Page 1
The 2200 virtually redefines the concept of dynamic head-room.
Its high power reserves are available, not only for the short 20-millisecond bursts used in the standard
IHF dynamic headroom test, but also for musical surges and climaxes lasting ten times longer. Even with
compressed recordings of rock music in which the peaks may be only 8 dB higher than the average
power, that may be enough variation to allow the 2200 to operate at high efficiency and maintain a
clipping level above 500 watts per channel into the 4-ohm impedance that is typical of real speakers.
The appeal of the NAD 2200 may be based mainly on its combination of high dynamic power and low
cost, but there are other noteworthy aspects of its design, too.
Transparent sound
The audio circuitry of the 2200 is based on the same principles that gained worldwide praise for other
NAD amplifiers. In the 2200 the circuitry is scaled up substantially in speed and power, using the finest
selected parts available today-individually tested filter capacitors and ultra high-speed transistors for
wide bandwidth and extremely low distortion. The output stage is a fully complementary parallel circuit
using high-gain 25MHz transistors with over 10 times the Òsafe operating areaÓ of transistors used in
typical 100 watts/channel designs.
High-voltage, high-current design
Current flowing through the voice-coil is what causes a speaker to produce sound, and NAD was the
first manufacturer to emphasise the importance of high output current capacity-unrestricted by so-called
protection circuits-to cope with the complex, reactive impatience that many speakers present. The NAD
2200 can produce peak output currents exceeding ±50 Amperes on demand, together with peak output
voltages of ±95 volts to handle the most dynamic signals.
Inverted channels for powerful bass
The greatest power demands commonly occur at low frequencies. Bass signals are in phase (and virtually
monophonic) in most recordings; thus when the bass waveform is strongly positive in the left channel, it
usually is strongly positive in the right channel at the same time. As a result both channels draw current
simultaneously from the positive half of the power supply, while the negative half sits idle. During the
negative half of the waveform, both channels draw from the negative supply while the positive supply
sits idle. In the NAD 2200 the right channel is internally inverted in polarity. When a bass waveform
causes the left channel to draw current from the positive supply, the right channel draws its bass power
from the negative supply, and vice-versa. This efficient usage halves the instantaneous drain on either
supply, allowing much stronger bass to be reproduced without draining the supply.
Bridging
The NAD 2200 is so powerful in the normal stereo mode that few listeners will ever need more. For
special situations the two channels of the 2200 can be bridged to form a mono amp of truly immense
power. Its rated continuous sine-wave output is 400 watts, while its dynamic power output exceeds
1200 watts into 8 ohms and 1600 watts into 4 ohms.
Two 2200s in bridged mode (delivering over 3 kilowatts into a pair of 4-ohm speakers) would cost about
the same as an ordinary 400-watt amplifier.
Quiet circuitry
The delicacy and purity of low-level musical information is as important for realism as the power to
handle climaxes. The signal-to-noise ratio of the 2200 (relative to its rated 100 W/ch output level) is
greater than Ill dB, No fan noise. In most power amps that are capable of the same 500 W/ch output on
musical signals as the 2200, a noisy fan must be used to dissipate excess heat from the circuitry.
The efficient 2200 is totally, blissfully silent in operation.
Close-tracking Soft Clippingª The newly improved Soft ClippingÓ circuit in the 2200 accurately tracks
the available peak power, regardless of speaker impedance. Older Soft ClippingÓÓ circuits tended to
reduce the available dynamic power by 1.5 to 2 dB in order to ensure that continuous output would
always be free from harsh distortion (even when the amplifier was over-driven).
With the new PowerTrackerª circuit, this limitation no longer applies. Now the amplifierÕs sound
remains subjectively clean and transparent right up to the maximum output level. It continues to sound
good even at levels 2 to 3 dB above the amplifierÕs rated maximum output, since the Soft Clipping
circuit gently rounds off the waveform corners and prevents any distortion due to power-supply
modulation as well as reducing high order harmonics which may damage tweeters.
Page 2
ÒAudible ClippingÓ Indicator
To enable the user to make the fullest use of the dynamic power of the 2200, a front-panel ÒOverloadÓ
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 canÕt hear it.) The indicator works by comparing the output signal with the input, instant by instant.
Ideally the amplifierÕs 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 ÒcommutatingÓ 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 (:Õ ?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 PowerTrackerª 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 amplifierÕs
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 doesnÕt 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 amplifierÕs dynamic range, when peak sound levels typically exceed 110 dB
(6 ohm speaker, 88 dB sensitivity).
