Class A Amplifier Heat Sink Requirements!?

[soldar]

Class A operation is solely determined by the quiescent current vs load demand, not the topology. In this amp the peak output current at which the amplifier comes out of Class A will be approx 2 x the idle (bias) current. This is where one side of the output stage cuts off. You could bias it into Class B, Class AB, or Class A on a continuum.

I think we are all in agreement. I do not think anyone here has suggested operation class (A, B, C, etc) is strictly directly determined by topology but it is a fact that topologies are developed for a certain class of operation and the complementary pair topology is designed for low quiescent operation. That is one of its main advantages and running a complementary pair topology in class A is kind of stupid. It is like running the motor in your car permanently at high RPM and then using the clutch to graduate the power sent to the wheels. You could do it but why would you?


E.T.A.: Just to be clear, I am referring to the complementary pair stage being designed to work in class B but being made to work in class A with excessively high quiescent current.  The previous stages, obviously, work in class A but that is not what we are discussing.

[Gary.M]

There is a good reason to use a complimentary pair in a Class A amplifier, it is more linear than some of the earlier were. I can demonstrate, double blind, listener preference for Class A level bias vs AB in this sort of amplifier. The harmonic structure changes.

[Robbie010]

Just going back to the original question regarding heat sinks.

The build guide for the amplifier states that the amplifier will run hot, almost to the point of being too hot to touch. As the operating parameters of the transistors alter at various temperatures, is that a purposeful part of the amplifier design? i.e. is it meant to run at a high temperature in order that the transistors operate within a specific range?

And, if so, that begs the question of whether over cooling the amplifier is a bad thing?

[Gyro]

No, it's a 'side effect' of the high dissipation. Running hot (certainly cycling from room temperature to hot) reduces reliability, so you want as much heatsinking as possible. Unfortunately heatsink performance increases with temperature so they are always going to get pretty warm.

The high temperature has another 'awkward' effect in that device characteristics shift with temperature. You normally need to adjust output stage standing current after it has reached full operating temperature, then wait until any temperature change resulting from the current adjustment and removing the lid has stabilized, then check/adjust again. It can take a while.

If you really want Class-A then valves are good. Radiative cooling is far more efficient than convection cooling, but that's a whole other tangent! 

[soldar]

You cannot have too much cooling. Come to think about it this would be a good candidate for liquid cooling or for building a huge metal sculpture to use as space heater plus amplifier.

I hate to think of the energy cost to run this thing but to each his own.