I'm trying to choose a heat sink for an audio amp I'm building.

In simulation the 4 output transistors dissipate about 27W each at maximum output (200W) with a sine wave input. It's unlikely I will run it at full output, and I'm assuming music has a lower RMS level than a sine wave with the same peak voltage, but I'm wondering about unintentional overloads (hooking up speakers incorrectly) and how fast it takes a transistor junction to go up in temperature. If I size a heat sink for the normal maximum output, but an overload condition that would raise the junction temperature too high lasts for just a couple seconds, could that still be enough to fry a transistor? I think if such an overload occurred, I would notice it immediately and stop the signal, which really just means stopping playing since it's an instrument amp.

My main goal here (after protecting the output transistors) is saving money on heat sinks.

 

With a heat-sink, the transistor thermal time constant should be many seconds, so you should be okay for momentary overload with a heat-sink that will handle the maximum music output you anticipate.

But note that heat sinks are rated as temperature rise from ambient, so you need to anticipate the maximum ambient temperature next to the heat sink when the circuit is operating.
A small muffin fan blowing on the heat-sink greatly reduces the size of heat-sink required.

 

OK thanks. I was hoping to find heat sinks whose fins can be outside the enclosure while the transistor is inside. Those might be more expensive though.

 

OK thanks. I was hoping to find heat sinks whose fins can be outside the enclosure while the transistor is inside. Those might be more expensive though.

I built a power supply with the heat sinks and power transistors mounted on the outside back of the enclosure.
You just have to run the wires to the transistors through the back of the enclosure to the transistor pins, which should have no significant effect on transistor or circuit operation at audio frequencies..

 

I built a power supply with the heat sinks and power transistors mounted on the outside back of the enclosure.
You just have to run the wires to the transistors through the back of the enclosure to the transistor pins, which should have no significant effect on transistor or circuit operation at audio frequencies..

I'm going to use TO-3 transistors, so the collector voltage would be exposed to fingers. How do you protect against that? Is there some sort of cover I could put over the transistor?

What I'd really like is a heat sink like this that's in an old stereo I have:





I just can't find one as a stocked part.

 

...........
so the collector voltage would be exposed to fingers. How do you protect against that

If the voltage is no more than 50V or so, it's not dangerous.
Otherwise you could use a cover like this.