Simple Phone to audio convertor

[delphijustin]

This here allows you to convert a telephone signal into a Standard Speaker Audio signal. By using a Transformer and a On/Off Switch and a Resistor

Here's what you need.
1 Transformer (for best quality use a 600Ω transformer)
1 1KΩ Resistor
1 4 to 4µF capacitor ( it may work with other ceramic capacitors )
A telephone cord
and something to cut-and-slice the phone cord.

Here's optional stuff you may want.
an Audio connector like( 3.5mm, RCA Jack, etc )

Here is the blueprint. If the transformer has 3 wires on oneside then wire the 2 middle phone wires up to the 1st and 3rd wire on that side and not using the middle wire.

**broken link removed**

What's neat is you can use it in reverse to transmit audio out trew the phone line. You can use any transformer you want so it can even be the cheapest transformer you buy.

 

[Externet]

Not exactly any transformer for use in parallel to the telco line as in your schematic.
The winding connected to the telco should have around 200 ohms DC resistance, 600 ohm impedance is fine there.

To use a "standard speaker" at the other winding, if a 'standard' speaker means 8 ohms; you need a 8 ohm impedance winding there; not 600. And the speaker should be a tiny one of around 0.25 Watt.

 

[MikeMl]

This has several problems. DONT DO IT!

First, you need a DC blocking capacitor in series with the transformer primary so that the DC current that would otherwise flow in the transformer primary doesn't saturate the core.

Second, the DC resistance of the primary is too low for the phone line. It effectively signals OFF-HOOK to the central office such that the phone-line current is limited only by the wire-loop resistance. The phone company hates people that do this! To signal OFF-HOOK, you need to connect a ~1000Ω resistance line-to-line. Then you can capacitively couple the audio to the transformer such that no DC current flows in the transformer.


This article shows how to do this the correct way

 

[delphijustin]

The idea of this circuit is to use less components and spend less money. I tried it with a 1000Ω and 500Ω on the phone side and 8 ohm on the speaker side and works well. This is like a "Science Project", a device that shows how something works. Not a use it all the time for every call. I even tried it with another small transformer and it works and i think its less than the other the 1000Ω and the 500Ω transformer.

 

[audioguru]

When you take the small signal level from a phone line and cut it 11 times with a transformer then its level will be very low in a speaker.
Where is the audio power amplifier?

 

[delphijustin]

The reason why to use a transformer is to protect the device you would hook it up to for example a Computer's Sound Card. If the phone line was directly going to the computer it could harm it because of the Voltage used to run the phones with. A speaker of course probly would not matter.

 

[transistor495]

It is one of the worst methods of coupling audio from telephone line. You mentioned sound card connectivity but I can't see any surge protection.

A very safe method is inductive audio pick-up using a magnetic loop and amplifying it using a pre-amplifier and get the output ready for soundcard or to power amplifiers+speaker.

 

[delphijustin]

If you need surge protection then you can buy a surge protector for the phone line. They are cheap to buy.

 

[delphijustin]

I added the capacitor to make a replacement for the switch because it turns on when you pick up the phone, it does not tie up the line, and you can still make calls with it hooked up.