Interesting effect of double filtering the phone - any thoughts

[#1]

I went over to help one of my referrals who was having trouble with noise on the phone line.
As usual I took along a spare router/filter/corded phone etc.
Using the quiet line test with my phone there was no noise with or without the broadband connected.
Trying with their DECT phone gave a similar result.
I then tried with their main corded phone which is a special high amplification unit for the hard of hearing (I think it isn't UK phone as it uses a USA connection with an adapter) and at normal levels no problems with or without router connected but once the amplification was increased above half way there was a lot of crackling when the router was connected. I checked this with two different filters (a Speedtouch and a D-Link) with no improvement but then tried with the filters in series and the noise went away completely.
I must admit that I don't understand it but at least the problem was solved.

[#2]

Strange one that as everyone and their dog always say double filtering is a big no as they are supposed to cancel each other out or something else equally detrimental.
Just goes to show that trying any off the wall idea sometimes unexpectedly works.

[#3]

No, double-filtering is GOOD!
Provided you do it right......
Many devices, typically SKY boxes, cause fewer problems when double-filtered.  
The rules are simple:
plug the phone cable (or other suspect device) into a filter
plug that filter into a 2nd filter
plug that filter into the wall socket
The broadband cable MUST be in the filter directly plugged into the wall socket (i.e. not shared with the double-filtered phone)

[#4]

Had crackling on my phone line for years kept having a go  at SKY because you couldn't hear it with the router switched off changed to plusnet was still there even in test socket.
I changed master socket because even with the faceplate off there was still some sort of resistor inside. No more problems £9.99 from Maplins
Sorry SKY  .
Steve

[#5]

Thanks HPsauce I get it.
Double filter phone = good
Double filter router  = bad

[#6]

Interesting, Oldjim. Not come across that before, but I'd guess that perhaps some part of the amplifier circuit goes into RF oscillation at higher gain settings, either through a fault, or bad design.
If so, then the level of the RF is presumably high enough to pass one stage of filtering but not two.

[#7]

Typically the filter that ISP's provide with their service aren't of a terribly good quality.
We use ZyXel quality filters for all our circuit installs at work, much better than the standard rubbish tbh. Not saying it's gonna make a huge difference but always better to be using higher quality components.

[#8]

Quote from: billnotben

Thanks HPsauce I get it. Double filter phone = good Double filter router  = bad

Well sort of.
The phone outlet is filtered, the router outlet is unfiltered.
So plugging a router cable into the router socket of a second filter means it's actually filtered once.
Which is usually enough to kill it. 

[#9]

I have a different reason for double-filtering: my two-way phone adaptor (tequired for the phone + sky box) blocks the ADSL port on my ADSLNation faceplate, a 'dangly' filter moves it out of the way.  I really should invest in a different shaped adaptor.

[#10]

Quote from: walker23

I really should invest in a different shaped adaptor.

Leave it alone. 
Double-filtering Sky boxes is a GOOD thing to do. 

[#11]

Quote from: HPsauce

Leave it alone.   Double-filtering Sky boxes is a GOOD thing to do.

The BEST thing to do with a Sky box is disconnect it from the phone line until you need to use interactive services. Sky don't care unless you have multiroom.
jim:green quote fixed mod:end

[#12]

I know nothing about SKY boxes, but from the fact that you are describing the connection being made to the phone line then I assume that they contain an old style dial up modem.
So rather than attempting to over filter a piece of equipment that is causing apparent interference, would it not be better to make the connection another way ?
Would a SKY box be able to communicate via a VoIP adaptor, so you plug the SKY box into the VoIP adaptor's phone socket (unfiltered), and plug the other end into you router.
If this works then it would have the following benefits -
1)  Potentially better ADSL as interfering device is no longer connected to the phone circuit.
2)  SKY calls would be cheaper or free as they would be going via VoIP, especially if you get free Plusnet minutes on your broadband account.
3)  Your landline phone will be free at all times as the modem wouldn't need access to the phone line.
4)  No dodgy wiring issues due to double filters, or long extension phone cables to the SKY box which will be prone to interference from nearby TV, DVD, VCR, PVR, etc.

Just a thought 

[#13]

Quote from: davidj60

The BEST thing to do with a Sky box is disconnect it from the phone line until you need to use interactive services.

I did have it (Sky+ HD) disconnected for a while but after a few months it stopped working (said 'no signal'), I thought it was broken but I just happened to try reconnecting the phone line, I guess it 'phoned home' to verify my account status or something and the picture came back.  I don't think these will work via a VOIP line as I'm sure the Sky small print says it must be connected to a BT landline.
The best thing to do with a sky box is probably to throw it out of the window (after disconnecting the telephone line) and cancel the subscription then spend the money on something more useful.

[#14]

Quote

Double filter router  = bad

The router side of a filter is just a pass-through so I can't see how that would cause a problem.

[#15]

Except that a filter connection wouldn't fit to the router side of the first filter so bad