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Old 17th May 2008, 10:12 PM   #1
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Default Thermal runaway question

I built this amp X2 (see attachment) as a two channel car amp

It works great, I've biased it to about 20ma

If I push the amp I keep having thermal runaway. Obvoiusly I've done something stupid somwhere...... and it's cost me a small fortune in output transistors. I've tried almost no bias as well (5ma)with the same result.


Is TR10 supposed to be attached to the heatsink?

Any ideas?
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Old 17th May 2008, 10:29 PM   #2
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I would surmise D1,D2, and D3 should be on the heatsink
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Old 17th May 2008, 10:35 PM   #3
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The bias components (transistor and diodes) need to be attached to the heatsink. You may be able to mount only the transistor on the sink for proper compensation but having all of them should provide better compensation.


Insert 5 amp fuses in the positive and negative rails between the supply and the amplifier. This will allow you to run the amp hard enough to make it heat up but should protect the power supply and the outputs. Drive it as hard as you can without blowing the fuses. Every couple of minutes, remove the input signal and re-check the bias current. Do this until it gets as hot as you intend to run it. If you have thermal protection, continue to check the bias current until it reaches thermal shutdown.
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Old 18th May 2008, 10:59 AM   #4
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Thanks Perry

Will try that.

I've realised again just how destructive thermal runaway can be. Fuses are essential.
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Old 18th May 2008, 11:12 AM   #5
luka is offline luka  Slovenia
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Hi

I would even try without fuses, + since they are so cheap and effective
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Old 18th May 2008, 04:29 PM   #6
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Default Is this circuit correct?

Is this circuit correct?

I have moved the transistors and diodes now, and it's a bit better.

What seems to be happening is both rails of output transistors turn on at high volume or high power transients, just below this level I can let the amp get very hot and it has no problem.

Only occurs with peaks
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Old 18th May 2008, 04:47 PM   #7
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If the bias current holds at all normal operating temperatures but it spikes for a few seconds just after a period of very high power output, the bias components are not close enough to the output transistors. The temperature of the output transistors is increasing but the temperature of the heatsink where the bias components are mounted is significantly cooler than the output transistors. Move the bias components as close to the outputs as possible.

This can also happen when the insulator material between the transistor and the sink is inefficient. The transistor sees a spike in temperature and it takes longer than it should for it to pass the heat to the sink. As the transistor returns to the temperature of the heatsink, the bias current returns to normal.
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Old 18th May 2008, 06:18 PM   #8
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Maybe I should those thermaltrak output transistors I have lying around.

It's awkward to move that whole circuit to the heatsink, what would you reccomend is the minimum components?

Thanks for the advice so far.
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Old 18th May 2008, 06:42 PM   #9
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You should try heating the bias circuit components individually to see which one causes the greatest change in bias current. Heat the diodes as a group. If heating the transistor alone creates a significant change in bias current, try relocating only the transistor. If both the diodes and the transistor make a big difference in bias current and moving the transistor alone doesn't provide enough compensation, you'll have to move all of them.

If you mount them on a small piece of proto-board, they will be easier to relocate as a group.
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Old 18th May 2008, 06:53 PM   #10
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I deserved that nice logical answer!

I could relocate most of it with two wires and proto board.

Once I get that right, I'd like to increase the VAS (not sure the drivers are being driven hard enough)

Once it's stable I will post the PCB I have done, which was designed to fit into a standard type car amp heatsink
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