OT (Audio) Hummmmm

I noticed this weekend that my subwoofer (active Kef thingy) wasn't
subwoofing. To be honest I can't recall how long ago it stopped, but now
I've noticed it's stopped I need to get it working again.

All the lights come on on the sub so that suggests the fuse hasn't gone etc.

To try and track down whether it was my av-amp not pushing out the right
stuff or the sub not working I moved the connection from my sub to the
pre-amp of my hi-fi (I run AV and hi-fi kit independently in the same
room) and got a rather loud, constant, distinct hum through the hi-fi
speakers. I've moved the cable at the AV end to the other sub pre-out
and the centre speaker pre out and the result is just the same.

The only thing I can think of off of the top of my head is some form of
earth problem; the AV rack and the hi-fi racks are powered off of
different sockets but on the same ring main (the sub being co-located &
powered with the hi-fi).

Should step one be to open up all the sockets (6) and plugs (about 25
odd) and dis boards (4) & extensions (1) and check the earths?

 

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STOP IT!

 

If I stopped the IT I'd not be able to read UKRM, surely?

 

And that would be a problem how?

Phil

 

I've just tried the sub with a different power lead and a different
phono cable (it did look silly sat on top of the amp). No joy.

I've checked the sub phono cable between the tuner and the amp and
that's fine.

Having changed the power to the sub with no avail, I'm going to check
the mains path on the AV amp

 

Which doesn't have an earth so that's that theory shot.

However, sub tried directly out the back of the DVD, and no movement in
the cone.

DVD connected to amp via sub cable and sounds fine.

AV amp connected to amp via sub cable hums.

Suspect sub is unwell (bugger), suspect pre-out on AV amp also unwell
(very bugger as it wasn't cheap)

 

Any signal earth returns will come down the signal cable, not via
mains earth, if that's what you'e suggesting.

Is it a phono lead? Kind of sounds like both the lead has a broken
outer connection and the sub have a problem. No multimeter?

One very crude test is simply to plug a known good phono-to-phono lead
into the sub and touch the exposed centre connector of the other end
of the lead (without touching the outer) - you should hear a hum.

Does the sub have its own volume control? Silly question but...

 

When you say no joy do you mean absolutely no output or just a buzz?

Very easy way to test the sub by itself:-

Plug it into mains, connect phono lead to sub, leave other end of phono
lead disconnected. Turn sub on. Pick up loose end of phono lead (pick it
up using the plastic/rubber shielding, not the metal plug) and touch
centre pin.

It should hum/buzz at you to some extent when you touch the centre pin.
Probably loudly so long as you're not touching any other metal.

If that buzzes, the sub and phono lead are probably fine, and the
problem is your AV amp.

Make sure you haven't turned off the sub in the configuration, and make
sure the signal that the AV amp is receiving actually has a sub channel,
i.e. its 5.1 or 3.1 or 2.1, where the .1 is the sub channel.

 

Ah, OK

I may play with that tomorrow

Yep, and I've played with it (oh and the sub output on the AV amp is
switched on)

 

No output; the buzz comes when I connect the cable to the other amp/speakers

Similar to Pip L's suggestion? One for tomorrow I think

Yeah, checked that, and am using the test tone generator in the AV amp too.

 

Did it (realised it was plugged in and the lead was to hand so got on
with it).

Nowt happened, so I think the sub is distressed in some way.

But from the hum, I don't think that fixing the sub is the end of the story

 

Sounds like its not, no.

I'd sort the cause of the hum out first, or you might blow up the
freshly fixed sub if you fix it and try it before fixing the AV
processor.

 

My thoughts exactly.

Now, where to get the AV amp fixed...