I am trying to find a way to record telephone conversations to my PC,
(legal, for training purposes only). I have an interface that plugs
into a cassette recorder and that works fine on the cassette. I have
tried this same interface plugged into my computer, either line-in or
the microphone jack, but when I record I get a loud hum noise and I
can faintly hear myself in the background, though I have to talk or
whistle loudly to do so. Apparantly I need a different interface. I
have found them for sale for around $50, but I would prefer to make my
on, usually these interfaces use fairly simply filter circuits. Any
suggestions?

Thanks