shocking find

I have a 40 watt Radio Shack amplifier and a standard hard wire telephone with a one earphone and microphone headset. The headset has a 1/8th TRS plug. I want to amplify the phone. Any ideas on how to do it?

Hero999

Why do you want to do that?

It should be more than loud enough, if you want to use a speaker then use the speaker output, I don't see the problem.

You could connect an LM386 amplifier to the jack but I don't see what that would achieve.

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audioguru

You need to use an audio isolation transformer between the phone's headphones jack and the amplifier's input so the phone line is still balanced and still cancels hum.
Get the transformer from an old modem.

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Uncle $crooge

Hero999

So it's a telephone, I should read posts more carefully!

You're right, an isolation transformer is what's required, they're normally 600 ohms to 600 ohms and only pass 300Hz to 3kHz.

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shocking find

Dude! OK! I heard that before. I can go out and buy a modem card a Fry's. I ASSUME you mean the card that comes with every store bought pc like a Hewlett-Packard. It is the card that has the phone jack in it for dial-up type of web access. But which wires go where???????? I have all four poles from the Radio Shack RCA input jacks exposed and I have isolated the tip, ring and sleeve from the telephone headset jack. The ring and sleeve are sound and the tip is the microphone. I WANT TO DO THIS!!! I want a clear sound. I may buy another amplifier and increase to 80 watts. I can pay a little money. The audio engineering companies all laugh at me or want $1,000 minimum.

shocking find

Oh Mannnnnnnnnnnn! What is up with you, dude? Steer me, don't veer me! You got an answer for me? I have a 40 watt Radio Shack amp and a hard wire phone and I need to HOW to hook it up and HOW to avoid squelch, hum and shrill whistles. Dude, I used caps to make a dynamic microphone work on the telephone line.

audioguru

An audio isolation transformer has 2 wires in and 2 more wires out.
Both sides are the same so it doesn't matter if it is backwards.
Use an ohm-meter to measure which wires are in each coil.

The transformer connects to a line input (not a microphone input) on the amplifier.

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Uncle $crooge

Hero999

I misread your posd and have alread apologised for it. I didn't realise you wanted to connect your amplifier to your phone line.

Please re-read my previous post!

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shocking find

What did you mean by this? "Use an ohm-meter to measure which wires are in each coil."

audioguru

The transformer has two coils. An ohm-meter will measure continuity from 1 wire to another wire to identify that those 2 wires are a coil. Then that coil can be the input. The other 2 wires can also be measured for continuity and be used for the output.

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Uncle $crooge

Hero999

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shocking find

The following took an aweful lot of time to find. There is almost nothing covering Audio Frequency Transformers. What you call "coils" the author calls "windings"?

"Audio-frequency (af) transformers are used in af circuits as coupling devices. Audio-frequency transformers are designed to operate at frequencies in the audio frequency spectrum (generally considered to be 15 Hz to 20kHz). They consist of a primary and a secondary winding wound on a laminated iron or steel core. Because these transformers are subjected to higher frequencies than are power transformers, special grades of steel such as silicon steel or special alloys of iron that have a very low hysteresis loss must be used for core material. These transformers usually have a greater number of turns in the secondary than in the primary; common step-up ratios being 1 to 2 or 1 to 4. With audio transformers the impedance of the primary and secondary windings is as important as the ratio of turns, since the transformer selected should have its impedance match the circuits to which it is connected."

shocking find

I Wikied transformer a long time ago. Actually it has been so long that I forgot where I found the following:
"Audio-frequency (af) transformers are used in af circuits as coupling devices. Audio-frequency transformers are designed to operate at frequencies in the audio frequency spectrum (generally considered to be 15 Hz to 20kHz). They consist of a primary and a secondary winding wound on a laminated iron or steel core. Because these transformers are subjected to higher frequencies than are power transformers, special grades of steel such as silicon steel or special alloys of iron that have a very low hysteresis loss must be used for core material. These transformers usually have a greater number of turns in the secondary than in the primary; common step-up ratios being 1 to 2 or 1 to 4. With audio transformers the impedance of the primary and secondary windings is as important as the ratio of turns, since the transformer selected should have its impedance match the circuits to which it is connected."

audioguru

Here is the schematic of an audio transformer that you should have obtained from the manufacturer of your transformer.

It has an input coil (winding) and an output coil (winding).
The coils each have a connection at their center called a "center-tap" that you don't use. The coils are made of wire so their low resistance can be measured with an ohm-meter for continuity. The two coils are completely separate.

Attached Images

audio transformer.PNG (4.2 KB, 6 views)

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Uncle $crooge

shocking find

I have not purchased any audio transformers because I was told to obtain an "audio isolation transformer" from an old modem card found in a p.c. (Personal Computer) . Just how in hell I'm supposed to do that is beyond me. I don't even know what to look for. Do I desolder the damn thing off of the modem card??? I have the rotten luck of removing and discarding all four modem cards from all four of my p.c.!!! To demonstrate that we are talking about the same thing, the cards I removed and discarded had the female phone jack incorporated. I needed the slots so I removed and discarded the phone modem cards. I am trying to connect my standard hardwired phone to a 40 watt a.c. amplifier to amplify my phone conversation. Trust me when I say that I am aware of current legal ramifications and I will not divulge anything to anyone. I have all input/output RCA and TRS poles on my amp and phone attached to copper wire so the connections are ready to go. All I need to do is get the components and attach them. I noticed in a department store that an employee had a cell phone that was connected to the store p.a. system so all he had to do was press a button and he was heard over the p.a. I do not know if he could broadcast his phone conversation over the store p.a. But his system pretty much parallels what I am attempting to do.