TD12 Hum and Cabling Question

[jtoynton]

Hello,

Been reading this forum for months now, and finally got around to posting.

I've recently treated myself to a Peavey PR-15P PA powered speaker, to use as a monitor for my TD-12 and one day something to practise/gig with.
Very impressed, very low-level hiss, and terrific punchy bass, unfortunately due to the neighbours, I can't turn the volume past 2, let alone up to 11!

The problem I've got is that when running the TD-12 though the amp, I'm getting an audible hum. I only get this hum when running the TD-12 in, so it's obviously not a problem with the Active Speaker. This just leaves the TD-12.
The amp is wired up via the master out, left into one input and right into the other input, each input is mixed and fed into the active speaker amp.
I get no hum (from my headphones) if I plug my headphones into the master out. I do get hum if I plug the patch lead into line in (just investigating..).
Measuring the AC voltage between the ground on the speaker and the ground on the TD-12 brings up ~8V, which is not enough to be audible, I believe.

The cables are standard "good quality" mono shielded cables, and I've played around with some others, no change..
I can obviously live with this hum (but why should I?), since the speakers are turned way down, but if I wanted to crank them up, this hum would be quite noticeable!
Any ideas what my next check is?

How do people connect their TD-12 to their active speakers? I've only got one speaker, so I only need one input, but I like the stereo separation in my headphones and rapidly switch between the two. So don't really want to compromise here.
Is running left and right into 2 of the active speakers three inputs correct, or will there be some phasing/distortion problems?

Cheers

Jonathan

[ige87]

Hello and welcome. Try searching for 'ground loop' in the search, or on google. It might also be worth using just 1 output. If you run a single cable from the L/Mono output on the module, it'll automatically send a summed mono signal to the speaker. Whether this would help in your situation or not is another matter.

[jtoynton]

Hello,

I've measured the AC voltage between the ground points again, it was actually ~6V not ~8V.

Earth pin socket on mains outlet -> Earth on speaker cabinet = ~6V
Earth pin socket on mains outlet -> Earth on TD-12 = ~0V
Earth on speaker cabinet -> Earth on TD-12 = ~6V

Huge resistance between Earth on speaker cabinet and Earth on TD-12, means minimal current flowing between the two.

I don't have any other mains powered audio equipment to run to the active speaker, apart from hifi, so stuck.

Cheers

Jonathan

[lopan]

I also get a hum sometimes when using the master outputs of my TD-12 or my TD-20. I bought a direct (DI) box with a ground lift input and that has solved the problem for me.

[Hercules]

The hum is probably indicating a poor ground connection

- try balanced cables from your master out to the powered speaker
- try taking an output from your headphones to your powered speaker and see if the hum is still there
- you could try putting one of those magnetic collars on the power cables (I doubt this will do anything but it's worth a test)

[jtoynton]

Hello,

Thanks for all your replies, I'm going to try the following unit this afternoon:

"Behringer ULTRA-DI DI100 Ultra-Rugged DI Box for Stage and Studio
Professional, multi-functional active DI box for all studio and stage applications."

£26

I'll let you know how I get on.

Cheers

Jonathan

[jtoynton]

Hello,

The DI box fixed my mains hum problem.
It has a button labelled "Ground Lift", when this was pushed in, the hum stopped.

Cheers

Jonathan