Ground loop between pre-amp and power amp?

[Simon_LDT]

Check to see if it is earthing properly with a meter( the socket on amp)

How can I do that? I've got a Voltmeter, is that the right tool? I'm not very tech-savvy when it comes to electrics.

[Simon_LDT]

Almost certainly not the problem, but just check all the 7 channels on the nak are switched to rca (not xlr). Can't imagine it would make a difference, but ...

Also, does your processor have any ground switches on the back? Some stuff I've had in the past has a ground lift switch, or one you flip for rca or xlr outputs.

Yep, flicked all those to rca soon as I got it wired up with my older AVR. No ground switch on the Emotiva.


EDIT: When adding a third (a final RCA for what I'm using it for - to power center and rears) the hum gets a bit louder. So when I've got a sole rca lead plugged in (and it doesn't matter which output I use from the pre) I get no hum, add a 2nd and I get some low level hum, and then a 3rd the hum gets louder.

[struth]

Initially i would check on ohms that the outer of rca is grounded ...if one is definately working then initially we can hope it is ok so try a reading from outer to outer between the 2 in case its gone hi resistance...could have a dry connection. If its an earthed chassis then you should get a reading to the earth pin of mains plug too..obviously switched off when on ohms..should read 0.00 or very close

Also you could try moving cablescand units about as you can often get a hum induced from a tx ,psu or mains cable etc.

[Macca]

Right, this is really strange. Unplugged everything and plugged back in the power amp and processor/pre (I've already confirmed that the power amp itself is not causing an issue as with all 3 channels I currently have connected to it are hum-free at this stage).

I then plugged in the surround right rca lead and no hum occurs. I then plug in the surround left rca lead and then the hum starts (and is in both rear channels). Why would plugging in a 2nd rca lead cause this? I even tried a different rca lead and the same thing happens. Even tried a different input (in case it was faulty) and again the same result. I haven't even plugged in anything else (HDMI, Optical, Coaxial, etc) yet the same hum is already there...

If you are confident that you have eliminated the power amp and the interconnect from the equation, and nothing else is plugged in then it would seem to be a problem with the rear channel output on the processor/pre-amp

[Simon_LDT]

If you are confident that you have eliminated the power amp and the interconnect from the equation, and nothing else is plugged in then it would seem to be a problem with the rear channel output on the processor/pre-amp

I've also tried all the other outputs (7) with each rca lead and the same results. Doesn't matter which output I use or which cable, each time I add a 2nd rca lead the hum begins. Very strange.


EDIT: I also just tried putting some speaker cable between the 2 amp's chassis and nothing happened there either.

[Oldpinkman]

I've also tried all the other outputs (7) with each rca lead and the same results. Doesn't matter which output I use or which cable, each time I add a 2nd rca lead the hum begins. Very strange.


EDIT: I also just tried putting some speaker cable between the 2 amp's chassis and nothing happened there either.

It doesn't need to be speaker cable - any conductor will do . Are you sure you got bare metal on the chassis? I'm sorry if this sounds daft - but just to check, you didn't try touching cable to painted chassis did you? You need to undo a screw and wrap a bit of mains flex round it to make sure you have a metal to metal connection between the 2 chassis.

[Simon_LDT]

It doesn't need to be speaker cable - any conductor will do . Are you sure you got bare metal on the chassis? I'm sorry if this sounds daft - but just to check, you didn't try touching cable to painted chassis did you? You need to undo a screw and wrap a bit of mains flex round it to make sure you have a metal to metal connection between the 2 chassis.

I undid a screw on the underside of each amp and wrapped the cable around and tightened it up securely. That no good?

I'm really not good at this kind of stuff.

[Oldpinkman]

I undid a screw on the underside of each amp and wrapped the cable around and tightened it up securely. That no good?

I'm really not good at this kind of stuff.

No - that should be fine. Puzzling. I hate fixing hum problems when they are in front of me, never mind trying to do it remotely.

[Oldpinkman]

You said you have a volt meter. I take it that is a simple multimeter and you can measure resistance - ohms. Just so I can let go of this bone, try unplugging both devices from the mains and measuring the resistance between the screw you undid and the earth pin on the plug. Confirm you got zero ohms (or very close to zero). You could also fit your chassis ground wire, and measure the resistance from the earth pin on one plug (preamp) to the earth pin on the other plug (power amp). Should be zero or near zero.

If the problem is absent when there is a single connection between the 2 devices, but adding a 2nd channel adds hum, it suggests a ground loop of some sort, but not an easy one!

[Simon_LDT]

You said you have a volt meter. I take it that is a simple multimeter and you can measure resistance - ohms. Just so I can let go of this bone, try unplugging both devices from the mains and measuring the resistance between the screw you undid and the earth pin on the plug. Confirm you got zero ohms (or very close to zero). You could also fit your chassis ground wire, and measure the resistance from the earth pin on one plug (preamp) to the earth pin on the other plug (power amp). Should be zero or near zero.

If the problem is absent when there is a single connection between the 2 devices, but adding a 2nd channel adds hum, it suggests a ground loop of some sort, but not an easy one!

This is the voltmeter I've got: http://www.amazon.co.uk/Skytronic-60.../dp/B000L0OHIK

Which setting do I need? I've currently got it on the 200 setting but I'm getting numbers jumping all over the place.