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Old 18th February 2013, 04:55 PM   #71
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yeah checked with multimeter short test yes base and collector are shorted... so how far the amp be biased in this case? I believe thermal runaway was started..

Last edited by rhythmsandy; 18th February 2013 at 05:07 PM.
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Old 18th February 2013, 05:09 PM   #72
llwhtt is online now llwhtt  United States
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Yep, that's the only way to get enought current through the resistor to burn it up, especially a base resistor. You probably experienced thermal runaway, too small of heatsink, too much idle current, and it just took off when it got hot enough.

Craig
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Old 18th February 2013, 05:16 PM   #73
llwhtt is online now llwhtt  United States
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In an AB amp the bias is "usually" set just enough to eliminate crossover spike as seen on an o'scope. But you usually never see that in the service manuals, usually a number is given, so many mv across a resistor or ma across an open fuse holder, whatever. These numbers will get you past the crossover spike but if you were to use the proper test equipment and set the idle current to just to eliminate the spike you will find the number is usually lower than specified.

Craig
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Old 18th February 2013, 06:42 PM   #74
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i did bias it at 500ma and found the heatsink hot the heatsink is an igbt one which measures 150mm length x 126mm width and 126mm height and weighs about 5kg...
but still it gets hot... I thought about 50w class A I can just imagine just with 500ma bias 4w how much hot it becomes...

so I measured across the emitter resistor .22 ohms so is this peak current or continuous?
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Old 18th February 2013, 07:00 PM   #75
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but altogether i enjoying the 4w class A at that time now just biased the other channel to 250ma and still able to get 2 watts of classA at lower volume the treble is very transparent and mids are very involving... i dont know how much precisely the wattage at the output but the heatsink is almost hot...

i am working on LME49830 with about 4 pairs of IRFP240 and irfp9240 which might give better current sharing altogether but I have underestimated the heat problems and thermal runaway with higher bias..
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Old 18th February 2013, 07:05 PM   #76
llwhtt is online now llwhtt  United States
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I don't think I would use either of those terms, peak or continuous, to describe setting the idle current. The idle current is usually described as quiescent.

How are you determining Class A output?

Craig
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Old 18th February 2013, 07:09 PM   #77
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im using the pot which alters the VBE and hence the current in the source resistor.
I=Vsourceresistor/Rsourceresistor
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