"I get hum when hooking a telephone across AB/AG."
You can't do this.. Intercom line cards are made for this. The trouble is simple. The A battery and A ground are two different applications. The Ag is the
ground in the crkt. Just as the B battery has a ground.
Grounds are not always common.
The AG completes the crkt. A batt B batt .
What you have is A battery then you have unfiltered B battery. Two separate crkts on the power supply. Don't confuse them. An don't make your grounds common when mixing them.
You need to understand that you are working with two different crkts. One is true DC and the other is supper imposed over a DC wave form.
