Trying to understand something...

[bassmannate]

I've starting trying to understand how active tone controls work. I think I'm starting to grasp it. My question is what is the advantage of a passive tone control over an active one? I'm looking more specifically at active Baxandall circuits. Seems like everything here has a passive circuit that bleeds some frequency off to ground to change the tone. I also realize that it's getting late (well, for me) on a Thursday and I may just be missing something obvious.

[Mike Burgundy]

Mostly it's simplicity - less parts and it works well enough.
If you need more control an/or specific specs (variable frequency, variable Q or specialised Q, specific phase behaviour, etc) things start becoming more complicated rapidly.
If a BMP-style tone control works (very) well in a pedal, why design a 100+ partscount constant-Q multiband EQ?

[MartyMart]

As above, if the design is well voiced and setup for it's purpose, then you can use minimal eq.
So much can be done at the amp/recording stage that I think complex pedal eq is a waste of time.
Get the circuit voiced correctly in the first place :-)

MM.

[bassmannate]

Ok. I figured it was for simplicity. The reason I'm looking into this is that I've been following the "forum amp" thread and I'm really interested in building the power amp and experimenting with some pre amps in front of it. Active seems to be the best way to go with that since it will be in with the amp. I still have a ways to go. I've got 2 projects on my bench right now that I'm going to start today.

[DougH]

Passive has more gradual rolloff, active is more abrupt. Depends on the application. Guitar distortion can sound smoother with more gradual filtering. Bass is typically more hi-fi and requires more dramatic eq contour. No set rules. Try things and see what you like.

[DougH]

duplicate post sorry...

[bassmannate]

Passive has more gradual rolloff, active is more abrupt. Depends on the application. Guitar distortion can sound smoother with more gradual filtering. Bass is typically more hi-fi and requires more dramatic eq contour. No set rules. Try things and see what you like.

And just before I saw this post I was thinking, "I bet bass amplification shares a lot in common with hi-fi amplification since most of the sites I'm coming across about the baxandall are hi-fi audio sites."

By the way, would just a typical Baxandall like the default in TSC work fine for bass or would I need to play with cap and resistor values? I know what frequencies the bass plays on but I don't know what frequencies work well in an eq or how to get them. I'm messing around with TSC a little but not quite grasping how changing different values is affecting the curve.

Thanks!

[DougH]

Yup

[Manny]

In my experience most boost type pedals with high outputs can easily accommodate passive tone controls.
Passive only seems to become a problem on pedals with low outputs because they come with a penalty of reduced output.

I agree with MartyMart though, a well voiced pedal often needs no eq at all.

[edvard]

Look up some bass amp schematics and look for the Baxandall 'signature' in the tone section.
Note the component values and go from there.
Also, remember the TSC circuit is but one example of a Baxandall arrangement; there are others...

[bassmannate]

Look up some bass amp schematics and look for the Baxandall 'signature' in the tone section.
Note the component values and go from there.
Also, remember the TSC circuit is but one example of a Baxandall arrangement; there are others...

Alright. I'll see what I can find. Ampeg uses Baxandalls pretty frequently, right? Might be a good place to start.

Yeah, I realize that there are lots of different arrangements of the Baxandall. It's making it a bit confusing figuring out how it works so I can figure out what caps/resistors do what.

[merlinb]

Doug Self is well worth a read:
http://books.google.co.uk/books?id=PvKPEFu2PVkC&lpg=PP1&dq=small%20signal%20audio&pg=PA259#v=onepage&q&f=false

[PRR]

Don't confuse the active Baxandall with the passive James.

[bassmannate]

Thanks! That made it a bit more clear.

[bassmannate]

Ok, if I understand this correctly, basically, an active Baxandall takes the output of a Opamp/transistor/tube/etc. and filters out certain frequencies to feed back into the input of the same device. At least, that's what I'm getting from Doug Self's book.

[edvard]

Exactly.
Theoretically, it is possible to do either with the same circuit.
Take an active circuit and route the feedback path to ground/power a passive circuit with an inverting amplifier and route the ground connnection to it's input.
Note that doing such a thing isn't exactly an either/or proposition.
Due to impedance and other issues, the values of some components will have to be tweaked to get the same cutoff ranges.
Like Doug said, active gets you higher order, but your ears will have to determine what's best.

To expand a little on what PRR said, here's links to the original articles by E.J. James and P.J. Baxandall:
Negative feedback tone control – independent variation of bass and treble without switches - P.J. Baxandall
Simple Tone Control Circuit: Bass and Treble, Cut and Lift - E.J. James