NAD 7140 - Manual (Page 2)

High-current output stage
Electrical power is the product of voltage and current, but current flowing through the voice coil is what
makes a loudspeaker cone vibrate and reproduce sound. In many amplifiers the out-put current is
deliberately constricted by current limiters (protection circuits), in order to allow the use of smaller,
cheaper output transistors. But NAD engineers have always known, and other manufacturers have lately
begun to realise, that to provide precise electromagnetic control of the speaker’s motion the amplifier
must be able to supply high peak currents upon demand. The NAD 7140 can produce peak currents in
excess of 30 amperes per channel.
Loudspeaker impedance matching
Standard lab tests of amplifiers use 8-ohm resistors in place of loud-speakers, but most loudspeakers
have a lower and more complex impedance that increases the required amplifier output current. (And if
you connect two pairs of loud-speakers, the effective impedance of the pair is halved.) For this reason
the 7140, like all NAD amplifiers and receivers, 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 7140’s amplifier circuit to deliver greater output voltage for maximum effective power delivery 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 output 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 of the 7140 remains clean and musical at
high sound levels, rather than being distorted as in other amplifiers.
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 7140, 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 inter modulation distortion and muddy bass in systems with-out filtering.
Design for real-world conditions
Tuner specifications are measured with a medium-strong signal (65 dBf, i.e., 1000 µV into 300 ohms),
but in the real world a tuner must perform well with signals of widely varying strength and quality. For
example, an FM tuner’s resistance to multipath interference depends on its ability to “capture” the de-
sired signal and reject weaker reflected signals; the lower the tuner’s capture ratio, the more efficiently it
rejects the interference and captures clean stereo. The capture ratio of the NAD 7140 is consistently
excellent, not only at the medium signal strengths where other tuners perform well, but also over the
100-to-1 range in signal level from 25 to 65 dBf, allowing many more stereo broadcasts to be received
without distortion.
Optimum gains and losses
In the I.F. section of any tuner the signal is both attenuated (by the high-selectivity filters) and re-
amplified, by high-gain ICs. But the final signal-to-noise ratio can never be better than at the point
where the signal IS weakest. By optimising the filter design to reduce losses, NAD’s designers were able
to eliminate an entire stage, reducing l.F gain, and still deliver the optimum signal level to the PLL MPX
decoder-obtaining an 80 dB stereo S/N ratio at 75 dBf.