Mains interference - culprit found (see later post)

Due to all sorts of compromises, I have my A set-up in one corner of my room, my V in another (4-5m apart), and my speakers in-between, with one next to the TV and one next to the hi-fi. Both speakers are magnetically shielded ProAc Studio 125s, and they act as speakers for the hi-fi and the AV (no 5.1 or 7.1 set up here...).

A stuff is all isolated and shielded: Audiophile Base platforms, LAT AC-2 power cords, VDH D102 MkIII interconnects.

V stuff is more patchy: mixed QED and stock aerial cables, QED P-CV1 component cables, VDH The Name audio i/cs, and stock power cables.

Speakers are on Nordost Blue Heaven cable (stunning IMHO).

Connecting the two corners is a 6m length of QED audio interconnect (HiFiCables.co.uk do off the reel lengths).

All is gorgeous on hi-fi alone. But when I watch DVDs/TV with sound through the hifi system, for up to half of the time I get a noticeable buzz from the speakers. I normally run the amp at around 40 (vs. max of 72) - please don't ask me to turn this into some more scientific measure! If I want to check the buzz is really there and not a figment of my imagination then I can crank it up to 60/72 and it is horrible.

Crunch question: has anyone any idea what could be causing this seemingly SPORADIC buzzing? If it were there all the time I would in some way be happier, as I'd know I had a fundamental problem but I can't understand why sometimes it is there and sometimes it is not.

Any advice gratefully appreciated.

Nigel

 

Lift the 6m run between the video stuff and the audio stuff off the floor. You could try using some VHS tapes to do so. It may be interference from the mains and so on under the floor that is triggered by people doing things in other parts of the house or even somewhere else on the street.

I only say this because it sounds like a long run of cable and if there is going to be interference getting in anywhere, then it is probably this. Especially if it only happens when using the Hi-Fi for video.

 

Experimenting...

Thanks Tenson. I tried unplugging and plugging a few things today and noticed:
- buzz was there from the start and did not go away. Maybe I have just not noticed it at times but it's really been there all along?
- when I unplug the TV end of the 6m cables, all is quiet; when I plug them back into the TV, the buzz returns. TV off at wall throughout experiment!

How can this be? Any further ideas?

Thanks again.

 

Sounds like a typical earth loop to me ie a 50hz HUM rather than buzz?! It would seem that [typically] your TV is causing this, as even switched off the earth is still connected. Try connecting up with the TV un-plugged at the mains and I bet the hum's not there.

 

Thanks John. I was so excited I almost tripped on my way to try this - you're right, a hum rather than a buzz.... Bingo! Unplugging the TV rather than just switching it off sorts the problem.

Now I'm just praying you have Answer Part 2 - because trying to watch a film with the TV unplugged is not going to be particularly rewarding.

Should I try using one of my LAT AC-2 power cords from my hi-fi on the TV?
Or do I have to move to a part of the country/world/universe which does not have mains hum....?
or is there another fiendishly simple solution?

Thanks in advance,

Nigel

 

You can try plugging the TV into the Isotek. It may help. Really you want an isolation transformer.

Another possible solution is to put a 100ohm resister between the earth connections of the 6m interconnect at one end. It should reduce the hum. A stronger resistance would reduce it more but make the impedance of the cable go up!

Otherwise the solution is to disconnect the earth of your TV from the mains! It will cure the problem but I don't need to tell you it is dangerous if an electrical fault develops. The 6m connection to your Hi-Fi would then the be earth for your TV so your Hi-Fi would go BANG if there was a problem and if that wasn't connection then any person touching the TV might be electrocuted.

 

100ohm resistor/Isotek minisub

Tenson, this is good stuff, thanks. I tried one of my LAT AC-2 cables last night and, as you would expect, it did nothing (but probably transmitted a cleaner mains hum ).

Moving the isotek minisub from the A to the V end of the room will not be a trivial job but I'm going to try a short extension (power) cable from the TV to the hi-fi end (a) plugged intothe wall to make sure the problem is still there and it is not just a rogue wall socket, and then (b) into the Minisub to see if it helps. If so, then I'll have to consider forking out for a second minisub at the TV end of the room... but at least life will be hum-free.

I would also like to try the 100ohm resistor idea. Are these readily available in the marketplace, and how much might one cost?

All the best,

Nigel

 

Ground loop isolator

 

Mosfet, what exactly does that do?

I thought about it and I guess it can either put a resistance in the earth line.. or does it link the earth connections at each end of the stereo pair together, but not across the lead. I am not sure that made sense so I drew a very crap picture. Would it even work or am I off my head?

Cheers

 

Mains interference - culprit found/solution not!

Much experimenting chez flash tonight:

1) Mains hum still present
2) Unplug tuner from Isotek Minisub ready for extension cable from TV
3) Plug TV via extension cable into different socket to usual - no hum
4) ditto another socket - no hum
5) ditto original/usual socket - no hum
6) plug tuner (switched off) back into Isotek Minisub... hum returns!
7) unplug tuner cable from back of tuner - hum stops

Conclusion: Arcam tuner is cause of TV mains hum. I don't know what the Isotek is doing to clean the mains on the way in but it's not stopping any dirt getting back out. And bypassing the Minisub does not solve the problem (before anyone who doubts the efficacy of mains conditioners starts to get excited about the idea that they might actually be introducing noise).

Solution: any ideas greatly appreciated. Unplugging the tuner from the mains every time I want to watch TV with audio via the ProAcs seems extremely uncivilised.

All ideas welcome, particularly from Tenson/Mosfet given previous suggestions.

Many thanks in advance,

Nigel

 

Nigel,

The problem is not noise getting in to the mains, thats why the Isotek doesn't stop it. The problem is basically a feedback loop in the earthling connections.

Try unplugging the RCA/phono leads of the tuner from the amp and see if that helps at all. I have a hunch it just might

If that works then disconnect the earth of the cable from the tuner to the amp at the amp end. It should still work and solve the hum. You will have to open the cables plugs and snip the earth connections, but leave the live wire connected. If you don't feel up to this then I'm sure someone here can make you a budget cable for a very small amount of money.

Another way is to put that box Mosfet suggested in the line. If you don't want to do that you could use a switch box to disconnect the tuner from the amp completely when not in use.

 

Do you have cable TV? If so, what happens when you unplug the coax cable (antenna) from the TV? Many ground loops are caused by grounding through the antenna AND mains. There are filters that break these ground loops. You can put them between the box in the wall and your cable.

 

Tenson - you're pretty damn hot with that etchasketch (or as my kids call it - the laptop)!

 

As I understand Tenson a ground loop isolator is similar to a passive DI box; it uses a small transformer to break the ground magnetically.

http://www.smr-home-theatre.org/Ground-Loops/

 

The Isotek, or any other mains filter, will work inwards, outwards and, in fact, for everything on the same ringmain/spur. Most pieces of equipment that have filtered mains inlets do so in order to stop interference getting out, in order for them to comply with appropriate EMC standards.

 

Oh well that does make a lot more sense than putting the isolation transformer in the mains path lol! Still I think Nigel can just disconnect the earth of his RCA's at one end to solve the problem.

 

You're getting and earth loop as your equipment is being grounded through the mains and signal leads at the same time. As the problem source seems to be the TV you need to disconnect the earth from the tv mains plug, however, this is a very dangerous and foolish thing to do - esp if you're not sure of what u r doing.

 

It sounds more like the loop is between the tuner, amp and TV. So disconnecting the earth of the tuner's phono leads to the amp should solve the issue just as well as disconnecting the earth of the TVs mains plug.

Doing the same to the 6m lead from TV to Hi-Fi would also fix it but as this lead is so long it probably needs the good shielding and grounding more than the leads from the tuner to amp.

Nigel, do you feel up to making your own cable to go between the Tuner and Amp?

 

Further tests this evening, starting with everything plugged in as usual:

1) tuner aerial cable unplugged - no change, still hums
2) tuner aerial cable back in, phono leads unplugged - nice and clean
3) tuner phono leads back in, TV aerial unplugged - still hums

So I need to do the earthless phono cable job. Not around from Thu to Sun so will investigate further on Sunday PM, but I suspect I may want to ask/pay someone else to hack into my VDH phonos (or have something made up) rather than do it myself.

To be continued....

 

J8hn - curious as to why you posted this... are you advising me to do something you would not advise me to do...?