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Old 8th September 2013, 12:56 PM   #1321
6L6 is offline 6L6  United States
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I see... It's the mains inlet.

Anyway, great build!
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Old 9th September 2013, 02:59 PM   #1322
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Everything going so well until...
I noticed a low level mains frequency hum on the right channel along with low level white noise. Left channel is silent as the night.
Haven't got a scope so any ideas? Difficult one this. My feeling is that it is the power supply but I have bypassed the diodes with a bridge and still the hum. Checked grounds and they are fine. Power transformers swapped and still same problem. Both power supplies are reading about 26V on each rail and no sign of AC.
Both channels are playing music quite happily and to high standard in fact I could live with it but it does suggest something ain't right. I am happy (not very happy really) to change the power supply rectifier circuit but has anyone experienced the white noise and hum that didn't come from the power supply? I haven't seen anyone mention this so far.
Incidentally offsets at Power Board and front end are fine as are all the other readings.
Moved transformers around chassis and this makes no difference.
Oh I forgot to mention; it is raining again!

Last edited by Backbones; 9th September 2013 at 03:01 PM.
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Old 9th September 2013, 07:04 PM   #1323
Eric is offline Eric  United States
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Two things come to mind quickly: 1) more uF in power supply, 2) move some wires around - bend/wiggle them in different directions to see if anything changes.
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Old 9th September 2013, 07:32 PM   #1324
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I've been bending and wiggling all day and nothing. uF level is ok on the other channel and on other builds so I don't think the problem lies there.
I'm going to have to power up the right channel from the left supply and see if the problem is still there. If it is I have a spare BA-3 front end to see if that makes a difference. Lots of work tomorrow I think.
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Old 10th September 2013, 11:25 AM   #1325
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Swapped rectifier circuits this morning and same hum/white noise present.
So bias board, power board and front end next.
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Old 10th September 2013, 01:30 PM   #1326
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Replaced front end with spare board and the hum and white noise have vanished. Does anyone have any ideas where hum and hiss could be coming from in terms of BA-3 front end components?
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Old 10th September 2013, 02:00 PM   #1327
Eric is offline Eric  United States
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Sounds like it was an open connection somewhere. A missed/cold solder joint? I would carefully inspect the board you removed and see what you can learn from it.
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Old 10th September 2013, 02:05 PM   #1328
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Agree with you Eric. Went over all the solder points with the soldering iron without any success last night. I could do it again of course.
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Old 12th September 2013, 06:03 PM   #1329
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Well after replacing the jfets, the caps, testing every trace and connection without success over a couple of days the very last things I got round to were the output mosfets, SJ313 and SK2013. I knew I was finally there when the replacement made the interference worse. I thought they might be phony components so I tried a different batch of mosfets and the hum and white noise have gone.

Last edited by Backbones; 12th September 2013 at 06:06 PM.
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Old 12th September 2013, 09:39 PM   #1330
bcmbob is offline bcmbob  United States
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Congrats! Happy you were successful at drilling down to the culprit.
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