LM3886 very prone to mains noise

[Andypairo]

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

 

[pinkmouse]

Try a 0.1uf cap across the input pins of the chip.

 

[demogorgon]

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

 

[carlosfm]

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/attachment.php?s=&postid=611337&stamp=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.

 

[Andypairo]

Try a 0.1uf cap across the input pins of the chip.

Are you meaning the OPA627 or the LM3886?

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

 

[carlosfm]

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.

 

[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.

 

[cjd]

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

 

[GregGC]

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

 

[carlosfm]

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.

 

[SpittinLLama]

My guess is that when a high inductive load, like a frige, there is a big spike on the power lines. The flourescent lights cause a high voltage spike. I have an amp built from 4 x LM3886 and when ever the flourescent light is switched off I hear a loud pop. The LM3886 has good PSRR at low audio frequencies, like less than 1kHz. It does not have much PSRR at higher frequencies. Try the cap and then if it is still bad you may want to look at adding some line filtering to you amp design.

 

[Nuuk]

Noisy fridge? Some more information here

 

[ingrast]

noise / hum

I came across this (in measurements, not really audible), and traced the problem to the (-) input network.

In the datasheet noninverting arrangement at least (or minor variations) the (-) input is DC blocked and AC returned to the V- supply. In this condition, it is easily seen any noise in the V- supply is a second signal source and correspondingly amplified. Hum levels up to -50 dB with respect to full output are common even with a zener regulated signal V- supply.

The cure, better regulation for this reference supply and / or better bypassing to signal ground at low frequencies.

Rodolfo

 

[carlosfm]

Noisy fridge? Some more information here

Thanks Nuuk, you have an answer for every occasion.😎
My fridge is 3~4 years old, and it always did that.
My wife lets me do everything with my system, but she's reluctant to see me fiddling with a fridge filled with food.😀
Maby when she goes shopping.😛

 

[carlosfm]

Re: noise / hum

The cure, better regulation for this reference supply and / or better bypassing to signal ground at low frequencies.

Rodolfo

It has nothing to do with regulation or bypassing.
Inverting or non-inverting, regulated or unregulated PSU, the "pop" is there.
In my case it was only the fridge, I don't have any problems with any electrical switch in the house.
The fridge only powers on from time to time, not a big problem.
But the cap solved it.
On the datasheet they recommend 220pf. In my case it was not enough. 300~330pf does it.

 

[jeff mai]

The chassis is probably quite important, but I think the ES is more important. You can put your amp in a nice chassis to shield EMI, but without the ES you give the EMI an easy way in - straight through the mains transformer.

 

[Nuuk]

The chassis is probably quite important, but I think the ES is more important.

A chain being only as strong as its weakest link, they are both important. It is no use locking the door and then going out and leaving the windows wide open!

Of course if you only have one problem, ie noise on the supply or RF then you only need to deal with that. 😉

But if you are making an amp for somebody else, you have to expect both problems and prepare accordingly. I guess that's one reason why most commercial amps come in a metal case and have one or two extra caps!

 

[jpl]

sorry heres the photo

 

[shusha]

Re: Re: noise / hum

It has nothing to do with regulation or bypassing.
Inverting or non-inverting, regulated or unregulated PSU, the "pop" is there.

In my case with RegPSU there are no pops. With only 1000uF per rail the pops are loud. With snubber the pops are here but very lightly hearable. In my case 220pF accross the inputs works excellent.

At the end I removed 220pF, because with snubber I hardly hear them.

 

[jpl]

sorry this is my first post ,the image was to big and did not post.Anyway this is a copper cased chipamp with dedicated mains plus rifa surpression caps in the amp before the transformers and it still hase mains problems.