old US Telephone hybrid ('Network' block)

In telephony, 'sidetone' means the signal in your receiver due to thr signal from your transmitter. If you are not careful, the sounds you hear the other hand, if there is no sidetone, the user is inclined to shout. If you subtact your locally-generated signal from the signal on the line (which, of course, ilncludes said local signal), you end up with only the signal from the other telepghone. If you do it perfectly, yuou have no didetone at all, by varying the rations you can get jsut enough sidetone to make the telephone easy to use. There are many ways of doing this subtaction. The older modems I work on tend to use a couple of op-amps. Modernt landline telephones often use a disctrete transistor circuit(2 or 3 transistors [1]). Older telephones both in the UK and US (and I assume elsewhere) use an audio transformer, often called (at least in the UK), the 'induction coil'. There is such a transformer in the 'Network' block I metnioned. [1] The cheap telephone I bought from Maplin to use with my line simulator uses 3 bipoloar transistors for this. You could also look at the manual for the Elenco telepohone kit (available on their website) to see a 2 transistor circuit. -tony