Quite recently,
Asbokid provided me with a link [1] to an interesting document. After reading it, I decided to examine the "FUN-JIN" branded plug-in microfilter that
TalkTalk had given me, when I first subscribed to their service five years ago. That microfilter was only used for a short period of time, when it was replaced with a filtered faceplate fitted at the
NTE5/A -- an
NTE2005.
Below are four images that I managed to capture with my flat-bed scanner. Notice the severed green wire? That was exactly how it was received. It had clearly been done as an afterthought, in the factory, maybe?
So there are four wires, yellow, green, red & black, that originate from the plug. The green and red wires just connected the corresponding pins of the plug and the (telephone) socket together. In terms of IDC#s, of which we are all familiar, they are IDC3 and IDC4 -- the "ring" wire and the earth wire. The severed, green, is the "ring" wire and the red, the earth wire. The other two wires, yellow and black, are IDC2 and IDC5 (the B- & A-wires) respectively. The yellow and black wires are connected directly to the inner pair of the
RJ11 socket. From there, they then continue to the PCB (the filter circuit) and then to corresponding pins at the telephone socket.
From the above, I deduce that there is just a low-pass filter (for frequencies from 0 up to about 3.5 kHz) between the line and the telephone. After a moments thought, I assume that every modem/router has a band-pass filter for ADSL/ADSL2/ADSL2+/VDSL2 frequencies, as appropriate, which block the low audio range and those radio frequencies above the top of the VDSL2 range.
The low-pass filter in the "FUN-JIN" device consists of three components. Two chokes (wound on a common core) and a capacitor. As of yet, I have not determined the exact circuit due to "gunk" on the PCB!
More to follow, once I've de-gunk'd the PCB.
[1]
http://www.rsars.org.uk/ELIBRARY/BBCONNECT.pdf