I snagged a PMC TLE1 subwoofer from UCSD Surplus not too long ago. This is a surprisingly light amp that can, even more surprisngly, get very low in response. Using two 6.5" woofers! Upon looking into the product page, this is due to the long folded transmission-line style enclosure that allows for a very long effective port length.

After using it for a while, I noticed that the speaker would make a farting noise a few seconds after shutting it off. From what I could gather online, some loudspeaker designs use a relay to disconnect the speaker while the supply capacitors discharge, and farting noises come up when these relays fail.

I could not find any schematics of the subwoofer amp. Time to take it apart!

Plate amp designed by Bryston, by the way.

There is indeed a speaker relay in the amp! Unfortunately, the relay is really nestled in there, and I couldn’t find a way to access it without seeming breaking apart sections that were glued in place.

The relay is the white box in the left of the photo, hiding behind the black capacitor and in front of the red capacitor. Look at those right-angle amp cards, by the way. I wonder if there's any cooling or interference reason for that arrangement.

While I’m taking this apart, I should check on the woofer drivers. These woofers actually also don’t have much a very high $X_{max}$, so it’s also hard to tell if they were both indeed working, or if one had burned out and is acting like a passive radiator. However, both voice coils are intact.

I guess I’ll keep the subwoofer as is for now. It doesn’t seem to impact the health of the woofer as far as I could tell.