audio interface for telephone

[audioguru]

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.

 

[pooh2006]

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 ???

**broken link removed**

 

[audioguru]

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

 

[pooh2006]

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

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 ??

 

[xheavenlyx]

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?

 

[audioguru]

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.

 

[ljcox]

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).

This is NOT a hybrid!

 

[ljcox]

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.

 

[mvs sarma]

i found this circuit before on **broken link removed** through google, and i think it have somethin' wrong coz after analyzing the op-amp subtraction circuit i found that it subtracts the hole mixed signal from the transmitter signal (or may be i'm wrong !), and also i'll require a dual op-amp ic beside the transformer in circuit which is not good for my desired circuit, anyway thanks.

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

 

[pooh2006]

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.

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.