This is an unusual piece of kit, even by outboard gear standards. The dbx 120A is a Subharmonic Synthesizer which generates low frequency subharmonics to increase bass output. It’s designed to enhance the low end by adding really low frequencies that doesn’t exist in the original signal. So, unlike enhancers or EQ which work with what’s already there, the 120A generates additional subharmonics to add to the signal
The 120A can be used on an insert – it has stereo inputs & outputs – or on a send with the return coming from the subwoofer output. It’s capable of generating large amounts of subharmonics so needs to be used with a degree of caution!
dbx has made a number of subharmonic synthesisers which are all basically similar. The 120A and 120XP are probably the best known and the 120A is a current product. They’re popular on film post stages for generating subharmonic rumbles for movie effects
I spent a while playing with the Subharmonic Synthesiser. It surprised me how effective it was and how much excitement it could add to a mix. It’s an easy effect to overdo, particularly when monitoring on small speakers when the true effect can’t be felt. Pulling the grilles off your speakers and switching the 120 in and out is instructive – with it switched in, you can see the bass cones flapping around like crazy if you’ve overdone it
To be used with caution on mixes, with care on sound effects, and with reckless abandon on big PA rigs. The 120A is one of the best toys I’ve played with in years. Useful for adding that ultra-low end in a mix, beefing up wimpy samples, or for adding rumble to a PA setup. The 120A will make things wobble!