Dead MJL3281A

jandr49 · 02:39 PM

Hi

I'm building Symasym 5.3 as a 1st major project - I've powered it up a few days ago with no drama - bias set up easily at 55mA - Its run continuously for 3 hrs (not loaded) with no problem. No oscillation on either channel. The only thing that was not quite right was the DC offset which was 7mV and 4 mV which I put down to poor transistor matching as I confess I did not pay enough attention to it.

I've spent some time last night matching some transistors and today powered it back up to remind myself which channel had the largest DC offset. It immediately blew the fuses in the left channel power supply. I've so far established that the MJL3281a is short circuit whichever way I measure but before I start dismantling it further could anybody point me in the right direction to find out why this has happened

Thanks

Richard

Idefixes · 3rd May 2010, 02:57 PM

7mV and 3mV are quite very low offset amount. Under 40mV it's generaly admitted as good.

Marc

jandr49 · 3rd May 2010, 03:07 PM

Thanks that makes me feel better about that aspect of it - I was judging it on MB's symasym page where he states good matching would achieve <2mV

Richard

3rd May 2010, 02:35 PM	#1
jandr49 diyAudio Member Join Date: Feb 2010	Dead MJL3281A Hi I'm building Symasym 5.3 as a 1st major project - I've powered it up a few days ago with no drama - bias set up easily at 55mA - Its run continuously for 3 hrs (not loaded) with no problem. No oscillation on either channel. The only thing that was not quite right was the DC offset which was 7mV and 4 mV which I put down to poor transistor matching as I confess I did not pay enough attention to it. I've spent some time last night matching some transistors and today powered it back up to remind myself which channel had the largest DC offset. It immediately blew the fuses in the left channel power supply. I've so far established that the MJL3281a is short circuit whichever way I measure but before I start dismantling it further could anybody point me in the right direction to find out why this has happened Thanks Richard Last edited by jandr49; 3rd May 2010 at 02:39 PM. Reason: Typo

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3rd May 2010, 02:57 PM	#2
Idefixes diyAudio Member Join Date: Dec 2003 Location: Mulhouse - Alsace	7mV and 3mV are quite very low offset amount. Under 40mV it's generaly admitted as good. Marc

3rd May 2010, 03:07 PM	#3
jandr49 diyAudio Member Join Date: Feb 2010	Thanks that makes me feel better about that aspect of it - I was judging it on MB's symasym page where he states good matching would achieve <2mV Richard

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