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Microfilters – things that go between telephones and the phone line

The following assumes that your phone line has ADSL ‘on it’ – in other words you use your phone line for broadband Internet.

If your phone line is busy, or you have unstable connection, or some of your telephone devices are not working or you need to know how many filters do you need, then please read this.

Why do I need them … ?

Your telephone equipment (phone/fax machine/answering machine etc) wasn’t designed for an ADSL signal. Therefore the phone or whatever may interfere with your Internet connection and vice versa. Microfilters were created to keep normal phone signals and ADSL separate.

Where do I need them … ?

You need to ensure that all your phones, fax machines, alarm systems, sky digi-boxes, analogue modems etc are on a line that has been filtered at some point. The actual positioning and number required will depend to a great extent on how your internal wiring is laid out.

The important thing to remember is that your modem (or router/modem) must NOT be on a filtered line or else it will not work!!

Very simple examples might be :

i) ADSL Filter built in to the BT Master Socket (Style 3)

Only one filter is required, but the modem/router needs to be close to the BT Master in this example.


ii) Inline ADSL Filter (Style 1)

In this example a single filter has been placed between the Master socket and the extensions. The modem/router is fed from the “ADSL” socket whilst the extensions are fed from the “Phone” socket.


iii) In-line ADSL Filter (Style 2)


In this example each phone is filtered separately. As the ADSL modem/router is connected to an extension, all the other sockets must have a filter.

Please note that although the examples only depict phones; they could equally be sky digi boxes, alarm auto-diallers, dialup modems, faxes etc.

Layout examples i) and ii) which have shorter DSL signal paths (within the home) will help minimise interference for people suffering from low SNR / high attenuation ADSL connections.

What sorts are there … ?

There are 3 main types (although there are different manufacturers in some cases)

Style 1 : Small plugin adapter – neat and compact, less obtrusive.


Style 2 : connects to existing socket by means of lead but looks a little awkward.


Style 3 : replaces existing master socket and should be the only filter required.


Styles 2 and 3 tend to contain higher quality builds (using more components) than style 1.

A comparison of different filters can be found here:

Last updated by Neo on Saturday, August 29th, 2009
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