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High Current and Headroom: Designing for Real-Use Conditions
Since amplifiers are usually rated in terms of a few standard specifications (e.g. 8-ohm power and THD),
many designers concentrate on optimising test-bench performance. But in the real world, amplifiers are
connected to loudspeakers with complex impedances, not to 8-ohm test resistors, and they are used to
play music, not sine-wave test tones. In NAD amplifiers every circuit is designed to deliver full
performance under real-use conditions. While many specific engineering goals follow from this principle,
two have been the cornerstones of every amplifier in NAD history (including the legendary Model 3020):
graceful handling of dynamic transients that exceed the amplifier’s rated power, and the ability to deliver
large bursts of output current to the loudspeakers. Electrical power is the product of voltage and cur-
rent, but it is the current flowing through the voice coil that causes a loudspeaker cone to vibrate and
reproduce sound. As NAD engineers have always known and other manufacturers have lately begun to
realise, to obtain precise electromagnetic control of the speaker’s motion the amplifier must be able to
supply high peak currents upon demand, unconstricted by transistor “protection” circuits. The NAD
2155 amplifier circuit produces peak currents of up to 40 amperes per channel.
Loudspeaker impedance matching
Standard lab tests of amplifiers use 8-ohm resistors in place of loudspeakers. But most loudspeakers
have a lower and more complex impedance that increases the required amplifier output current. (And if
you connect two pairs of loudspeakers, the effective impedance of the pair is halved.) For this reason
the 2155 amplifier is designed to deliver its maximum power into low impedances of 4 or even 2 ohms.
But the exclusive NAD impedance selector allows you to re-optimise the amplifier circuit to produce
greater output voltage, for the most effective delivery of power to loudspeakers whose true impedance
is 8 ohms or higher.
Soft Clipping
NAD’s trademarked Soft Clipping circuit gently limits the waveform when the amplifier is driven beyond
its maximum power rating. By preventing the out-put transistors from being driven fully into saturation,
the Soft Clipping reduces the harshness that is normally heard when an amplifier is over driven. Because
of this and the amplifier’s high dynamic headroom, the sound remains clean and musical even at very
high sound levels, rather than becoming harsh as in other amplifiers.
Exceptional Performance and Value
The NAD 2155 is a superb general-purpose power amplifier with an unusually attractive ratio of
performance and power to price, and it is also the core of the unique NAD building-block concept that
allows the stereo system to evolve with your needs or your budget, now and in the future.