I recently had problems with my broadband connection and while trying to track down the source of the problem I swapped the ADSL microfilter on the telephone socket supplying my BT broadband router.
These ADSL microfilters are the splitters that separate the telephone voice signal from the broadband signal so that they do not interfere.
The filter that I had been using was a few years old so when swapping it for a new ADSL microfilter didn’t solve the problem I decided to leave the new one in situ anyway.
Rather than just chuck the old filter in the bin I thought I would crack it open to have a look at the circuit inside. I took a photo which is shown below.
You will see there are a couple of coils (the round black things), two resistors mounted vertically and five assorted capacitors. The two wires leading off to the bottom right go to the BT telephone wall plug.
I then dug out another old ADSL microfilter I had hanging around and decided to open this one up too. The design of this one was different, larger with a socket on the input side as well as the two output sockets. The cover was clearly marked with what plugs in where.
As can be seen below this is a rather simpler affair. Just a couple of capacitors and the two square coils.