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Old 10th December 2002, 02:08 AM   #1
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Default problem with Aleph 3

I noticed that with my home built Aleph 3, one of the output mosfets (not the current source mosfet) is much hotter than the others. I had matched the mosfets, but evidently did not do a good enough job.

The solution is to take the mosfets out, and put in a better matched set of mosfets....But in the mean time, is there anything I can do, like changing a resistor, to make the output mosfets share the load better?

I'm afraid that running the amp the way it is, will cause a failure in the hot mosfet leading to DC current finding its way to the speakers causing an expensive accident.

Any advise is greatly appreciated....
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Old 10th December 2002, 02:10 AM   #2
BrianGT is offline BrianGT  United States
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Default Re: problem with Aleph 3

Quote:
Originally posted by audionut
I noticed that with my home built Aleph 3, one of the output mosfets (not the current source mosfet) is much hotter than the others. I had matched the mosfets, but evidently did not do a good enough job.

The solution is to take the mosfets out, and put in a better matched set of mosfets....But in the mean time, is there anything I can do, like changing a resistor, to make the output mosfets share the load better?

I'm afraid that running the amp the way it is, will cause a failure in the hot mosfet leading to DC current finding its way to the speakers causing an expensive accident.

Any advise is greatly appreciated....
Have you checked the values of your source resistors connected to the output devices, are they fairly close in tolerance? Have you compared the value of the voltage over the source resistors? They should all be the same.

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Brian
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Old 10th December 2002, 02:43 AM   #3
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You don't say how much hotter. I suggest that you
measure the DC voltage across the source resistors
of all the Mosfets and report back.
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Old 10th December 2002, 02:49 AM   #4
halojoy is offline halojoy  Sweden
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Default problem with Aleph 3

Quote:
Originally posted by audionut
I noticed that with my home built Aleph 3, one of the output mosfets (not the current source mosfet) is much hotter than the others. I had matched the mosfets, but evidently did not do a good enough job.

-----------------------------------------

Any advise is greatly appreciated....
You could match the MOSFETs under conditions closer
to the working condition they will operate UNDER

Otherwise you can measure and adjust
until they fits fairly good together
as the boys already have told you

this is my any advice
a lot of good ideas, not only my idea
but Nelson and the others, in the thread

here, I searched and found him
an old technique

there are several schematics
ask before you use them, just in case
Nelson's will work safely to 100%

matching of mosfets

there are several schematics
ask before you use them, just in case
Nelson's will work safely to 100%

FUZZY on Matching FETs
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Old 14th December 2002, 03:08 PM   #5
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The hot mosfet is quite a bit hotter than the others...On its source resistor I measured a voltage drop of .95v the other resistors had voltage drops of around .45v. I measured the current going through the hot mosfet and it was 1.75amps while the other mosfets measured around 1 amp.

Until I rematch the mosfets, is there anything I can do to bring the current draw down for this one mosfet?
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Old 14th December 2002, 06:21 PM   #6
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I think you could place another source resistor in series with the one on the hot FET. This should reduce the current of the FET.
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