So if you cut the bass and treble on a 2 band EQ onboard preamp does it boost the mids ?
Or vice-versa if you boost the bass and treble does it cut the mid ?
Technically, the answer is no, because the mids usually stay put, depending on how close the treble and bass centre frequencies are. You'd really need to adjust the gain
as well as the treble and bass to boost or cut the mids. But I think I know what you mean, and yes, with a baxandal filter you can get a smiley face or frowny face frequency response.
Also, technically speaking the original james-baxandall filter is actually passive (complete with log taper pots). With the controls in the middle, you have a flat response but with an overall cut of around 24dB. With gain recovery, you get back to zero, then the treble and bass controls give you +/- 12dB or thereabouts.
Around the late 70's, with cheap and small ICs, the baxandall was adapted to be wrapped around an opamp, and (IME), this acounts for many treble and bass preamps since then - onboard, rack-mount, amp, hifi, mixing desks etc etc etc.
Around the late 80's a third control was often added as part of the same filter. A mid control done this way has a low Q and therefore often only has half to two-thirds the cut and boost of the treble and bass controls. This is what's going on with the 3-band stingray pre from the early 90's. The 3-band fender bass preamps I've seen, and lots of 90's acoustic guitar preamps have also been like this.
I am especially wondering about the musicman 2 band EQ preamp (cut/boost) and the Aguilar OBP-1 (only boost)
These are both notable preamps that
aren't baxandall filters.
The MM 2-band is quite unique. If you google, I'm sure you'll find some response curves online. (If I dig around I'm pretty sure I modelled this myself years ago.) This is a preamp that has been talked about at great length. It's not flat with the controls in the middle, but it is cut and boost, and yeah to a certain extent you can get a smiley or frowny face. The preamp has only one opamp, so it's input Z is quite limited. (virtually all more recent preamps have a 2nd opamp that serves as an input buffer.) That's why it works so well with just one pickup that is wired in parallel mode (IOW, low Z), and directly coupled (IOW, no vol or blend pot between the pickup and the pre).
AFAIK, the treble and bass filter in the obp1 (as well as the sadowsky pre, and probably the old bart tct) is simply a scaled/tweaked fender tone stack. (I'm happy to be corrected here.) This is a filter that is in virtually all fender valve amps in one form or another. It's a passive filter (complete with audio taper pots), and like the original baxandall, needs around 20-24dB of gain recovery. With the controls in the middle, you get a big smiley face, and with both controls all the way down, you get pretty close to a flat response. A frowny face is usually not an option. Be aware that these controls don't act in a symmetrical way, and also interact quite a lot. Plus their centre frequencies shift. The treble is kind of a shelf, and the bass is kind of a peak type response. Kind of...
If you aren't into spice (electronics circuit modelling software), there used to be a freeware tonestack modelling programme made by someone-or-other. If memory serves, this covers 4 or 5 common tone controls, and you can tweak the pot and cap values and see the freq response curves. Pretty cool really for the hobbyist, and worth tracking down if it's still out there.
FWIW, it's important to not just look at the one element of filtering in its own right. It's very common to have anything but a flat response in one part of a system to account for the shortcommings of another part of the system. Cassettes and records have a pretty radical EQ applied at the mastering stage, then your boombox or turntable applies the opposite (RIAA) eq to improve the noise response. Dolby and Bose are more examples of a similar thing (...well sort of). It's common for guys to desire a flat response from a bass preamp when their pickups and speakers especially, often have huge peaks, troughs and roll-offs in frequency response. Not to mention the fact that our instrument is a piece of wire tensioned on a plank of wood. Have a listen to your bass unplugged in a quiet room. Sounds pretty honky, right? OK, I'm getting OT now...
