Abstract Passive systems for the isolation of machinery vibrations can be supplemented by active techniques. It is shown that using secondary force inputs in parallel with a passive isolation stage has considerable potential advantages, both in the improvement of isolator performance and in the stability of the mounted machine. Simple analytical models are used to demonstrate the inherent frequency limitations of active methods, and finite element methods are used to investigate the detailed dynamics of a typical raft-isolator-receiver system supplemented with active control. Results are presented which demonstrate substantial improvements in isolation efficiency of a laboratory system consisting of a raft mounted via four passive isolators from a resonant receiving structure. The passive isolators are supplemented with four electrodynamically generated secondary force inputs driven by a multi-channel feedforward digital controller.