If the rigs are more "modern" in design... usually all solid state...you can
utilize a multiple electrolytic capacitor - I suggest 100 MFD 25 volt -
input design in the positive line (usually positive cap lead to rig speaker
output and negative cap lead to "common speaker", with the negative speaker
lead going to ground or common of each of the rigs.
This design will require each separate rig volume control to control its
specific volume level, with no "master volume level" control... simple,
simple, simple.
Like good friend fred - K5QLF loves to remind us all, KISS
Bill K5BDZ
P.S. Older rigs or those that use audio output transformers will likely
require a separate multiple winding audio transformer.... that's another
design.
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