The image below features the EQ section of a DJ mixer with -26db gain reduction on each frequency band (hi/mid/low). My goal is to modify this circuit so that the op amp filters are "full kill" (aka isolators) - meaning that when you turn the VR all the way down, it kills the frequency band completely. What can I do to accomplish that (or at least increase the gain reduction beyond -40db)?
My goal is to modify this circuit so that the op amp filters are "full kill" (aka isolators) - meaning that when you turn the VR all the way down, it kills the frequency band completely.
Redesign using tunable notch filters as opposed to gyrator inductors and series capacitors. Gyrators are great for emulating inductance but it comes at a price of series resistance (R108, R112 and R116).
However, killing a signal completely is largely unfeasible - you might be able to get down to maybe 100 db reduction with careful circuit layout and decent components but you have to ask yourself if this is really what you want.
Your immediate question is answered, but I wanted to add some thoughts:
- You cannot achieve "full kill" across a band of frequencies, even in theory -- at least not if you restrict yourself to filters that don't have an infinite amount of delay. There is always a delay vs. completeness of attenuation in play. This isn't just defeatism -- this is a direct consequence of the underlying mathematics.
- Even for something theoretically possible, like a notch filter (which gives "full kill" at exactly one frequency) or a collection of them ("full kill" at a finite number of frequencies) you cannot practically achieve total cutoff -- in the real world, signals always leak through.
- The closer you push a real-world system to the ideal, the more nasty artifacts you'll create. In the case of audio, this will take the form of delay, or phase distortion, ringing, or pre-ringing. You may not mind how these artifacts sound -- or they may drive you (or some fussy recording engineer) up the wall.