[mkjustuk]

I have a Rotel power amp which has a three pin plug. I also have a Sony receiver doing duty as a processor until I can afford a Rotel.

The Sony has a two pin plug with a UK adapter that came as factory issue. The Sony is double insulated according to the back panel.

The Rotel hums quietly through all speakers when on regardless of the Sony being on or off. However, if I disconnect say the centre phono lead at the Rotel end the humming in the centre speaker stops completely with the Rotel still on.

So, I know the amp is fine and it is something to do with the physical connection between the Rotel and Sony. My hunch is it is to do with the Sony not being properly earthed, but how can I solve that?

My only thought so far is to buy an open three pin plug and wire up just the earth. Plug it in and connect the other end to a case screw on the Sony - would this work or is it too simple?

[grimep]

Sound like an earth loop, pretty common in home recording studios. It occurs when there is more than one ground path between two items of equipment.

http://www.soundonsound.com/sos/Nov0...lation1102.asp

If you can't be bothered reading all that, try sticking the mains leads into different power outlets...

[mkjustuk]

Thanks for that, I will have a wade through it.

In the meantime is there any risk of damage to anything if I carry on using the setup as it is?

[Ianfromnotts]

I used an impediance matching transformer to cure a similar problem the only trouble is it isnt a cheap solution. The transformers have to be high quality if sound quality isnt going to suffer. The ones destined for audio use like audio innovations from the 80s come as stereo units and sell second hand for about £100/£120 each on ebay. If you want to try a cheap solution to see if it helps before spending lots of cash this might help. I am sure if it affects sound quality very badly or doesnt work you can return it

http://www.maplin.co.uk/Module.aspx?...=SEO&U=strat15

[mhuk05]

If you're handy with a soldering iron you can try soldering a 10ohm resistor between one of the RCA inputs and the amp chassis.

also:

http://www.subwoofer-builder.com/earth-loop.htm
http://www.epanorama.net/documents/g...oop/index.html
http://www.audionote.co.uk/articles/art_balanced.shtml
http://www.jaycar.com.au/images_uploaded/humloop.pdf

[mkjustuk]

Thanks for the info Ian - alas that link doesn't work (looks like a session link) do you recall the product name/number?

mhuk05 - pretty handy with a soldering iron - do I solder inside the amp from the shield of the RCA input to the back panel (apologies if it is discussed in one of the links I will review those tonight)?

[mhuk05]

These are the exact words told to me (to help me fix hum on my power amps):

Also we can send you a small 10 Ohm resistor(N.C.)
so you can solder it at the RCA Input to the chassis ground to
kill any ground loop that might cause the hum.

You may also try to loosen the bolt that holds the toroidal
transformer and rotate the toroid left or right a bit(movement
restricted to less than 0.5 inch) and avoid the peak of the magnetic field
that might invade the PCB.

Saying that, I bought a 10R resistor from Maplin and it's huge!

The Maplin link above is for Ground Loop Isolator:

http://www.maplin.co.uk/Module.aspx?ModuleNo=33172

[JudgeBaxter]

Out of interest, which model of Sony receiver have you got, there was an issue with a couple of models from 2 or 3 years back that gave a constant hum through the speakers........?

cheers,

dave.

[mkjustuk]

The receiver is an STR-DB1070 but it doesn't hum itself, just when hooked up to the amp.

I figure the easiest way to progress is to unhook everything from the Sony (except the 5 phonos) and then plug things in until I get the hum back. If I get hum just from the Sony and the 5 phonos then I guess I will have to go down the ground loop isolator root, but I don't like the idea of some cheap cabling in the way.

[Buckster]

I tried everything to get rid of my ground loop hum on my Rotel Power Amp (caused by my BFD ...) - and today finally did it- bought a Shark "Car Audio" Ground Loop Isolator - put it between my sub pre-out on amp - and the BFD - and all ground loop noise now gone - just hope it hasn't effected the quality of the audio to the sub now ...

Mark.