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Old 20th May 2004, 08:26 PM   #1
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Default Looking for successful Zobel cases

Since it seems like Zobel issue becomes more and more controversial, I'm looking for cases when implementing a zobel network in a chip amp helped to stop oscillations.

Please post here a link to any posts (on this forum, or the other, Chip Amps forum) that reported fixing the oscillation problem when Zobel was added.

I'm offering Brian's board to anybody that submits such link.
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Old 20th May 2004, 08:40 PM   #2
grataku is offline grataku  United States
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Are you "collecting the facts" about zobel? Fred's gonna get you, too.
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Old 20th May 2004, 09:12 PM   #3
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Well, I'm making sure if my warranty isn't too generous. And my research is not "for free"
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Old 20th May 2004, 09:15 PM   #4
moamps is offline moamps  Croatia
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Hi,
on the picture below there is a characteristic example of LM3875 instability (oscillations within the range of a few MHz) when driven a bit harder. When the Zobel network is applied, the instability disappears. It would be a good idea for the people who have apparently never seen this to say what equipment they are using for their measurements. That is, it may well be the case that they have not been able to detect this instability due to the poor quality of the equipment.

Regards
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Old 20th May 2004, 09:24 PM   #5
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It would be also interesting to know what circuit, and how implemented, they use. I had also problems with instability, but it was fixed after changing grounds layout.
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Old 20th May 2004, 10:13 PM   #6
grataku is offline grataku  United States
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Sorry Mo, but no boards for you!
That a textbook oscillation that I can even successfully produce on the AlephX under certain circumstances...and when I do it doesn't go away with a zobel.
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Old 20th May 2004, 10:31 PM   #7
moamps is offline moamps  Croatia
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Quote:
Originally posted by Peter Daniel
......what circuit, and how implemented, they use.....
p2p, IGC, metalfilm resistors 22k/220k, Panasonic elcos 1000uF/100V +100nF on IC, standard bridge, 300VA/48Vsec CT transformer

If anything, I believe I know how to implement grounding correctly.

Regards
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Old 20th May 2004, 10:40 PM   #8
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Quote:
Originally posted by moamps
Hi,
on the picture below there is a characteristic example of LM3875 instability (oscillations within the range of a few MHz) when driven a bit harder. When the Zobel network is applied, the instability disappears. It would quipment they are using for their measurements. That is, it may well be the case that they have not been able to detect this instability due to the poor quality of the equipment.

Regards

Moamps,

Your scope trace didn't come out very clear.
I think increasing the intensity of your trace, or turning the background lighting down a bit would give a clearer picture..

So anyway, are you referring to the blurry region at the peak and trough of the sinewave as the MHz oscillations???

If yes, then what are your timebase and sensitivity set to on the 'scope, and what frequency is your input signal??

I'd like to try and reproduce this 'problem' on my GC, and would appreciate some information on your testing parameters.

SteveM
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Old 20th May 2004, 10:41 PM   #9
moamps is offline moamps  Croatia
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Quote:
Originally posted by grataku
Sorry Mo, but no boards for you!
That a textbook oscillation that I can even successfully produce on the AlephX under certain circumstances...and when I do it doesn't go away with a zobel.
Hi:
Board or no board, the Aleph doesn't need a zobel because of its topology. If it oscillates, this indicates the presence of a positive feedback (which is independent of the load) somewhere in the circuit. IMO

Regards
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Old 20th May 2004, 10:48 PM   #10
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Quote:
Originally posted by moamps


If anything, I believe I know how to implement grounding correctly.
Seeing your previous work, I don't doubt that. I was rather referring to other people amps.
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