Adcom GFA-555MS, Operating Instructions Manual

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without repetitive destruction of the loudspeaker load.
To achieve the above, eight sets of specially selected TO-3-type, metal-cased bipolar output
transistors are used in the GFA-555ms in a triple-Darlington configuration of its driver and
output stages. The safe operating area of these transistors, along with their circuit topology,
insures very efficient power delivery to reactive loads, regardless of the phase angle of
voltage and current. In addition, no protection or current-limiting circuitry is used in the
GFA-555ms which could degrade the Signal.
Electronic protection, or current-limiting circuitry, would impede the delivery of large amounts
of current (power) to low-impedance, highly reactive loads, thereby constricting dynamics
and deteriorating sonically the performance of the amplifier. Therefore, only fuses on the DC
rails are used for protection.
The power supply in the GFA-555ms has enormous reserve power capability with an
extremely large transformer feeding a storage bank of 60,000uF of filter capacitance. The
transformer itself was designed for extremely good regulation, insuring stable voltages
regardless of the power demands from the amplifier. Its higher efficiency was insured by
using a toroidal design. Additionally, thermal and dynamic tracking of the bias for the
triple-Darlington driver and output stages is provided to insure that the amplifier operates in
its optimal range regardless of the length of time and the level of operation.
The GFA-555ms's circuitry is direct-coupled internally to minimize the sonic deterioration
which capacitors, when used between stages of amplification, can cause. As a result, the
GFA-555ms can amplify DC and has no limit as to low-frequency or peak-bass energy
delivery. In particular, the driver stages are designed to provide all the current required by
the output devices without limiting, compressing, or otherwise distorting the signal coming
into the power amplifier.
All internal point-to-point wiring is OFC and the highest-grade parts, consistent with their
application and voltage requirement, have been used in its construction. In especially critical
circuits, the finest quality film capacitors have been used. Among its other design
improvements are the following:
Larger potted transformer provides better overall regulation for driving lower
impedances at high power levels; also provides greater peak-current capability into
loudspeaker loads.
No electrolytic capacitors in the low-frequency signal path or feedback-loop path. Only
precision non-polarized capacitors are used.