This is a beautiful dbx subharmonic sequencer, in excellent working condition other than some minor rack rash that is invisible when racked.

In the studio I ran the unit mono using one sub out and the unit receives signal from a send or buss. It is then returned to the console on a channel where it is further EQ'd and compressed. The unit only has two controls. First there is a Frequency Boost knob which boosts the level of the fundamental frequency and then there's the Subharmonic Level which determines how much subs is being synthesized by the unit. There's a green LED which lights up when subharmonic synthesis occurs. When using this unit be very careful of the level of the synthesized subs as it can easily blow your speakers. I usually put a limiter on the output of the dbx 500. Dbx also made other "Boom Boxes" such as the dbx 100, 110 and the 120a/120x/xp/x-ds series which has more controls. The dbx 500 was released in 1978 and was produced for only 2 years and is very difficult to find nowadays. 

Manufacturer description: BASS- LIKE YOU'VE NEVER HEARD BEFORE

The Model 500 is a signal processor that performs two related functions, both of which enhance low frequencies.

1. It synthesizes bass at ultralow frequencies by sensing the mid-bass frequencies present in the program and using them as a guide to recreate corresponding amounts of ultra-low bass.

2. It boosts low bass frequencies already present in the program, but in a manner unlike common bass tone controls.