuner section.

Fig. 2--FM quieting characteristics.

Fig. 3--THD + N vs. r.f. signal level for mono and stereo, in wide and narrow i.f. modes.

Fig. 4--THD + N vs. frequency.

Fig. 5--Frequency response (solid curves) and separation in both wide and narrow i.f. modes, and with "FM NR" in wide mode.

Fig. 6--Separation and crosstalk components for a 5-kHz modulating signal.

Control Layout

The power switch and a stereo 'phones jack are at the extreme left of the matte-black front panel. Speaker selector pushbuttons "A" and "B" and the "Bass Ea" button are to the right of the 'phones jack. Further to the right are the bass and treble controls, a pair of three-position lever switches that select bass or treble turnover frequencies, a pushbutton that bypasses the tone controls altogether, and a "Copy" lever switch with positions for copying from tape 2 to tape 1, and vice versa. Next are the two tape-monitor selector buttons; the "Phono," "Video," and "CD" input selectors; a button marked "Low Level," usually called muting, which reduces volume by about 20 dB; a rotary balance control, and a rotary volume control. All the button switches are of the push-on/push-off type and are surmounted by indicator lights so you can tell which are activated. Along the upper half of the front panel are seven numbered buttons for the preset radio station frequencies, as well as a "Bank" button which selects between two banks of presets and an "Enter" button used for memorizing the frequencies selected. Pushbuttons for "Mono," "AM," and "FM" come next, and near them are the buttons for selecting the narrow i.f. mode and "FM NR." Pressing this last button reduces weak-signal FM stereo noise by moderately "blending" left- and right-channel signals. The digital frequency display, in addition to showing the tuned-to frequency, also has five LEDs below the numerals that show relative signal strength and a single LED that illuminates when center tuning has been achieved. Above the display are three more LEDs; these light in the presence of an FM stereo signal, when the "Soft Clipping" switch on the rear panel has been activated, or when protection circuitry has been triggered.

A pivotable AM loopstick is on the rear panel, adjacent to the 75-ohm F-type FM antenna connector and the spring-loaded terminals used for hooking up an external AM antenna. Further to the right are two pairs of color-coded speaker-cable binding posts and two convenience a.c. outlets, one switched and one unswitched. Under the speaker terminals are slide switches for optimizing the speaker impedance match, for mono bridging, and for activating the infrasonic filter, plus a "Soft Clipping" switch for altering the way in which clipping occurs when amplifier overload is reached.

The "Impedance" and "Bridging" switches are normally locked in their preferred positions by means of small plastic strips which must .be unscrewed in order to alter those switch settings. "Preamp Out" and "Main Amp In" jacks are centered beneath the two sets of speaker terminals. At the lower left, beneath the loopstick and the other antenna terminals, are the various phono and high-level inputs, a switch that selects MM or MC cartridge preamplification, and a ground terminal.

Tuner Measurements

Figure 1 shows the frequency response of the FM tuner section, measured across the speaker loads, with tone controls bypassed. Response was considerably better than claimed, never deviating by more than 0.15 dB. The right-channel response has been deliberately offset for clarity Actual channel balance was virtually perfect at the volume setting used in this test.

Figure 2 shows how noise decreased with increasing signal strength for mono and stereo FM signals. At 65 dBf, S/N measured 78 dB in mono and 73 dB in stereo--just 2 dB short of the claimed 80 and 75 dB, but still excellent results for any tuner section. In mono, 50-dB quieting required only 14 dBf of signal input. This level is also the point at which stereo threshold occurs; to reach 50 dB of quieting in stereo, a signal level of only 23 dBf was needed. This result is all the more impressive since the "FM NR" circuit was not used when obtaining this measurement. There was an improvement of about 2 dB in S/N when I switched to the narrow i.f. mode. However, since the shape of the quieting curves--and of the audio output, represented by the top curve in Fig. 2--remained essentially the same, I did not plot a second graph for this operating condition.

In measuring THD, on the other hand, choosing the narrow i.f. setting made a substantial difference compared with operation in the normal (wide) i.f. mode, so THD + N versus signal level for 1-kHz signals modulated 100% was plotted for both modes (Fig. 3). Usable sensitivity for mono-the point at which THD + N rises to 3%--measured exactly 10 dBf in the wide i.f. mode and about 12 dBf in the narrow. At the strong-signal reference level of 65 dBf, THD + N in the wide i.f, mode decreased to 0.045% for mono and an even lower 0.04% for stereo. The effect of narrowing the i.f. bandwidth is clearly evident; THD + N rose to 0.13% for mono and 0.32% for stereo. While these figures are significantly higher than those obtained in the wide i.f. mode, they are nevertheless quite low, considering the benefits obtained when this mode is needed for rejecting adjacent-channel interference.

Fig. 7-AM frequency response.

Fig. 8-Frequency response, preamp and amp sections, via high-level inputs.

Fig. 9-THD + N vs. power output for 8and 4-ohm loads.

Figure 4 shows THD + N versus frequency for strong (65dBf) signals in both the wide and narrow i.f. modes. Here, THD + N in the wide mode hovered around the 0.04% mark at 1 kHz for both mono and stereo. At 100 Hz, it measured 0.04% in mono and 0.046% in stereo; at 6 kHz, the wide-mode results were 0.045% in mono and just a bit below 0.06% in stereo. When the narrow i.f. reception mode was used, THD + N in mono remained low, reading only 0.033% at 100 Hz, and increasing to about 0.13% at 1 kHz and just over 0.3% at 6 kHz. Stereo THD-N in the narrow mode measured 0.5% at 100 Hz, 0.3% at 1 kHz, and just under 0.3% at 6 kHz. The minor differences between the readings at 1 kHz and 65 dBf, in Figs. 3 and 4, are possibly caused by very slight detuning of either the generator or the tuner between readings. When THD levels are this low, it is extremely difficult to take two successive plots and obtain absolutely identical results. The important conclusions that can be drawn, however, are that this tuner section provides remarkably low distortion plus noise when operated in the wide i.f. mode and offers tolerably low THD levels even when it becomes necessary to employ the narrow i.f. mode.

In Fig. 5, I plotted FM stereo separation for three conditions. The bottom dashed curve represents the best separation, using the wide i.f. setting and no blend--or, as NAD calls it, "FM NR." Under these conditions, separation came close to 50 dB at 1 kHz. Even more remarkably, it measured about 37.5 dB at 10 kHz and 48.5 dB at 100 Hz. Next, I repeated the test, this time using the narrow setting. Separation remained high-about 42.5 dB at 1 kHz, 41 dB at 100 Hz, and 34.5 dB at 10 kHz. Finally, I returned to the wide i.f. mode but turned on the "FM NR" circuit. Unlike with other blending arrangements, which usually degrade separation levels down to 10, 15, or at best 20 dB, I still obtained about 37 dB of separation at 1 kHz, 36.5 dB at 100 Hz, and 33 dB at 10 kHz with this circuit activated.

Figure 6 is a spectrum analysis, from 0 to 50 kHz, showing a 5-kHz output on the left channel (the tall spike near the left end of the sweep) as well as crosstalk products and noise appearing at the output of the unmodulated channel. A small amount of second-harmonic distortion also can be seen (the first major peak to the right of the main peak), but the only other significant crosstalk products evident are small sideband components near the 38-kHz subcarrier frequency--well outside the audio range.

The NAD 7400's SCA rejection was an excellent 72 dB. Alternate-channel rejection in the narrow i.f. mode measured 85 dB, while in the wide mode it was still a relatively high 75 dB. Image rejection and i.f. rejection both measured in excess of 100 dB; capture ratio measured 1.4 dB at 45 dBf, and AM rejection was 65 dB, exactly as specified.

In their descriptive brochure, NAD maintains that the AM section has "unusually wide audio bandwidth ... that is in striking contrast to the muffled AM reception that many tuners provide." Well, I suspect that judgment is relative, because my sample, at least, didn't do all that well in the audio bandwidth department, as shown by Fig. 7. Response was down 6 dB at 65 Hz and 3.5 kHz. Some of the other claims made for this AM circuit did seem justified, however.

The AM tuner section appeared less susceptible to noise than most, and did have lower THD-about 0.45% for a 400Hz signal, 30% modulated, at an r.f. level of 50 mV.

Fig. 10--THD + N vs. frequency at rated output (100 watts per channel) into 8 ohms and at 100 watts per channel into 4 ohms; see text.