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Couple audio source to telephone line

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couple apr9600 with mt8880

Hi,
I am doing a project in which I require to couple output of digital voice recorder IC APR9600 to telephone line. The purpose is to play recorded messages over telephone. I have already interfaced output MT8880 DTMF dialer chip to telephone line via 1:1 transformer.
Can anyone show me simple schematic to connect output of this IC to 1:1 transformer while MT8880 is already connected to it?
 

mt8880 transformateur

Hello.
You must use ANY audio output transformer; connect the low impedance winding ~8 Ohm in SERIES to ONE of the telephone line wires.
Connect the other winding to the audio circuit.
This can be left permanently in circuit; will not affect any telephone operation sharing the same line or other branch; is undetectable by telco office; will not prevent ringing from working; works two ways, as to inject or to source audio from the telco line, it will not cause hum or unbalance the line.
An old transistor radio is a good donor source of the miniature transformer.
Miguel
 

how link mt8880,apr9600

Hi,
Thanks for your reply.
Isn't there any direct way of coupling without using transformer? Is it possible to use transistor or some other device as using one more transformer will make my ciruit bulkier.

My current circuit is similar to as shown in this schematic (only the telephone interface part is similar)
**broken link removed**
 

Hello.
The circuit shown in the link uses a relay U5 driven at its pin 6 to connect 'red line' to the transformer.
That way is used to seize the line (go off hook) besides just coupling audio. It's called "wet" transformer, carries DC and couples AC.
You can ALSO connect your audio (in or out) to the SAME node the 8880 is at, across the diode 1N4148, keep the level less than 0.6V p-p.
Miguel
 

Thanks. I'll try that out and see if it works.
 

You can consider to add a common collector driver between the transformer and the dialler. Then connect the audio to the driver input.
 

Hi "buta",
Do you mean the connection as shown in attached schematic?
 

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