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Joined: 9/20/2012(UTC) Posts: 13  Location: london
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Hi All, I have not long built my Buffalo Dac, which is playing music directly into my power amp and sounds fantastic! I do however have a couple of things I need some help with. I'm using SE out of the Legato but would like to try balanced out. The right channel is trimming down to 0v across + and - but left channel won't trim below .520v. Single ended outs both trim down to 0V (though I notice if i measure it a few days later it may drift up to .002v - is this normal or anything to worry about?) I've also noticed a very faint high pitched noise on this left channel (I've tried swapping outs wiring but just moves noise to right speaker) This is only audible when music isn't playing and when close to the speaker, but it's annoying enough. Could this be related? I've tried re-flowing the solder in case there is a dry joint but no luck. Any other suggestions please? Thanks Andrew Edited by user Wednesday, June 5, 2013 1:45:40 PM(UTC)
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Rank: Member
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Joined: 9/20/2012(UTC) Posts: 13 Location: london
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Things have changed for the worse since my tinkering this morning - all though it may help identify problem. I now have 3.3v across the SE left channnel and 4.6v across Balanced and I can't trim either. The faint high pitched sound has gone - it's now a low but louder hum. I can still play music but need to get this sorted. Please help.
Andrew
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Rank: Member
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Joined: 9/20/2012(UTC) Posts: 13 Location: london
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Things have changed for the worse since my tinkering this morning - all though it may help identify problem. I now have 3.3v across the SE left channel and 4.6v across Balanced and I can't trim either. The faint high pitched sound has gone - it's now a low but louder hum. I can still play music but need to get this sorted. Please help.
Andrew
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Can we get a picture of the rig ?
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Rank: Member
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Joined: 9/20/2012(UTC) Posts: 13 Location: london
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Hi, Here are some pics of my build. Right channel is working like a dream, left has DC on SE and Bal that[img]null[/img] I can't trim along with some low level mains type noise. Plays music very well otherwise. Thanks Andrew andy360 attached the following image(s):  IMG_2509.JPG (110kb) downloaded 31 time(s). IMG_2510.JPG (100kb) downloaded 21 time(s). IMG_2511.JPG (104kb) downloaded 28 time(s). IMG_2513.JPG (84kb) downloaded 25 time(s).You cannot view/download attachments. Try to login or register.
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Rank: Administration
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my first guess is a damaged FET (QP1-QP12) on the left channel. They are tough to check while installed, since they are in parallel. You can compare pin to pin resistance readings between the two channels though (with the power off).
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Rank: Member
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Joined: 9/20/2012(UTC) Posts: 13 Location: london
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Hi Brian,
I checked resistances, there were some slightly higher resistance readings in the mosfets on the left channel, but these could be influenced by surrounding components like the trim pot maybe?
Would I be best to replace the six mofets on the left channel do you think or is there anything else I can check?
I anticipate they will be difficult to remove!
Thanks
Andrew
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Rank: Member
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Joined: 9/20/2012(UTC) Posts: 13 Location: london
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Hi, I replaced all six FETs along with the 3 x BC550 transistors on the left channel. Still have DC offset and it seems to be getting worse - cannot trim below .950v on balanced and I can no longer trim SE to 0v. There is a high pitched electrical noise and a deeper mains hum, along with an occasional rustling sound from the speaker. Also the occasional pop. Plays music otherwise. I also noticed while checking voltages that my Placid HD voltage dropped down to 3v on that channel for a short period then later went back to normal (maybe something shorting on the legato?) Not sure what to do next? Andrew Edited by user Saturday, June 15, 2013 1:38:33 PM(UTC)
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Ok, that sounds like a bad solder joint. You can dab a little flux on each one and reheat with your soldering iron.
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