[Asterisk-Users] Re: Hum on a TA750

[Rich Adamson]

> I read your excellent post on the subject.  I too am hearing the 60 Hz 
> hum on the FXO ports of my TA750.  One  Interesting thing is that it 
> goes away if there's an analog phone on the same POTS line, and you go 
> off-hook on it.  I guess the analog phone has the repeat coils built 
> into it, and they balance the line for the FXO port.

That's exactly correct; the analog phone "is" providing the balanced 
tip-ring termination that apparently is not being provided by the TA750. 
(Sure wish I had a TA750 to prove that. Anyone want to ship me one for 
a week or two?)
 
> In any case, I'd be very interested to see your schematics.

I'll through something together and post it on our web site soon, and
then post something to the list.

I'll have to find a reasonable example of a repeat coil (or transformer)
that can be used to try this. It's actually a 600 ohm to 600 ohm audio
transformer with two primary and two secondary windings and a 1:1 
winding ratio.

Rich