Subwoofer hum question.

shankly1985 · 7 Apr 2015 at 02:19

Is it normal for an active subwoofer to hum when the receiver is switched off? The hum will stop once the sub enters sleep or I power back on the receiver?

lucid · 7 Apr 2015 at 09:53

Striclly speaking, no. But it can depend on the quality and condition of the circuitry in the sub. If it's a half reasonable design and there's nothing faulty in the circuitry, then it shouldn't hum.

The most common causes of hum are ground loops and poor cable shielding.

Ground loops are responsible for fewer than 10% of hum problems. They are caused by a difference in earth level at two places in an electrical circuit. i.e. your sub is powered from a wall socket some distance away from the gear in the rack. However, just pulling power from two different sockets won't automatically create a ground loop. Certain other conditions need to be satisfied too; so don't assume the worst.

You can check for earthing issues with a couple of simple tests: Move the sub so it's powered from the same socket as the AV Receiver. Connect the signal lead as normal and make sure it's laid out rather than coiled. If the hum is still there the unplug mains to any other device connected to the receiver. If you still have hum then unplug the receiver. At this point it should be impossible for a ground loop to exist, so there should be no hum from a ground loop either.

By far the most common cause is poor cable shielding. Inadequately shielded cables act as an antenna for electromagnetic magnetic (EMI) - and radio frequency interference (RFI). This is noise from poorly shielded power supplies, mains cables, other signal cables, radio frequency transmissions etc etc. When those signals reach a poorly shielded sub lead the two things happen. First there's inadequate shielding to trap and shut the intererence to earth, and second, the overspill reaches the centre core conductor and then appears as an audio signal to the sub... which hums. i.e. you hear the interference.

The solution is simple in theory: buy a well shielded sub lead. But in practice it's not so simple. Very few manufactures show you how well their cable is shielded. What you really need for audio shielding is two high density woven shield braids.Go on to amazon and type in "hum killer sub lead" You'll find exactly what you need to cure the issue.

shankly1985 · 7 Apr 2015 at 10:55

Thanks
The speaker system is Tannoy SFX5.1 and I using the speaker wire that come with it. And a rca cable from maplin.

I just ordered a better sub cable Fisual 3 m Pro Install Series Subwoofer Cable.
I also plan on using there speaker wire also.

lucid · 7 Apr 2015 at 15:27

The Fiscal S-Flex 2.5mm speaker cable is decent enough. However, I strongly suspect that the Fiscal sub cable is no better shielded than any £2-£3 Ebay cable. It has nice plug ends, but that does naff all for shielding. You pays your money and takes your choice.

shankly1985 · 7 Apr 2015 at 15:51

It was highly reviewed on the rainforest.. Surely it be better than maplin special? I only paid £5.50

wonko · 7 Apr 2015 at 15:55

I'm struggling to understand how it could be a shielding problem on the RCA cable if the sub doesn't hum with the receiver on. Surely that implies the source of the hum is coming from/via the receiver?

flame696 · 7 Apr 2015 at 21:31

I have the sub from the Tannoy SFX package and it doesnt hum at all im using it with a cambridge audio RCA cable

Woden · 7 Apr 2015 at 21:42

I have this sub and what may well be the cheapest came ever made. It hums.

lucid · 7 Apr 2015 at 23:29

shankly1985 said:
It was highly reviewed on the rainforest..

With respect, they were reviewing a £5 cable, so it's not like the bar was set very high.

shankly1985 said:
Surely it be better than maplin special? I only paid £5.50

I can't comment on that specific Maplin cable, however the general point is that with the vast majority of these cables there's insufficient information on the seller's web sites to make a proper informed choice. They give you vague descriptions and pointless marketing hype...

The Maplin Range – Premium construction and specification, with marketing leading cable technologies.

...but says nothing about what these technologies are, or how they work, or the benefits of them.

In fact, it's common to find there's no information on the impedance of the cable, what the capacitance value is, how much signal loss there is (dB/100m) and at what frequency. You're buying blind on the electrical performance specs. Similarly, there's nothing substantive about the construction e.g. shielding can range from foil only (completely ineffective at audio frequencies) to plain spiral wrap to foil and braid to double braid (the most effective). Along with that is what percentage of the cable is covered by the shielding. A sparse braid shield covering 30% of the cable is far less effective than two overlapping braided shields each providing 90%+ coverage; yet all could be accurately described as "shielded".

There are three reasons why high density overlapping braided shields are effective:

1) they provide near total shield coverage even when the cable is bent

2) foil shields work at gigahertz frequencies (radio/TV/satellite) but you need braided shields to work at the kilohertz range used for analogue audio

3) the shield is the 'path to earth' for interference. Foil shields are poor because of high resistance. Braided shields have lower resistance. The higher the braid coverage (%) then the better it is as a conductor to earth which makes it a more effective shield

Your Fisual cable is £5.50 including P&P and Amazon's fees to the seller. We can presume that the seller still makes a profit after paying for the stock and the staff to process orders. So think about it, how much do you really think that cable is actually worth?

If the Fisual works for you then great, but it's more by good luck than planning.

lucid · 7 Apr 2015 at 23:30

wonko said:
I'm struggling to understand how it could be a shielding problem on the RCA cable if the sub doesn't hum with the receiver on. Surely that implies the source of the hum is coming from/via the receiver?

Which is why I gave him a set of tests to do.

shankly1985 · 8 Apr 2015 at 00:49

lucid said:
With respect, they were reviewing a £5 cable, so it's not like the bar was set very high.

I can't comment on that specific Maplin cable, however the general point is that with the vast majority of these cables there's insufficient information on the seller's web sites to make a proper informed choice. They give you vague descriptions and pointless marketing hype...

...but says nothing about what these technologies are, or how they work, or the benefits of them.

In fact, it's common to find there's no information on the impedance of the cable, what the capacitance value is, how much signal loss there is (dB/100m) and at what frequency. You're buying blind on the electrical performance specs. Similarly, there's nothing substantive about the construction e.g. shielding can range from foil only (completely ineffective at audio frequencies) to plain spiral wrap to foil and braid to double braid (the most effective). Along with that is what percentage of the cable is covered by the shielding. A sparse braid shield covering 30% of the cable is far less effective than two overlapping braided shields each providing 90%+ coverage; yet all could be accurately described as "shielded".

There are three reasons why high density overlapping braided shields are effective:

1) they provide near total shield coverage even when the cable is bent

2) foil shields work at gigahertz frequencies (radio/TV/satellite) but you need braided shields to work at the kilohertz range used for analogue audio

3) the shield is the 'path to earth' for interference. Foil shields are poor because of high resistance. Braided shields have lower resistance. The higher the braid coverage (%) then the better it is as a conductor to earth which makes it a more effective shield

Your Fisual cable is £5.50 including P&P and Amazon's fees to the seller. We can presume that the seller still makes a profit after paying for the stock and the staff to process orders. So think about it, how much do you really think that cable is actually worth?

If the Fisual works for you then great, but it's more by good luck than planning.

So true about luck I just went with reviews and you made a very strong point about price etc. If this doesn't work then, what would you recommend for Sub cable? Is Fisual speaker cable also worth buying?

lucid · 8 Apr 2015 at 01:10

This is very good. It has the double braid shielding and lots of other good attributes too.

shankly1985 · 9 Apr 2015 at 12:15

That cable seems to have done the trick