The BassAmp design came about many moons ago when I started doing loudspeaker design. Turns out most speaker cabinets roll off low frequencies (bass) notes all too early unless they are positively huge. Most graphic EQs are limited in frequency coverage and have some funky sound problems inherant in their design. So I set out to build a easily adjustable (simple part changes) analog bass EQ ... and put some pretty lights along with it. (yey - blinky) Later on it was easily adjusted to accomodate the frequency response of my new dipole subwoofers.
DAPB - Dual Active PassBand
Ended up settling on a more complex filter design than required for one main reason. The Q or sharpness of the filter and the frequency point - both could be adjusted independantly. Some time delay issues happen as a result of this flexibility - but for bass frequencies, its a negligable problem.
Linkwitz Riley Crossover
Since the box was going to need to be able to merge a subwoofer in with regular speakers, a crossover circuit was also needed. This could be a more conventional LR 4th order filter circuit. Nothing too fancy, just some quality Burr Brown (TI) OPA2134 Opamps and ceramic caps to keep things as clean as possible.
Of course one of the most important features of the design are the VU meters! This design is easy since National makes a VU display driver IC that is pretty much plug and chug. Of course one extra caveat is that the audio circuitry and display circuitry use their own linear (Talema transformers based) power supplies to prevent any switching noise bleeding thru. The end result is a bit overkill current budget wise, but its certainly plenty silent.
In case you are curious - I've done my best to fully document the circuit here under schematics. Everything but the basic power supply circuitry should be here.