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Old 12th April 2005, 09:46 PM   #1
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Default LM3886 very prone to mains noise

I've just completed my second gainclone - the first was a VBIGC with LM3875 - with LM3886.

This one is regulated (LM338) and uses the OPA627 as buffer 'a la Carlos'.
No pot, star ground in the middle point of a thick wire that joins the gnd of the amp boards.

The problem is that if I switch lamps in my flat I hear a pop in the speakers.. nothing huge but still annoying.

Did anybody experience that?

Any suggestion (apart from getting a mains filter?)

Cheers

Andrea
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Old 12th April 2005, 09:49 PM   #2
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Try a 0.1uf cap across the input pins of the chip.
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Old 12th April 2005, 10:04 PM   #3
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I had a discussion with a friend of mine on the subject, though this was a commercial amp.
what decided to try was to use a snubber, yes, a 1uf 400V cap in series with a 1ohm resistor. it worked somewhat, the problem was reduced, though, not abolished.
we should maby have tried without the resistor, but i was very set on the whole snubber subject at the time.
we should altso have tried to use two caps, from each rail to ground.
this was done on the mains, as we didn't bother to open the amp.
we came to the conclution altso that the amp sounded a litle cleaner and had less noise idle. it's worth trying. wont cost you much anyways.

regards
Marius
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Old 12th April 2005, 10:12 PM   #4
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I don't have problems with electrical switches, they are in shape, but the fridge made a "pop" in the speakers when it turned on.
I always tend to attack the source of the problem but... my wife wouldn't like to see me fiddling with the fridge.

A 300~330pf cap across the inputs of the LM3886 will solve the problem, or attenuate it to practically inaudible levels, it depends.
Check it out:

http://www.diyaudio.com/forums/attac...amp=1112539310

PS: I use a styroflex cap. For small values, they are the best IMHO.

And BTW, you need to replace those faulty switches too.
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Old 12th April 2005, 10:20 PM   #5
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Quote:
Try a 0.1uf cap across the input pins of the chip.
Are you meaning the OPA627 or the LM3886?

Quote:
Originally posted by carlosfm

And BTW, you need to replace those faulty switches too.
The house is new... if the switches are that bad I wonder how will they become with use

cheers

Andrea
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Old 12th April 2005, 11:19 PM   #6
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Quote:
Originally posted by Andypairo
The house is new... if the switches are that bad I wonder how will they become with use
Bad quality switches.
Pretty from the outside, nasty on the inside.
I'm sure they spark inside when you turn on the lights.
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Old 12th April 2005, 11:28 PM   #7
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I have two chip amps that do this and another that doesn't. The main differences are that the one that doesn't is in a metal case and that it has an electrostatic screen between the primary and secondary of the mains transformer.
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Old 13th April 2005, 05:54 PM   #8
cjd is offline cjd  United States
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The house is new... if the switches are that bad I wonder how will they become with use
You could get lucky and they wear in to improved performance. But so much of what is used in homes these days is pure cheap junk. Sad but true.

C
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Old 13th April 2005, 07:20 PM   #9
GregGC is offline GregGC  Canada
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Quote:
Originally posted by jeff mai
I have two chip amps that do this and another that doesn't. The main differences are that the one that doesn't is in a metal case and that it has an electrostatic screen between the primary and secondary of the mains transformer.

I think good shielding box is mandatory for any amp. Otherwise it's easy to pick up all kind of EM junk.

Greg
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Old 13th April 2005, 07:26 PM   #10
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Quote:
Originally posted by GregGC
I think good shielding box is mandatory for any amp. Otherwise it's easy to pick up all kind of EM junk.

Greg
I always use metal boxes, grounded.
But occasionally the fridge made "pop" on my speakers.
Maby an expen$ive all-copper chassis would be better than aluminium or steel.
But a 300pf cap does it for me, and I couldn't notice any effect in the sound.
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