JL 500 /1v2 Blown PS still plays - diyAudio
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Old 23rd October 2011, 09:28 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default JL 500 /1v2 Blown PS still plays

I put this amp on the bench to test before opening it up. it powers and plays very very softly; like 30db or so before the amp distorts badly. The power supply doesnt drag down and is always able to supply full v14v voltage at very low amperage draw.

So I pop the back cover, and find all the PS fets and gate resistors have gone bad. All IRFZ44N PS Fets are shorted from legs 1&2, and all 47ohm thru-hole resistors are burnt beyond recognition.

Two questions:

1. I have IRFZ44E which are rated higher than the N verson. I'm sure they are OK to use in most amps but this being a JL Audio Class-D I wanted to make sure.

2. Why did this amp even power and play anything at all? It is not like any proper rail or regulated voltage was anywhere to be found (I didnt look actually; since the PS was frankly blown to bits).

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Old 23rd October 2011, 09:32 PM   #2
timwebb is offline timwebb  United States
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there could be one or two irfz44 still operating.
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Old 23rd October 2011, 09:34 PM   #3
Dr Zeus is offline Dr Zeus  United States
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Half answer to #2, it appears this amp does make +-14.5v regulated even after I remove all the failed PS fets. The rail voltage is still garbage and appearing to only be +-0.4v.

With all the failed PS Fets clipped off te PCB, the amp powers and plays with massive distortion. I really dont understand why it would power or play anything?
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Old 24th October 2011, 12:36 AM   #4
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The amp doesn't have any way to tell that the power supply is blown. As long as there is no critical fault (excessive current draw or DC offset), it powers up. This includes the low voltage/driver power supply.

The low voltage supply produces enough voltage to allow some audio to be produced but nothing of significant power. I don't remember this happening on the class D amps but on the class AB amps, I know they will produce low level audio with a blown power supply.

The 44E should be OK. The problem with class D and transistor substitution is mainly related to the output transistors, not the power supply. That doesn't mean that you can drop any FET into the power supply but it's less critical than the outputs.
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Old 24th October 2011, 06:41 PM   #5
Dr Zeus is offline Dr Zeus  United States
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I totally fixed this amp with new IRFZ44E and gate resistors.
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