Unconventional EQ Circuit

[Tim Age]

I wanna preface this with saying that I have only a very basic idea of what I'm doing. Thanks.

I tried to figure out how EQ / tone circuits worked, and what I saw in schematic was mostly a bunch of filters chained together in ways I didn't understand (example, a Marshall amp EQ). And I wondered if it wouldn't also work a different way. What I came up with was a three-way voltage divider, feeding each output through an opamp (with a gain knob), and then a filter for each. Basically splitting the signal into three areas (low shelf, hi shelf, mids) with variable gain for each, and mix them back together afterwards.

Would that work, am I missing something obvious, are there any examples of that being done?

[Rob Strand]

Quite a few of those have come-up over the years.  One that comes to mind is the Sunn Coliseum Bass amp.


EDIT: to save some searches,
http://www.prowessamplifiers.com/schematics/sunn/Coliseum_Bass.html

[Boner]

Not trying to sound rude but its like asking "How do I make sandwiches without understanding the concept of ingredients and proportions."
Understanding the idea of filtering is pretty important in EQ design. You really should read up and try to figure out the idea of filtering, you WILL understand if you dedicate some time.

http://sim.okawa-denshi.jp/en/Fkeisan.htm
Is is a nice site that goes over different filtering ideas. Theres also calculators which is amazing if youre like me and not wanting to do "too much" math.

Do you have access to any simulation software? Simulating different circuits will provide gobs of needed experience. See what works and doesnt, understanding the idea of capacitance in an RC circuit and how it effects cut off frequency etc etc.

[Tim Age]

Not trying to sound rude but its like asking "How do I make sandwiches without understanding the concept of ingredients and proportions."
Understanding the idea of filtering is pretty important in EQ design. You really should read up and try to figure out the idea of filtering, you WILL understand if you dedicate some time.

http://sim.okawa-denshi.jp/en/Fkeisan.htm
Is is a nice site that goes over different filtering ideas. Theres also calculators which is amazing if youre like me and not wanting to do "too much" math.

Do you have access to any simulation software? Simulating different circuits will provide gobs of needed experience. See what works and doesnt, understanding the idea of capacitance in an RC circuit and how it effects cut off frequency etc etc.

I've done a bunch of stuff with filters in the past (mostly passive tho, modding my guitars and bass for low-cut knobs etc) and know the page you linked, but so far, I'm lost with the whole 'interacting in ways I never planned' part. So I'm both asking for a way to make a sandwich without understand sandwiches, and for feedback on a curious idea. At some point I'll deffo have to learn thru all that stuff, sure.

[idy]

In your post you described your curious idea two different ways:
the second I think is better:
... splitting the signal into three bands, giving each variable gain and then mixing. Sounds like a lot of things that exist in the real world.

The Sunn Bass schem shows four amplifiers (transistors) each with its own band filter. Each has a passive volume control and they mix though resistors into another amplifying stage. This could be done with opamps. Here the filtering is part of the three amplifiers, not a separate thing.

You are I think contrasting this with the "tone stack" you see in "FMV" amps; a "stacked up" filter with three outputs then being sent into the next stage, often through the wiper of the treble pot. They tend to be very interactive, one big filter with three knobs that affect one another.

Another 2 popular types are the passive "James," and active "Baxandall" control. Often two or three bands, the Bax uses a single active element (transitor or opamp).

[Boner]

when you have issues with one filter working perfectly by itself and another working perfectly by itself, but when you hook the two up in series and everything goes tits up with gain and frequency response, its because of loading. Put a buffer in between the two stages and maybe at the beginning and ending of your EQ stage and all will be well again

[Rob Strand]

when you have issues with one filter working perfectly by itself and another working perfectly by itself, but when you hook the two up in series and everything goes tits up with gain and frequency response, its because of loading

There's a few basic idea on these pages.      One thing not shown is what happens when you change the value of the tone pot.   Large pot values are going to separate the two filters.   The "everything goes tits up" case is when you make the tone pot too small.

http://www.muzique.com/lab/tone3.htm

This one independent controls.  It stops the filters interacting but there's still some loading of the filters due to the pots to ground.
http://www.muzique.com/lab/atone.htm