It’s an “Earth loop breaker”; A common method of providing some isolation between star ground and chassis, lowering noise in an amplifier. I designed it for the GFA-555, but it will also work in many amps.
The wire on the left goes to a bolt attached to an earth-grounded amp chassis. The wire on the right goes to the amp’s star ground. In between is the 47R resistor to couple the chassis to the star ground, with a parallel 0.47uF capacitor to conduct RF. The bridge rectifier is configured as a diode clamp. It does nothing in normal operation, but if a fault develops between star ground and earth ground—say a power supply wire touching the chassis—the voltage will be clamped hard at 1V, and either the power supply fuses or mains fuse should blow.
I used to build these devices from old-fashioned square bridge rectifiers, mounting the cap and resistor to the bridge with solder and shrink-wrap.
It was a lot of fuss. The new board is easier, neater, and less expensive. (The flat-pack bridges are cheaper than the old square ones for the same performance.)
I am selling the bare boards for $10ea, but please be sure you thoroughly understand the changes you’re making. These boards are not meant for all amplifiers. The amp must have an earth-grounded chassis for safety. Do not insert this device between safety earth and chassis. Safety earth must always connect directly to the chassis. Please read Rod Elliot’s excellent article on ‘Earthing your Hi-Fi” for more information. He describes the use of earth-breakers such as this.