The NAD 3140 integrated amplifier, like the Model 3020, carries a moderate power rating—in this case, 40 watts per channel from 20 to 20,000 Hz into 8-ohm loads with no more than 0.03 per cent total harmonic distortion. The NAD amplifier (the initials stand for New Acoustic Dimension) does not have the usual current-limiting protective circuits for the output transistors, which have earned an unsavory reputation in some quarters for their undesirable behavior when driving some types of reactive loads or very low impedances. Instead, it uses high-power-rated output transistors that are capable of delivering far more than the amplifier's rated power; instantaneous program peaks can thus be reproduced without clipping even at levels far above the amplifier’s continuous-power ratings. The NAD 3140 is protected against prolonged overdriving by thermal circuit breakers that shut down each channel if its output devices become too hot, by d.c. supply fuses that will blow if excessive load currents are drawn, and by a power-line fuse that protects the entire amplifier in the event of some serious internal failure.
The dark chocolate-brown front panel has a large volume knob concentric with a detented balance ring. Smaller knobs, with center detents, operate the bass and treble tone controls. There are independent selector switches for the input source to be heard through the amplifier and for the source that is to be channeled to the tape-recording outputs. The input selector has two magnetic-phono-cartridge inputs (one with three switchable values of input capacitance and the other with a fixed low capacitance but a 10-dB higher gain for high-output moving-coil cartridges or low-output moving-magnet cartridges). There are high-level tuner and aux inputs as well as playback inputs for two tape decks.
The tape rec selector has the same input sources (but only the moving-magnet lower-gain phono input is available for tape recording). Since it has tape i and tape 2 positions, it can be used to dub from either deck to the other while the operator is listening to the playback from either deck (or to any other program source) via the input selector. At the left of the panel is the speaker selector knob for controlling two sets of speaker outputs, singly or in combination. It has an off position for headphone listening through the front-panel jack.
The speaker eq knob is a unique feature of the NAD 3140; it is designed to extend the low-bass response of many acoustic-suspension speakers whose output normally rolls off at a 12-dB-per-octave rate below the bass resonance frequency. Its center off setting is flanked by 45-Hz and 70-Hz positions. These introduce a 12-dB-per-octave boost below the indicated frequencies, which correspond to the resonance frequencies of many popular small- and medium-size acoustic-suspension speakers (typically those using 8-inch woofers). The total boost is 12 dB, effectively extending the bass range of such speakers by an octave. Usually, such a boost in an ordinary low-power amplifier would simply lead to output-stage overload, but (as we shall see) the NAD 3140 is not an ordinary amplifier.
Pushbutton switches control power, an infrasonic filter cutting off at 12 dB per octave below 15 Hz (which should always be used in conjunction with the speaker eq), mono mode selection, loudness compensation, and a low level mode that drops the amplifier gain by 20 dB. At the top of the panel is a LED display showing the approximate instantaneous power output of each channel into 8-ohm loads. It has green lights marked 0.5, 5, 25, and 50 watts, plus red lights marked 100 watts (these last will not be lit except under extraordinary conditions). To their left are three status lights: a power light and two others marked soft clipping and slc.
The “soft-clipping” system was a feature of the NAD 3020 that has been carried forward into the 3140. It modifies the manner in which a waveform is clipped when the amplifier's power limits are exceeded, the usual “sharp corners" of a clipped waveform being rounded off somewhat by special circuits. This reduces the amplitudes of the higher-order harmonics responsible for most of the irritating sonic quality associated with moderate clipping. The soft-clipping circuit is activated by a pushbutton in the rear of the amplifier, which also turns on the green front-panel indicator.
The SLC (for “speaker-lead compensator”) is a current-feedback circuit intended to compensate for the undesirable effects of excessively long or high-resistance speaker leads. Among those negative effects are a reduction of power delivered to the speakers, a slight frequency-response variation reflecting the speaker’s impedance variation over the frequency range, a reduced effective damping factor, and (according to NAD) the generation of considerable distortion at the speaker due to a nonlinear variation of its voice-coil inductance at extreme cone excursions, causing a nonlinear current in the speaker circuit.
The SLC alleviates all these problems to some degree, depending on the length and gauge of the speaker wires. It has been designed to compensate for a speaker-lead resistance of about 0.5 ohm, corresponding to a 12-foot length of 24-gauge wire (an undesirably small gauge, but it is nevertheless sometimes used in speaker installations) or the equivalent lengths of larger wires (38 feet of 18-gauge, for example). According to the amplifier’s instruction manual, if your speaker lines are long enough or thin enough to fit these guidelines, the SLC button in the rear of the amplifier should be pressed (this also turns on an amber light on the front panel). The effect of the SLC is subtle, and with adequate speaker wiring the feature need not be used.
On the rear apron of the NAD 3140 are jacks for the two phono inputs, the high-level inputs, and the inputs and outputs for two tape decks, plus a DIN socket for one of them. The preamplifier outputs are brought out to separate jacks, joined externally by jumpers to either of two pairs of power-am-plifier input jacks marked normal and lab. Through the normal inputs, the amplifier response is limited by low- and high-pass filters to a range of 15 to 35,000 Hz. If the jumpers are installed in the lab jacks, the amplifier bandwidth is 10 to 70,000 Hz. Above one set of phono-input jacks is a slide switch that selects input capacitance values of 100, 200, or 320 picofarads to suit almost any combination of cartridge and tone-arm wiring capacitance. Both phono inputs have a 47,000-ohm resistance. The speaker outputs, are through insulated spring-loaded connectors, and one of the two a.c. outlets is switched.
The NAD 3140 has yet another feature that greatly enhances its potential utility. By means of a rear-panel switch, its channels can be bridged to form a mono power
amplifier of more than double the stereo power ratings for both channels. A single 8-or 4-ohm speaker can be connected across the two “hot” outputs and will be driven (via the right-channel input) by an amplifier power of 150 to 160 watts, depending on the load impedance. The NAD 3140 is 16-1/2 inches wide, 11-3/8 inches deep, and 3-3/4 inches high. It weighs 17 pounds. Price: $348.
Laboratory Measurements
The power transistors of the NAD 3140 (four in all) are within the case, and their finned heat sinks are ventilated though a grille on the top cover. The 1-hour preconditioning period left the top of the amplifier only moderately warm, although our subsequent tests (especially driving low load impedances) caused it to become quite hot. The clipping output at 1,000 Hz, with both channels driving 8-ohm loads, was about 73 watts per channel. The 4- and 2-ohm clipping powers were, respectively, 94 and 112.5 watts per channel (the internal d.c. fuses blew on several occasions when we drove full power into 2 ohms). The 8-ohm IHF clipping headroom was 2.61 dB, evidence of the very conservative power rating of the amplifier. With the 20-millisecond burst signals of the IHF dynamic-headroom test, the maximum outputs into 8, 4, and 2 ohms were, respectively, 103, 152, and 201 watts per channel, giving an IHF clipping-headroom rating into 8 ohms of 4.11 dB, one of the highest we have ever measured. In the mono bridged mode the clipping-power output was 176 watts into 8 ohms and 200 watts into 4 ohms.
The harmonic distortion at 1,000 Hz, driving 8-ohm loads, was less than 0.002 per cent up to more than 10 watts output, increasing gradually to 0.0028 per cent at 60 watts and more suddenly to 0.013 per cent at 70 watts just before clipping occurred. With 4-ohm loads, the distortion was 0.004 to 0.005 per cent at most power levels up to 50 watts, reaching 0.016 per cent at 90 watts. Into 2 ohms, the distortion rose from 0.005 per cent at 1 watt to 0.011 per cent between 30 and 90 watts (at the two lower impedances we could not make distortion measurements any closer to the clipping-power levels because of the unit’s protective fuses).
Intermodulation distortion, using 19,000-and 20,000-Hz input signals of equal amplitude with a peak level equivalent to that of a 40-watt sine-wave signal, was —84 dB for the third-order component at 18,000 Hz and —87 dB for the second-order component at 1,000 Hz. The amplifier recovery from a 10-dB overload was essentially instantaneous (less than the 2-microsecond resolution of our test setup). The slew factor exceeded our measurement limit of 25 (it is rated as being greater than 50).
The high-level-input sensitivity was 26.5 millivolts for a 1-watt reference output, with an A-weighted output-noise level of — 86 dB referred to 1 watt. The phono sensitivity for 1 watt was 0.41 and 0.097 millivolt for the two inputs, both of which had noise levels of —82 dB. The phono overload at 1,000 Hz was 66 millivolts for the high-gain (moving-coil) input and 210 millivolts for the moving-magnet input. The equivalent overload levels at 20 and 20,000 Hz were within a few per cent of the 1,000-Hz values. The phono-input impedance was 48,000 ohms in parallel with nominal capacitances of 100, 225, or 360 picofarads, depending on the setting of the switch in the rear of the amplifier. The capacitance at the high-gain phono input was fixed at 100 picofarads.
The tone controls had the familiar Baxandall characteristic, with the bass-turn-over frequency varying from less than 100 Hz to about 400 Hz as the control was turned and the treble curves hinging at about 3,000 Hz. This type of control gives a useful range of adjustment with little risk of grossly disturbing the program’s frequency balance. The loudness compensation boosted both low and high frequencies, but to such a moderate degree that the sound was never objectionable. The RIAA phono equalization was accurate within ± 1 dB and was changed by less than 0.5 dB at any frequency by the inductance of a phono cartridge. The speaker eq curves were as represented, with their +3-dB response points at 40 and 65 Hz and a maximum boost of 11 dB at 20 Hz from the 70-Hz curve (the 45-Hz-curve response was +8 dB at the lower measurement limit of 20 Hz).
The power-display lights came on abruptly and without the ambiguity we have seen on many such displays. There were some differences between channels, and at low powers the actual output was from 30 to 100 per cent higher than the lights indicated. At higher levels (25 and 50 watts) the calibration of the display was more accurate, with errors of about 10 per cent.
Comment
When we reviewed the NAD 3020, we were highly impressed by its ability to outperform any similarly rated amplifiers we had seen or heard. The NAD 3140 is a logical extension of the philosophies expressed in the 3020 and with roughly twice that unit's power capabilities (plus a few niceties that could not have been included in a budget-price amplifier). Anyone familiar with NAD amplifiers wid recognize that a “NAD watt” is substantially more effective in-the real world than most other manufacturers’. The NAD 3140 is called a “40-watt” amplifier, but measurements and listening leave no doubt that it can easily hold its own against most conventional “100-watt” amplifiers.
As with the 3020, we subjected the 3140 to the severe test of driving all the available speakers in our laboratory simultaneously. The combined load impedance was about 2.3 ohms. We drove the amplifier to its maximum output, as evidenced by the “50-watt” indicator’s being lit much of the time. The sound level was high, as might be imagined, but there was rarely any audible evidence of clipping or other distortions. Comparison with other amplifiers having considerably higher power ratings (60 to 100 watts) confirmed that the NAD 3140 could indeed play louder than any of them, and it always sounded cleaner under these admittedly severe and unreasonable conditions.
The “soft-clipping” feature, as we had found with the 3020, offers a rather subtle improvement. Its effects cannot be heard under any ordinary listening conditions (unless you usually listen with the amplifier driven into clipping). When clipping does occur, either deliberately or accidentally, turning the soft clipping on reduces the harshness somewhat. It is not a miracle panacea for clipping (a more powerful amplifier or a more efficient speaker is the only real cure), but it does no harm and we preferred to leave it on at all times.
The SLC, we were told, would have no significant effect unless the d.c. resistance of the speaker lines were about 0.5 ohm. Ours are 30-foot lengths of 14-gauge wire and their resistance is negligible. We simulated longer leads by adding sections of smaller-gauge wire to the speaker lines. Under these conditions we could hear a slightly increased high-frequency response when the SLC was turned on, but the other claimed advantages were not audible in any clearly identifiable form.
The speaker eq is one of the most novel and useful features of this amplifier. Although it probably adds little to the cost of the 3140, it provides a bass extension with many speakers that can be felt as much as heard. As we pointed out earlier, such a boost would be foolhardy in most 40-watt amplifiers, which would soon run out of headroom with a 12-dB low-bass boost, but with the NAD 3140, especially in combination with the infrasonic filter, neither the amplifier nor the speaker is likely to be strained. We were also pleasantly surprised to find that the high-gain phono input yielded very lis-tenable noise-free levels even with some low-output moving-coil cartridges.
In our closing remarks on the lower-power NAD 3020 we speculated as to what a higher-power NAD amplifier might be like. Now we know; it is surely as much a bargain as its junior partner, and it represents a genuine advance in design. To assess the NAD 3140 fairly, do not consider it as merely a 40-watt integrated amplifier.
Think of it instead as an 80- to 100-watt amplifier of exceptional quality and with versatility and features found nowhere else, including an ability to drive low-impedance speaker loads that would confound most other amplifiers on the market, even some rated at many times its power. In that light, it seems to be as much a bargain as the 3020. Dare we wonder what a still more powerful NAD amplifier might be like?
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