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Author Topic: Acceptable/Best practice for getting extensions to ring - DSL central filter  (Read 1589 times)

dougcuk

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I have a technical question regarding acceptable/best practice when dealing with ADSL/VDSL  filtering.
I have just swapped over to a new BT Master Socket 5C with Mk4 VDSL/ADSL filter
Previously I had been using seperate plugin ADSL filters on all devices (I have ADSL2+ broadband)
- and I had also disconnected the bell wire (contact 3) at all BT sockets.

Now that I am using the filtered Master socket I have a problem that I need to resolve.
It turns out that all of my wired phones are old enough to require the third (bell) wire to ring
- most are BT branded phones from around 1990-2000 and I love them (so I'm not scrapping them).

Previously the ADSL filter at each socket was providing the capacitor for the bell to sound
- but now without the bell wire or plugin filter none of my extension connected phones ring.

So what technically would be the best solution? (Ignoring costs and labour time)

1. Reconnect the bell wire around the whole network (5 extension sockets plus several junction boxes)
2. Use a plugin ADSL microfilter at the extension socket to allow the bell to ring
3. Replace the extension sockets with a BT socket that has the bell capacitor
   - either the LJU2/1A (Basic old style PSTN Master) or LJU2/2A (PABX Master - No surge arrestor)   

I did a quick forum search and only found one related response from @4candles back last year:
".... A rat's tail filter or filtered extension socket will enable it to ring. The fact that it will in effect be 'double filtered' is of no consequence."
But I would be interested in a bit more detail.
   
Which solution is technically the best?
I think each solution will work - but not sure of the technical pros and cons of each.
Which is the best with regard to the cleanest ADSL or VDSL signal (no nasty signal distortions)
- and/or the function of the phones for voice calls and for ringing (4 sometimes 5 phones).

Live Long & Prosper
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Black Sheep

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Each solution would indeed work.

I'm not certain exactly what it is you are technically after ?? The VDSL signal has been split-off at the SSFP, so it doesn't really matter what method you use with the remaining PSTN signal ??.

I suppose with 4/5 phones plugged in, you would need to know their combined REN number (found on base of device usually). PSTN circuits have a max REN of '4' and a 'Ringer booster' would have been installed in years gone by for anything over this. However, in todays modern world 4/5 plugged in devices are a thing of the past with the advent of 'Base stations + add-ons' telephones.

Save yourself some time and effort and just plug the filters back into the sockets that need them. They worked that way before, so they will work again.
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dougcuk

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My main concern was that the OpenReach engineer that fitted the new Master socket advised me to remove all the plugin filters as they could interfere with the electrical properties of the line and cause some degradation of the ADSL signal. Obviously the high frequency filtering shouldn't be affected by double filtering - but things like reflected signals or impedance and capacitance effects were my first thoughts.

So I assume from your answer that you believe that the plugin filters will not cause any detectable problems (assuming none are faulty in any way)?

The plugin filters obviously contain many more filter components (coils, capacitors, diodes) attached to the 2+5 pair than the Option 3 (the Master socket) solution - where you have just a capacitor, resistor and optional surge arrestor.
Would having multiple full spec Capacitors connected on the line be an issue - (as provided by the sockets I listed)? 

I prefer hardwired solutions for both phones and routers where possible - more reliable and less to go wrong - and they work during power cuts too!
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Black Sheep

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You can indeed get increased capacitance by using either sockets with caps in-situ, or plugging in the filters again ..... but with the greatest of respect, are you not 'over-thinking' the situation ??

We know the circuit worked fine pre-engineering visit when all the filters were plugged in, they will continue to work the same if you plug them back in now the SSFP is installed.

I'll be honest, I have no idea of the scientific calculations regarding the impedance/capacitance effect ..... I'm just a firm believer in simple solutions.  :)
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tubaman

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My personal preference would be to reconnect the bell wires to the extensions as it is the most aesthetically pleasing solution (no dangly filters).
But you can just put the dangly filters back and it would put you in a no worse (or better) situation than you had with ADSL.
Either should work just fine.
 :)
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4candles

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Agreed - any which way you like.
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sevenlayermuddle

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Personally, I’d start by reconnecting all the bell wires, and seeing whether problems arise.

If the filter at the master socket were perfect, which is impossible, the bell wire would make no difference whatsoever.   But as long as the filter is quite good, which it probably is, the effects of bell wire should be minimal.

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benji09

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  If my understanding of this is correct, the bell wires should already be connected, and the problem you have, is that of too many phone bells....... The impedance of the  capacitor in the FFP is too high to allow all bells to ring. If this is the case, then put one or more of the filters back into the phone sockets until  all the phones ring?




 
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dougcuk

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Many thanks for all the opinions.
So far it seems there is no clear winner on technical grounds.

Even before the complications of broadband and filters I never had a REN issue.
With the phones I am using - plus the traditional single master socket capacitor and bell wire -  all the phones would ring perfectly. With the coming of broadband I added plugin filters on all phones - and then later disconnected the bell wire to reduce "aerial" induced noise on the circuit.

Obviously the plugin filters contain a "ring" capacitor as part of their design (at least the good ones do) but also a lot of extra filtering components that are not required now I have the central filtered master socket. So I have to assume that using extension sockets with capacitors (PSTN Master or PABX Master) would have the least electrical complications - compared to adding back all the plugin filters. All largely academic I will agree - most likely no detectable difference in real life.

I was reluctant to reconnect the bell wire for two reasons:
- it seemed like a backward step to a configuration with known electrical interference issues
- and also because I was thinking I might make use of the spare twisted pair (on one of the extension runs)   
  to fit a DSL extension using the existing (4 core) cable, so I can relocate the router if required.
   
So on balance for the moment I think I shall leave the bell wire disconnected (and available to repurpose for the DSL extension socket) and use some spare PABX Master units as extension sockets - neater than having plugin filters and electrically less complex. Anyone have any additional thoughts on this?
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tubaman

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...
   
So on balance for the moment I think I shall leave the bell wire disconnected (and available to repurpose for the DSL extension socket) and use some spare PABX Master units as extension sockets - neater than having plugin filters and electrically less complex. Anyone have any additional thoughts on this?

If you are going to do that it would probably be best to remove the resistor and surge arrester from the extra masters so that they are as simple as possible electrically.
 :)
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dougcuk

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The PABX version doesn't have the surge arrestor anyway, that was why I specified that version
- but I believe the resistor is required for the capacitor to function correctly as per the circuit design.
Feedback invited! 
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runitdirect

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Many thanks for all the opinions.
So far it seems there is no clear winner on technical grounds.

Even before the complications of broadband and filters I never had a REN issue.
With the phones I am using - plus the traditional single master socket capacitor and bell wire -  all the phones would ring perfectly. With the coming of broadband I added plugin filters on all phones - and then later disconnected the bell wire to reduce "aerial" induced noise on the circuit.

Obviously the plugin filters contain a "ring" capacitor as part of their design (at least the good ones do) but also a lot of extra filtering components that are not required now I have the central filtered master socket. So I have to assume that using extension sockets with capacitors (PSTN Master or PABX Master) would have the least electrical complications - compared to adding back all the plugin filters. All largely academic I will agree - most likely no detectable difference in real life.

I was reluctant to reconnect the bell wire for two reasons:
- it seemed like a backward step to a configuration with known electrical interference issues
- and also because I was thinking I might make use of the spare twisted pair (on one of the extension runs)   
  to fit a DSL extension using the existing (4 core) cable, so I can relocate the router if required.
   
So on balance for the moment I think I shall leave the bell wire disconnected (and available to repurpose for the DSL extension socket) and use some spare PABX Master units as extension sockets - neater than having plugin filters and electrically less complex. Anyone have any additional thoughts on this?

You concerns over the bell wire are incorrect. The bell wire can only cause problems IF the DSL signal is travelling down the same cable. If the extensions are for telephone ONLY then you can wire it all up correctly, ie, connect the bell wire and see no performance difference. The extensions are electrically isolated from the DSL signal.

If you wish to relocate the router using the existing cable you have no choice other than to leave the bell wire off and use PABX sockets for the extensions. Don't use PSTN, you don't need the extra components these have which can actually cause problems for the DSL signal.

The ideal solution is to connect all of the bell wires & leave the router/modem at the master socket. If you really must move it the second best solution is to run a dedicated extension for this purpose.
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tubaman

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My understanding of the resistor is that it allows remote line tests to be conducted even if no phones are connected, by giving a path across the line via it the capacitor. I believe it's known as the 'Out of Service' resistor.
Your filtered master socket will have one, and one is all that is needed.
I'm sure more learned members - Mr B*cat or Mr Sheep perhaps - will confirm.
 :)
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burakkucat

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My understanding of the resistor is that it allows remote line tests to be conducted even if no phones are connected, by giving a path across the line via it the capacitor. I believe it's known as the 'Out of Service' resistor.
Your filtered master socket will have one, and one is all that is needed.

You are absolutely correct.

The shunt across the pair, comprising of a series connected 1.8uF capacitor and 470K resistor, is a requirement to ensure that the circuit is correctly terminated when no equipment is connected. The existing master socket, be it NTE5/A or NTE5/C, provides the required shunt.

My suggestion to dougcuk is to modify each LJU2/2A by removing the unnecessary (in this particular usage-case) 470KOhm resistor. You will then have just the 1.8uF capacitor between between pins 2 & 3 of any telephone then may be plugged in. The circuit will then be complete for any applied "ringing" current and the telephone's "bell" or sounder will then operate. I.e. B-wire, capacitor, transducer ("bell" or sounder) & A-wire.
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sevenlayermuddle

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I only mention this as OP has expressed an interest in keeping old phones working.  Does this extend to electro mechanical pulse dialling...?

Thing is, the often overlooked fact that a single continuous ‘bell wire’ serves not just to make the phones ring, it also serves to stop other extensions’ bells from ‘tinkling’ when any phone pulse-dials a number.

When a pulse dial is operated, it creates pulses that will be propagated by a ‘ringer’ capacitor, and may cause the bell on other extensions to ‘tinkle’.   With a continuous bell wire this is avoided, because each dial clamps the common bell wire whenever it is moved away from rest, preventing other bells from tinkling.

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