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Old 30th June 2005, 03:29 AM   #1
Sheldon is offline Sheldon  United States
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Default buffer noise

Hi,

The info I seek may be buried here somewhere, but I couldn't find it. The problem is a slight amount of hum. By slight, I mean about 3mv ac on the output of the amp (both inverted and non-inverted). I didn't care much before as the speakers I were using were not real efficient and the hum was low enough to be inaudible until very close. I now have an application with more efficient speakers and would like to minimize hum.

The chip-amp part, built on Brian GT's original boards doesn't seem to be the problem. Both amps have discreet jfet buffers, ala Pedja. If I short the buffer output to the amp, I get a fraction of a milliamp hum at the ampt output. No change (that is, the same 3ma or so)if I short the input to the buffer. So, the slight hum I get is from the buffer itself. Any obvious layout issues anyone is aware of, that I should check before I dig in and start probing around? One buffer is powered by three-terminal regulators on the board the other set is powered by a discreet ps separate from the buffer board. Both amps use Pedja's discreet regulator supply as the main supply.

Sheldon
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Old 30th June 2005, 03:44 AM   #2
SY is offline SY  United States
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Is it 50/60Hz or 100/120Hz? Different causes. Yes, 3mV is much too high.
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Old 30th June 2005, 05:22 AM   #3
Sheldon is offline Sheldon  United States
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Sounds like 120 to me. Inadequate ps filtering?

Sheldon
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Old 30th June 2005, 05:27 AM   #4
SY is offline SY  United States
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It could be (I don't know what the power supply rejection is like for that circuit), but more likely, it's a grounding issue. Power supply ripple is being impressed on the signal.
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Old 30th June 2005, 02:39 PM   #5
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by SY
It could be (I don't know what the power supply rejection is like for that circuit), but more likely, it's a grounding issue. Power supply ripple is being impressed on the signal.
You're probably right on that score. I think the psrr is fairly good. And in any event, there ain't much ripple. If I use my multimeter and put a 0.1uf cap in series with the probe, I measure about 3mv. Plus that supply on the one amp then supplies 7815/7915 for the buffer, I measure about 1mv there. I put caps after the regulators (about 10uf, I think) and I wonder if that might be a source of the ground noise. I'll play today. I can test with 9v batteries too.

It's a bit mysterious, as I have the grounds lifted from the mains ground with a 25ohm resistor, but maybe I've made some other error. Time to dig in.

Sheldon
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Old 4th July 2005, 09:04 PM   #6
Sheldon is offline Sheldon  United States
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Well, I dug in. Checked out everything I could think of. The grounding scheme seemed fine. One last test. I disconnected the power supply and powered it up from another clone power supply. Noise went from 3mv to 0.2mv.

Conclusion? I've just got too much stuff in too little space and I'm getting inductive coupling somewhere. Probably doesn't help the case is aluminum (one of Peter's nice cabinets). It's the buffers that seem to pick up the noise. In the second amp (the one I used for power) I get about 0.6ma noise, but the buffers in that one are further away from any ac source. Looks like the solution will be to make a separate case for the power supply.

sheldon
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Old 5th July 2005, 06:11 AM   #7
fallow is offline fallow  Finland
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Could you put shematic and layout pictures here?
It would be much easier to track the problem with the grounding.
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Old 5th July 2005, 06:42 AM   #8
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If you're picking up mains signals couldn't you just shield the entire input buffer with some copper foil? Or if you want another way build the entire input inside an altoids tin.

Tell us how it works out. I'm planning on using the JFET buffer myself.
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Old 5th July 2005, 08:21 AM   #9
Nuuk is offline Nuuk  United Kingdom
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Sheldon, following this thread, and reading Pedja's pages again led me to the thread where I think it was you who first put forward the idea of shunting the output of the buffers to ground while they warmed up.

So this is just to say thank you for that idea which works well. I have written up all the details here .
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Old 5th July 2005, 05:24 PM   #10
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Quote:
Originally posted by Nuuk
Sheldon, following this thread, and reading Pedja's pages again led me to the thread where I think it was you who first put forward the idea of shunting the output of the buffers to ground while they warmed up.

So this is just to say thank you for that idea which works well. I have written up all the details here .
Instead of that fancy 555/relay circuit how about a simple capacitor charging circuit to hold on the mute feature of the chip until everything is warm? I mean, as far as I know all this bad noise during power up won't hurt anything, it just sounds bad.

Please correct me if I'm wrong.
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