NAD 3155 User Manual

3155

Integrated Amplifier

Date of manufacture : Jan 85 - Jan 87

Please note that this document contains the text from the original product brochure, and some technical statements may now be out of date

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 3155 integrated amplifier and the matching 2155 power amp are
the finest in a long tradition of highly praised NAD amplifiers, with increased power, improved reliability,
and numerous small refinements in both sound quality and ease of use.

The 3155 is two products in one. As a preamplifier it matches most separate audiophile preamps in
sonic performance and surpasses them in operating flexibility. As a high-current high-headroom power
amplifier it drives even “difficult” loudspeakers to surprisingly high volume levels with clean, solid, full­bodied musical sound that remains refreshingly free from distortion even in transient peaks. The
combination of pre- and power amp, sharing one heavy-duty transformer and chassis, offers another
traditional NAD virtue: an impressive ratio of performance to price. The 2155, which is simply the power
amplifier section of the 3155 packaged on a separate chassis, is NAD’s “building-block” amp, offering a
variety of convenient and economical approaches to upgrading a stereo system for higher performance
and power. The 2155 is an obvious choice if you want to step up 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 3155 and 2155 deliver substantially more than their conservatively rated 55 watts/channel into the
complex and varying impedances of real loudspeakers. In the bridged monophonic mode each 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, the 3155 and 2155 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 3155’s preamplifier section, makes the 3155/2155 combination a
remarkable best-buy system.

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 3155/2155 amplifier circuit produces peak cur-rents of up to 40 amperes per channel.

Preamplifier Features (3155)

Digital-ready design.The amplifier’s line-level inputs will not be overloaded by high-level signal peaks
from a Compact Disc player or digital tape recorder. The 103 dB signal-to-noise ratio of the 3155
exceeds that of any analogue or digital program source. The low-impedance, low-noise design of the
volume and tone control circuits guarantees that the transparent clarity of the finest analogue and
digital recordings will be preserved.

The phono preamplifier, too, is audibly (and in some cases dramatically) quieter than most other preamp
circuits, not only with moving-magnet pickups but also with low-output moving coil cartridges. The
entire amplifier circuit has ample dynamic range for every program source, including the widest-range
digital Compact Discs. Ultra-quiet MM and MC phono preamp circuits. As long as LPs remain a principal
music source, there must be no compromise in the phono preamp. The phono circuit in the NAD 3155
employs ten selected transistors per channel to obtain the widest possible dynamic range with
outstanding freedom from noise and distortion at all signal levels. A differential FET input circuit ensures
ultra-low-noise operation at the high impedances of MM cartridges, and its linearity in the MegaHertz
range, aided by a custom-wound input filter, provides exceptional rejection of all interference from TV,
CB radio, and computers. A separate high-gain MC pre-preamp stage uses newly developed transistors
optimised for lowest noise with low MC impedances. A push-pull high-current output circuit drives the
precision RIAA equalisation network without slewing distortion, and its 107 dB dynamic range comfort­ably accommodates all digitally-mastered LP recordings with room to spare.

Bass EQ

A special equalisation circuit provides 6 dB of boost at 32 Hz in order to strengthen and extend the
deep-bass response of closed-box loudspeaker systems.
A typical bookshelf speaker that rolls off below 50 Hz will have strong output to 30 Hz when used with
the NAD 3155, providing the sort of authentic bass “feel” that might otherwise require a costly separate
subwoofer system. Infrasonic filter, Precise infrasonic filtering is included to eliminate signal
contamination from turntable rumble, record warps, tonearm/stylus resonances, vibration and acoustic
feedback. This guarantees the cleanest possible handling of signals within the audible range and
eliminates the excessive woofer-cone excursions that can cause intermodulation distortion and muddy
bass in systems without filtering.

Power Amplifier Features (3155 and 2155)

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 3155 and 2155 amplifiers are designed to deliver
their 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 loud-speakers 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 overdriven. Because
of this and the amplifier’s high dynamic headroom, the sound remains clean and musical even at very
high sound levels, rather than being distorted as in other amplifiers.

Bridging

If still more output power is needed, a bridging switch immediately converts the stereo amplifier into a
monophonic unit conservatively rated at 150 watts. A matching NAD 2155 power amplifier (also
bridged) can be used for the second stereo channel. With an IHF dynamic headroom of +2.5 dB in the
bridged mode, the 3155 and 2155 can produce peak levels of over 250 watts per channel for musical
transients. The combination of the 3155 and 2155 in bridged mode becomes, in effect, the world’s most
powerful integrated amplifier while retaining all of the operating simplicity of the 3155 amplifier alone.
Since the 3155 has independent preamp outputs and power amp inputs, it can also be used with an
electronic crossover for bi-amplification with either full-range speakers or a separate sub-woofer.