How to quiet humming speakers?

You can check if it's pick up on the lead by shorting out the input jack. If it still hums, there's likely inadequate or failed smoothing in the power supply. Other possibilities are hum pickup from something else in the room - does moving the entire unit make any difference?

Thank you for the suggestions, Dave!

I'm not sure I understand what you mean. If I short the minijack then it buzzes LOUDLY. I'm careful not to do that.

Can I fix it?

No, it hums happily wherever it is.

But if I come back before I return, please ask me to wait.

-Torben

It isn't the speakers or the speaker cable. Swapping new ones would prove that.

If you short both the tip and ring to the sleeve it should not buzz loudly.

You're right. I now unplugged the minijack and shorted all three parts of it, and the humming WENT AWAY completely. When I removed the metal, it started humming again.

Then I plugged it into my (cheap old) MP3 player instead. It still hummed. I powered the player ON. The humming STOPPED. It also didn't hum while playing. I stopped the player. No humming. I powered the player OFF. The humming is there again. I unplugged the MP3 player. Still humming. I plugged the speakers into my audio unit again, and now it doesn't hum!

I also noted that plugging/unplugging the minijack in my audio unit creates that brief, loud buzzing, but plugging/unplugging the minijack in my MP3 player gave no loud buzzing.

Does this indicate a poor connection inside my audio unit??! I hope not because it was expensive, but if so then I would get in touch with the manufacturer

Thank you all for your help so far! I hope you can answer this too.

-Torben

Short the tip and ring to sleeve with some wire.

That suggests that when the power is off the player presents a high impedance, similar to no connection.

That it sometimes hums and sometimes doesn't suggests the jack in the audio unit might be a bit weak -- these things have little pieces of springy metal that make the actual connection, and they can sometimes lose their springiness.

I'd suggest getting a pair of resistors of appropriate value (this will depend on what the player is designed for, but something like 200 ohms might be generally suitable) and putting them across the input of the speaker amplifier -- unless, of course, you also need to be able to use it with a high impedance device.

But before doing that, you might also consider another possibility -- are you sure the plug and jack are the same size? There shouldn't be much "wiggle" if both the plug and jack are the same size. If they're not, adapters are available.

Yes, the plug and the jack are the same "walkman-type" size. But apparently the jack in the audio unit is weak, as you suggest. A real disappointment in a $300 device.

I got the humming stopped for now, at least until someone wiggles the cable again. Thanks for your help.