The amp on the portable speaker I’ve been using for years for basement square dancing felt exceptionally hot after a dance recently. Sound was still fine, but even after sitting for a few minutes with no audio input, the fan was blowing dangerously hot air.
Originally designed to run off 12V for a square dance float for a local parade, the amp is a 2x20W TDA 8563 with a little heatsink and a fan on a home made PCB, 
mounted on top of an old speaker enclosure hacked to (barely) hold two big ugly speakers. (The speakers were originally both coax, but I pulled the tweeter on one because it sounded too harsh overall. Each channel of the amp drives one speaker.) It’s OK on 12V, but it’s better with a couple more volts. I got a little 4V gel battery and wired connectors to hook it in series with the 12V that lives on the speaker when I needed a little more volume. But since most of its use is comfortably indoors, I found a 16.5V laptop supply and use that most of the time.
When I got it home I poked a thermocouple junction into a corner between the amp IC and the heat sink. Running with the AC supply and cranking it up, I watched the temp hit 230F after a few minutes. That’s about the absolute max temp, so it’s pretty bad. A little playing showed that if it ever got above around 120F, it would fairly quickly hit 230, regardless of whether there was any audio input. Thermal runaway! That must have been what I saw (felt) at the last dance. If I ran from a 12V battery (at 12.6V) thermal runaway didn’t occur. Hmm – I really want the greater volume a higher supply voltage gives, but I can’t let it get that hot. What to do?
I found a pretty blue heatsink in the junk box that provided maybe 3 times the surface area and could be made to fit. But as I was taking the old one off, I noticed it was loose – providing poor thermal contact. Was that the problem? I’m quite sure it never used to get that hot.
Since I mostly wanted a working system, I went ahead and mounted the bigger heat sink. I did make sure it was bolted down nice and tight this time 🙂 You can see the thermocouple probe on the left.
But the higher voltage supply still spooked me a little. Abs max voltage is 18V, so the 16.5 should be fine, but I could afford to cool it down a little. I made up a little M-F extension to go in series with the AC supply with two several-amp rectifier diodes in series. Let’s see – 1.6V at maybe an amp is a watt or two. I’ll expect them to get fairly warm, but they should handle that no problem. They dropped the 16.5V to 14.9 under load, and I felt better. Test time.
To my delight and amazement, I could crank the newly-heatsinked amp up to full (undistorted) volume and even with the AC supply and without the little diode voltage dropper, I could never get it above about 95F! I still don’t know how much the better heat sink contributes, and how much getting rid of the bad thermal connection helps, but I’ll take it.
While that little speaker/amp has been a real workhorse and has never failed, since I’d had it all apart, I did bring a spare setup to the next dance. (The spare was not needed.) I kept a close eye (well, finger) on the new heatsink, but it never got more than a little warm. Success!