Quad 33 and FM3

[Synchrodyne]

As far as I know, that low-power DC output was intended for the remote switching of connected power amplifiers, although I do not have a "hard" reference to back that up.  Although not used for the 303, perhaps Quad thought that with possible future and larger power amplifiers, on-board relay switching of their power inputs would be preferable to having the 33 do the switching.  But it didn't turn out that way, and the 405 followed the 303 pattern, being switched by the 33.

Cheers,

Steve