In many areas of the world, keeping an amplifier cool is no big deal, because of generally low average temperatures. In Sydney (Australia), we can guarantee at least a few days every year when the temperature will be over 40 degrees C, and we are not alone in this. With Global Warming, we might all have the same problem in a few years
The project described is not intended to allow you to skimp on the heatsink, but to ensure that the amp is never allowed to exceed a preset temperature. Class-A amplifiers and very high power amps will benefit the most, but the thermal controller principle described can be applied to almost anything.
It is extremely sensitive, and can easily be set so that a few degrees change is enough to turn on a fan, and reduce the power (or turn it off again) as the amp cools.
As of 02 Jan 2000, there is a second version, designed to operate directly from the amp’s +ve supply rail, and is specifically for amps that use a supply voltage greater than 30V. The circuit is almost identical, using one additional low cost transistor and an extra resistor.
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