Adcom GFA-555MS - Manual (Page 11)

correct stereo phasing with optimal bass response, you must observe these connections
precisely.
Although the GFA-555ms can generate a substantially greater amount of power in the
bridged mono mode than when it is in its normal stereo mode, it requires the use of
loudspeakers the nominal impedance of which does not drop below 4 ohms. It is not
recommended that the GFA-555ms be used in the bridged mono mode into loudspeakers, or
multiple loudspeaker loads, which drop in value substantially below 4 ohms. Otherwise, you
may trigger the THERMAL PROTECTION 0 or blow one of the DC RAIL FUSES e. Please
refer to the section RIGHT/LEFT STEREO SPEAKER OUTPUT for further clarification. A
little known fact is that when any amplifier is operated in the bridged mode, the load is "split"
between the two amplifiers in the bridged configuration. Therefore, an 8-ohm loudspeaker
will be seen by the amplifier as if it were a 4-ohm load; a 4-ohm loudspeaker load will be
seen by the amplifier as a 2-ohm load.
NOTE
If the connections described above are followed exactly, the GFA-555ms will be polarity
correct, that is, it will not invert "phase". Any positive-going signal at its input will appear
as a positive-going signal at the loudspeaker.
DC RAIL FUSES
The DC RAJ L FUSES provide protection for the output stages and power supply in the
event of excessive current demands from the amplifier, either long-term or short-term.
If the amplifier ceases to operate, either on one or both channels, particularly during
high-level passages, or long-term high-volume playback, and the POWER LED glows while
the THERMAL PROTECTION LED is out, the chances are that one or both of the DC RAIL
FUSES on that channel, or both channels, are blown.
Your ADCOM amplifier is designed to activate its protective devices reliably, particularly
when the amplifier is carelessly operated well beyond its limitations. Other types of
protection circuits, beyond the methods used in the GFA-555ms amplifier, such as current
limiting, etc., usually result in deterioration of the audio performance of the amplifier.
While the GFA-555ms will operate dependably in every normal situation, no amplifier is
impervious to abuse. There are conditions which must always be avoided if the amplifier is to