I copied this simple setup between 2 old analogue telephones: **broken link removed**
It works perfectly, however, it is very quiet when listening. This makes no real difference over 10 meters or 100.
Is there a simple way I can increase the volume perhaps by increase voltage or changing the resistor? I know our local telephone network actually runs on 12v - I am just nervous to try and break the phones.
I know this is very basic - if I am in he wrong forum pls point me to a better spot.
Hi.
My suggestion is to eliminate the resistor and make the battery 12V. That will give 6 V to each telephone.
Let me know if you get happier with the results then.
Well first thing I think of is using a 12VDC battery! You won't break the phones, because they are sent up to 90VAC when someone rings in. (A lot more than 12VDC).
If you remove the resistor then the battery will short-circuit the audio.
A real telephone line does not limit the current with resistors because then the audio level will also be limited as in your circuit. Instead they use an electronic circuit that limits the current but passes the audio perfectly.
The circuit in the article has the two phones in series. Removing the resistor will not short-circuit the audio.
I suggested leaving the resistor in place so that non standard telephones won't draw too much current. I suggested a capacitor across the resistor to allow all the audio to pass from one phone to the other. A capacitor across the battery would also help.
The battery does not short-circuit anything. It is in series to audio, presents negligible impedance.
With 12V, eliminating the resistor allows proper 30mA for telephones to function properly. Adding 300 Ohms to the loop decreases the current to 17mA, marginal operation.
Calculations for a telephone are done with a desirable standard of 200 Ohms DC resistance.