new complete installation on pi3

Sure we are reading - but you are not asking precise questions. Instead you are presenting som ethings that you want to do, and asking what we think. Sorry, but I think this is not proper way to address the forum.

Instead, you should ask more precisely. Just for once, let me show you how.

"Can I attach the audio out signal of my existing door station to an USB sound card input ?" Most likely not, it will be much too strong and could even damage that card. However, since nobody has measured this in your special case, it has a slim chance to be working.

"Can I attach the audio out signal of an USB sound car to the input of my existing door station ?" Could work, but one must be careful when the existing input to the door station is kept at the same time.

Regards

pah

Hi,
I think it's ok!
The connection of an inductivity is a little bit difficult. You have to put a catch Diode in parallel of the relay.
Regards
HeinzA

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Hi mamato,
look here for the catch diode:
http://www.forum-raspberrypi.de/Thread-gpio-relais-schalten
A nice evening to you!
HeinzA

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Generally your existing door station has an amplifier for the microphone => strong signal comes out of the door station, tailored to long cables running through the house.
USB sound cards have a microphone amplifier, tailored to get a weak signal from a microphone capsule => will be completely overrun by the strong signal from the door station. Ugly noise, if anything.

"Speaker" may involve a local amplifier as well - then it could be ok, as I said. USB sound card output needs such an amplifier to hear anything - if you couple it directly to a speaker, you won't hear much.

HeinzA is talking about SWITCHING the amplifier via a relay.

Regards

pah