NAD 7140 Stereo Receiver Review

In its design and performance, NAD’s 7140 AM/FM stereo receiver is an exceptional value. Both its amplifier and tuner sections incorporate valuable circuit features that place the unit far ahead of similarly priced competition.

Some of this can be gleaned from the published specifications. Although the amplifier section is rated at a modest 40 watts per channel into 8-ohm loads (between 20 and 20,000 Hz, with no more than 0.03 per cent total-harmonic or inter-modulation distortion), it follows the NAD tradition of a current-out-put capability far exceeding that of most comparably rated amplifiers. Since it also has a relatively high dynamic-headroom rating of 3 dB, in actual use the power amplifier section of the Model 7140 is much more powerful than its specs would imply.

The NAD design principles recognize that the minimum impedance of most speakers is somewhat lower than their nominal ratings. It is not uncommon for the impedance of a “4-ohm” speaker to fall as low as 2.5 ohms at some frequencies. If two sets of such speakers are operated in parallel, an amplifier with limited current capability may produce distorted sound or even overheat and shut down under prolonged or high-level operation. To help deal with this problem, NAD provides a switch in the rear of the Model 7140 receiver to reduce the operating voltage on its output transistors when driving loads of 4 ohms or less (this is designated the “normal” setting). When the switch is set to its “8-ohm” position, the increased power-supply voltage enables the amplifier to deliver its maximum output to nominally 8-ohm speakers.

In some ways, the FM-tuner section of the NAD 7140 is even more impressive than its amplifier. It is highly sensitive and has less noise, both in mono and stereo, than most of the top-quality separate tuners (let alone receivers) that we have tested. The tuner features a novel “Dynamic Separation” circuit designed by Larry Schotz and adapted to this receiver by NAD engineers. This circuit reduces hiss on weak stereo FM signals without subjective loss of either channel separation or high-frequency response. It is essentially a signal-controlled high-frequency channel-blending circuit whose operation is determined by the signal strength, the FM modulation level, and the frequency content of the program. For signal levels under about 150 μV (48 dBf) and low modulation levels, the. Dynamic Separation circuit blends the channels at high frequencies to reduce the noise by 3 dB. But since the separation at or under 1,000 Hz does not drop below 10 dB, a full stereo effect is maintained and there are no unnatural shifts of the stereo image as the circuit operates. When the FM signal is modulated at normal levels, if it contains much high-frequency information, or if its strength exceeds the threshold, full stereo separation is restored.

Like many receivers, the 7140 has antenna inputs for 75- or 300-ohm FM antennas, but unlike most others, the 7140’s coaxial fitting for the 75-ohm input carries the signal directly to the r.f. amplifier stage. According to NAD, this gives higher tuner sensitivity than that achieved by the usual method of connecting the 75-ohm antenna feeder across one half of the internal 300-ohm input transformer. A nice touch.

The all-metal cabinet of the NAD 7140 is finished in dark gray. It measures 16-1/2 inches wide, 13-3/4 inches deep, and 4 inches high, and it weighs 20 pounds, 5 ounces. Price: $478.

Lab Tests

The standard one-hour preconditioning at one-third rated power did no more than make the top of the NAD 7140’s cabinet moderately warm (its heat sinks are within the cabinet). The 1,000-Hz clipping-power output into 8-ohm loads was 66 watts per channel at the 8-ohm high setting of the speaker selector switch. This setting was used for all our tests, even driving 4-ohm and 2-ohm loads. The Model 7140 distinguished itself in these tests. Not only was the 2-ohm clipping output a full 98 watts per channel at 1,000 Hz, but with the 20-millisecond tone bursts of the dynamic-head-room test it produced an astounding 233 watts per channel! The only indication that we might be pushing the receiver too far came during our distortion measurements with both channels driving 2-ohm loads, when the a.c. line fuse blew at 90 watts output—and this probably would not have happened if we had used the “normal” 4-ohm setting of the speaker-impedance switch. In all other respects, however, the 7140 amplifier handled the abuse of our most severe tests with ease.

The distortion characteristics of the amplifier were equally striking. In fact, at any power up to clipping, at any audio frequency, and with any load impedance from 2 to 8 ohms, the distortion never exceeded 0.01 per cent. Most of the time it was between 0.002 and 0.003 per cent.

FM tuner performance was equally gratifying. The Dynamic Separation circuit performed exactly as described by NAD. When the circuit was on, the 50-dB stereo quieting sensitivity of the Model 7140 was 32 dBf (22 microvolts), a figure rivaled among receivers we have tested recently only by two others, one of which was the NAD 7150— whose FM tuner, not coincidentally, was also designed by Larry Schotz. The noise reduction afforded by this circuit on weak stereo signals was plainly audible and worthwhile, and we were never able to detect any signs of its dynamic operation.

Comments

The performance of the NAD 7140 is difficult to criticize. In fact, it is so outstanding, in so many respects, that there is a real danger of our lapsing into fulsome praise instead of offering genuine criticism. Let us say only that its FM tuner is one of the most sensitive we have had the pleasure of using—in a meaningful sense, such as its exceptional stereo 50-dB quieting sensitivity. The signal-to-noise ratio is also exceptional. The tuner’s other characteristics are equally impressive, ranging from good to outstanding, and it had no weaknesses that we could discover. Even the AM section had an unusually wide and flat response.

The amplifier section of the NAD 7140 recalls our reaction to the NAD 3020 amplifier that we tested several years ago. At the time, we remarked to the effect that an “NAD watt” is equivalent to at least two of anyone else’s. This commentary on a NAD amplifier’s conservative ratings and high current-delivering capacity (made at a time when that quality was much less common than it is today) applies in full measure to the new 7140. Considering how it performs with low-impedance speaker loads, it seems incongruous to call it a “40-watt” amp.

The 7140’s great power reserve also makes its bass eq feature truly usable, since even slight boosts in the low frequencies can require substantial output power. It gives almost any reasonably good small speaker an added low-bass solidity with not a trace of heaviness or boominess. This circuit also provides a steep infrasonic cutoff at a rate of about 36 dB per octave below 30 Hz, making it safe for use with vented speakers that might otherwise be damaged by excessive infrasonic input.

The few problems with the NAD 7140 are mostly in the area of additional operating flexibility that is offered by some of today’s receivers and amplifiers. For example, there is provision for only one tape deck (and it is not possible to record from one source while listening to another). On the other hand, separate CD and AUX inputs are a definite plus. There is also only one phono input, although it can be switched to give a 20-dB higher gain for use with moving-coil cartridges. Since the amplifier’s input impedance is always a nominal 47,000 ohms, some people may wish to load their moving-coil cartridges with 100 ohms or whatever they feel to be a suitable value. As a rule, this is unnecessary with moving-magnet cartridges.

We appreciated the overall concern for “human engineering” and logical control functions that is apparent on the front panel as well as elsewhere in the NAD 7140 receiver. For example, in “normal” operation, when none of the pushbuttons on the panel are pushed in, the Dynamic Separation circuit and the infrasonic filter are functional, the bass eq and loudness compensation are inactive—providing what we consider to be the logical set of basic operating conditions.

One of the front-panel features that is not immediately obvious, but is unique to this receiver, is the concentric arrangement of the volume and balance controls. The bal-ace knob is partly recessed inside the volume control, opposite to the arrangement on many other amplifiers and receivers. It is just about impossible to shift the balance setting accidentally when adjusting volume, yet the control is instantly accessible and its bar shape shows its setting at a glance. This is a small but revealing example of the kind of thinking that has gone into most aspects of the design of this receiver.

By any standard, the NAD 7140 is a very fine receiver. At its price, its overall performance is unmatched. Unless you need features this product doesn’t have, its impressive combination of performance qualities, at a price only slightly higher than that of most ordinary receivers, makes it a bargain.

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