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Old 10th February 2007, 01:52 PM   #11
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can you try 22pf across the inputs and 100nf from rail to rail as close tot he chip as you can?
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Old 10th February 2007, 02:47 PM   #12
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Quote:
Originally posted by markiemrboo
Both at the same time? Possibly... I have two probes. It'll be kinda tricky though. Are you thinking that maybe it's the regulators oscillating?
one at a time would do.
Yes, I wonder if there is a little oscillation in the regulator that suddenly increases when the amp starts to do it's thing in sympathy.
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Old 10th February 2007, 02:49 PM   #13
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Quote:
Originally posted by neutron7
can you try 22pf across the inputs and 100nf from rail to rail as close tot he chip as you can?
I can't try the 22pF, as I don't have one. I have put 100nF between the -ve and +ve rail and cut the traces as Andrew suggested. The oscillation appears to have gone!

In fact, either I am missing something or it's now very clean indeed. On the old board with 1uS and no music playing I saw some sorta very tiny amplitude wave, but I put that down to just noise from the scope at the time. Now it just looks like a straight line
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Old 10th February 2007, 03:04 PM   #14
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Quote:
Originally posted by markiemrboo


I can't try the 22pF, as I don't have one. I have put 100nF between the -ve and +ve rail and cut the traces as Andrew suggested. The oscillation appears to have gone!

In fact, either I am missing something or it's now very clean indeed. On the old board with 1uS and no music playing I saw some sorta very tiny amplitude wave, but I put that down to just noise from the scope at the time. Now it just looks like a straight line
Now, that's very odd.
The chipamp experts and users have been designing their PCBs with the grounds connected together and telling me I was talking balderdash, although not quite so bluntly.
Maybe running the different types of returns to an off board audio ground is the better way to do it.

I can confirm that off board star (audio) ground will make multi-channel easier.
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Old 10th February 2007, 03:21 PM   #15
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Quote:
Originally posted by AndrewT

Now, that's very odd.
The chipamp experts and users have been designing their PCBs with the grounds connected together and telling me I was talking balderdash, although not quite so bluntly.
Maybe running the different types of returns to an off board audio ground is the better way to do it.

I can confirm that off board star (audio) ground will make multi-channel easier.
Well, I can definately confirm that the oscillation has now disappeared. The dummy load no longer starts to smoke after being on for all of ~3 seconds with the input shorted either So thanks for the suggestion!

I dont actually doubt that having the grounds connected together works fine.... on a well laid out board - something which my board is obviously not

I may well try connecting the Zobel return back to the power ground, but leaving the input return separate and seeing if it comes back.
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Old 10th February 2007, 04:10 PM   #16
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Is the ground of the chip (pin 7) even connected to ground? If it is it looks to be a rather poor connection.
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Old 10th February 2007, 05:54 PM   #17
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Originally posted by BWRX
Is the ground of the chip (pin 7) even connected to ground? If it is it looks to be a rather poor connection.
Yes. I was using 'thermals' for the polygon fill, but you probably can't see it very well in the screenshot.

While I am here, I dont suppose anyone would be nice enough to explain the rough effect of the big 3.3uF cap between the +ve and -ve rail? I'm thinking of removing it. I can make the PCB's quite a bit smaller without it...
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Old 10th February 2007, 05:59 PM   #18
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You certainly don't need that cap and probably won't notice any difference with it gone.
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Old 10th February 2007, 06:04 PM   #19
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Originally posted by BWRX
You certainly don't need that cap and probably won't notice any difference with it gone.
Good enough for me! Thanks!
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Old 11th February 2007, 12:27 PM   #20
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Just thought I would report back. I tried with just the NFB return separate from power and zobel, but reconnected the zobel return to power ground (if you follow) and it's no worse than having them all separate, so I think I am going to keep it like this!

I tested with the 3.3R and even a 0.47R and it was fine. In fact, for some reason a 40Hz square wave seemed to 'perform' better in to the 0.47R, it didn't seem to slope quite as much (it probably shouldn't slope at all? and if it shouldn't I think I am going to blame that on my sound card for now..) ... which doesn't seem right, but oh well
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