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Old 18th February 2006, 03:56 PM   #1
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Default Noise in one channel of Ono

I am assembling the Aleph Ono, with Bakmeel’s boards.
The amp is playing nicely but one channel has a rather high amount of noise.
It is best described as the effect of a microphone picking up wind.
One channel has a noise figure of –65/-60db, the other one –57/-53 with peaks( the ‘wind’) up to –40db. (inputs shorted)
The scope tells me that only the output stage is affected, being collector Q6 and Q17, and sources of Q7 and Q18.
I marked the noisy area in green on the schematic detail.
I swapped the mentioned transistors and fets from the good channel to the bad one with no result.
I will replace some, if needed all resistors in that area unless someone can come up with a fresh idea.

/Hugo
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Old 18th February 2006, 09:21 PM   #2
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suspicious devices:

pot (first on the list -always)

caps (smaller ones)

resistors




after replacing pot,if that don't cure your problem,replace caps ,one by one;
do not bother with exact value,put whatever you have close-just to isolate exact part
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Old 19th February 2006, 08:26 AM   #3
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choky,
Thank you, I think I narrowed it to a bad soldering somewhere, probably in the supply line. I recall having a hard time to solder the board. They are quite old and travelled a lot. I should have cleaned it beforehand as some areas seems to have picked up light corrosion.
Tapping the board or lightly wringing it cures the fault but not permanently.
All parts are brand new, pots are good multiturns so I'll stick to a major resolder session for the moment.
Will keep in touch.

/Hugo
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Old 19th February 2006, 10:52 AM   #4
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Some parts are indeed tricky to solder.

Also check that the drain pads of the output fets (Q7 and Q18 in your drawing) do not touch the power supply rails.

Q6 has a solder connection on both top and bottom side.

R53 is even more critical in that aspect. One of its pins acts as a connection between top and bottom, make sure you have that one right.

Hope this helps.

Cheers,
Bouke
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Old 19th February 2006, 12:55 PM   #5
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Bouke,
Thanks, indeed the board is tricky at some points but that's part of the fun.
I found the mischief and choky was right. I put a multiturn in place of R4 to control the output DC. Replace that part with a fixed resistor solved the problem.
It is however beyond me why only one channel behaved like that.
I even swapped the multiturns but the same channel kept misbehaving. Nothing wrong with the pot after all.
Anyway, problem solved, thanks to both of you.

/Hugo
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Old 6th March 2006, 02:18 PM   #6
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Aiming for more perfection (don’t we all) I measured the Ono and wonder if the noise seen on the graph could be further reduced. Peaks occur on 50, 150, 250, 350Hz and so on. I played with different ground settings connections but can’t reduce more than this. Any ideas? Or is this inherent to the Ono?
Inputs are shorted.

/Hugo
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Old 6th March 2006, 02:19 PM   #7
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This is the soundcard’s noise floor:
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