
This is a crowbar type protector in the rare event of power amp PCB failure.
It is passive, which is to say it need no power supply. There are simply two speaker wires in and two speaker wires out. The 5A onboard fuse protects the 16 amp triac and is quite transparent to audio. As simple fuse only protector would need to be as low as 2 Amps and could compress the low notes and cause some distortion. It is usually fitted inside an amplifier but can be housed in a small box behind a loudspeaker.
See Speaker Protection 1 Tested PCB for Single Channel Audio Amplifier | eBay
The following pictures and video show the speaker protection device working in a live test with a fractured cone Mission bass driver as the load. The circuit is connected to my power amp test rig, This uses the same PSU as do the Crimson amplifiers with 10,000Uf reservoir capacitors. The protector and speaker is fed via a 4 microhenry inductor as almost all amplifiers incorporate something similar in series with the speaker output. Initially the PSU positive rail is switched via a 100 mA current limiter. The meter shows the protector is working as the needle goes to full scale. Next the current is switched to unlimited mode to simulate an amplifier failure. The loudspeaker makes a “tick” noise and within 1 second the onboard fuse burns out. A very low level tone of around 1kHz continues through the speaker indicating the fuse in now open circuit and that the amplifier has failed. The speaker is unharmed despite the 36V DC at around 15 amps being applied.

The protector itself worked fine once the fuse was replaced. It successfully disconnected 2 more times and the triac switch was still intact and workable.