Jazzy -
Series EQ controls are a little more complicated, but it's fairly easy to visualize.
On your garden variety bass, the tone knob is a simple passive low-pass filter. Beyond a certain frequency (the corner frequency), any higher freqs are attenuated or filtered off. When you turn the knob down, you lower that corner frequency and more and more of your output signal is getting filtered off. The corner frequency and how it changes is based on the relationship between the capacitor in your control cavity and the resistance value of the potentiometer. In the tone control, the pot is wired so the resistance increases as you turn the knob down.
This is the simplest low-pass filter you can make (called and RC filter, because it's just a resistor and capacitor). When it cuts frequencies, it does so pretty gently. Any time you put a capacitor across the signal lines, you create an RC filter. A long guitar cable can have considerable capacitance, which is why you might lose highs with a long cable in a passive setup (you won't hear this, but your guitar player will).
The Alembic EQ is a more advanced kind of filter. With the the Q-switch in the up position, the EQ pretty much acts like a traditional low-pass filter. There's an active preamp in the circuit, which takes the cable capacitance out of the picture, and the starting corner freq of the EQ is also a little higher than a Fender. But it pretty much acts like a regular tone control.
In the other two positions of the Q-switch, Alembic is playing an interesting trick. This control is tweaking the frequency response of the preamp so that there's a sharp peak in the gain right at the resonant frequency. Below the corner frequency that you're picking with the knob, the sound is unaffected. Above the corner, it's being cut like the regular tone control. But right in the vicinity of the corner frequency there's a big boost happening.
This size and shape of resonance can't easily be reproduced by passive circuits, so positions 2 and 3 of the Q-switch are something special. Position 2 (middle) creates a small resonant boost and Pos 3 makes a large one.
Turning the tone knob is now both boosting and cutting frequencies. You can sweep the peak through both the harmonics and high fundamentals of the bass and create a lot of sounds that wouldn't otherwise be possible. I like to tune my bridge pickup to get more pick snap in pos 3, but would normally run the neck pickup in pos 1.
You may notice that if you select Pos 3 on the Q and turn the filter knob while repeatedly plucking the string, it almost sounds like a wah-wah pedal. This is very similar to what a wah pedal is doing - moving a resonant peak through the freqs. Most wah pedals are set up as a different kind of filter (bandpass) which creates a resonant peak and cuts everything above and below the center frequency. Although you can simulate the Series resonant peak with a wah, it would probably wipe out your low end.
In a Series II, the circuit is the same, but now you can adjust the height and sharpness of the resonant peak with the CVQ knob.
So, the best way to approach the Series EQ is to think of Pos 1 as a regular tone control. To tune in some extra snap, you can use Pos 2 and 3. The switch setting will control the amount of snap, and the knob will let you tune where the boost is happening.
David Fung
