Possible earth loop frustration

[eamon]

Hi all

 

I am a bit stuck in problem solving a really annoying issue that I cannot seem to cure.

 

I am doing a gallery install that has kept changing leaving me to get it all working the night before the opening....it being the typical way. I have now a very simple install but I have a persistent hum from my set up.

 

 

set up:

 

a) 40" sony Tv mounted on wall.

b) Media Player (small USB) connected to TV with HDMI.

c) Active 4" speaker mounted directly above the tv.

 

TV, media player and speaker are all fed from the same supply.

 

 

I am coming out of the TV headset outlet and running into the input of the speaker. I am using a mini jack to 2 x phono plugs to connect straight into the active speaker. The input of the speaker is phono. There are no other inputs on the back of the speaker.

 

I am getting a hum which (I believe) is stemming from the unbalanced cable I am having to use. I understand the science of the matter and am used to fixing this in a proper theatre environment. I cannot, somewhat frustratingly seem to solve this earth loop.

 

Both the TV and Media player are class 2 and do not possess an earth.

 

 

I have tried a) laptop humstopper from Maplins, b) active DI w/earth lift, and a variety of other methods to eliminate the aforementioned hum.

 

I am wondering if anyone has any possible pointers or advice on how I might solve this fun problem.

 

 

I appreciate this is an earth loop but all 3 x units are class 2 with no earths available at the plug end! For the purposes of testing, I lifted the main earth and still the problem persisted. Please note, this was for a test only and even if this did work, I would not allow this to be left unearthed. The earth has been re-connected. Please do not start a flame thread on the what for's and why's; I do understand the reasoning and believe strongly that only fools do this as a method of practice.

 

Hopefully some one has a possible idea or has being in this situation before?

 

 

Thanks in advance.

 

Eamon

[KevinE]

have you tried earthing one of the signal grounds/cable screens? A fully floating system can hum if there's no path at all for the screens to earth.

Obviously not an earth loop if there's no loop formed by the earth....

[eamon]

Hi Kevin

 

I tried earthing on the back of the speaker through one of the metal screws in the vain hope that might cure it.

 

I am not sure if I am reading you correctly re your idea:

 

Do you mean to run a link from an earth (13 amp plug etc) to one of the cables and splice with it the ground of the phono lead? Sorry, if I am being a bit thick but this wrecked my head etc.

 

 

Eamon

[jamessmith2k]

Have you tried the speaker from a seperate source to make sure its not a speaker fault?

[Whiskers]

Is your hum 50 Hz fundamental or in any way picture related?

 

is there an audio out from the media player?

 

What happens if you remove the HDMI cable?

 

if you use a battery amplifier or very carefully headphones do you still have hum then?

[KevinE]

easy to wrap a bit of wire from one of the phono screens to a mains earth, just to see if the hum disappears. I assume it is actually hum, and not some switching noise from one of the devices

[Shez]

I was also thinking about a separate audio output from the media player but even if it has one, you may get sync issues. Surely the TV has a proper line output? That would be much better than using the headphone output that really isn't intended to be connected to anything other than headphones.

[pmiller056]

If everything (player, speakers, screen) is class 2 unearthed, try connecting the screen of the audio cable (at one point only) to mains earth. I've been in similar situations and it has worked for me. Use a spare 13A plug plugged into the same socket strip as the rest of the stuff and wire some green/yellow wire from the earth pin to some exposed metal on the audio equipment. Take care if you are doing this whilst everything is plugged in, the combined leakage current can give you an unpleasant tingle if you get yourself in the way.

[Ynot]

You may have already done this, but the essence of any fault finding exercise is pretty much isolate each element first by way of alternate sources etc.

 

Speaker - send it something from a laptop to prove it's not humming on it's own on receipt of a signal - and ideally to start, do that via a proper phono pair (I'd use the small USB to phono gizmo we have here). If the the phono feed is clean, go back to your mini-jack to phono adaptor into the laptop and try again (though I'd echo Shez in saying that really isn't the best option as a source). Or even try that mini-jack into your mobile's output.

 

The TV itself - as already suggested, what do you get on actual headphones out of that socket?

 

And the media player - what do you get if that's plugged direct into the speaker?

[timsabre]

A lot of headphone drivers now use bridge tied load (BTL) outputs which give a DC offset on the signal. This is not good if you connect them to something else, even when everything is class 2. I'd agree with the others to try and get a phono output.

[revbobuk]

If the audio is coming from the HDMI output of the Media Player to the TV and thence to the speaker, try using an HDMI audio breakout box, if the Media player has no separate audio out, to bypass the screen altogether as far as audio is concerned. I've got a spare one sitting on my desk at the moment!

[eamon]

Hi all

 

Many thanks for the responses and helpful advice.

 

The late reply is due to the aforementioned install and the looming deadline.

 

The original setup for audio was much different than the actual way it ended up- aint that a surprise....

 

I was worried about a possible earth hum but I originally had a zone controller in the mix due to a greater audio desire but the plans changed late in the day and I ended up with the situation I did not wish to have!

 

Many thanks to Kevin & Pmillar who suggested the solution. Their method of grounding the shield work a treat. Problem disappeared and people think I am now a genius...!!

I had often wondered about doing something like that but was never sure of the ramifications etc.

 

Once again thanks for all the help.

 

Hopefully the willingness to share information and help each other out will never leave this industry.

 

Eamon

[kgallen]

Shunting the problem to mains earth like this doesn't feel like an ideal solution to me, more like a workaround. Mains earth is not a "for free" problem solver, it's there for the specific purpose of creating a high fault current path at mains voltage to trip a circuit protective device. In addition many devices create a functional earth current through the leakage of e.g. EMC suppression devices - X and Y caps and the like (*). Creating a path for this current back into the relatively sensitive audio circuits of an install doesn't seem like a wise move. Further introducing a mains earth into appliances qualified as class 2 would surely imply a violation of that qualification.

 

Can't you isolate the real root cause here - going back to Tim's thoughts on output stages and the audio return paths that exist in the system? Thinking along the lines of audio isolation transformers instead if there is an identified audio ground potential difference caused by the required audio connections?

 

Thoughts?

 

Regards,

Kevin

 

(*) pmiller already cautioned this issue. A "tingle" from an audio circuit? That can't be right, good or acceptable.

[pmiller056]

Mains powered entertainment equipment made to EN60950 (or similar), Class 2 (double insulated, no earth connection required) is permitted to have a small earth leakage current (off the top of my head about 0.5mA) from any exposed metalwork to earth. You probably won't notice this level of leakage current in normal use. The problems start when you have multiple class 2 items directly interconnected because the leakage currents will add up - this is when the combined leakage current gets noticed as a tingle. This IS an acknowledged shortcoming of this standard, but I don't know of what is being done to remedy this. In this situation, the reference potential of the entire system (ie the cable screens) ends up with some portion of the mains voltage impressed upon it, causing the hum and buzzing noted, particularly if audio circuits are poorly screened internally. By adding the earth connection as suggested, you are providing the whole system with a stable reference point. Note that this is NOT a safety earth connection intended to pass a fault current - it is a signal/reference earth only.

 

Peter

[kgallen]

Note that this is NOT a safety earth connection intended to pass a fault current - it is a signal/reference earth only.

 

Understood, but it is introducing an "earthy" potential into the equipment and such reasoning was behind the introduction of main and (as was) supplimentary equipotential bonding in electrical installations. A fault current can't tell the difference between a intended earth fault path and one not intended as a safety earth connection.

 

To the wiring regs, functional earth currents can be up to 5mA from which alternate earthing strategies and protections have to employed. As you say, these functional earth currents add up from the whole final circuit into which the equipment is connected.

 

A fully floating system can hum if there's no path at all for the screens to earth.

Kevin - this is not correct. The return path current for any circuit wants to get back to its origin, the general mass of earth doesn't come into it. Hum arises because a return current takes divergent paths to get there and some of those paths are through the audio stage.

 

 

Thanks,

Kevin