Are you using the original volume pot ? The way it is connected is subject to crackling noises when the pot is turned, and if it is dirty then intermittent crackles during operation too. There should be series capacitors in the input side to prevent any DC reaching the pot. The 6.2k ohm resistors are not needed and only contribute to small amount of extra hiss.

Yeah, I'd figure on just replacing the potentiometer, make sure you get an audio one for best results, though if absolutely no other choice for fit, logarithmic that doesn't mention audio suitability, but those may start crackling again in a year or so.

Damn, I hoped that wasn't the pot.

Unfortunately, since the board must fit into the original case, I'm forced to reuse the original pot and the original power switch.

I found some replacement for the pot, but only on aliexpress, so I doubt about the quality.

The pot is like the one on the pics:

Tiido wrote on 2024-11-12, 12:40:

There should be series capacitors in the input side to prevent any DC reaching the pot. The 6.2k ohm resistors are not needed and only contribute to small amount of extra hiss.

Ok thanks, so can I replace R1 anr R2 with capacitors? Which value?
And what about C4 and C5? I should keep them or remove them?

That pot type is luckily very very common. You can probably revive the original pot by putting some cleaner+lubricant into it. Ideally some pot specific stuff but even WD-40 does wonders and makes a crackly pot be ok for a long time.

R1 and R2 can be replaced with capacitors, value depends on how low the cutoff frequency has to be. Since these are small speakers it doesn't have to go very low or even shouldn't unless you want premature clipping. With 50kohm pot, even 1µF capacitor gives 3Hz -3db point. C4 and C5 are ok since the input impedance of the amp chip is very high so there's essentially no additional cutoff from it. If you have any of the same caps to spare they'll be a good choice in place of R1+2, giving 14Hz -3db point.

Ok replaced R1 and R2 with 0.22uF caps.
And, tried with some holy water (WD40) on the pot.
Things improved a lot, but still have some scratch.

I will search for a new pot and a specific cleaner.

Thank you very much.

That sounds promising. It is also possible to actually take apart these pots and use something like a soft pencil eraser to go over the contact surfaces but it can only be one once or twice before the metal tabs that hold stuff together break off.

There's a chance the resistance track is actually having a groove worn in it and in that case the only proper solution is replacement of the pot itself. Little bit of more life can be extracted by pushing the brushes to a new vertical location along the track but it only delays the inevitable.

A little update.

I ended up by replacing the pot with an equivalent one found on aliexpress, that I opened and cleaned up.
It's not perfect, but for now I can stay with it.

The sound quality is pretty good, still a pair of cheap speakers, but much better than the original.

So, many thanks to everyone for the help! 😀

But let me sum it up for a moment.
The original pair of speaker I spot, was a replacement for an Harman Kardon soundbar. They sound very good, but they was 5mm to high, so they didn't fit into the speaker case.
Fortunately I spot another pair that at the end don't sound bad at all.
I made 2 revision of the PCB, because I done some little mistake on the first one (inverted the volume pot direction).
In the second one I also managed to fit an heatsink for the amplifier. Which is totally useless, because it never drive such high power.
And now to fit the board must cut a little piece of the plastic slots inside the case.

At the end, with all the time and money I spent with prototypes, components, etc. This costs me more than a pair of good speakers.
And yes, that's one of those times, in this hobby, that you fell a bit stupid 🙁