Question about ground loops

[FennerMachine]

I have a ground loop.

It is not very loud but slightly annoying. I could probably live with it as only occurs on the input I use for movies. When watching movies I don't notice it except for almost silent scenes.

The hum increases in volume with the volume control on the Quad 44.

Where should I start looking or is it just a process of elimination?

I'm fairly good at eliminating faults logically, step by step, but have found ground loops to be very time consuming so would appreciate any advise!

 

[dumbledore]

Try connecting the source to another input to confirm that the ground loop is on the input and not on the source.

 

[FennerMachine]

Swapped the input – hum moves to the new input with the cable from AVR.

Swapped the cable – still hums.

I've moved some equipment around so interconnects are not running past the Quad 303 – still hums.

The hum starts when the AVR is turned on.

I have the Denon, Quad 44 & 303, 751BD and TV all plugged into an Isotek Cleanline but I think the only mains earthed items are the TV and Quad 44.

Next to try – disconnect the outputs from the 751BD to the AVR & use different interconnects if that makes a difference. I could also try the Quad 44 in a standard surge protector.

If I can't eliminate it I might have to put up with it. I can only just notice it when sitting but because I know its there...

 

[scene]

Couple of questions:

1. Have you got a TT connected? If so, and you've got a ground wire connected between the TT and the amp, try disconnecting it and see if the hum goes away

2. Have you got the AM aerial connected to the back of the amp? If so, try disconnecting it and see if this helps.

Do you get hum from ALL inputs connected to the AVR?

 

[FennerMachine]

1 – no

2 – no

3 – good question! I could try the pre-outs of the rears maybe to see if they hum.

 

[dumbledore]

I have the Denon, Quad 44 & 303, 751BD and TV all plugged into an Isotek Cleanline but I think the only mains earthed items are the TV and Quad 44.

Try removing the TV from the socket to test if the hum is still there. If so then the ground of TV and Quad44 is your loop. If this fixes you could disconnect the ground from the plug on the Quad.

 

[John Duncan]

Seems the 751 is partial to ground loops...

I solved it with a five quid isolator from eBay.

 

[dumbledore]

Seems the 751 is partial to ground loops... I solved it with a five quid isolator from eBay.

This should fix it. But can he live with the degradation in sound caused by the cheap isolator? It may be better to live with the hum!!!

 

[John Duncan]

Can't say I've noticed any difference tbh (having compared with and without)

 

[Mooly]

Is it physical ? Are the units in close proximity to each other (one sat on another). If so try moving them apart.

 

[FennerMachine]

Mooly – the equipment has fairly good spacing with nothing on top of anything else but thanks for the suggestion though.

I tried several things with no change:

Disconnected the 751BD completely

Tried other pre-outs from AVR

Disconnected the power to the TV

Tried Quad 44 and AVR in different surge protector to a different mains socket (both done separately).

Disconnected and reconnected cables

Could it be something other than an earth loop? Could it be the Denon AVR is developing a fault?

Bit of a coincidence if it has as I noticed no hum until moving things around for the 751BD.

I don't notice the sound with my PC on, I have to get really close to the speakers to hear it but with the PC off its noticeable from where I sit. Maybe its been like it ages and I just didn't notice!

If its not a ground loop then an isolator won't work will it?

I'll leave it for now and maybe have another go another day.

Any more suggestions welcome!

 

[John Duncan]

Oh hang on, so you still have a hum with the 751 not even plugged in?

 

[FennerMachine]

Yes, still hums, so its not the 751.

I may have had the hum for a while and just not noticed as the PC was always on when using the Denon before. Makes me think its a fault with the Denon.

Just disconnected loads of bits leaving just the 751, Denon & Quads connected. Any other cables still connected to these devices did not go anywhere. Still hums.

The Denon is nearly 10 years old now but it would be a shame to replace it if there is something else causing it. There are other devices plugged in to wall sockets upstairs, fridge, freezer, router, telephones, but the four items I had plugged in are all running from an Isotek Cleanline which should filter noise from the mains and provides a common ground. Nothings perfect though so I can't depend on that.

 

[dragon76]

Do I understand your set up properly - do you use AVR's preouts to your Quad preamp for movies? Do you have TV antenna anywhere near your setup? If so, try disconnecting it from the socket. I had very similar issue as yours sometime ago and that helped in my case.

 

[John Duncan]

You're using the Denon's preouts into the Quad right? What happens if you plug the 751 straight into the Quad or straight into the Denon? I have heard issues with unity gain channels where the noise from preouts (or from the cranked up stereo amp) was quite high...

 

[dumbledore]

Yes, it looks like the denon AVR has had it. Is possible the hum is introduced by the AVR power supply going a bit out of spec. Refurbishing the unit may do the trick. If the noise is introduced by the powesupply a isolator will not work. However, this is not certain so you could still use one as is not expensive and will help you debug the hum issue. If the hum goes than you know the issue is a ground loop. You could attach the AVR to another amplifier to see if the noise is still there.

 

[FennerMachine]

Denon pre outs to the Quad 44 causes hum when that input is selected. I have tried it on another input and it still hums.

The 751 to the Quad with the dedicated stereo out is OK.

The hum occurs even if the 751 is completely disconnected so it must be related to the Denon.

I think dumbledore may be right. The Denon may be on its way out.

I have a friend who can get it serviced for me.

I will order a ground loop isolator to try just in case. They are cheap, should be handy for future use.

 

[FennerMachine]

Update.

Tried a ground-loop isolator.

Isolator cable from Denon pre-outs to cable plugged into Quad 44.

Still have a hum.

I have done a bit of searching for a solution and it looks as though a few people have similar issues with Denon AVR's (searching for other specific makes finds similar problems with those too, so not a Denon specific problem).

It could just be the Denon is failing and needs a good service.

I have Emailed Denon to ask them what they think.

I have no other amps to try with the pre-outs and don't want to strip out my whole system unless I absolutely have to to find the cause.