Solid State Loudspeaker Relay


On big power amplifiers, conventional electromechanical relays leave much to be desired when it comes to their ability to reliably switch fault level currents in the event of a catastrophic amplifier fault. In a worst case situation (and I speak from experience), one of the output devices fails short circuit, placing the full amplifier rail voltage across the loudspeaker terminals. Relay switching capability falls off rapidly with the increase in DC levels, and in the scenario just posited, will usually fail short circuit, with the contacts welding together. Most speakers will only tolerate this type of abuse for a few 10’s of milliseconds before they are irreparably damaged.  The design presented here covers the relay function, and some ideas on overcurrent  protection and related circuits, overcomes all of the conventional EM relay shortcomings by employing low Rds(on) mosfets arranged as a a bi-directional (i.e. AC) switch. The design can switch fault level currents of 50 A.  Furthermore, unlike relays, there is no wear out mechanism.

Speaker Relay V1.03.pdf