TABLE OF CONTENTS The protection of solid-state amplifiers against overload is largely a matter of safeguarding them from load impedances that are too low and endanger the output devices; the most common and most severe condition being a short across the output. This must be distinguished from the casual use of the word overload to mean excessive signal that causes clipping and audible distortion.
Overload protection is not the only safety precaution required. An equally vital requirement is DC-offset protection though here it is the loudspeaker load that is being protected from the amplifier, rather than the other way round.
Similarly, thermal protection is also required for a fully equipped amplifier. Since a well-designed product will not overheat in normal operation, this is required to deal with two abnormal conditions:
The amplifier heatsinking is designed to be adequate for the reproduction of speech and music (which has a high peak-to-volume ratio, and therefore brings about relatively small dissipation) but cannot sustain long-term sinewave testing into the minimum specified load impedance without excessive junction temperatures. Heatsinking forms a large part of the cost of any amplifier, and so economics makes this a common state of affairs.
Similar considerations apply to the rating of amplifier mains transformers, which are often designed to indefinitely supply only 70% of the current required for extended sinewave operation. Some form of thermal cut-out in the transformer itself then becomes essential (see Chapter 8).
The amplifier is designed to withstand indefinite sinewave testing, but...
