STB + AV Receiver = Subwoofer Humming?
 

Whenever my VirginMedia ScientificAtlanta 4200DVB is connected to my brand new A/V surround receiver, the subwoofer starts to hum loudly. :confused: 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!

Re: STB + AV Receiver = Subwoofer Humming?
 

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.

Re: STB + AV Receiver = Subwoofer Humming?
 

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

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

Re: STB + AV Receiver = Subwoofer Humming?
 

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

Re: STB + AV Receiver = Subwoofer Humming?
 

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

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?

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).

Re: STB + AV Receiver = Subwoofer Humming?
 

The Humbug is not an RF isolator.

Re: STB + AV Receiver = Subwoofer Humming?
 

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

Re: STB + AV Receiver = Subwoofer Humming?
 

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?

Re: STB + AV Receiver = Subwoofer Humming?
 

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?

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

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!

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

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. :shocking:

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,

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

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!

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! :shocked:
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!

Re: STB + AV Receiver = Subwoofer Humming?
 

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?

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

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


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 ....................

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 ----------

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.

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

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.)

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

I did reply to two posts individually, when this happens the forum glues the two replies together all on its own...

I have read the first post in more detail D'Oh..Sorry must spend more time reading and understanding less time typing !!

You state in your first post you have a Scientific Atlanta box, AFAIK this is called a V+ box, sorry if I have contributed to any confusion :confused:

Just to be clear;
-the hum is only from the cable box, no other sources?
-the hum will follow the cable box if it is swapped from one channel to another on the AV Amp.

These should eliminate you AV system from any suspicion ?

If all the cables are OK I am at a loss for an explanation short of a problem in the network or cable box. Sorry over to someone more qualified than me :dunce:

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

A Scientific Atlanta 4200DVB is not a V+ box, it's a standard V box. The V+ is an 8300.

STurtle:
You could well have a faulty V box as it sounds like the ground for the audio is missing (could be dry joint or blown track on the SCART socket.) I used to work in the car audio repair trade and we used to get loads of stereos back for making an external amp 'Hum' just as you describe and it was usually an open circuit track on the PCB between the RCA sockets and the main ground in the circuit. Some manufacturers used fuse links to ground the RCA's so these would blow instead of a copper track.

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

I am a muppet :dunce:

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

Yes: If only the AV receiver, the speakers (unpowered) and the subwoofer are connected to each other and connected to the socket, the hum is audible.

No and Yes: The hum is heard already heard with the AV receiver in standby. It does not matter how the receiver is switched, it does not matter which cable is connected to where: Any cable, be it video from the Scart or one of the two analog Audio RCA connectors to any input port port on the AV receiver (video or audio) will cause the Subwoofer to start humming (unless the subwoofer has switched into standby itself).

Connecting the TV to the above changes the tone of the Hum somewhat, but without the TV, Av Receiver and Subwoofer alone produce no hum.

So this pretty much pinpoints to the STB.

---------- Post added at 20:40 ---------- Previous post was at 20:38 ----------

In reply to Misterguitar: This is very interesting. Thanks!

---------- Post added at 20:48 ---------- Previous post was at 20:40 ----------

However, how do I explain this to the VM Tech support?

I sort of expect them to say something like "Your STB box works with a plain Tele. Connecting it to a surround receiver is your problem!" Furthermore convincing them that something is their fault is usually very difficult, especially since I probably cannot show them my "fix", because I don't think VM appreciates me tinkering with their STB, faulty or not. Sure, it is just a grounding wire, but most people do have a healthy respect for electricity and would cringe seeing an extra cable leaving the lead (albeit I did it neatly, taking a proper three-wire plug and a spare C7 plug, separating the gound wire just 5cm before the case, including a pull-relief and fastening it around one of the STB's screws that hold the C8 socket). I doubt that communication with them will be easy...

Maybe I just stick with it and don't change my running system...?

Re: STB + AV Receiver = Subwoofer Humming?
 

What he said^^ Try disconnecting the CATV feed (screw on F-connector) on the back of the STB. If the hum stops then this is the case and we need to visit to fix the fault.

Re: STB + AV Receiver = Subwoofer Humming?
 

Quote:

Originally Posted by Feenix (Post 34899190) What he said^^ Try disconnecting the CATV feed (screw on F-connector) on the back of the STB. If the hum stops then this is the case and we need to visit to fix the fault.

I am not sure I understand?

I already wrote that the hum is entirely unaffected whether or not the cable that comes from the outside, from VirginMedia, is connected to the STB or not.

This cable is the only F-connector that I can see anywhere, so I am not sure what you are referring to. Could you please explain in more detail?


Also, as I had written before, the hum is produced by the STB already if it is only connected to a power supply and any single audio/video cable running from the STB to the receiver, connected anywhere, regardless of the state of either device (i.e. both in standby, except for the subwoofer itself.)

(Also, the STB is fairly hot at 48⁰C. I was told by VM phone support that this is normal for the ScientificAtlanta 4200DVB. However, the part of the NagraVision SmartCard that is inside the STB has turned to well-done brown color. There is nothing on top of it, it has about 5cm room to the next shelf above and to either side, and room temperature is at at 20⁰C. The STB is constantly burning 30W, regardless of standby.)

Re: STB + AV Receiver = Subwoofer Humming? (SOLVED)
 

You have to convince them this 'Hum' is ONLY present when the V box is connected. Don't go mentioning the extra ground wire you've fitted though!

If they do agree to send an engineer then connect up another source to the amp like a VCR in the same way to prove to him (or her) that it is DEFINATELY the V box.

Re: STB + AV Receiver = Subwoofer Humming?
 

sorry I got the idea that you had a V+ box but my mistake but the advice I gave you will work on both boxes just so you know.

Re: STB + AV Receiver = Subwoofer Humming?
 

Ah missed that bit, my bad.

Re: STB + AV Receiver = Subwoofer Humming?
 

what i said in my last post was complete crap, try a new scart lead, it sounds like its the scart lead, if not, then the best thing is to ring virgins faults line, it could just be a low signal, virgin will need to fix it.

Re: STB + AV Receiver = Subwoofer Humming?
 

I already get the hum by connecting the analogue audio out of the STB to the AV Receiver (i.e. the non-optical Cinch/RCA coaxial connection). Thus without any scart lead at all. However, for a different reason, I swapped the Scart-lead out to another cable, with exactly the same effect of obtaining the same hum.

Any electrical connection between STB and Receiver yields the hum.

Re: STB + AV Receiver = Subwoofer Humming?
 

:clap: It seems that you have done all that could be reasonably expected to prove it is the VBox. Its call out time, I think.

If it was me, I would have a separate source on hand to demonstrate to the engineer that the same cables, and amplifier channel do not hum with other sources.

There can be no argument surely.

Hope you get it fixed,

Re: STB + AV Receiver = Subwoofer Humming?
 

Well, I am still not sure it is worth the trouble. After all, I already fixed the hum myself - and why change a running system?

Worse, I am afraid that I get a different type of V-Box. This would mean mean at best that would I need to re-program my Harmony Universal Remote. Worse, the new box could lack the digital audio out, or have a different quirk that I might not be able to fix.

So unless there is some danger in damaging my other equipment with the extra grounding (or an unlikely health risk - well I did add an RCD), I will stick with the devil I know.

Re: STB + AV Receiver = Subwoofer Humming?
 

Hi,

I am getting a Hum issue with my V+ HD Samsung 3110 box I had it installed on the 27th October and from then on I get a Hum through all speakers attached to my AV amp. Before reading the posts I had tried earthing the splitter box that Virgin had out in place and reduced the buzz by about 70% but I can still hear it and its driving me mad.

Has anyone called Virgin? If so have they sent a replacement box or replace cables? If so did this resolve the issue?

Regards Scott

Re: STB + AV Receiver = Subwoofer Humming?
 

No, I took no further action since earthing the ScientificAtlanta. I am too scared that they will replace it by another model, which might have unknown consequences (e.g. missing digital out, different IR requiring me to reprogram my Harmony Universal Remote, etc.), especially since I am working during the day and could not explain anything to the engineer myself. Also, there is the pain in convincing the VirginMedia Telephone Support to send an actual Engineer - after all, that is the task of the Telephone Support Team, to reduce the number of call-outs.

So a lot of pain, but no possible gain (since there is no more hum after earthing).

Re: STB + AV Receiver = Subwoofer Humming?
 

I have had an engineer out and they simply installed a ground loop isolator which according to the Virgin media engineer should have been installed in the first place.

I have no Humming at all now :) nothing else needed changing and the chap i spoke to on the phone booked me an engineer as soon as I mention the problem.

Re: STB + AV Receiver = Subwoofer Humming?
 

Hi there:)


I have a ground loop hum from my subwoofer - it dissapears as soon as I unscrew the actual coax cable from my Virgin v+ set top box .

The virgin engineer is coming out tomorrow between 9am -midday - can you let me know what they fitted to yours - or e-mail me a pic of it which cured your humming noise please - i would be very grateful.:)

Then I can hopefully get the virgin Engineer to do the same to mine.

I have PM'd you.