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
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
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
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
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.
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.
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.
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
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
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 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.
ingrast said:
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.
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!
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