My sub-woofer hums...

[Tim Downie]

... and no, it's not because it can't remember the words. ;-)

All the time my KEF PSW2000 subwoofer is powered up, there is a constant background hum.

sure it's just a mains frequency hum coming from its power supply (it doesn't change whether any input is connected or not).

Does this sound like a fairly basic/simple repair job? Is it likely to be just the smoothing capacitors in the power supply or might it be more complicated?

I know which end of a soldering iron to hold and recognise the dangers of big capacitors. Could I fix it myself?

Tim

[The Natural Philosopher]

Is it a gritty him or a fairly 100zh hum or a really 50hz hum..

I am surprised..how long has it been doing it, and how long have you had it?

You MIGHT have an earth loop.. unplug it and see whether the audio input earth is connected to the mains earth.(use a continuity tester).if so try a quick test with the mains earth disconnected...

[John Rumm]

It could be a problem with the unit, or it could lay outside... I had similar problems with my surround system picking up hum. In the end I managed to eliminate it by the simple expedient of changing the socket it was all powered from! (nothing immediately obviously wrong with the socket either - although we are on a TT system, so getting a good earth reference is not easy). Perhaps some experimentation with wire routing, power outlets etc may be worth while before getting too deeply into the insides of the sub.

[Tim Downie]

I'm pretty sure it's 50 hz. I thought somesome somewhere would have recorded mains hum on the net for comparison but I haven't found it yet.

Had the sub for 5-6 years now. Hum has slowly been getting worse.

Don't think so. Doesn't matter where in the house I plug it in and nothing has changed with regards to its wiring in donkey's years. I'll get the multimeter out after I've walked the dogs.

Tim

[Adrian]

There's (allegedly) a sample of 50hz hum in Wikipedia en.wikipedia.org/wiki/Mains_hum

Now that sounds like one or more large electrolytic caps slowly drying out.....? As they dry out their capacitance decreases, and their effectiveness at 'smoothing' decreases.....

Should be fairly obvious which the culprits are (assuming it's a transformer-type 'linear' power supply). If you can identify the culprits, then you should be able to find replacements from the usual suspects....

To confim the diagnosis you could try adding a couple of (suitably-rated) caps across the suspected duff caps on short plying leads - taking due care to safely discharge the stored energy, which, athough it's _probably_ not lethal might well make you jump, or even weld your screwdriver to something ! BTDT !

Hope this helps

Adrian

[Dave Plowman (News)]

First call would be the main smoothing caps.

[The Natural Philosopher]

Its probably 100hz.

Ok. Its possible that the main capacitors are going high impedance-ish

It's also possible that as components are ageing its power supply rejection is worsening.

Ok, thats ruled out..

Check first by switching it off while running..there should be a few seconds of power ..if the hum vanishes as soon as the PSU is off the mains, but it still works, its definitely self generated.

[Bob Mannix]

All the responses so far have assumed an amplified hum signal. This unit has a built in amplifier and it's possible the mains transformer for the power supply is physically humming/vibrating as it is loose or has become so and it's nothing to do with leads, connections etc. This would sound like it was coming out of the speaker.

[The Natural Philosopher]

That is entirely possible. Though quality kit will generally use a toroid that should not be too bad in that respect.

[meow2222]

3 likely options are

- bad psu reservoir lytics

- bad capacitor connected to the amp IC that helps it reject psu hash

- a worsening poor connection on a ground wire somewhere in the unit

- mechanical transformer hum is also possible, but far less common than the others.

Capacitors can be piggybacked to check them Bad connections can be pressed/prooded to check them

NT

[Dave Plowman (News)]

In this context they don't usually produce 50Hz but more of a buzz. They can produce low frequencies when attached to a large resonant panel but I'd say a speaker maker would avoid this.

[Tim Downie]

Yep. Hum stops as soon mains cut.

Tim

[Tim Downie]

It is a toroid. Is it safe to power it up with the speaker disconnected to check that it's not coming from the transformer?

Tim

[Tim Downie]

Yep. Hum stops as soon mains cut.

Tim

[The Natural Philosopher]

Almost certainly yes. Only older valve amps would over voltage on no load.

[Dave Plowman (News)]

Higher capacity or working voltage won't matter - but it will likely be bigger.

[Tim Downie]

Hunting down suitable capacitor replacements for my sub-woofer is turning out a lot harder than I was expecting.

I've googled oodles of sites but can't find anything *near* the right spec.

It's a 12000 micro farad 63v electrolytic jobbie. A "radial" type (stubby cyclinder, two pins at the bottom for soldering to PCB). Needs to be no more than 35 mm diameter and preferably 45 mm tall (although there is room for more height).

Other info, it has LSM, 85C(M) and VENT printed down the side (and it was made by LICON).

I can't find anything with those values and even if I over-spec, I can't find anything close enough.

Can anyone help?

Tim

[John Laird]

Farnell have a selection of 10000uF and 15000uF examples. (Start at

and try searching for capacitor. All the result URLs I found include a sessionid so that'll time out and not be generally useful.) The slightly under-spec part is much cheaper...

Another common recommendation is CPC.

Maplin seem to have thoughtfully discontinued their 10000uF stock. They have almost completely morphed into a purveyor of consumer crap. I struggle to work out how to buy some of the stuff they sell and/or choose a sales droid who looks like they might be able to help.

-- "If it works, tear it apart and find out why!"

[stevelup]

Hi

RS Part Number 2508869138

12,000uF 63V 35mm Diameter, 50mm Length

Regards,

Steve

[stevelup]

CPC have a 10,000uF part in stock