(Info on the seemingly rare Sunfire Laboratories Subwoofer. )
This Subwoofer appears to be a “pro” version of the early “true subwoofer” that started shipping about 1995. Unlike the consumer version, there are no speaker inputs and no RCA high pass outputs. And the AC line cord is hardwired, not the usual IEC power inlet.
Instead, there’s a pair of XLR inputs, a remote mute feature, and some sort of daisy chain connection.
I also like the robust speaker “guards” that double as carry handles!
Inside, there’s an extra PCB for the balanced inputs.
Here’s a closeup of the input board. The IC MC3400 is an op-amp, so I’m guessing this unit has a good grade active balanced input system, rather than the oft-used cheezy “just ground pin 3, no one will notice” XLR input setup.
Schematics and cautions
Looks like the Super Junior or the Mark II are the closest schematic matches.
Anyway, these schematics give me a good idea about the various power supplies, which is most likely the place where the slight hum is originating.
The power amplifier looks like a pretty standard Darlington comp design, so a dummy load should be fine for debugging the backplate.
Biggest caution is to use a diff probe on my scope, as there is no AC line isolation, and typical of high power amps, a rather high B+/- 160 volts.
While I was in here initially, I replaced the 30 year old 1500 uF mains filter caps as preventative maintenance. The replacement is a slightly larger value (2200 uF) and is the high ripple current, high temp version. The new ones are the same can diameter, just slightly longer (DigiKey PN 493-8528-ND). No problems with fit.
Interestingly, once pulled the original mains filter caps still measured within spec, although the capacitance was a bit low and the leakage was somewhat elevated. Didn’t fix the slight hum, but did make it a tad quieter so moving in the right direction.
Last Updated on 2026-03-11 by Daev Roehr