“Audible Clipping” Indicator
To enable the user to make the fullest use of the dynamic power of the 2200, a front-panel “Overload” LED indicator illuminates whenever the amplifier is driven into clipping (or exhibits any other distortion) for a long enough time to be audible. Its calibration is based on psycho-acoustic studies showing that the audibility of clipping depends not only on the severity of the resulting distortion but also on its duration. (If an intense but brief transient overloads the amplifier for less than a thousandth of a second, you can’t hear it.) The indicator works by comparing the output signal with the input, instant by instant. Ideally the amplifier’s output signal should be an exact replica of its input, scaled up by a factor of 40 in voltage. The comparator circuit divides the output signal by 40, subtracts it from the input signal, and flashes the LED if there is any potentially audible deviation from perfect linearity.
The NAD 2200 is a “commutating” power amplifier, I.e., It has two power supplies, switching to the high-voltage supply when maximum power is needed, and switching to the lower-voltage supply for cooler operation at average power levels. (The switch is called a commutator; hence the name for this type of amplifier.) By itself, the basic idea is not new. What makes the NAD 2200 unique is how dramatically it overcomes the two problems that other commutating amplifiers suffer from: (1) poor efficiency, resulting in high heat dissipation in the power supply and consequently high cost; and (2) switching transients, which can become a form of audible distortion.
Basically, any power amplifier consists of two parts: a power supply and an audio circuit. The audio circuit functions as a valve, opening and closing to feed current from the power supply to the loudspeaker in accordance with the demands of the audio signal.
In the case of the 2200, the audio circuit is a fully complementary DC-coupled class A/B circuit designed for 500-watt output, operating in class A for distortionless sound at low levels and moving into class B for clean, efficient operation at higher levels. It has a fast, high-capacity output stage equipped for the high peak cur-rents (:’ ?I 50 Amperes) and the large peak-to-peak voltage swing (190 volts) that are appropriate to a well designed 500-watt amplifier.
The high-voltage power supply in the 2200 provides the 190-volt swing needed for full-power operation, but it is deliberately designed to be self-limiting, able to supply high current for only a brief period. The lower-voltage supply provides a 125-volt swing and has ample capacity to run the amplifier comfortably at 150-watt levels all day long, If the amplifier were built for a continuous 500 watt output, it would require an enormous power transformer, special-order high-current filter capacitors, plus an elaborate system of heat sink fins and ventilating fan to dissipate the resulting waste heat.
The manufacturing cost of the amplifier would be doubled or tripled, for no purpose. Music rarely requires an average power much greater than about 50 watts (even for very high volume levels), and very few loudspeakers can absorb a continuous output of 500 watts for more than a few seconds. Music is dynamic, requiring high power only in bursts.
The NAD 2200 PowerTracker™ circuit is designed to reproduce music. Its high-voltage power supply contains a solid-state memory device that stores information on the recent history of the amplifier’s output current and consequent heat dissipation. If the output has been fluctuating up and down (I.e. playing music), then the average current is modest; and the high-voltage supply continues to operate at full capacity, ready to supply high power when needed. But If the average goes up! reflecting a constant output of several hundred watts for more than a few seconds, then the high-voltage supply gradually shuts itself down, forcing the amplifier to derive its power mainly from the lower-voltage supply.
Thus while the NAD 2200 functions as a 500-watt amplifier with musical signals, it cannot be made to overheat. And when fed continuous sine-wave test tones it becomes, in effect, a 150.watt amp. Its operation is so efficient that the size, heat dissipation, and manufacturing cost of the 2200 are nearly the same as an ordinary 100 to 150.watt amplifier, If commutator switching occurs at low power levels, the switching transients can become a form of audible distortion.
This doesn’t happen in the NAD 2200, for two reasons, (1) The changeover to the high-voltage supply occurs only at rather high power levels (around 140 watts). Relative to this level, even an ordinary switching transient would represent an inaudibly small percentage of distortion. In most music there is no switching at all; the high-voltage supply is used only during those brief transients and climaxes that demand the top 6 dB of the amplifier’s dynamic range, when peak sound levels typically exceed 110 dB (6 ohm speaker, 88 dB sensitivity).
