NAD 2200 Amplifier
Despite its conventional appearance, the NAD 2200 is radically different from other stereo power amplifiers in its design and performance, some aspects of which border on the spectacular. It carries a relatively moderate power rating of 100 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.03 percent distortion. In size, weight, and price, it is similar to a number of other good 100-watt amplifiers. Not surprisingly, it follows the NAD tradition of giving its amplifiers conservative power ratings and the ability to drive low-impedance speaker loads without difficulty (the clipping power output is specified as 140 watts into 8 ohms or 200 watts into 4 ohms).
However, the NAD 2200-which the manufacturer calls the "Power Tracker"-has some remarkable dynamic power capabilities. Its dynamic headroom is rated at 6 dB, which means that it can deliver-in 20-millisecond bursts, twice per second-400 watts per channel to 8 ohms, 600 watts to 4 ohms, and 800 watts to 2-ohm loads. If that is not enough, the amplifier can also be operated in a bridged (mono) mode, in which it is rated to deliver up to 400 watts of continuous output into 8 ohms-or, in terms of dynamic power, 1,200 watts into 8 ohms and 1,600 watts into 4 ohms! It is also said to have a wide "dynamic power envelope," which means that it can maintain these high levels for longer than the standard 20-ms bursts.
The importance of high dynamic power output for realistic music reproduction has been widely recognized for some years. One way to achieve it is by using a signal-controlled, or "smart," power supply, in which the output voltage is controlled by the instantaneous signal level and automatically adjusts itself to accommodate the brief high peak levels found in most musical and vocal waveforms. Since the average power requirement is usually one-tenth or less of the peak power demand, the switched-voltage design approach offers the attractive possibility of doing without a large, expensive, heavy-duty power supply whose full output will be needed for only a small fraction of the time. Instead, the amplifier can use a relatively compact, inexpensive, low-voltage power supply almost 100 percent of the time, augmented by a second, higher-voltage supply that comes into action only as needed.
The output transistors of the NAD 2200 amplifier are high-powered, fast-switching devices capable of delivering some 60 amperes of peak current for brief periods. One key difference between the 2200 and most other switched-voltage amplifiers is the choice of the power level at which the changeover occurs. In some amplifiers it occurs at a fairly low power output, such as 15 watts. This arrangement offers economies in manufacture, since most of the time the amplifier generates little heat and requires a small heat-dissipating surface. On the other hand, the switching often produces a transient "glitch" on the waveform, which could conceivably be audible, at the point where the supply voltage increases.
NAD chose to design the 2200 as an inherently powerful amplifier, with the size, weight, and heat-dissipating ability (as well as the price) of a typical 100-watt unit. Because of its conservative design, its clipping power output, even in the "low power" mode, is about 140 watts per channel. Since the switch to a higher supply voltage occurs at the 140-watt level, any switching transients are masked by the high acoustic level.
A number of protection systems were built into the 2200, one of the most interesting of which is the "thermal feedback" that protects the high-voltage supply and the output transistors. NAD studies indicated that actual high-level music transients are likely to last much longer than the 20 milliseconds of the standard EIA dynamic power measurement, and that considerably more than the continuous power level might be required for several hundred milliseconds. However, a conventional single-voltage amplifier would be prohibitively large and expensive if it were designed to deliver several times its normal rated output for that length of time. The answer was a higher operating voltage, the value of which is ultimately limited by the temperature of the output transistors and, to a lesser degree, of the power transformer.
Under normal operating conditions, the full voltage of the NAD 2200's power supply is made available. As the temperatures increase, the level of the second voltage is reduced sufficiently to prevent damage to the amplifier. In the limiting case of continuous high-power drive, the second power supply effectively shuts off, leaving the amplifier operating as a "100-watt" unit on its lower-voltage supply. Because of the very conservatively operated output transistors, the amplifier cannot overheat, no matter how long it is run or at what level. And all this without a cooling fan!
The 2200 also features NAD's Soft Clipping circuit, which is slightly modified to come into play only about 0.5 dB before the amplifier is about to clip. The circuit rounds the abrupt waveform edges that normally accompany clipping, thereby removing the high-order harmonics that sound so harsh and can also damage a tweeter-no small consideration for an amplifier as powerful as this one. The 2200's front-panel overload light is activated by a circuit that senses when audible clipping is about to occur.
In order to make the maximum possible power available at low frequencies (which are normally present in both channels in the same phase and thus present an additive load to the power supply), NAD has connected the two channels (internally) in opposite phase. The signals are reversed to the correct polarity at the speaker binding posts. This circuit configuration prevents the amplifier from being used with speaker switching systems in which both channel outputs have a common ground.
The NAD 2200 measures 16-1/2 inches wide, 14-3/4 inches deep (including the rear-panel connectors), and 4-7/8 inches high. It is finished in dark gray and weighs 29 pounds. Price: $448.
Lab Tests
Preconditioning the NAD 2200 for 1 hour at 33 watts output into 8-ohm loads left its top cover (over the internal heat sinks) only moderately warm. With both channels driven at 1,000 Hz, one channel clipped at 158 watts output. Into 4 ohms, the power at clipping was 240 watts, and into 2 ohms it was 365 watts (the last measurement was made with only one channel being driven, since an internal d.c. fuse blew when we drove both channels to clipping with 2-ohm loads).
The 1,000-Hz distortion into 8 ohms was about 0.005 percent at very low power (0.1 watt). It decreased to 0.0035 percent at 10 watts and reached 0.0082 percent at 150 watts, just before clipping occurred. With 4-ohm loads the distortion was very similar at most power outputs and reached 0.0071 percent at 200 watts. The 2-ohm distortion was also very low at most power levels, reaching 0.0225 percent at 300 watts. Into 8 ohms, the distortion was lowest, between 0.001 and 0.002 percent, from 30 to 100 Hz; it rose into the 0.01 to 0.02 percent range from 1,000 to 20,000 Hz at power outputs of 100, 50, and 10 watts.
Dynamic power measurements were somewhat complicated by the thermal feedback system of the amplifier. As it heated up the maximum power decreased, so we were never able to repeat our measurements exactly on successive attempts. Nonetheless, the results were very impressive. With the standard 20-millisecond test signal, the maximum output was 450 watts into 8 ohms (for a dynamic headroom of 6.5 dB), 685 watts into 4 ohms, and 870 watts into 2 ohms. We repeated these measurements with 4-ohm loads and with longer-duration tone bursts.
The output of 700 watts did not change for bursts in the 20- to 50-ms range, but it decreased slightly to 570 watts with 100- and 200-ms bursts and to 285 watts with bursts of 300 ms and longer. (The output did not drop significantly with a 1-second burst, and, in fact, it remained at 240 watts in a "continuous" measurement lasting perhaps 30 seconds.) We repeated this test (in part) using the bridged mode, reading outputs of 1,100 watts for 20 milliseconds, 800 watts for 50 milliseconds, and 700 watts for 100 milliseconds.
The frequency response of the NAD 2200, measured through its "normal" inputs, was down 3 dB at 12.5 and 45,000 Hz, and it was flat within +0, -0.3 dB from below 20 Hz up to 20,000 Hz. Through these inputs the bandwidth of the amplifier is limited by internal filters.
Using the "lab" inputs, the lower - 3-dB frequency was below our 5-Hz measurement limit, and the upper - 3-dB frequency was 135 kHz. The A-weighted noise of the amplifier was 93 dB below 1 watt, or 113 dB below its rated output. The amplifier was stable when driving simulated reactive speaker loads, and its reactive load factor was 1.6 dB at 63 Hz. The slew factor exceeded our measurement limit of 25.
Comments
Our measurements speak eloquently about the performance of the NAD 2200. We also used it as the power source for some of our pulse-power tests of loudspeakers (1 cycle of a sine wave followed by 128 cycles off) and found that its bridged-mode output was just short of 2,000 watts into a 2-ohm impedance! For listening to music in a home environment, the NAD 2200 may well be the most powerful amplifier you can buy, especially if you use a pair of them in the bridged mode (which probably would be a case of "overkill" for most people). If ever an amplifier could be said to be "digital ready," this one can. Its readiness is not merely a matter of being louder than other amplifiers, although not many others could match it in sheer decibel level, but of its virtual immunity to overload with whatever program peaks an amplifier might encounter, even on Compact Discs.
The Soft Clipping circuit works well, although the chances that anyone will drive the NAD, 2200 to within 0.5 dB of its clipping point are slim indeed. The circuit has no measurable effect on the amplifier's distortion or other characteristics, so it can be left on at all times. We found that the overload light did not flash until the output waveform was heavily clipped, but this is a minor matter. No matter how you look at it, the NAD 2200 is a superb amplifier and an outstanding value.
