may I suggest a slightly different approach?
If you are going to use the Radio Shack Audio Isolation X-former ,
then try experimenting and wiring it as follows:
Red to Battery + , Yellow to Either Amp or RX (but not both)
Blk to Battery +, White to Either Amp or RX (but not Both)
using this schematic from radio shack:
http://support.radioshack.com/support_supplies/doc9/9885.htm
the reason, is that with the connection mentioned previosly, you are introducing a voltage drop in the return ("ground") side, and sometimes that will cause a ground loop, whcih can cause feedback problems. OTOH, it may be you are getting good common mode rejection /ubbthreads/images/graemlins/smile.gif I Dunnno.
the connections and use of the "isolation transformer" are actually being connected, and electrically operated as a series choke, in both the + and - battery supply lines.
you might also check the ohmic wire resistance of that xformer. It is listed as 60 Ohms on the secondary. if the primary is the same ohmic resitance ( possibly, since it is 1:1 for AC) then the total series resistance could be as much as 120 ohms.
that would be introducing a voltage drop, reducing voltage to your setup.
other possible replacments for the x-fromer would be a low resistance choke and filter cap on each power line to the E-Caller units (AMP, RX)
and heck, even a small value of series resistance, say 3.3 to 10 Ohms.
the choke (inductor) value would be approx: .15H or 150mh, to achieve the same 600 ohm impedance at 300hz. with the iso-xformer's fairly high ohmic resistance though, it may be doing more of the "isolation work!! /ubbthreads/images/graemlins/smile.gif
the most rejection can be had by using a 3 terminal regulator to one of the units (the RX) it can provide up to 70dB of rejection to noise/ripple, blah, blah, but your RX will have to bale to work at the lower, regulated voltage.
best regards,
bryan