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'Loudmouth' Class C 12V power amplifier

Although not intended for 'hi-fi' applications this portable design was capable of pulling a fair punch with excellent intelligibility for speech applications and music.

To reduce crossover distortion, an HF astable running at 30kHz helped to average and thus fill-in the transition period. To maximise power output, the speaker was fed by two output stages, each anti-phase to the other and a transformer winding across the output was used to give a back EMF 'kick' of 24V across the load. Efficiency at maximum output is about 55%.

If, for higher outputs, 40411 output transistors were not available, other 30A types like the 2N3772 could be used instead. Upgrading the transformer secondary is also recommended. Compare to Class D operation.

These were used very successfully with a LM381 preamp and a Technics SL-10 driving a quantity of various speaker types including a Bessell array, sometimes in the remotest of sites, the author being attracted by the limitations set by battery supplies. For safety, fuse to suit load. An automotive bulb can be ideal.

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