Recommend a ground loop isolator
I'm after a good quality/value ground loop isolator that works, to place between the preamp and sub on the stereo line-in's rca phono cables. Any recommendations?
Ta.
The AV receiver is connected through the preout to the LFE on the sub. The stereo preamp is connected to the stereo line in on the sub. When I use the stereo with the 2 channel setting on the sub everything is fine. But if the AV is on for TV or DVD and the stereo preamp is on standby their is a low quiet hum. I have to disconnect the stereo line phono cables to stop the hum.
It certainly sounds like a ground loop. Is the equipment plugged into the same mains source or into different sockets? If the latter try plugging them all into the same socket (via a mains extension lead but watch you don't exceed 13 amps). If you can't do that, at least plug the two amplifying devices into the same mains source and the other kit elsewhere.
If you can get all your kit attached to the same earth (ground) the problem should go away. Good luck!
Chris
They are all plugged into a 8 socket surge protector. Preamp, power amp, the sub, AV receiver, TV, Sonos ZP90, DVD player, HD recorder. I didn't get the hum with previous amps.
Sounds like there is a poor connection in one of the amplifier mains leads then. Have you tried swapping mains leads?
If you have then I may give up!
Chris
Do you mean the mains power lead? I haven't tried that. If so I'll swap out mains leads for others and see what happens.
Ta.
Yep a ground loop occurs when there is a difference in the connections to ground (earth). If you have all the kit plugged into the same mains source then the problem must be between the source and the kit. That means the lead or worse in the internal connections.
Chris
Tried swapping to different mains leads. No joy I'm afraid. I shall try an isolator next.
The isolator I tried was from Amazon and had loads of excellent reviews. I plugged it in and instantly the hum got about 3 times as loud. I sent it back.
My system hums if the CD player is plugged in, whether turned on or not.
Could you tell me what isolator it was that didn't work so that I can avoid that one and get another type.
Tried swapping to different mains leads. No joy I'm afraid. I shall try an isolator next.
Oh dear! If it were me I'd open up the boxes and resolder all the mains connections but I can't recommend that unless you really know what you are doing. 
If you know a friendly electrician you might get them to try this for you.
Isolators are pretty cheap so it's worth trying it. Remember however that they are just filters and they will filter your signal as well as the hum!
Chris
PS Beware of "ground lifters"! These work effectively to eliminate ground loops BUT they do it by disconnecting the earth on one of the pieces of equipment and this is DANGEROUS and carries the risk of electrocution.
PPS Have you spoken to the dealer you bought the kit from?
Could it be worth disconnecting the shielding on the 2 RCA leads that go from the 'pre amp out' to the 'line-in' on the sub to see if that cures the hum? Or is there a risk attached to that?
I don't think that will help I'm afraid.
Chris
Don't really know your overall system setup, but when I had such a problem with my system (AV receiver into HT bypass of the stereo amp) I determined the hum was coming from the TV antenna, which socket was just behind the equipment rack. When disconnected from the socket the hum dissapeared. I then used my power filter to significantly reduce this ground loop - antenna cable from TV to power filter and then another cable from the filter to the wall socket. Maybe this helps.
I assume you have some noise, maybe 50Hz hum, arising from a ground loop? If so I'd try to get rid of the ground loop before putting anything in the signal path.
Chris