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Old 6th October 2008, 09:38 AM   #1
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Default Fault with MOSFET based headphone amp

I have recently built a headphone amp which has developed a fault.

Ono - 2SK170, Which Idss?

I have tidied up the cables within the case and bolted the boards down since first testing it and now the left channel is noticeably lower than the right. It also seems lighter in the bass department (although it's dificult to tell with the lower level).

I have done all the usual checks, visual and gounding checks with no mismatch between the channels. I have checked the pot and also switched the connections on the CDP. Both channels measured exactly the same give or take the odd .1 of an ohm.

Am I looking at component failure? Could it be the CCS network(LM317T)?
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Old 6th October 2008, 03:30 PM   #2
h_a is offline h_a  Europe
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Maybe you haven't notived, but your link points to a phono-preamp thread - not to a headphone-amp.

Have fun, Hannes
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Old 6th October 2008, 04:18 PM   #3
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Oop's, no wonder I haven't had many replies. Thanks for pointing that out. This is the correct link (I hope), post 16 is the CCT digram.

OP Amp based headphone amplifier.
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Old 7th October 2008, 07:51 AM   #4
h_a is offline h_a  Europe
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Well, if both channels measure the same (at DC) I would replace the mosfet.

You already checked that R1 and the 100k resistor have the correct values?

Have fun, Hannes
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Old 7th October 2008, 12:18 PM   #5
Gordy is offline Gordy  United Kingdom
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So it measured equal before 'bolting' it down?

So un-bolt it and measure it again...
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Old 7th October 2008, 12:53 PM   #6
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Done that, no difference. I have also changed the LM317 on the affected side, which did lift the level slightly, but they are still not matched. The original LM317 did show signs of overheating. While the boards were out, I reheated all the solder joints just to be on the safe side, as it could have been a dry joint causing the problem. I will try swapping the MOSFET later. Is there any way to check the MOSFET's with a meter before fitting them (i.e matching them)?
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Old 7th October 2008, 01:57 PM   #7
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Changed the MOSFET and now the inbalence has gone the other way. The right channel is now much lower than the left
(The right channel is much louder than before)
I'll change the right channel just to be on the safe side.
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Old 7th October 2008, 02:16 PM   #8
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Can you report the working points you have? Voltage across each resistor and across the mosfet?
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Old 7th October 2008, 03:29 PM   #9
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I'm starting to think this project is doomed. I am now getting +ve rail voltage at the junction of R2/R3/RG on the right channel (which has a lower output), which would lead me to suspect O/C R3 or short CCT R2. Maybe I have a faulty component in the bias CCT which has caused the MOSFET breakdown. I do hope this hasn't damaged my last MOSFET. I fear the worst............
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