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Old 26th January 2013, 03:35 AM   #1
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Default How can you tell when you pushed a heatsink far enough?

Wondering if I can get anymore bias current without affecting long term reliability and stability. About 1.2A bias current for this class A amp using SC5200's. Heatsink is is extremely hot, can only stand touching it for about 5 seconds, but I have be playing music for about a hour straight and the amp hasn't died.

Do I need to stop at 1.2A or is there a way to tell if I can dial up the bias current more without killing the output stage?
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Old 26th January 2013, 03:38 AM   #2
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Also, 18V rail.
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Old 26th January 2013, 03:52 AM   #3
gootee is offline gootee  United States
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How high does your scope go? With long untwisted wires, and lots of enclosed loop areas, and no bypass or decoupling caps, maybe the high temps are partly due to high-frequency ringing and/or oscillation.

Anyway, I would not go much higher without better cooling. (You could at least stand the heatsink up, to get some more help from convection.)
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Old 26th January 2013, 04:56 AM   #4
Jay is offline Jay  Indonesia
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Nooo... the heatsink is already too far from the pcb
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Old 26th January 2013, 06:00 AM   #5
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General rule is keep the trannies below 60 degrees for long term reliability.

You can operate them much hotter but then you are trading reliability.

I'll re-iterate what has already been stated above, (1) The heatsink fins need to be vertical. (2) Check that amp isn't oscillating.
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Old 26th January 2013, 06:45 AM   #6
JMFahey is offline JMFahey  Argentina
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Do I need to stop at 1.2A or is there a way to tell if I can dial up the bias current more without killing the output stage?
Only sure way to know is to actually kill it, repair it, and next time never surpass, say, 60% of that dissipation.
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Old 26th January 2013, 06:58 AM   #7
Mooly is offline Mooly  United Kingdom
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I'd say that touching a heatsink for 5 seconds on an amp thats been running for 60 minutes is OK. It's class A, its reached thermal equilibriam and there should be no transients to push dissipation in the junctions higher.

Remember worst case for pure class A is no signal. The louder you turn it up the cooler it runs
A simulation free zone. Design it, build it, test it.
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Old 26th January 2013, 12:23 PM   #8
Calvin is offline Calvin  Germany
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the standards and norms may give a hint.
For audio-video the DIN EN6065 may apply.
It sets for a maximum surrounding temperature Ta of 35°C and 45°C for tropical climate.

max temperature rise Tc in K above Ta for touchable parts:
Knobs, Handles etc, metallic: +30K (normal use) +65K (defunct)
Knobs, Handles etc, nonmetallic: +50K (normal use) +65K (defunct)

casing, metallic: +40K (normal use) +65K (defunct)
casing, nonmetallic: +60K (normal use) +65K (defunct)
values of TC need to be reduced by 10k for tropical climate.

For nontouchable parts basically Ta depends on the specific parameters of the part.

I wouldŽnt want any touchable part above 60°C.

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Old 26th January 2013, 12:57 PM   #9
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Default Heating my Livingroom

I have 5x Yamaha MX-1000 amps. They are Class A & 330 RMS @ Low distortion.

When I lived in Germany I used them to Heat my livingroom in the winter. The heatsinks were hot enough, that it was UNcomfortable to keep your hand touching them. The Heatsinks wouldn't actually burn you, but Almost.

Back to your Ques. You can push your heatsinks UNTIL, the sound degrades To YOUR Ears.

OR get Better Cooling for those Heat Heatsinks.


Last edited by MLStrand56; 26th January 2013 at 12:59 PM. Reason: spelling
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Old 26th January 2013, 01:30 PM   #10
DF96 is offline DF96  England
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I think post 6 has the answer. Unfortunately it is a two-pass algorithm.

Alternatively, get out datasheets and calculator and do some detailed electronic and thermal design.
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