Question about speaker sensitivity and residual noise

[declans3]

Hi,

I have a query about speaker sensitivity and I hope someone can clarify my understanding. I currently have a Sonos Connect Amp that I use for music and also to for my TV. The TV is connected to it using a stereo analog output and this works fine and allows me to stream the TV audio to other rooms in the house. The downside to this is that there appears to be a ground loop between the TV and the Sonos (even though they are fed from the same mains point) and so I get a hum when I turn the TV off. I have added a Behringer Hum Destroyer to the signal path and while this has helped it hasn't eliminated the problem entirely - there is still a very slight hum with the TV off which I can
live with. I am now looking to upgrade my speakers to a pair of Q acoustics 3050 floorstanders and as these have a relatively high sensitivity of 92dB I am concerned the hum will be more noticeable. My understanding of speaker sensitivity is that for a given signal level the perceived volume level will be higher (for a higher sensitivity speaker) but if the overall volume levels will be higher I will just turn down the volume level on the Sonos to normalise them to their current level. If this is the case then I will be no better or worse of then I am currently - can anyone confirm this will likely be the case ?

Also if anyone has any suggestions for removing the hum altogether then I'd love to hear them.

Thanks,
Declan.

 

[Mark.Yudkin]

You ask about solving the hum and about speakers. The sensitivity of the speaker will have no effect whatsoever on the relative level of the hum compared to the rest of the signal - you will be neither better off nor worse.

However, you should not have a ground loop hum and definitely do not have to put up with it. it is almost always resolvable, but can be a pain in the neck to track down.
I am assuming you do not have a PC, Macintosh or laptop in the equation; if you do, remove it completely and report back.

The most common cause of TV-related ground loops, especially in UK with Sky etc., is that many installers do not bother to do the job right. So let's check this first. Please remove all antenna feeds / signal feeds / cable TV / satellite / radio / etc. connections from your TV ,etc. You should now be unable to watch TV or listen to the radio, but what we want to know is if it also removes the hum. If it does, call in whoever installed whatever source(s) you are using and get the installation's grounding corrected.

 

[noiseboy72]

I would suggest that larger, more sensitive speakers might make the problem more pronounced, as the greater efficiency and wider frequency response will both increase the hum in terms of volume - when the system is idle and no other signal is present.

I completely agree that you need to find the source of the hum, but this is not always easy. Things can be correctly installed and hum loops and functional earth paths still an issue. Many TVs do not have an earth connection, nor do Sky boxes, so the drain may well be via the audio equipment. I am quite surprised the humbucker did not solve the problem however. Ideally you want to find an earth path that is separate to the audio ground, but again, this can be very tricky. A cable run from a radiator and screwed to the case of the Sky box has been known to work in the past, but you could also try to the TV.

 

[Mark.Yudkin]

Larger, more sensitive speakers will have no effect on the relative level of the hum (unless the current speakers are ripped out of a tinny transistor radio, in whch case that is likely the source of a range of problems ). Satellite installation must have an earth if they are not to set your house on fire in the first electrical storm. You may safely ignore noiseboy72's contradictory post above.

The first thing you need to do though is check if it's the TV signal feed. If not, I'll walk you through tracking down the source of the hum - it might drive you nuts but we will definitely get there.

 

[noiseboy72]

Try reading the post next time.

I did not suggest the relative level of the hum would increase, but the perceived level when the system is idle. Hum is a low frequency component and larger more efficient speakers will therefore reproduce the hum at a higher level. Of course, the relative level will not change.

Satellite installations very rarely have an earth to the LNB / cable feed, as in the event of a lightning strike, energy would be dissipated down the cable into your AV kit. The dish is naturally connected to the house - as it's metal and screwed to it, but the LNB and quite often the arm is plastic and is designed to insulate. Satellite feeds with earths to dishes are usually limited to transmission systems, so we need not concern ourselves with these. Show me a Sky installation with an earth wire from the dish to an earth spike.

Most hums and buzzes are caused by either ground loops or functional earth drains - where the noise created by the switch mode power supply is not adequately connected to a solid earth. In order to avoid hum loops, satellite receivers are generally double insulated and do not have an earth connection. They usually rely on either the TV or AVR being earthed in some fashion, but with some TVs and AVRs also now being insulated, there is often not a good earth path. Ideally the audio ground path and mains earth would be kept separate - or balanced signals employed, but in domestic AV, this is usually not possible.

If the issue is functional earth noise, then the drain wire from the radiator is a safe earth trick that quite often works. It is also worth checking the settings on the hum bucker, as it may have an earth lift - or simply require turning around!

Whatever you do - and whoever's advice you take, please do not disconnect any mains earths or attempt to earth via a mains socket.

 

[Mark.Yudkin]

Until the OP comes back with some answers, we are merely speculating on the cause and possible remedies.

Where I live, grounding the satellite setup is mandated by law; interesting that in the UK people like to live so dangerously.

As you say, "cutting" any mains earth is a very bad idea, even though there seem to be a number of posters on this board who seriously suggest it.

 

[declans3]

Hi Guys,

Firstly many thanks for your replies and my apologies for the delay in responding. I now have the new speakers and as expected there is no difference in the hum volume. This is good as it is at a tolerable level but I would much rather if it were gone. My setup is as follows:

A Samsung UE55H6400 TV connected to a UPC (now VirginMedia) Horizon Cable STB and a PS3 (both connected via HDMI). The headphone output of the TV is connected to my Sonos Connect Amp with the Behringer Hum Destroyer inline. The TV also has a Cat5 connection to my LAN.

Regardless of which cables I connect or disconnect to the TV the hum persists - even when I disconnect the mains from the TV. The hum only disappears when I disconnect the cable connecting the TV audio out to the Sonos. I previously tried using the TVs optical audio out to connect to the Sonos (via an optical to analog audio converter) and this removed the hum but the TV only provided line out audio (i.e. fixed volume level) on the optical output meaning I could only control the volume from my Sonos rather than my TV remote control (a bigger inconvenience than the slight hum when the TV is off). If you guys have any suggestions on how I can eradicate the remaining hum I'd much appreciate it.

Thanks,
Declan.

 

[noiseboy72]

Have you tried a different cable? I wonder if it is picking up noise from somewhere or has a faulty shield.

My other thought is that there is a fault on the Sonos, as if the hum is present even if the TV is off, then the only active component is the Sonos itself. Is it in warranty or can you borrow one from someone else to prove this?

 

[Mark.Yudkin]

A Samsung UE55H6400 TV connected to a UPC (now VirginMedia) Horizon Cable STB and a PS3 (both connected via HDMI). The headphone output of the TV is connected to my Sonos Connect Amp with the Behringer Hum Destroyer inline. The TV also has a Cat5 connection to my LAN.
...
I previously tried using the TVs optical audio out to connect to the Sonos (via an optical to analog audio converter) and this removed the hum

I would have expected these two cases to have had the same effect, so could you please confirm that the hum occurs when both the cable TV lead and the CAT5 to the LAN are simultaneously disconnected - that is there is no electrical connection between the TV (or PS3) and the cable lead(s) or LAN (only power). What about when you additionally disconnect the PS3? (Leave the Behringer out of the system during tests).

 

[declans3]

Hi Guys,

The problem is sorted - turns out it was a faulty cable connecting the TV to the Sonos and when I swapped this out the hum disappeared. Delighted as this has been bugging me for ages and never expected a simple cable to be the culprit. I can also remove the Hum Destroyer from my signal path which is an added bonus. Many thanks for your help.

Regards,
Declan.

 

[arb70sok]

Enjoyed this thread! Two guys working together to help get someone's problem sorted. Interesting background knowledge for the rest of us to pick up!