Ischultz posted:

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?


The simple answer is that you must use a transformer.

Your telephone line is a "balanced" line. Your sound card has an "unbalanced"
input. When you connect between the telephone and the PC's line input using
your interface (device) you are essentially placing a ground on one side of the
telephone line, unbalancing it and causing it to be a great noise and hum
receptor.

The interface (device) you now have works with the cassette recorder because
the interconnecting cable is short and the cassette recorder is not grounded.
If it *were* grounded, it would cause noise and hummmmm too.

You cannot fix this by using more capacitors anywhere. You must use a
transformer.

Tell us more about your interface device and we can help with specific details
of the transformer you need.

Don