Got it. I wondered why the requirement for the low hum AG AB. Now it makes sense. It's been a while, but I distinctly remember that on "some" of those intercom units, you could not use AG AB and tie it to the BG BB. I tore apart an old TT unit once and found the commons were not "common."
I scoped the power supply AG AB on a 601 we have here and found the AC ripple to be around .2 VPP. I could hear it directly off the AG AB terminals, using a butt set, however, I hooked up a Melco IC unit and there was no detectable hum.
We do a lot of stage intercom work and I have used telco power supplies to replace the puny switch mode supplies that are installed with the ICs. On several occasions, I have used an auxiliary filter circuit, much as the one I described in this thread, to eliminate all the hum. If Matt1964 is still reading this, you may want to go to 5,000 mfd. on the capacitors and lead in with a 50 mH choke from the AG to the first capacitor.
Rcaman
