Ground lift?

[vinyl_junkie]

Hi, I have a old disco mixer from the 70's that I imported from America. It obviously runs on 110v, has a American 3 prong plug with the earth and so on...

Now here is the thing. As soon as I put the plug from the mixer into my step down transformer even with the mixer and step down turned off (even from the wall) it causes a hum on my amplifier.
As soon as I unplug the mixer, hum goes away.. If I unplug it from the step down (with the plug off) hum goes away.. If I only touch the earth pin of the mixer to the step down hum appears again.

Having had a look at the mixer the earth goes straight from the plug to chassis which also seems to be the common earth for other bits.

I have disconnected everything else from the mains other than the amp and said mixer. The amp is a class II device.

Is there a way to isolate the earth for this device or can solve this issue?

[Hugh Robjohns]

It certainly sounds like a classic ground loop problem, but it can't be a ground loop if only one device is connected to the mains ground... So either the amp isn't class 2 or you've got a secondary mains earth connection via some other circuit path or physical contact.

Anyway, the usual solution is a line isolation transformer between mixer out and amp in, something like an art Cleanbox 2 or art DTI box.

H

[vinyl_junkie]

Thanks Hugh.

I think you may be right about the amp, my cock up writing this up.

The plug to the amp is a standard 3 prong kettle lead with a earth pin but inside the amp the earth pin from the IEC socket isn't connected to anything.

[Hugh Robjohns]

That is very unusual. Most power amps and hifi amps are earthed. It's the sources like cd players, cassettes and tuners that are normally class 2.

But if it really isn't grounded,then you need to look for a ground path elsewhere. Is the metal case of the mixer touching anything else made of metal that could be grounded?

H

[vinyl_junkie]

But if it really isn't grounded,then you need to look for a ground path elsewhere. Is the metal case of the mixer touching anything else made of metal that could be grounded?

H

It's not touching anything at the moment and I have nu-plugged everything from it other than the main output rca going into the amp.

Forgot to mention but the amp only hums on the input on which the mixer is plugged in to. Thing is it only hums as I say when the earth is connected to the plug socket which I find odd.

As soon as I unplug it hum goes away albeit mixer is turned off.

[vinyl_junkie]

Also in a mad dash I went and unplugged everything in the house other than the fridge and those bits which made no difference hahah

[Hugh Robjohns]

Thing is it only hums as I say when the earth is connected to the plug socket which I find odd.

As soon as I unplug it hum goes away albeit mixer is turned off.

Not odd at all. It's exactly what a ground loop does.

If you have a multimeter, unplug the amp and measure the resistance between the earth pin on its mains plug and the sleeve of the RCA input socket. I'd expect it to be close to zero ohms...

H

[vinyl_junkie]

Thing is it only hums as I say when the earth is connected to the plug socket which I find odd.

As soon as I unplug it hum goes away albeit mixer is turned off.

Not odd at all. It's exactly what a ground loop does.

If you have a multimeter, unplug the amp and measure the resistance between the earth pin on its mains plug and the sleeve of the RCA input socket. I'd expect it to be close to zero ohms...

H

Just tested this and it's actually it's quite high! 412 ohms I get. Tested the mixer and that's close to zero ohms.

[vinyl_junkie]

Not a good day today but wrong again, read on hahah

Ok took the lid off the amp, I was thinking how the hell can I get a reading if there is no ground connected inside.. Well... The pin from the IEC socket isn't connected to anything if you look at it from the rear but the socket is soldered on the PCB board cough earth pin soldered, the protruding pin at the back threw me off!





So now on the amp from the earth pin to the chassis resistance is low (close to 0) but earth pin to sleeve of RCA inputs it's high as stated in prev post.. About 414 ohms

Would this high resistance be the cause of the hum?

[Hugh Robjohns]

Okay, so the amp is class 1, and the problem is a classic ground loop. As I said the most practical solution is a line isolating transformer in the audio signal path to break the loop via the unbalanced cable screen. Since you're using RCA connectors, the ART DTI box would be very convenient, or you could try the cheaper car hifi ground isolators you'll find in Maplins.

I'm slightly surprised that the RCA ground to chassis resistance is measuring 400 ohms... But that's not the issue. The fact is that ground loop currents are getting into the amplifier ground reference and becoming audible as hum.

H

[vinyl_junkie]

Silly question but is there a way of remedying the fault without using line transformers?

[Hugh Robjohns]

Not sensibly and safely.

You could try cutting and isolating the RCA cable screens to break the ground loop... but that risks picking up unwanted RF interference instead of hums. I would recommend cutting at the amplifier end if you want to try it. But the line isolating transformer is by far the best way.

I'm assuming the power amp and mixer are plugged into adjacent mains sockets. If they aren't, try it -- it might reduce the ground loop hum enough to render it inaudible (although it will still exist).

This kind of issue is endemic with unbalanced connections, I'm afraid.

H