10+ year old 2.1 speakers - 60/120Hz BUZZ getting QUIETER when I increase the volume?

[ElectrocutedBaboon]

Hello,

I have a 2.1 speaker system I bought back in the early 2000s. I bought them new and am the original owner.

They worked great for a while. But lately I've noticed that there's more 120hz (I think) buzz coming from them. The buzz does NOT get louder when I increase the volume. In fact, it gets quieter. As the volume further goes up, I hear a hiss (thermal noise, I think--nothing can be done about that).

With more time, the buzz got worse and worse. Today I can hear the buzz, in a quiet room, from about 5 or 6 feet away. Volume's all the way down, and nothing's going into the speakers.

VOLUME - <---> +

< Buzz --- buzz gets quieter --- buzz almost gone, faint hiss is audible --- hiss is clearly audible --->

After much research and experimenting, I am 99% sure that the filter caps need to be changed out.

Everything points to the caps. But what's confusing me, yet might be revealing to somebody with experience in electronics, is the fact that the buzz gets QUIETER as I increase the volume.

Am I on the right track in assuming that the buzz gets quieter because more current is drawn to amplify what is supposed to be the signal (nothing plugged in, remember)? So this leaves less current for the buzz?

Would anyone point me to a page explaining why the buzz doesn't get louder even though the volume control is increased? I realize it's because the buzz doesn't make it into the amplifier IC, but still manages to find its way to the speakers... but what route does it take?

I'm again taking an interest in electronics after a long hiatus, but I never got past those Radioshack experimentation kits... lol... So I hope you could pardon what may be my incorrectly phrased questions.

Thank you.

 

[ElectrocutedBaboon]

Also, if this helps in any way:

- The transformer is NOT shielded at all! It's just a donut-shaped mess of coil, and looks like it's wrapped in plastic. I don't know what its core is, but the thing is rather heavy... so I doubt it's air. So...

- Between the transformer and circuit is air, about half an inch of sawdust wood, and more air. When I moved the circuit away from the transformer, the buzz got noticeably quieter.

- The whole circuit is on just one, single PCB. This means that the bridge rectifier, the filter caps, the amplifier, and other misc. chips are all on the same board. I don't know whether or not this makes an appreciable difference? Can I get better results if I MOVE AWAY the power supply onto another board, physically farther from the rest, and just use low resistance wires to power the amplifier?

 

[Harald Kapp]

Most probably the electrolytic capacitors in the power supply have lost capacity. Replace them by new capacitors.

 

[ElectrocutedBaboon]

Thank you for the very quick reply!

Am I correct in assuming that I can simply connect replacement capacitors to the same junctions on the PCB, without having to take out the older ones? This is just to test to see if the hum goes away, of course. I just don't want to unsolder if unsoldering isn't necessary.

I understand that this will increase the capacitance past the new capacitor's rating, but how much more capacitance does the configuration have to give for it to screw with the AC side of things? How would this affect the ripple?

 

[davenn]

as a test ... yes you can just temporarily parallel them up

its an old trick us TV techs used to do in years gone bye
we didnt even solder them in ... used side cutters to sharpen the ends of the legs of the capacitor to points then just hold them across the PCB solder joints of the suspect caps. Found many a faulty cap that way

JUST BE WARY of the voltage and uF value of the cap you are testing with. Make sure that its as least the same ratings of the incircuit cap

Dave

 

[ElectrocutedBaboon]

Hello again,

I haven't yet tried to replace the caps because I am trying to research as much as possible before I desolder the first joint. This will be my first electronic repair project, and the idea is to understand what it is, exactly, that I'm replacing, and why.

So far I've studied about how capacitors are used to smooth out the ripple in DC after the rectifier. I also learned about how an inductor (of sufficient characteristics) can be used after that first capacitor in order to further smooth out the DC.

If I wanted to go crazy, I'd mod the circuit to filter and then filter some more. But that's getting ahead of things--I suspect the original designers did a fairly good job of that.

Considering that the buzz I'm hearing matches the frequency I hear coming from the transformer (when I put my ear close to it), I'm willing to bet my arm and leg on that the noise is 120hz.

So my question...

I get that it's most likely faulty capacitors. But in an audio amplifier circuit, is there ever a situation in which this noise ISN'T the result of a poor power supply filtration in place? Basically, if I can smooth out the DC considerably, will this go away?

I know about ground loop issues, but that isn't the case here. The subwoofer has only two prongs and is not connected to anything else.

There's also the issue of the transformer's magnetic field coupling with the circuit board. BUT... I don't think this is something that would get progressively worse over the years. My options are to either shield the transformer and/or circuit board, or simply to move the transformer far enough away (probably bringing it into an external box and running wires).