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I have an old pair of self powered speakers. They're 400W and great with the sound and back in the day for large events were fine, BUT, they seem to have a low signal-to-noise ratio; even at rest with nothing plugged in aside from power they each give off a distinctly audible hum (louder than a whisper and clearly notable in a room). The hum doesn't change when any speaker dials are changed.

From reading up on this I believe this is down to the Amp inside each speaker (but I'm not certain).

I believe I need to decrease the noise floor by tweaking the Amp to amplify less (?) or some other means. Possibly by also perhaps shielding internally in the speaker?

I'm a bit stuck as I can't afford new speakers, and feel I can't sell these ones until I've resolved (or at least, improved) the issue.

Speaker details:

  • Date New: 2012
  • Power: bi-amp 300W (peak) PER SPEAKER (according to literature; box claims 400W)
  • Frequency Range: 50Hz --> 20,000Hz

Speaker internal: Internal View of Speaker

Speaker back plate: Back plate of Speaker

What can I do, if anything to improve or remove the background hum from these?

(I don't think I can sell these like this and without selling them I have no way of buying anything newer/better)

Advice welcome.

EDIT: I believe possibly that the cause is that AC power in the UK is 50 Hz so that the speaker amp is able to pick up the electricity power supply (other speaks I see all have their frequency range threshold at 60Hz rather than 50Hz) .... Is this likely? Can this be improved by shielding? Again how easy would this be to do?

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Usually with older powered speakers the hum that creeps in is caused by old electrolytic capacitors in the power supply. Electrolytics give a huge capacitance value for their size but they have a liquid 'dialectric' substance inside that dries up with time. When mains voltage, at 50Hz in the uk, comes in a rectifier flips over the negative portion of the wave cycle so you get a bouncing 100Hz wave. The power electrolytics are used to absorb the peaks in this rectified AC and smooth over the power waveform. Once they become old they are not able to do this so well and you get so-called 'ripple' in the power supply. Is the frequency of the hum you detect 100Hz? If so this is probably ripple caused by bad power caps.

If this is the case, replacing the power caps will probably help, this is the most likely culprit though there are a few other things it could be. Shielding is unlikely to help as from your description it doesn't sound like it's a 'picked up' signal. It's likely the power supply to your amps power output has a bit of 100Hz ripple on it which is being amplified in the power stage regardless of what signal is being fed into it.

EDIT - that said, 2012 is not that old for caps to go bad, did you notice the noise getting worse over time?

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  • Thanks for the feedback. The speakers used to be used a lot but - me getting older - they're far less used now so not too familiar with the hum "increasing"..
    – Martin
    Commented Aug 27 at 9:48
  • Agree with @OwenM. If you can beg, borrow, steal - or if need be, buy - a similar electrolytic to that (or those) used in the power supply, you could clip it in across the existing. If the hum drops noticeably, that is your answer. If it makes no diff, it's not. NOTES: You can parallel a good cap on a bad one without removing the bad one first. Only the power supply caps are relevant. If moving the speakers relative to nearby wiring changes the hum level, then look to relocation and possibly shielding (otherwise, shielding won't help).
    – MadMonty
    Commented Aug 31 at 20:06

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