Weird slow oscillation

[edm]

I have a lm3886 amplifier, which is based on the application note. The amplifier has given me a hard time to operate correctly, first i had an uncurable hum. Which I thought was a ground loop but was caused by the trafo.

Now i have some sort of very slow oscillation. I already have a sollution but frankly I don't completely understand how my own sollution works . So I will give a try to explain my cure and hope somebody can explain why this problem arises in the first place.

The amp is being fed through an elco bank (30.000uF) and then on the pcb by a small cap (1000uF) near the chip. If the main elco bank is connected directly to the pcb, my speakers reveal a very slow oscillation. If I connect my scope, the oscillation is gone. So at first I thought i had some sort of earthing oscillation thing going on. But then I thought I could also have some sort of oscillation between the cap bank and the 1000uF cap on the pcb, so I decided to try damp it by inserting a 1 Ohm resistor in between them and it appears that the oscillation is gone.

But now the explanation??? Can this behaviour be attributed to an oscillation between to a cap bank and a local cap and if so what would exactly happen. And what is the role of the scope, at first I thought it was the high input impedance, which stops the oscillation but only leaving the earthing connector in place seems to be sufficient (the amp is not connected to safety earth, should it be?).

What makes it hard to figure this thing out is that as soon as I connect a scope all things wierd things dissappear. So I have to figure this thing out in my head, which is hardly functioning anymore after a long debugging session .

In my frustration I also blew up a gainclone by mistakenly connection the +Vcc to the minus and vice versa, so I cannot reproduce if the gainclone 'suffers' from the same effects. (and also cannot check the difference in sound between both amps and the effects of a big cap bank on the sound )

I hope somebody can shine some light on my rainy day

[moamps]

Hi,

which schematic you use, with asymetrical or symetrical power supply?
Oscillation is gone when you connect ground from scope to ground of amp or when you connect sonde to output?
What you mean with "slow oscillation"?
Some picture maybe help to.

Regards

[edm]

The schematic is based on the one on page 1 of the lm3886 pdf, with the difference that the input is not constituted by a pot and a 1k, but instead the signal passes through a 1k res then a 2u2 cap and then through a node where one arm goes to the +In and the other arm goes trhough a 47k resistor to the ground. So nothing fancy. For the power supply a 25V trafo is used, then a double bridge configuration going to the previously describe cap bank and the cap on the pcb.

The oscillation is gone if the scope is connected to the amp ground.

What I mean by slow oscillation is that if the 'damping resistors' between the cap bank and the local cap near the chip is not present I see the conus of the speaker moving rather slowly back and forth, it takes a couple of seconds for a complete cycle. And the amplitude is increasing continously, I don't think the speaker is enjoying this low level large amplitude movement a lot......

From what do you want to see a picture? The slow oscillation is only visible on the speaker and that would take a movie to see it.... It cannot be seen on the scope.

Emiel

[moamps]

Originally posted by edm
and the other arm goes trhough a 47k resistor to the ground. So nothing fancy. For the power supply a 25V trafo is used, then a double bridge configuration going to the previously describe cap bank and the cap on the pcb.
The oscillation is gone if the scope is connected to the amp ground.
From what do you want to see a picture? The slow oscillation is only visible on the speaker and that would take a movie to see it.... It cannot be seen on the scope.
Emiel

Hi,

did you try usual input circutry? With same results?
Are on your trafo 2 secondaries or one?
This oscillation is very dangerous for speaker, replace with some resistor when testing.

Regards

[AudioFreak]

It indeed sounds like you have grounding problems and perhaps the 'oscillation' is motor boating .... what does it sound like? I doubt that this problem will be particularly harmful to your speakers as it seems from what you are discribing, the problem is in the bass region not the treble where destructive oscillation often occurs ... It may be best to test on a $5 full range driver until the problem is rectified.

EDIT: Just reread the above posts. It sounds as though the ground reference of the chip is shifting relative to the power ground that the speaker is connected to. Where is the -ve of the speaker connected to in your circuit? Best keep your good speakers disconnected for the time being.

[edm]

Originally posted by moamps

did you try usual input circutry? With same results?

I do not understand what you mean, could you please refrase?

Originally posted by moamps

Are on your trafo 2 secondaries or one?

2 secondaries, which are connected to two bridges and from those bridges two are paired and connected to ground.

Originally posted by AudioFreak
EDIT: Just reread the above posts. It sounds as though the ground reference of the chip is shifting relative to the power ground that the speaker is connected to. Where is the -ve of the speaker connected to in your circuit?

I have tried to connect the -ve of the speaker to the central star ground and to the local ground on the pcb, both with identical results.

Originally posted by AudioFreak
Best keep your good speakers disconnected for the time being.

Although I would rather like to keep them disconnected the only way I could see the instability is to keep the speakers connected... Because connecting the scope across the speaker posts 'solves' the problem....

But why is the subsonic oscillation gone after the insertion of a 1.2 ohm resistor between the main cap bank and the local cap bank????

Before i started i never would have thought that wiring everything together would be the difficult part...... But Murphy likes playing games with me....

[AudioFreak]

Originally posted by edm
Although I would rather like to keep them disconnected the only way I could see the instability is to keep the speakers connected... Because connecting the scope across the speaker posts 'solves' the problem....

That's exactly why I suggested that you get a cheap full range driver for testing purposes.

[edm]

Right

[moamps]

Originally posted by edm

I do not understand what you mean, could you please refrase?

Hi,
did you try with 1K and pot (or trimmer for test purpose) on +input?

Regards

[pinkmouse]

Where is your 'scope grounded? is it the same outlet or a different one?

Is your chassis grounded?

Is your amp circuitry grounded to the chassis?

If so, have you checked the ground in your lead/supply/socket?