How thermal drift occurs in amplifier?

Answer 1

As with all things, temperature does change things.

With amplifiers, the changing temperature in the power transistors running the speakers can become a ruining experience.

The louder you play, the more heat is generated in these transistors.

The hotter they get, the less current do they stand.

Some transistors may be able to push forward 160-200 Watts of music at 25 degrees Celsius. As they get warmer, their level of 'performance' must be put down according to temperature. At 50 degrees Celsius, they may be able to withstand 120 Watts of music. at 100 degrees, they may only be able to give 60 watts of music. At 120 degrees, they are close to the transistors internal breakdown temperature and only 10 watts of music may be destructive.

Good cooling of power transistors in amplifiers help avoid destroying an amplifier but it does not fully solve the problem of temperature drift.

A mayor issue is that an amplifier that does not consume power (like when no music is played) will not be able to start instantaneously to play good music. We will experience a delayed action that is very well audible. In a class B or a class AB amplifier, this is fixed by always having the amplifier to work, they just work on making silence, constantly having a small current over the power transistors.

This small current however is dangerous for the transistors. The transistors try to 'amplify' and in this process, all of this current is transformed into heat. The big problem is that as transistors get warmer, their ability to amplify increases.

A small current of 25 mA may generate 2 Watt of heat at 25 Degrees Celsius, but at 75 Degrees Celsius, the same small current of 25 mA may generate 20 Watt of heat. The process is self destructive. As transistors heat up, they generate more and more heat and get more and more out of the little start current of 25 mA. In the end, they burn up internally.

It is important that the small current decreases with the increase of temperature, otherwise your amplifier will quickly be destroyed. This can happen in a matter of minutes only, even with quite a decent cooling profile and even a fan attached.