subwoofer hum

evulboo

evulboo

evulboo

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Cornwall / Helston
Hey, I was wondering if anyone could help me. I bought a new 5.1 speaker package on the weekend which consisted of monitor audio mr4, mr1, mr centre and the mrw-10 sub. It's all set up and working fine, but when it's quiet I can hear a low level hum coming from the sub. Now I've tried a couple things to try and sort it but it still remains and I'm a bit of a loss.

The hum remains if I pull the rca cable out, turn the volume down, try any of the phase crossover dial/switches. I've tried different plug sockets around the house, I've unplugged the cooker & fridge lol I know but I hear they can create a earth loop?

I'm now booked in for an exchange on Saturday but maybe I'm missing something?

Cheers
 
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Had this problem myself on a previous sub. At the time it really did drive me crazy. I bought all kinds of fancy cable, group loop isolators etc. nowt worked. In the end I sold it and upgraded to a monolith.

Seems like you have done pretty much all the main checks. It does sound like a faulty sub but one last ditch check. Can you pop round a friend or family's house with it. Just plug it in, power cable nothing else. If it hums its faulty.
 
If the sub isn't connected to anything except the mains socket then it can't be a earth ground loop since there's nothing for it to "loop" with. TBH, ground loops are not as common as people believe. That's not to say that they don't occur because they certainly do. But often the finger is pointed at that when the cause is something else.

A lot of electronics no longer has a dedicated earth connection. It is "double insulated" instead. That means that there's no direct path to earth except via the signal cable, and then only if it's connected to something in your system that is itself earthed.

I'm not surprised about the slight sub hum when there's no signal cable connected. That's because the earth is free floating. However, I would expect the sub to be quiet once the signal cable is properly connected.

The main cause for hum is interference being picked up by the subwoofer cable. The reason for that is the cable having inadequate shielding. The type and quantity of shielding, along with how the cable is routed (sharp bends affect poor cable shielding more) and what it's running near (mains cables in particular, but poorly shielded PSUs and other signal cables too) all play their part. A properly shielded sub lead will deal with many of these issues by protecting the inner conductor from interference and dumping that interference to the system earth.

Cost of the cable is no indicator of quality. What you need to see is the cable's construction; in particular you're looking for a cable with high density woven shielding, and preferably two overlapping layers. Cables with foil shielding don't work as well for isolation at audio frequencies. Foil becomes more effective at the frequencies used for TV and satellite signals (MHz to GHz range and up) rather than the kHz range for audio.

Another thing that is useful where interference is an issue is proper coaxial phono plugs. These are the type that maintain the cable shielding properties right the way through the plug to the chassis. Ordinary soldered phono plugs don't do this.

If you're having the sub changed then that might cure the issue if the sub is faulty. But if you find there's still an issue then have a look on the rain forest for "subwoofer hum killer" in subwoofer leads.
 
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Thanks that made quite a bit of sence, Could you recommend me a cable please it's worth trying before I exchange the unit

Cheers
 
New cable didn't make any difference, going to make a trip to Richer Sounds on Saturday and see what they think
 
I'll be keeping the cable as it is a huge improvement in quality over the 1 I was using, perhaps my ears are just a bit sensitive? My girlfriend can't hear it unless she has her ear almost touching but I can hear it 4m away when it is quiet. It's not like it's loud but when u hear it it really bugs me.

Do subs always produce a slight hum?
 
It can depend on how loud the sub is turned up and the quality of the electronics. Also how quiet the house is. I have a big REL. That's silent even at max volume. I have installed other brands which had a slight hiss or hum if on max but only audible up close and when it's quiet. I have come across little Missions, Yamaha and Wharfedales that were audible from a distance.
 
The thing is, this slight hum doesn't change in volume if it is turned up or muted, its just constant, i'll post what happens tomorrow after a RS visit
 
That's actually a really important bit of info. It suggests to me that this is an issue affecting just the high voltage (power amp) part of the circuitry rather than the more normal interference that might affect the low voltage pre-amp stage.

This could either be a fault or just a limit of the circuit.
 
Quick update, just got home from RS who did confirm it was a faulty sub albeit only slightly. I've now upgraded to a bronze bxw-10 which is hum free, I've yet to push it in a film. Will have a play later

Many thanks for you help Lucid
 
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