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Old 4th September 2007, 11:53 AM   #1
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Default Mains pops and clicks

My new parallel LM3886 amplifier seems to be suffering from mains pops and clicks. It happens when I flick the switch on my oscilloscope and also my soldering iron.

I have removed the mains cord whilst it's still on, running briefly on the caps charge, and then quickly flicked the soldering iron switch off and the pop / click is still there, so I guess it's not actually coming through the mains cord!

I have also today fitted an input RF filter, 1k in series and a 470pf capacitor to ground. The pops / clicks are still there. They're actually still there when I remove the input from my preamp! I think it may have helped somewhat with the mobile phone blips though... possibly....

I have a Zobel on the output, but no inductor / resistor in series (yet). I may try this now though...

I have read that a 220pf - 470pf capacitor between the non inverting and inverting input may be a fix for this. A simple and possibly somewhat obvious / silly question then:

Can I still do this with LM3886s in parallel? I would just need to put the same value capacitor over each chips non-inv / inv input pins, right?

Thanks!
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Old 4th September 2007, 02:02 PM   #2
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When you do this un-powered test are the input interconnect wires still connected to the amplifier? Are they still connected to the pre-amp? What kind of enclosure is the amp in? Is any test equipment connected to the amp?
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Old 4th September 2007, 02:17 PM   #3
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Quote:
Originally posted by Kevin Graf
When you do this un-powered test are the input interconnect wires still connected to the amplifier? Are they still connected to the pre-amp? What kind of enclosure is the amp in? Is any test equipment connected to the amp?
Oh, I didn't think of that!

The pops do indeed disappear when I disconnect the input and then do an un-powered test. Must be the preamp then I guess? I may attempt to fit an RF input filter in there too later on. Maybe that will help....

Although... the pops still happen with the input disconnected, so maybe it is actually just getting in through the mains. An X2(?) capacitor from mains live to neutral perhaps?

The preamp is in a horrible looking ABS plastic box and the power amp is in a steel case, with aluminium front plate.

No test equipment is connected.
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Old 4th September 2007, 02:57 PM   #4
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Default POPS and CLICKS

If all else fails connect a 100pf ceramic cap between the inverting and non-inverting pins of the LM3886's.
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Old 5th September 2007, 02:43 PM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
all these amplifiers have two inputs.
In a non-inverting amplifier, the non-inverting input is connected to the input jack. The inverting input is connected to the hot speaker terminal.

Are you letting interference in via the speaker terminal?

In an inverting amplifier, both the speaker terminal and the input jack are connected to the inverting input.
The non-inverting input is connected to audio ground and may in turn be connected to safety earth.
Are you getting interfence entering via either the safety earth or via the speaker terminal?
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Old 6th September 2007, 09:16 AM   #6
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I used to get pops from my amp when the fridge turned on and off in my house, but once I had fitted a disconnecting network between the ground and the safety earth it massively reduced this.
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Old 6th September 2007, 10:26 AM   #7
Nordic is offline Nordic  South Africa
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appart from ground isolating network... try the following filtering...

1R 17W in series with 200nf X2 cap rated at appropriate voltage for your mains... + some margin..... this get connected in parallel with the transformer primaries... on my preamp with this filtration... I can't even hear when i flick the switch, it just keeps playing off the PSU caps, till the relay grounds the output...

you can also add 100nf at appropriate voltage for the secondary side of your transformer... from bridge rectifier AC to AC...
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Old 7th September 2007, 12:35 PM   #8
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Quote:
Originally posted by AndrewT
Hi,
all these amplifiers have two inputs.
In a non-inverting amplifier, the non-inverting input is connected to the input jack. The inverting input is connected to the hot speaker terminal.

Are you letting interference in via the speaker terminal?

In an inverting amplifier, both the speaker terminal and the input jack are connected to the inverting input.
The non-inverting input is connected to audio ground and may in turn be connected to safety earth.
Are you getting interfence entering via either the safety earth or via the speaker terminal?
I don't know. I guess I might be, as I have no inductor / resistor on the output. Well, I do have the 0.1R on the output for current sharing, but I don't think that would be enough to prevent interference, right?

A mix of the preamp and output cables I suppose then. I might fit the inductor / resistor on the output later and see how that goes. I'm redesigning my preamp (again)

Quote:
Originally posted by ianpengelly
I used to get pops from my amp when the fridge turned on and off in my house, but once I had fitted a disconnecting network between the ground and the safety earth it massively reduced this.
I've always used a disconnecting network, so I guess it isn't that

Quote:
Originally posted by Nordic
appart from ground isolating network... try the following filtering...

1R 17W in series with 200nf X2 cap rated at appropriate voltage for your mains... + some margin..... this get connected in parallel with the transformer primaries... on my preamp with this filtration... I can't even hear when i flick the switch, it just keeps playing off the PSU caps, till the relay grounds the output...

you can also add 100nf at appropriate voltage for the secondary side of your transformer... from bridge rectifier AC to AC...
I've got a 100nF over the secondaries before the bridge rectifiers, AC to AC

Is the 1R necessary? I only ask because I have looked in a computer switching PSU and it had just 100nF.. or maybe 10nF, I can't remember, from live to neutral...

Thanks all
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Old 7th September 2007, 02:40 PM   #9
Nordic is offline Nordic  South Africa
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The 1R 17W + 220nf goes on the mains wire, before the transformer... 1 leg of resistor to L.. other leg of resistor.. solder to one leg of the cap... free leg of the cap to N.

A 17W resistor can cost a good dollar... 1 more then the accountants would like.......

"do not be daunted by the power rating of the resistors shown, depicted as 1 Ohm 17W here; the actual required rating of the resistor will depend on many factors, but with a 17W rating, you're not likely to go wrong with any amplifier up to some 150W/8 Ohms and corresponding increases of power with lower loads"

http://www.tnt-audio.com/clinica/ssps1_e.html

http://www.tnt-audio.com/clinica/ssps2_e.html

http://www.tnt-audio.com/clinica/ssps3_e.html
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Old 8th September 2007, 11:13 PM   #10
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Quote:
Originally posted by Nordic
The 1R 17W + 220nf goes on the mains wire, before the transformer... 1 leg of resistor to L.. other leg of resistor.. solder to one leg of the cap... free leg of the cap to N.

A 17W resistor can cost a good dollar... 1 more then the accountants would like.......

"do not be daunted by the power rating of the resistors shown, depicted as 1 Ohm 17W here; the actual required rating of the resistor will depend on many factors, but with a 17W rating, you're not likely to go wrong with any amplifier up to some 150W/8 Ohms and corresponding increases of power with lower loads"

http://www.tnt-audio.com/clinica/ssps1_e.html

http://www.tnt-audio.com/clinica/ssps2_e.html

http://www.tnt-audio.com/clinica/ssps3_e.html
Thanks Nordic. That looks like a good read too! I will come back to this one I finish redesigning my preamp though For now this new power amp is playing music wonderfully except for a small clicking when I turn on the soldering iron. That doesn't exactly happen every day, so I can live with it for a while yet

Oh and if AndrewT reads this, I used the light bulb trick for this project. It's pretty neat and actually told me, by lighting up very brightly, that one channel was oscillating because I had the feedback resistors the wrong way around by accident! i.e 22k where 1k should have been and vice versa. So... thanks very much for bringing that trick to my attention
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