Possible grond loop problem

[Overdose]

I have noticed (quite some time ago) that my speakers eminate a 'hum' from the main drivers. I did initially think that the fault lay with the speakers, or that it was just inherent in this model, however after trying out a few things, it appears that the hum is only evident when there is no connection from the preamp (audio interface). The UA-25 is USB powered and so needs the computer to be on, but when it is switched on, the hum almost disappears completely.

I am now using a dedicated ring circuit for my AV, but the problem was evident prior to this being installed.

So what is going on here? Is this a ground loop problem? If so, what is the solution?

 

[Andrew17321]

Not a ground loop. In fact the very opposite - no gounding when the input wires are not connected to anything. The input connector is picking up mains hum that pervades the room and the amplifiers are amplifying it. Quite normal.

As there is no problem when the pre-amp is connected, forget it.

Andrew

 

[Overdose]

If it is mains hum that is being picked up, why does the hum disappear when the preamp section is turned on? :?

Would the hum not be persistent?

 

[Andrew17321]

I would assume that the reason is that when the preamp is turned on, it puts a well controlled voltage on the connecting wire; when it is off removes this voltage - it probably internally disconnects the output leaving it floating, so that the wire picks up the hum. Alternatively, it might even earth the connection, in which case the hum could indeed be from a ground loop, but you would probably get it all the time then. Depends on the equipment.

As there is no hum when all is connected up there isn't anything to worry about. However, if you are curious and want to investigate further you could try the following.

1 Remove the interconnect from the speaker, short the input socket with a short bit wire, eg bent paper-clip, between the inner and outer connection, so that there is definitely 0 signal in. If there is no hum when you increase the volume, then you can be sure that your active speaker is not responsible for the hum.

2 Plug the mains input for the speaker into the same socket you use for the preamplifier using a two way adapter. If the hum has gone with the preamp off, then you do indeed have a ground loop (when the preamp is off). If that is the case, and you really want to eliminate it, you can cure it by disconnecting the earth lead from the pin in the mains plug for your speakers. While I have done this to cure hum in my sub, I cannot recommend it, especially if you have young kids.

Andrew

 

[Overdose]

OK, thanks. I'll have a closer look when I get a chance.