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Inject sound to a phone landline

linningck

May 18, 2015
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Hi,

I would like to inject sound to a phone landline. I mean, I would like to link my music box output plug to phone landline to let people listen to the music by telephone. I found that transformer 1:1 600 ohms can help to inject sound to landline. I put two diodes to protect against high voltage. However, two problems occur:
1) I would like to send voltage (signal) from audio to phoneline without any voltage going to the audio output from phoneline to avoid issue.
2) I use a transformer 1:1 600 ohms but it is heavy and huge. Is there any other way to send signal on landline carrying voltage.

any idea on point 1and 2 ?

Thank you ~~~~
 

Externet

Aug 24, 2009
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To inject or extract audio to/from a telephone line, use any 8 ohms to 1Kohms audio output transformer, can be obtained from RadioShack or pulled from an old transistor radio..

The 8ohm winding connected in series to only one of the telco pair wires. The 1Kohm winding for audio source.

----> http://comingsoon.radioshack.com/radioshack-audio-output-transformer/2731380.html#.VW8dUDdq3WU

Telco R--------------------------------------------------------------------------R to telephone
Telco T----------------------8 ohm winding---------------------------------T to telephone
 

linningck

May 18, 2015
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Thanks for reply. Actually, what I am trying to do is called a phone patch. It is very common but no one can explain it to me, lol. I still try to figure how it works.On your proposition, you forgot that phone line is 42 v to 100 V and it can damage any electronic device connected to it. I am not sure a 8 ohm audio output can protect correctly.
But, I will check your info into further investigation.
 

Externet

Aug 24, 2009
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..."you forgot that phone line is 42 v to 100 V"...

No. I did not forget. A telco line pair will have near 50 VDC when on-hook; about 8VDC when off-hook; and about 86V AC at 20 Hz when ringing; and around -9dBm (0.25VRMS) audio during talk; and the DC load presented by the telephone apparatus should be around 200 ohms.

The arounds and abouts depend of your particular telephone brand and the distance from your premises to the Telco substation.
And as you say on your first post, voltage clippers (counterparalleled diodes) should be fitted on the 1K winding of the transformer to protect any further circuitry from spikes and ringing. The transformer will isolate any DC on the telco pair and will couple only AC to your patch circuit.

A telephone patch uses a 2-wire to 4-wire converter (confusing name) hybrid coils which allows bidirectional audio ( to and from ) cancelling one from appearing in the other.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Sounds like @Externet has some experience with the telco service and 'phone patches (maybe as an amateur radio operator, back when hams did phone patches?). A true bi-directional phone patch requires a hybrid coil connection to separate the incoming and outgoing voice signals, with just a little bleed-through to allow you to hear your own voice in the receiver. You don't need this! Follow @Externet 's suggestion and use the transformer. Try to avoid blasting the telco lines with a signal greater than -9 dBm, assuming you actually know what you are doing.
 

AnalogKid

Jun 10, 2015
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I've used both transformer types. The transistor amplifier output transformer can have better frequency response, but small ones saturate with the telco DC line current. This can be 100 mA or more in some situations; a lot of uncancelled flux. A 600:600 modem transformer is built to handle this, the the frequency response and distortion are poor. Why do you (TS) say yours is "huge"? Is it an audio interstage or old vacuum tube type?

ak
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I agree with @AnalogKid with regard to placing any kind of transformer across the telephone line, thereby completing an "off hook" DC circuit that the TELCO will probably recognize, even if it doesn't provide enough current to trigger a dial tone. The TELCO automagically and periodically tests all its circuits for leakage and will investigate anomalous activity. Better to use a largeish capacitor (to preserve audio quality) to DC isolate the transformer from the line and use a relay to connect it to the line-pair only after you have established an "off hook" telephone connection with the other party on the line. Even then, the TELCO may detect something amiss and send someone to investigate. And most especially they will investigate if you screw up and put too high an audio level on their lines.
 

linningck

May 18, 2015
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Hi all, my previous reply doesn t appear. I dont know why.
I read all you replies. Thank for your interest and help. Sincerely.
I took your replies into account.ANd I am glad to move forward with you on this.
to Analogkid, I said huge because the size of transformer is almost 1.5 cm x 1.5 cm.
Actually, it is feasible and legal. it is called a phone patch.
I just dont know how to make a simple phonepatch.
any idea or blue print ? It will be a phonepatch just to inject sound to landline and from a weak source (around 1 V)
 

Colin Mitchell

Aug 31, 2014
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All you need is 2 x 100n caps and a few resistors.
I have sold hundreds of these items.
 

linningck

May 18, 2015
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Sure, but what about the connection between them ? Which circuit configuration should I use ?
A simple autopatch (radio phone patch), everybody tell me: easy. But when it is time to have a real blueprint with exact circuit components... People are usually not so precise.
if you can help me on this part. You are welcome :)
 

linningck

May 18, 2015
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Thank you. So, it should be like that.
I still have few question. Which voltage should I inject into the patch then into the phone ?Untitled.png
 

Colin Mitchell

Aug 31, 2014
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The top wire of my circuit goes to the top wire of your drawing and the bottom wire of my circuit goes to the bottom wire of your circuit that has the arrow in one direction and the arrow in the other direction.
 

linningck

May 18, 2015
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My request is between handset and phone :)
Thank you for your help.
I still keep your idea in mind.
 

Colin Mitchell

Aug 31, 2014
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This is what you asked for: "I would like to inject sound to a phone landline."

You can connect the circuit to other wires but you wont get the advantage of the amplifier circuit in the phone
 

AnalogKid

Jun 10, 2015
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Now that we finally have a drawing, we see that the original question was poorly stated, and unintentionally misleading. There are two basic types of telephone handsets, one with a carbon microphone or equivalent, and one with an electret microphone. The more modern the phone, the more likely it has a straight electret mic. Injecting audio into this is like adding an external microphone to a iPhone. There is a small DC voltage present on the input that has to be accounted for, but other than that it requires an input attenuator because the normal microphone signal is about 1% of the Line Out signal from a music player.

More details, better answers.

ak
 

linningck

May 18, 2015
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Sorry for misleading. I made it too simple at the beginning.
Any idea about an electronic circuit I can use to inject music to phone by passing between handset and phone is welcome.
Analogkid, why an attenuator ? It is the first time I read about this for my project ? I am curious.
Most people talk about amplifying before injecting.
 

linningck

May 18, 2015
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I made this one. but three questions raised:
1) which specifications must have the transformer ? 100 ohms ? is it a 1 (audio) : 5 (phone), 1:5, gain ? Source is around 0.6 mV. Or, should I use a transistor ? which one ?
2) source is weak and not protected against voltage input. How to avoid voltage from phone to pass the transformer (even diminished by reverse transformer gain) and "attack" the audio source
3) blueprint is too easy. I think I need to put some resistance and capacitor to stabilize the signal and reduce the noise, for example. But, how ?




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