Distortion at low volume disappears with changing the volume


I have a Marshall Acton 1, I'm very fond of, but a while back I started to notice some distortion from what I think is the subwoofer. At first I noticed the distortion only occured on low volumes. With a little bit of searching, I found that the distortion only started after the music was playing for a while. I also noticed that when I opened up the volume and brought it back to the original more silent setting, the distortion was gone.

I have opened the speaker up but saw no damage to the woofer speaker itself. I fixated some wires so there was no way the woofer would be vibrating against them.

Now I think I start to notice that increasing the volume doesn't always work (anymore??.)

I have to say the speakers is often left with the power on....

I've searched the web for this issue but didn't find a thread describing the exact same problem. I did shower find that a capacitor could likely be a suspect...

I've placed the tread here because I found the amplifier would be a D-class one..

I am in fact a technical guy with a good understanding of electrics and a little knowledge on electronics, but I have no idea what to do to identify nor how to fix this problem.

I appreciate any form of help someone offers me. I throughly hope I can fix my beloved speaker.

Thanks for your time and any suggestions you might make,

Could be a mechanical issue, like rubbing of a moving part, voice-coil ie. against something stationary, like pole pieces.
It could be electrical too, if the amplifier suffers from the equivalent of crossover distortion, typically excessive dead-time for class D
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Hello Elvee,

First of all thanks for your reply!

I've checked rubbing of moving parts, but only on the outside of the speaker, I will open the speaker back up and check the inside as well.

For the 'distortion' issue, is this an aging issue? And more important is there anything I could do about? Or verify this diagnose...

Kind regards Dieter.
The simplest way to determine whether the problem is electrical or acoustical is to temporarily connect another amplifier to the speaker. It doesn't need to be high-end or powerful, just functional.
If the sound remains acceptable at any level, chances are that the amplifier has a problem.
You can do the same the other way round: connect any suitable speaker to the class D amp, and make listening tests at various levels.
This should help in narrowing down the origin of the problem. Once it is done, we can begin examine the possible remedies
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I also noticed that when I opened up the volume and brought it back to the original more silent setting, the distortion was gone.
That is typical behaviour of a poor mechanical contact in the speaker chain. In 'hifi' amps it is usually the contacts in the speaker relay. Really common issue. Any mechanical contact that is poor would do the same, a switch for example or even a plug/socket connection.
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Occuring way more since lead free solder was introduced.

Anyway, in this particular case it may be the volume potentiometer. The average quality of those is so so and the described phenomenon is a regular complaint. Often scratching is also heard and channels may fail at certain positions.
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