Question Speaker hum

Russ_64

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Hi, I know this subject has been raised before and I have read all the threads but still a bit stumped.

So, I noticed that on the power strip I was using for my AV (Masterplug power-surge protection type) which has a green earth light, the green light was dead. I decided to replace it with a Tacima CS947 AV strip. It works fine but now I have a hum on my left and right speakers only.

My left and right (floorstanders) are connected to the Stereo Amp (M3i) which is hooked up to the AVR pre-outs. Both the M3i and X3300 (and Oppo) are plugged into the Tacima. There is no hum on the other speakers connected to the AVR (rear surrounds, center and Atmos).

To test it I tried my old bookshelf speakers and is same. I also unplugged my BK sub and no difference.

How is this happening if they are both connected to the same power strip? It is only audible when there is no sound and I am close to the speakers. Should I be concerned or is this nothing?
 
It certainly sounds like a ground loop hum. Disconnect everything from the plug except for the amps. Start with the M3i, if no hum then add the receiver then the other bits of kit until the hum appears. You should also knock off any appliances in the house, adding them back one at a time. Could be something as really simple as an earth wire in a plug not being tight enough.

If it persists get an electrician to check out the grounding of the house, it's a five minute job. My mates got his own electrickery business and he checks my circuits on a regular basis for me gratis.
 
You can test if your mains sockets have an earth connection by using a tester such as this:
61QFLhtBjEL._SL1000_.jpg

However, if you really want confirmation earth goes to earth you need a loop tester (which pushes some current from the live into earth to confirm it is good connection) such as this:
71df%2BSJl5FL._SL1500_.jpg

Either a great addition to anyones toolbox to ensure safety in the home.

Also be sure the amp is not picking up hum from a nearby transformer of the other components. Disconnect the input to the amp and see if the hum goes. If not, try moving the amp away from other components to see if that helps. Given the M3i is earthed at the main plug this would be the way a ground loop is being formed with one of your other components of their connections to other earths (e.g. via the screen of coax cables).
 
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Thanks - I will try to find it. But is it serious - what damage will it do?
 
The earth in a mains supply is designed to discharge electrical energy safely to earth in the event of a fault. So if equipment with an earth (called class I) has a failure of the primary insulation the earth will safely discharge this to earth (and blow the fuse). In double insulated equipment (class II) there is primary insulation and secondary insulation in case of a fault in the primary. If your socket-outlets had no earth then you have lost the protection form electric shock, should a fault occur. Both testers shown will indicate if this problem exists so you can take action.

Ground loop hum is something else, and is due to the normal earth leakage currents (typically 0.001 of an Amp but could be more) flowing from one earth to another via the screen of cables connecting components together. This low current AC signal enters the amplifiers low level inputs and becomes amplified causing the hum.
 
So, I noticed that on the power strip I was using for my AV (Masterplug power-surge protection type) which has a green earth light, the green light was dead.

Are you sure it's an earth light. I've got a Masterplug power strip and the green light on mine is labelled "surge" and isn't lit.
 
Are you sure it's an earth light. I've got a Masterplug power strip and the green light on mine is labelled "surge" and isn't lit.
All my protectors have the surge light lit in green, I've got six in all dotted around the house. If the green light goes out there has been a surge and the protector has done it's job.
 
Are you sure it's an earth light. I've got a Masterplug power strip and the green light on mine is labelled "surge" and isn't lit.
I guess it must be a surge protection light. Guess I put 2 + 2 and got 5. I think that over time my cables behind the rack are in a bit of a mess and will need sorting. The only inputs into the Amp are RCA from AVR and Oppo. Could be a power lead is making contact with these.
 
Surge protection plugs really don't do very much. The little protection component will blow with the first transient and offer no further protection. To combat a large power surge you need much more complex protection, as the transient could be many thousands of volts and potentially 10s of thousands of amps.

The hum is probably due to the Oppo player and its subsequent connection to the TV. If you can only just hear it, I would not worry. It won't do any damage and won't be audible or having any detrimental effects on the audio. Most current AVRs do not have a ground connection to the issues that TVs and receivers can cause. It's normally due to poor power supply design dumping noise on the audio earth and ground - as they are usually connected together within the TV.

It's quite possible that the power strip had a protection component that reduced the measured impedance between earth and neutral and this reduced the hum. If you replace the "failed" power strip, does this improve things?

Missing earths are dangerous in some circumstances and should never be intentionally removed, so if you can remove the plug top, check all the terminals are tight. Those little socket testers can be useful, but can sometimes indicate faults that don't actually exist. If it brings up a fault, get an electrician to test the circuit with more advanced test gear.
 
It's always best practice to keep power leads and signal leads apart, especially if the shielding breaks down to age. Easier said than done, the rear of my cabinet is like a rat's nest with at least 30 cables and leads, 10 of which are power leads.
 
I have done a bit of sorting out of cables behind my rack. Not easy and pulled a muscle in process but the result is that the hum/hiss is very soft, I can only hear it if I put my ear up to the speaker.

Just strange that it started with new Tacima power strip. And as my speakers are new I am a bit over sensitive. Think it is fine and music sounds great. Thanks for the advice and reassurance.
 

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