I have an A/V system with two active sub-woofers.
One is close to the amplifier connected with a 1m RCA cable. Its mains connection is into the same power strip as the amplifier. There will be an earth loop but it is short and it does not add a significant amount of noise. Update, I now know that the amp is not directly earthed but it may be through one of the other devices.
The second sub is further away and is connected with a 10m RCA cable. It is not actually that far but the cable needs to go around the wall and not across the floor. 7m would be sufficient but that is not a commonly available length. I tried a 5m cable with a 2m extension but it was not noticeably better. The mains for this sub is close to the sub and hence not close to amp. This earth loop is much larger and picks up a significant hum which I would like to eliminate.
Further evidence that the problem is an earth loop:
The hum is stopped if the signal cable is disconnected. I tested by disconnecting the at the sub end. My initial concern was that there was a fault within the sub. I have tried disconnecting the signal cable at the amp end. Initially, I got a worse hum but when I shorted out the loose end, this went away.
If I feed a signal from a battery powered iPod, there is no hum. The iPod was connected with a short cable. I'll test with a longer cable along a similar route to the usual signal cable and update later.
Temporarily breaking the supply earth while connected to the amp also stops the hum.
All plugs are three pin and connected to grounded sockets. It is possible that the earth is not used but test 3 suggests that the sub is certainly grounded. I could repeat test 3 but break the earth at the amp end as an additional test. Most of the plugs are sealed and not easy to inspect.
So, a simple and effective but maybe not safe and legal solution is to break the supply earth. I would like to find a better solution.
I found this item on Amazon: AV Link Ground Loop Isolator. Cheap and easy to fit. I feared that it might attenuate the signal and cause harmonic distortion but a quick test using test tones from my iPod and my ear to judge suggested that it was not too bad. However, when I connected it with the amp, it actually made the problem far worse rather than better. I guess that it contains a transformer which was picking up more hum than the earth loop.
I found other items, e.g. PAC SNI-1 Noise Isolator which did not seem to be transformer based, but the reviews were terrible so I did not even try them.
So, I am still looking for a solution and preferably not a very expensive one. The devices above are passive but I would consider a powered device.
It is common advice to keep the signal cables away from the mains and that is already the case here but I was considering the exact opposite: power the sub from the same power strip as the amp and the other sub. Hence, it would need an equally long power cable as the signal cable which I would run together. The idea is that both the signal and power cable would pick up the same hum but out of phase and hence they would cancel over. Is this worth trying or have I missed something?
I am in the UK so the supply is 230VAC 50Hz. Since this is the UK, I expect that everything in the house is on the same phase. In this case, both sockets are on the same breaker in the consumer unit.
The AVR is a Denon X3200W - manual.
The subwoofers are BK Gemini IIs. I cannot find an online manual but even the supplied paper manuals are very simple.
Other devices connected to the amp are: the TV, a Tivo (digital video recorder), two disc players, a digital iPod dock, and the other sub. The sub uses an RCA cable, the iPod dock uses an optical cable, and all others are HDMI.
All devices except the Tivo and the distant sub are powered by the same power strip. The Tivo goes to another power strip but both strips go to the same double socket. This is partly just a matter of sockets on the strips but also one strip contains devices that can be off when no one is present and the other is for devices that should remain on: Tivo, internet router, phone base station, etc.
I have now checked the amp end of its power cable and it has a two pin connector. So, it is not earthed and my loop must be through one of the other devices, possibly even the other sub.
Thanks to the comments, I have more ideas to try. Annoyingly, repeats of tests that I have made before don't seem to entirely consistent. Possible explanations of the inconsistent results are that my hearing varies and other noises vary. E.g. sometimes I hear the hum from a fridge two rooms away more clearly than the hum from the subwoofer. More research is required.