An audio isolation transformer will isolate the ground on your amplifier from causing bad hum on the telephone line.
The cell phone uses air to isolate it from the telephone wires.

If you can't simply unsolder 4 pins on a transformer on a modem card then do gardening instead.

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Uncle $crooge

 

That's a bit harsh audioguru, how would he know what an audio isolation transformer looks like if he's never seen one before?

The kind of transformer used in a modem isn't as good as a high quality audio transformer, it'll only pass frequencies between 300Hz and 3kHz instead of 15Hz to 20kHz.

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I measured the horrible frequency response of one telephone line to another telephone line:
3kHz was down about -12dB. I complained to BELL and they said it was normal. Their limit is -7.5dB one-way which is -15dB round trip.

I didn't bother measuring the low frequencies because there weren't any at the receiving end.

The response was flat from about 850Hz to about 950Hz.

For a few years I installed and programmed (DSP acoustical echo cancellers)high quality boardroom telephone conferencing systems. I made a transmit equalizer that had a peak of +12dB at 3.5kHz. Every time I demo'd my equalizer the people said the sound was much crisper and clearer and it was sold.

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Uncle $crooge

 

The guy in the store probably had a paging amplifier (5W) attached to his phone system. If you're not concerned about legal ramifications, you can use capacitors to isolate the line. If you don't want shrieks and whistles get a frequency shifter or cut the mic off.

 

The best thing would be to connect a 1:1 transformer at the points of Reciver (Also called ear piece) at the instrument 4 way cable comming to handset. use the secondary with a resistive divider for matching purposes)

At this point the side tone is well suppressed-thus the microphone signals will be minimal.

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Regards,
Sarma.

 

I thought he was going to use the headphones output of the phone, not connect to the earpiece. An isolation transformer is needed to avoid hum.

A phone makes sidetone so that you can hear how loud or soft you are speaking. Then you will have acoustical feedback howling if the mic can hear the speaker.

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Uncle $crooge

 

when we try a Mic and a amplifier system ina closed accomodation without proper accoustic arrangements, this howling effect is immennt- unless special care is taken . In case of telephones, i have rarely seen the mute facility when the MIC supply is cut. really very few instruments.

I had occassion to couple telephone istrument to a Public address system while inaugurating a LDD/ILD in the telececom deparment ,i was working for.

ther i had to do all the rehaursal to ensure smooth and clear broadcast of the speech contesnt of an inaugural call. In this case i can't mute the MICof the telephone.
Ofcourse , it went on well.

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Regards,
Sarma.

 

Some PA systems in stores avoid acoustical feedback by using a recorder then playing it into the PA when the mic is turned off.

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Uncle $crooge

 

The local OSH hardware store equipped the managers with cell phones that also interfaced with the store p.a. What WILL they think of next?

 

I've found a Radio Shack 1:1 isolation transformer for 3.99USD with 600-900ohms,300-5KHz response and 100 mega-ohms insulation resistance at 250vdc! Did I do good, boss?

 

You did very well, boss.
I hope it works (RadioShack parts have garbage quality) and I hope they say which wire is which.

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Uncle $crooge