STB + AV Receiver = Subwoofer Humming?

[STurtle]

Whenever my VirginMedia ScientificAtlanta 4200DVB is connected to my brand new A/V surround receiver, the subwoofer starts to hum loudly. How do I fix this?

So I eagerly unwrapped my surround receiver today - and was immediatly greeted by this bad ground humming. So I disconnected my equipment piecewise to determine the culprit. Expecting some kind of bad ground, I started pulling power plugs, especially unrelated light equipment connected to the same socket, and to my surprise the humming only immediately stopped once I unplugged the set-top box. This is surprising, the SA 4200DVB only has a two wire connection, without any ground! The STB, receiver and powered subwoofer were the only devices connected to that mains outlet.

Of course, it has grounding through the cable, so I unplugged this, too, but this did not affect the humming noise.

So all I need to re-produce the annoying humming in the powered subwoofer is to connect the STB to the mains and run one cable from the STB to the AV-Receiver. Everything else unplugged, including the outside cable from VirginMedia. The cable from the STB is a SCART to Composite+Stereo adapter. I tried three different adapters. It is enough to connect any single RCA-plug (video, left or right) to produce the humming, so it is the shielding somehow, but I wonder how, since there is no other ground going to the STB.

Of course, TOSLINK cable for digitial audio does not affect the humming at all, but how do I get the picture across? Are there any galvanic isolators for RCA or SCART plugs?

Or is my STB simply

Any help greatly appreciated!

[Dazza124]

The sub is one of the most sensitive of speakers.

I purchased a good quality mains filter for all AV equipment including the V+ and try where possible to lay cable routes so the mains and speaker cable is as separate as possible.

I bought a decent sub cable with good shielding as well.

[altis]

http://tecton-cctv.com/humbug.html

http://tecton-cctv.com/pdfs/humbug-CCTV.pdf

[Nedkelly]

faulty isolater can do this i have seen it before or there might not be one the drop.

[STurtle]

I have bought a double-shielded special subwoofer cable already. Doesn't help.

Originally Posted by Nedkelly

faulty isolater can do this i have seen it before or there might not be one the drop.

I don't understand what you are saying. Can you please explain further what I should do or where I should look for what to determine the cause?

Originally Posted by altis

http://tecton-cctv.com/humbug.html

I've got a similar device, but as far as I can see, these are for antenna-like connections (RF connectors) and not the composite video connection that I am using (RCA connectors).

[altis]

The Humbug is not an RF isolator.

[Dazza124]

As i said in my post i think its either a earth problem or mains spikes.
All mains have certain "noise" but some are worse than others depending on numerous factors.A mains filter, in my opinion would work.get get from maplins, comet etc etc.

also is the sub placed to near the tv or other devices.

http://www.mainsfilters.co.uk/

Here is a similar unit.

http://www.maplin.co.uk/Module.aspx?ModuleNo=46830

[STurtle]

Originally Posted by Dazza124

As i said in my post i think its either a earth problem or mains spikes.
All mains have certain "noise" but some are worse than others depending on numerous factors.A mains filter, in my opinion would work.get get from maplins, comet etc etc.

I have a netfilter for my computers, so I tried that one without any success: Regardless of whether the STB or the Subwoofer or both are connected throught he netfilter socket, the humming persists.

---------- Post added at 10:46 ---------- Previous post was at 10:29 ----------

Interesting Observation:
All humming of the subwoofer stops, if I connect the casing of the STB to the ground pin of a socket.

Is this safe to do?

[hedgie]

Originally Posted by STurtle

I have a netfilter for my computers, so I tried that one without any success: Regardless of whether the STB or the Subwoofer or both are connected throught he netfilter socket, the humming persists.

---------- Post added at 10:46 ---------- Previous post was at 10:29 ----------

Interesting Observation:
All humming of the subwoofer stops, if I connect the casing of the STB to the ground pin of a socket.

Is this safe to do?

I would expect the external case of any appliance to be grounded, especially a metal case such as the SA STB. In fact I am pretty sure it is a requirement for small appliances to be case grounded. Sounds like a loose connection in the STB which could have safety consequences.

Before you get a tech out is the earth pin in your STB lead connected and tight?

[STurtle]

Originally Posted by hedgie

I would expect the external case of any appliance to be grounded, especially a metal case such as the SA STB. In fact I am pretty sure it is a requirement for small appliances to be case grounded.

No, not anymore nowadays:
The Scientific Atlanta Explorer4200DVB set-top box bears the "nested squares"-symbol of IEC Protection Class II Device and as such it does not require a connection to electrical earth. Therefore it features an unpolarised two-wire IEC-60320 C8 socket only. I also somehow recall an old rule-of-thumb that devices with metal cases always required grounding and hence could not achieve a Class II rating; but I know that this is not true any more since at least a decade.

Cutting it short: No, the STB is not grounded
- apart from the ground provided by the "cable" coming from VirginMedia, which is actually connected to the case (the plug is screwed onto a part of the case which deliberately lacks paint for that reason).

Before adding an extra ground wire, I measured about 1.6V DC and 0V AC between the STB-case and the earth-pin in my electrical socket. I also measured a DC current of a few Milliampere. I forgot the actual number, but it was small. It must be, since it does not trigger the RCD that I bought and plugged in between the socket and the STB now.

I wonder whether I should just continue to enjoy hum-free bliss or whether I should talk to VirginMedia about it. However, I doubt any replacement STB would do better anyway, so I guess I ignore it for now.

However, my limited electrical knowledge is restricted to "White Goods" only, so I would highly appreciate any input on the issue from anyone with actual expertise in "Brown Goods" or electronics. Thanks!

[hedgie]

Originally Posted by STurtle

No, not anymore nowadays:
The Scientific Atlanta Explorer4200DVB set-top box bears the "nested squares"-symbol of IEC Protection Class II Device and as such it does not require a connection to electrical earth. Therefore it features an unpolarised two-wire IEC-60320 C8 socket only. I also somehow recall an old rule-of-thumb that devices with metal cases always required grounding and hence could not achieve a Class II rating; but I know that this is not true any more since at least a decade.

Cutting it short: No, the STB is not grounded
- apart from the ground provided by the "cable" coming from VirginMedia, which is actually connected to the case (the plug is screwed onto a part of the case which deliberately lacks paint for that reason).

Before adding an extra ground wire, I measured about 1.6V DC and 0V AC between the STB-case and the earth-pin in my electrical socket. I also measured a DC current of a few Milliampere. I forgot the actual number, but it was small. It must be, since it does not trigger the RCD that I bought and plugged in between the socket and the STB now.

I wonder whether I should just continue to enjoy hum-free bliss or whether I should talk to VirginMedia about it. However, I doubt any replacement STB would do better anyway, so I guess I ignore it for now.

However, my limited electrical knowledge is restricted to "White Goods" only, so I would highly appreciate any input on the issue from anyone with actual expertise in "Brown Goods" or electronics. Thanks!

Well there you go ! Something is obviously not right I have an optical connection to my AV Amp and a composite out using a SCART Splitter on the STB. I have no noise or pickup at all. As an observation my Yamaha AV Amp has a specific earth point on the case.

I must do a bit more research on this, apologies if I gave a bum steer. For anyone else reading this be VERY CAREFULL when measuring voltages around equipment, mains voltages can, and will kill.

I am intrigued by this, certainly every device I have opened up in recent years with a metal case has had an earth lead to the case. It may not require it, but what's the harm ? It may even help with EMC Shielding. Even my PC case has an earth point on the chassis, then again that has three pin IEC "Kettle Lead" so no surprise really !! D'oH.

Good luck, hope you get it sorted,

[STurtle]

Originally Posted by hedgie

Well there you go ! Something is obviously not right I have an optical connection to my AV Amp and a composite out using a SCART Splitter on the STB. I have no noise or pickup at all.

Yes, same configuration for me: Optical audio connection and Scart to Composite(RCA) from STB to the AV Receiver, but lots of noise. The receiver features a grounding, like yours. Now I even connected the analog audio RCA connections from the Scart too (needed for 2nd headphone pair, since it does not transform digital input to its 2nd analog audio out plugs), and still no noise!

...

I am intrigued by this, certainly every device I have opened up in recent years with a metal case has had an earth lead to the case. It may not require it, but what's the harm ? It may even help with EMC Shielding. Even my PC case has an earth point on the chassis, then again that has three pin IEC "Kettle Lead" so no surprise really !!

Yes, thanks for your input: I thought exactly like you did, when I took a look at the Scientific Atlanta 4200DVB and noticed that it only has a two-wire connection! Humming is usually caused by a bad ground, but to my surprise there was no ground at all!
This then naturally led to my idea of "adding" a grounding, because what could be the harm if almost all other devices are grounded too? Of course, I first checked what would happen by measuring the difference in potential, rather than blindly adding a wire!


However, since I thus connected the "cable"-mass to my household grounding, I wonder whether this affects the "cable" part outside my house. Could it mess up my neighbour's TV reception? I have no idea about antenna/cable stuff and transmission of TV & Internet through a single coaxial cable - which is why I am asking for help here!

[neilboardman]

I also have a problem with my cinema surrond sound kit, every time I connect my V+ to the subwoofer it sounds odd, my speakers are in the right place but it still sounds odd, I have tried every thing to sort it out, I have no opition but to use the optical port on the back of the V+ box but cant fix the problem, has anyone had the same problem and can offer some pointers to help me out?

[hedgie]

Originally Posted by STurtle

Yes, same configuration for me: Optical audio connection and Scart to Composite(RCA) from STB to the AV Receiver, but lots of noise. The receiver features a grounding, like yours. Now I even connected the analog audio RCA connections from the Scart too (needed for 2nd headphone pair, since it does not transform digital input to its 2nd analog audio out plugs), and still no noise!

Is there noise on the digital channel with the RCA Stereo Jacks disconnected ?

Originally Posted by STurtle

Yes, thanks for your input: I thought exactly like you did, when I took a look at the Scientific Atlanta 4200DVB and noticed that it only has a two-wire connection! Humming is usually caused by a bad ground, but to my surprise there was no ground at all!
This then naturally led to my idea of "adding" a grounding, because what could be the harm if almost all other devices are grounded too? Of course, I first checked what would happen by measuring the difference in potential, rather than blindly adding a wire!

I could appreciate you took care and had knowledge by the tone of your post. I thought I would emphasise the safety aspects as not everyone has the same respect for electricity ....................

Originally Posted by STurtle

However, since I thus connected the "cable"-mass to my household grounding, I wonder whether this affects the "cable" part outside my house. Could it mess up my neighbour's TV reception? I have no idea about antenna/cable stuff and transmission of TV & Internet through a single coaxial cable - which is why I am asking for help here!

I would ASSUME that the cable distribution equipment isolates one feed from another.

Final diagnostic thoughts & suggestions;

1-Does your Optical lead have a sheath and metal connectors or is it a plastic cable. I have a Cambridge audio lead, which has metal connectors. Can you check that none of the connectors on your amp are touching each other.

2-Do you still have the sound problem if you disconnect the video lead and only connect the V+ digital signal ?

3-Can you try swapping to a different channel on the AMP DVD in perhaps just to try and isolate the amp from the equation....

4-Do you get the humming with any other external source DVD or games console perhaps ?

---------- Post added at 09:23 ---------- Previous post was at 09:17 ----------

Originally Posted by neilboardman

I also have a problem with my cinema surrond sound kit, every time I connect my V+ to the subwoofer it sounds odd, my speakers are in the right place but it still sounds odd, I have tried every thing to sort it out, I have no opition but to use the optical port on the back of the V+ box but cant fix the problem, has anyone had the same problem and can offer some pointers to help me out?

When you say odd what do you mean, echoes ? Only HD content will output Dolby Digital, most of the channels just put out PCM Stereo(I think). My Yamaha amp then defaults to the last used sound profile, I just default to stereo. If your amp is on a pseudo surround effect this can cause some odd effects.

Have you tried the BBC HD "Barker Reel" that is a good test for AV Setups. The test card even has specific 5.1 channels I think.

[STurtle]

Originally Posted by hedgie

Final diagnostic thoughts & suggestions;

Despite these being posted underneath a quote from me, I guess this was rather directed at neilboardman's added topic, rather than me? (Since I do not own a V+)

(Anyway, in my case it was only the power cables and the Video-RCA connection between STB and Receiver that produced the humming, adding the plastic TOSLINK or the VM-cable to the STB had no effect, nor any other wirings of other devices connected to the AV-Receiver. Clear case.)