i think that u totally misunderstood the situation, this is the explanation diagram for what i hope to develop, and sorry for annoying you if my explanation was not obvious enough.

 

Since you are not using the public telephone system and are not using a speakerphone then why don't you re-wire the phone so it has one pair of wires to transmit and a second pair of wires to receive?

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Uncle $crooge

 

thx, what i need to do is creating a universal circuit that simulate the re-wiring so it could be used with any telephone (with rj-11 connection), not only one telephone !, then what will be the reasonable scheme for this circuit ??.

 

A telephone has separate wiring internally for transmit and receive then uses a hybrid circuit to combine them into a single pair of wires.
Your circuit must do the opposite. It must use a hybrid circuit to convert the single pair into separate transmit and receive for your sound card. It also must power the phone.

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Uncle $crooge

 

but somethin' like this circuit (it's not my design) can be called a hybrid circuit (as it simulate the transformer ??), because i tested it and i have got a good quality (but for sure the mic handle a mixed signal) with a loop current for about 15mA (the resistance of phone == 500ohm).

 

Hi Pooh,
In your circuit a lot of the signals from the SPKR output of your sound card go directly into the MIC input. If it had a hybrid circuit then only the sounds from the person on the telephone go into the MIC input.

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Uncle $crooge

 

thx, i'm now agree with you about designing a hybrid circuit will be better and safer, now i have some questions about this circuit (which i think is the basic design):

1- for providing the loop current, i'll need a dc path or better as mentioned before using current source circuit for about 20mA with a transistor (then i'll put a dc-blocking capacitor before the transformer) and if there is another modification to provide the power to the phone without causing any damage to the dry transformer or adding any extra noise ???

2- is this circuit will be enough to give me a nice audio separation for about 75-80%, or there is any other modifications that u can suggest ???

 

The link you posted has a different circuit for a hybrid transformer:

Attached Images

Hybrid transformer.PNG (8.2 KB, 7 views)

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Uncle $crooge

 

but the author of the article mentioned that the 150 ohm resistance coming out from the centre of the sec. can varry, and the last posted circuit is a complete tested one but from another site, and what about the power issue for the dry transformer ??

 

I dont know if this will help here but a few months ago I put in a FM transistor inside a normal telephone, only problem with this way is that you goota open it up! If you think this might help then ask for the explaination and Ill post that (not posting now fearing it might be out of topic completely).

HOWEVER, i realized one thing, the output from the telphone speaker (into your ear) has BOTH the signals, the talker and the talkee! So on my set channel (89.5 MHz) could hear the conv, from about 5 meters. The problem of providing ample/stable power to the FM transmitter was not sorted, since I made this in a day. was this useful?

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www.crazyengineers.com www.scienceforums.net www.Hackaday.com

 

A telephone adds some of your transmitted voice to the earphone called "sidetone" so that you can hear if you are talking loud enough.

Some telephones must be powered. Use a high impedance current source or current sink. The line to the phone should be balanced to avoid mains hum or a shielded cable must be used.

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Uncle $crooge

 

This is NOT a hybrid!

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Len

 

This is a hybrid.

T2 is the hybrid. It is essentially a Wheatstone Bridge.

IF Zl matches the line impedence, there is infinite loss (in theory) between the Mic & Speaker.

The loss from line to Mic is about 3 dB, and the same from the Speaker to line.

Attached Images

Hybrid.png (71.6 KB, 8 views)

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Len

 

Yes sir, it is adopted from the same site , an year back for explanation purpose. Near equivalents are very much used for telecom applications. They work well.
this is simple as the transformer winding is not having any taps

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Regards,
Sarma.

 

thx for your help, and thx to all of u, i'm working now on a design of the hybrid using only a subtraction op-amp, and i'll post my results after testing here.