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Old 13th Nov 2020, 3:30 pm   #1
Ken Taylor
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Default Recording telephone audio

I would like to record the audio from some of my telephone instruments via a computer application. I understand I will need a battery and a transformer to isolate the DC from the PC, but what voltage battery and turns ratio transformer would I need? The telephone I will probably use will be a GPO type 300 or similar.

Note the telephone is not connected to any network.

Any advice gratefully received.
Ken
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Old 13th Nov 2020, 4:09 pm   #2
Dave Moll
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Default Re: Recording telephone audio

Of course, the official approach is to use a pickup coil to intercept the magnetic field from the induction coil or earpiece. This avoids any additional direct electrical connection to the telephone. If the PC includes a microphone (as opposed to line-in) socket, the output from the pickup should be suitable to feed into that, otherwise a pre-amplifier will be needed.

From your description above, I'm not clear what function the battery is providing, but the best point to which to connect a transformer would probably be across the receiver (red and green wires of the handset), as there shouldn't be any DC present. As to turns ratio, others may be able to advise.
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Old 13th Nov 2020, 4:49 pm   #3
TonyDuell
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Default Re: Recording telephone audio

There is/was a Velleman kit, K4900 :

http://www.velleman.co.uk/contents/en-uk/p242.html

that may do what you want. I am not sure if it's still available (some sites say it's discontinued) but it should be possible to make something similar. The transformer (at least when I built it about 10 years ago) is a good old LT44
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Old 13th Nov 2020, 5:20 pm   #4
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Default Re: Recording telephone audio

A 9V battery in series with a resistor in series with the instrument will work, the output will be across the resistor and instrument, the choice is yours. At a guess a 1k or so will give line levels. There is probably no need to add a capacitor as the input will have one already.

Last edited by Guest; 13th Nov 2020 at 5:22 pm. Reason: Added a bit
 
Old 13th Nov 2020, 6:10 pm   #5
dagskarlsen
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Default Re: Recording telephone audio

It is a lot of info here, but I would have made a pretty simple circuit pretty equalt o what merlinmaxwell suggested.

The traditional transmitter element (microphone) need a dc current of around 20 mA so a 9V battery with a resistor of 220 ohms, or even better a coil or transformer with high ac impedance but not to high DC resistance will be ideal.
A 12v relay coil will usually do the job perfect.
Then you could just take the signal from the phone directly. If the phone has this diagram you could do as scetched in red.

Link to lots of circuits: https://www.epanorama.net/circuits/teleinterface.html
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Old 14th Nov 2020, 3:01 pm   #6
Ken Taylor
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Default Re: Recording telephone audio

Thank you all for the suggestions and links. Pretty certain there is a LT-44 in the bottom of a drawer somewhere too, otherwise, the 9v battery and series resistor looks like a no-brainer.

All the best
Ken
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Old 14th Nov 2020, 6:01 pm   #7
Ken Taylor
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Default Re: Recording telephone audio

I have now made the recording. I used a (nearly) 9v battery, a small transformer and a 10k pot to set the output amplitude. Thanks again to all, especially Dagskarlsen for the document - fascinating reading on for a wet weekend!

Ken
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