THE FM section of the NAD 7150 stereo receiver features the Schotz Variable Bandwidth PLL detector, whose announcement last year aroused much interest in audio circles. The detector of an FM tuner the circuit that converts the varying frequency of the received FM signal to a varying-voltage audio signal--is one of the tuner's more critical elements. Its bandwidth must be sufficient to accommodate the maximum frequency deviation of the broad cast signal (up to at least ± 75 kHz) with out significant nonlinearity (which would result in distortion of the audio signals). However, the noise in the converted signal increases with the detector's bandwidth.
The Schotz tuner circuit maintains the detector's bandwidth at the minimum value required by the reception conditions (signal strength. noise level, and amplitude modulation of the FM signal caused by multipath interference) in order to keep the noise level in the detected signal as low as possible.
This automatic bandwidth control is made possible by the PLL (phase-locked loop) detector, whose internal oscillator locks onto the FM signal and tracks its instantaneous frequency. The low-frequency voltage that controls the PLL internal oscillator is an accurate analog of the demodulated audio program; modifying the characteristics of the PLL's filter with a composite signal-quality voltage (derived from circuits assessing signal strength, noise, and AM content) allows the effective detector bandwidth to be varied without increasing distortion.
The most obvious benefit of this detection system is greatly enhanced tuner sensitivity for both mono and stereo signals. At moderately high signal levels the NAD 7150's tuner section offers performance comparable to that of top-quality FM tuners, but under weak signal conditions the Schotz circuit can deliver clean audio with markedly lower noise and distortion than conventional tuners. There is no sacrifice of other desirable tuner characteristics such as capture ratio, channel separation, selectivity, or frequency response. On the whole, NAD's Schotz tuner is unquestionably one of the most advanced FM tuners available to the consumer.
For example, its rated usable sensitivity in mono is 7 dBf (1.2 microvolts, or uV).
The stereo 50-dB quieting sensitivity is rated at 33 dBf (25 AV). Both figures are considerably better than those claimed for (or achieved by) even the best conventional tuners. In addition, the NAD 7150's Schotz tuner has distortion and signal-to-noise (S/N) ratings that help place it among the top FM tuners of our time. This caliber of performance would be unusual for a deluxe separate tuner (the Schotz tuner is offered separately in the NAD 4150, incidentally) and was unheard of in a receiver until the NAD 7150 made its appearance.
NAD emphasizes utility and function in its products. NAD amplifiers, for example, have very high dynamic-headroom ratings and are able to drive very low load impedances (such as 2 ohms) at surprisingly high power levels without clipping or other distortions. The amplifier section of the Model 7150 is an excellent illustration of the NAD design approach. It is rated to deliver 50 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.03 percent distortion. Good performance, certainly, but hardly suggestive of any unusual power-output capabilities. The first clue to d that quality comes in the clipping-output ratings: 65 watts into 8 ohms, 75 watts into 4 ohms, and 85 watts into 2 ohms. Next we come to the dynamic-power figures: 100 watts into 8- or 4-ohm loads and 130 watts into 2 ohms. Finally, the Model 7150 can be operated as a bridged (mono) amplifier rated at 125 watts (150 watts clipping power or 225 watts dynamic power) into an 8-ohm load. A companion amplifier (the Model 2150) can be used to supply the second stereo channel, making the NAD 7150 into a veritable "super receiver." The tuning of the NAD 7150 is digitally synthesized, with 100-kHz steps for FM and 10-kHz steps for AM, the frequency being displayed by large LED numerals in the center of the panel. Also in the display window are five LED's that light progressively to show relative signal strength and small green arrows that light when a station is mistuned by one increment (100 kHz) to show the direction of the required tuning correction. Tuning is done with UP and DOWN pushbuttons, and a large button be low them engages the scanning mode, which causes the tuner to stop on the next receivable signal it encounters.
The tuner has five station-preset memories (each usable for one AM and one FM frequency), and a button selects either AM or FM reception. Lights next to the tuning display show when a stereo signal is received and when the SOFT CLIPPING switch in the rear of the receiver has been engaged.
This latter feature, previously introduced in other NAD amplifiers, causes clipping to occur gradually when the amplifier's power limit is exceeded, reducing the harshness of the resulting distortion.
Square buttons below the frequency-display window activate a SPEAKER EQ 55-HZ boost circuit (for improving a system's low-bass response), an infrasonic filter with a 12-dB-per-octave slope below 15 Hz, mono operation for the amplifier and tuner, and FM interstation-noise muting. Another pair of buttons (Low LEVEL and LOUDNESS) re duce the audio gain by 20 dB and turn on the loudness compensation. The volume and balance knobs are concentric, and the latter is lightly detented at its center. The bass and treble tone controls are also center detented. Other knobs operate rotary switches. The speaker selector connects either, both, or neither of two pairs of speaker outputs to the amplifiers (the front-panel PHONES jack is always "live"). The RECORD and LISTEN switches provide independent control over what a user is recording from and what he is listening to. Each has positions for aux, tuner, phono, tape 1, and tape 2 (the record control also has an off switch position).
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FREQUENCY IN HZ (CYCLES PER SECOND) CONTINUOUS WATTS/CHANNEL
The rear apron of the NAD 7150 contains phono jacks for the various signal inputs and outputs as well as spring-loaded insulated connectors for speakers and antennas. The PRE OUT and MAIN IN jacks are joined by removable jumpers; when the jumpers are removed, a signal-processing accessory or speaker equalizer can be inserted in the system or the 7150 can be used in the bridged mode to power one channel while an external power amplifier supplies the other. There are two a.c. outlets (one of them switched) and a pivoted AM ferrite-rod antenna. Rear-panel slide switches connect the amplifiers in the bridged mode, switch in the SOFT CLIPPING circuit, and set the output-transistor supply voltage for optimal operation with speakers having either HIGH (8-ohm) or NORMAL (lower) impedance. Finally, one slide switch changes the phono-preamplifier gain for use with either moving-magnet (MM) or moving-coil (MC) phono cartridges, and another selects a phono-input capacitance of 100, 200, or 320 picofarads across the fixed 47,000-ohm phono-input resistance.
The NAD 7150 is finished in black with white markings. It is 16 1/2 inches wide, 12 1/4 inches deep, and 4 1/4 inches high, and it weighs 22 pounds.
Price: $598.
Laboratory Measurements.
After pre conditioning, the amplifiers of the NAD 7150 clipped at 81.3 watts per channel with both channels driving 8-ohm loads at 1,000 Hz. The 4- and 2-ohm clipping outputs were 110 and 146 watts per channel, respectively. The dynamic-power output was 100 watts into 8 ohms, 178 watts into 4 ohms, and 225 watts into 2 ohms. The 8-ohm clip ping headroom was 2.11 dB, and the dynamic headroom was 3 dB, as rated.
The 1,000-Hz harmonic distortion with 8-ohm loads was between 0.002 and 0.003 percent at most power outputs from 1 to 70 watts, reaching 0.038 percent at 80 watts.
With 4-ohm loads the distortion was even lower at most output levels, reaching as "high" as 0.0075 percent only at 100 watts and 0.25 percent at 110 watts. The 2-ohm distortion was roughly 0.01 percent from 1 to 100 watts, 0.28 percent at 140 watts.
At the rated 50 watts into 8 ohms, the distortion was less than 0.002 percent from 50 to 3,000 Hz, rising to 0.0045 percent at 20 Hz and 0.018 percent at 20,000 Hz. At lower output levels, the performance was similar, with only slightly higher distortion readings. The IHF intermodulation (IM) distortion with 18- and 19-kHz input signals was a low-87 dB at 1,000 Hz and -88 dB at 20 kHz. The amplifier was stable with high- and low-frequency reactive loads. The slew factor was greater than 25.
The auxiliary-input sensitivity was 21.5 millivolts (mV) for a 1-watt output, with an A-weighted noise level of -83.5 dB. The phono (MM) sensitivity was 0.3 mV, with the very low noise level of-81.1 dB. The MC switch setting increased the sensitivity to 11.5 microvolts, with a noise level of-64.4 dB. The moving-magnet phono preamplifier overloaded at very high inputs, 230 to 260 mV over the 20- to 20,000-Hz range. The impedance for both MM and MC inputs was 47,000 ohms in parallel with the indicated capacitance values of 100, 200, or 320 pF.
The tone controls had a moderate but ad equate range, about ± 10 dB for the bass and ±8 dB for the treble. The bass turnover frequency varied from less than 200 Hz to over 400 Hz, and the treble curves were hinged at about 3,000 Hz. The loudness compensation boosted the low frequencies moderately and the highs (above 10,000 Hz) only slightly as the volume setting was reduced. The infrasonic filter reduced the response by 2.5 dB at 20 Hz, and the 55-Hz EQ boost peaked at 38 to 40 Hz with an amplitude of about 10 dB (it varied some what with the volume setting). The EQ circuit also attenuated the response rapidly be low 30 Hz (presumably to prevent amplifier overload from turntable rumble and other infrasonic disturbances) and had little effect above 70 Hz. The phono equalization was almost perfectly flat (0.5 dB overall variation from 20 to 20,000 Hz) and was not affected by cartridge inductance.
The FM-tuner sensitivity of the NAD 7150 was indeed extraordinary. The usable sensitivity in mono was 7.8 dBf (1.3 micro volts, or µV). The 50-dB quieting sensitivity in mono was 10 dBf (1.7 µV), and in stereo it was 31.2 dBf (20 µV). The stereo thresh old was 15.6 dBf (3.3 µV). The tuner noise level at 65 dBf (1,000 µV) was-77 dB in mono and-70.5 dB in stereo, with corresponding distortion readings of 0.11 and 0.15 percent. The IHF intermodulation distortion (IM) of the FM tuner was very good in mono:-78 dB for the 1,000-Hz difference component with the 14- and 15-kHz modulating signals we used and-66 dB for the third-order distortion at 13 and 16 kHz.
In stereo, both distortion readings increased substantially, to -49 and -50 dB, and there were the usual numerous spurious IM products throughout the audio range. (Also as usual, none of these distortions were audible or identifiable with complex musical program material.) The stereo frequency response of the tuner was virtually ruler-flat, varying only ±0.5 dB from 30 to 15,000 Hz. The channel separation was about 50 dB in the mid range, falling to 42 dB at 30 Hz and 45 dB at 15,000 Hz. Other tuner performance parameters also measured very well. Although the capture ratio (1.86 dB at 65 dBf or 1.12 dB at 45 dBf) did not quite match the tuner's ratings, it was certainly good, and the 70-dB AM rejection was outstanding. The image rejection was a very good 92 dB, and the alternate-channel and adjacent-channel selectivity measurements were excellent at 86 and 15 dB, respectively. The 19-kHz pi lot-carrier leakage into the audio was a low -72 dB, and the tuner hum was-66 dB.
Even the signal-strength lights are designed to show meaningful differences in received signals: the first four lights came on at intervals of about 20 dB, from about 20 dBf (5 µV) to 82 dBf (7,000 µV); the fifth light was only 3 dB above the fourth, at 85 dBf (10,000 µV). The AM-tuner frequency response was unusually flat over most of its range, varying about ±2 dB from 26 to 2,200 Hz and down 6 dB at 3,200 Hz.
Comment. Since the Schotz PLL detector circuit is always active, its effect may not be obvious from merely listening to the NAD 7150 receiver. Clearly, the tuner is far above average; indeed, there is no other we know of that can match its overall measured performance, and everything we heard from it in our use tests was consistent with our laboratory findings. The SPEAKER EQ 55- HZ feature was one of the more useful receiver features we have encountered, since its effect is to give a solid "bottom end" with little or no obvious bass coloration.
Even with speakers having a good bass response, we found it beneficial. If the speakers are at all bass-shy, it is invaluable.
We did not make any measurements of the SOFT CLIPPING circuit of the NAD 7150, having done so with other NAD amplifiers we have tested. For the type of listening we do, it offers no particular advantage, but if you like to push an amplifier to its limits, this could be worthwhile. The loudness compensation was good, although we rarely use that feature with any amplifier. In this case, the Low LEVEL button allows the volume control to be operated at a considerably higher setting than usual, which in turn makes it possible to add a slight amount of loudness compensation with no ill effects.
Both in the performance of its various subsections and in its overall capabilities, the NAD 7150 is an exceptional receiver.
You'd have to try a receiver with substantially higher amplifier power (more than double) before you'd have any possibility of bettering the sound of this one, because the 7150's high dynamic headroom makes it perform and sound like a unit with a much higher power rating. The NAD 7150 reflects few, if any, of the usual compromises in performance or features found in receivers, and it is difficult to imagine a combination of separate tuner and amplifier units that could match this receiver's functional and listening qualities, let alone surpass them to any significant degree.
-Julian D. Hirsch
[Equipment Test Reports Hirsch-Houck Laboratories: Julian D. Hirsch and Craig Stark]
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