Bryston OBSOLETE 4B NRB - Manual (Page 2)

GENERAL DESCRIPTION
Bryston's 4B stereo power amplifier consists of two amplifier modules, each with its own ±85v
power supply, which can be operated independently or combined (MONO mode) to form a single
800 watt amplifier (8i2 load) in a 3 rack height chassis. Operated independently (DUAL mode)
each channel can deliver 250 watts into SQ (or 400 watts into 4Q). Each of the two amplifier
modules use 8 of Bryston's custom power transistors for a total of 16 output devices per 4B.
POWER CONSIDERATIONS
The 4B employs truly massive power supplies which can draw prodigious amounts of current from
the line when the music demands it. In addition, every Bryston amplifier contains exceedingly
high quality dedicated circuitry in its power supplies to reject RF, line spikes and other power line
problems. For these reasons the 4B should NOT be connected to the accessory power outlets in
a preamp NOR into such devices as power bars or line conditioners. It should be plugged into its
own socket in the wall outlet (with the power switch in the OFF, or OUT, position).
In general, if your preamp has a three prong grounded line cord you can avoid local ground loops
which could cause hum or noise in the system by plugging the preamp’s line cord into the same
wall outlet next to the power amp.
CLIPPING INDICATORS
The 4B amplifier uses a pair of 3 colour LEDs with green indicating power on and red indicating
actual overload or distortion. Yellow appears dynamically at the threshold of clipping to indicate
caution in proceeding to higher power levels. As long as the LED is green, or green and yellow
with only very brief red flashes, the waveform is reaching full power only at the highest musical
peaks. Once the indication contains a substantial red content it should be considered as a sign
that the level is too high and may cause speaker damage if allowed to continue.
The clip-sensing circuit in the 4B uses an AC comparator to detect any source of signal distortion
including clipping, short circuits in cabling, excessive DC or supersonic signals, whether at input
or output, etc. Most conditions which could cause the red LED to light for more than a moment or
two can be dangerous to your speakers and should be corrected immediately.
SPEAKER POWER HANDLING & FUSE PROTECTION
Most speakers will safely absorb an undistorted musical signal up to the full power capability of
even large amplifiers like the 4B (DUAL mode). However, once the signal starts to be clipped the
distortion by-products generated by the overdriven signal can result in very high power levels fed
to the smallest and most fragile of the speaker's components (it should be noted that an amplifier
is quite capable of distorted power levels of twice the undistorted rating and that all the excess
power goes to the high frequency driver). It is also quite possible to damage the woofer by
causing it to exceed its excursion limits and impact against the back of the magnet by doing such
things as turning up the bass control on a preamp and playing music quite loud. This is especially
true with amplifiers which can develop full undistorted power down to extremely low frequencies
as all Bryston amplifiers can. Many speaker manufacturers recommend series fuses to protect the
drivers in case of accidental (or even wilful) overload. If you wish to use fuses follow the
manufacturers recommendations since even what seems like a small increase in current rating on
a fuse will result in large increases in power allowed. Example; A 3 amp fuse will allow about 70
watts but a 4 amp fuse will nearly double this to about 130 watts and a 5 amp fuse will allow 200
continuous watts.
It should be emphasized that almost any amplifier is capable of inflicting damage on
almost any speaker if used in a thoughtless or abusive manner and that Bryston cannot be
responsible for speaker damage caused in this way.