The heavy weight of the older bipolar transistor power amplifiers was due to the fact that they had to incorporate a heavy "iron" AC power transformer. Since bipolar transistors are Current devices, that meant being able to supply the amplifier itself, which typically weighs less than a pound or two (driver circuit board plus output transistors and lightweight aluminum heatsink) with a voltage lower than the AC power input, transformed to high Current. At 50-60Hz, that requires a lot of iron laminations in the power transformer.

The weight savings in today's designs come from using modern Switching Power Suppy desings, which, while still needing to supply the lower voltage at a higher current, can do so without the need for the big mass of iron because the Switching Supply first rectifies the incoming AC to DC, then "chops" it up into AC at a much higher frequency, typically around 40KHz or higher. They still use transformers, but at that higher frequency the transformers are very small indeed by comparison.

The use of the FET (Field Effect) Power Transistor intead of the older Bipolar Silicon transistor also contributes to the lowered cost and weight of the modern power amp. Means we can control more power with less devices. And a few other good things.

An amplifier of any kind is but a modulated power supply.


--Mac

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