Trouble with sending audio signal over phone line

Thread Starter


Joined Mar 17, 2013
This is in regard to sending audio from a 70 volt line audio amplifier output to the regular telephone company line.
I have a method that has been in place and worked fine for years. I will include a link to show you the product
being used:

It is just a simple audio transformer with 200K ohm input impedance and 600 ohm output impedance. We have it connected to
a switch with options to have it "Off" (no connection), "ON" (you can hear dial tone with the local phone connected), "BOTH"
this allows the person operating the system to hear that the audio is being carried on the phone line by listening in on the local phone,
and "Audio Only" which disconnects the local phone from the system but lets the audio continue through the phone line to the
destination dialed up. BTW, the application is to broadcast audio of church services onto the phone line to the local radio station.

For no apparent reason, you can hear the dial tone but when switching over to "BOTH" the dial tone goes dead. This would be where
the phone remains connected and now the audio signal is combined to send to the radio station. I thought it was something going on
with the selection switch but finally discovered that when disconnecting the transformer secondary, the dial tone returns! Strange thing
is that according to the specs I've found, the primary and secondary DC resistance is still what it is supposed to be. I was suspecting
shorted turns but have no way to test for that... there is not a short in the secondary and this still happens with the primary disconnected.

Now I'm wondering if there may be a change by the phone company that makes this method no longer work... like if something is now
digital rather than analog... I don't have a grasp of how the phone system works (or had worked) vs. anything new that may have been
done to result in this problem.

Any suggestions? BTW, the original device is no longer made but have located an old stock of the actual transformer... a Calrad 45-703
I want to avoid ordering this rare part in case it is not really the reason for the problem.