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tranny problem, looking for advice on a power amp
post Mar 23 2013, 08:47
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Hi and thanks for reading this post.

My amplifier has been playing up on me and i noticed a swollen capacitor which i replaced with a new one of the same voltage and capacitance.

When reassembled the amp would not work at all, after a few hours of experimentation i noticed a row of transistors which are clipped onto the metal chassis of the amp, obviously this was done by the manufacturer to keep the transistors cool and i had to unclip these to get the board out.

When the transistors are clipped back onto the chassis of the amp so in effect they are all sharing one large heatsink the amp does not work, i gave each transistor its own heatsink and now the amp works fine.

Can anyone tell me what is going on.

Many thanks in advance. John.
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Glenn Gundlach
post Mar 23 2013, 10:39
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You do NOT want individual heatsinks for the transistors. Parallel bipolar transistors are not exactly matched and one of them will run higher current than the other(s). The higher current will increase its temperature which increases its sensitivity and cause it to take MORE of the load current which will make it get hotter and hotter and hotter until it fails from over temperature. BTW even if you had perfectly matched transistors, the warmest one will eventually take the load and heat up more. The only solution is to thermally link them and add degeneration resistors in the emitter circuits to distribute the load currents. I'm not sure what happens in FET power transistors but if the manufacturer had it working on a common heatsink it should continue to work that way.

Did you replace the heat sink grease (if used) and get the devices securely anchored?

As for capacitors, there is more than just uFd and Voltage. You want low ESR (Equivalent Series Resistance / highest ripple current) to minimize internal heating as heat is what kills electrolytic caps. This is mildly a problem at 60 Hz but critically important in switching (high frequency) power supplies. The DigiKey site lets you easily sort for things like ripple current / ESR.

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