Nad 2155

• Power output conservatively rated at 55 watts per channel into 8 or 4 ohms.

• +3 dB IHF Dynamic Headroom for peaks: 110 W/ch at 8 ohms, 130 W/ch at 4 ohms.

• High-current output stage, able to deliver peak currents up to 40 amperes for precise control of voice

coil motion with speaker impedances as low as 2 ohms.

• Bridging circuit yields 150 watts continuous power, 250 watts IHF dynamic power.

• Exclusive NAD impedance selector optimises power delivery to either high or low speaker impedance.

• Soft Clipping™ minimises distortion at output levels above the rated power.

NAD leads the audio industry in producing amplifiers with high-headroom circuitry and high-current output

stages, precisely the qualities needed for musically accurate reproduction of today’s wide-range

analogue and digital recordings. The 2155 power amp is one of the finest in a long tradition of highly

praised NAD amplifiers, with increased power, improved reliability, and numerous refinements in sound

quality. As a high-current high-headroom power amplifier it drives even “difficult” loudspeakers to

surprisingly high volume levels with clean, solid, full-bodied musical sound that remains refreshingly free

from distortion even in transient peaks.

The 2155, which is simply the power amplifier section of the 3155 and 7155 packaged on a separate

chassis, is NAD’s “building-block” amp, offering a variety of convenient and economical approaches to

up-grading a stereo system for higher performance and power. The 2155 is an obvious choice if you

want to upgrade from a low-powered receiver or integrated amp, or if you are using a separate

preamplifier. If you need still higher power, a pair of 2155s in the “bridged” mode deliver nearly three

times as much power output for only twice the cost.

The 2155 delivers substantially more than its conservatively rated 55 watts/channel into the complex and

varying impedances of real loudspeakers. In the bridged monophonic mode this amplifier is rated

conservatively at 150 watts continuous output. And these amplifiers maintain a full 3 dB of IHF dynamic

headroom (2.5 dB in bridged mode), meaning that they deliver twice their rated power in brief bursts:

over 110 watts/channel in stereo and about 250 watts in bridged configuration. This headroom for

peaks is crucially important for reproducing the uncompressed transients in modern digital and DBXencoded

recordings.

When combined as a bridged stereo pair, these amplifiers function as a 150 W/ch stereo amplifier with

250 watts per channel of short-term output. That power, together with the remarkable flexibility and

flawless sonic performance of the 7155 or 3155’s preamplifier section, make these combinations

remarkable best-buy systems.

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 current,

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