baxandall differences


2010-09-28 1:19 am
Hi, there are two variants of the baxandall filter here:

I have a formula, involving the cap and the pot values:
Fb0 = 1 / (2 * π * C * Rv) .
(interested in the low freq. tone control)

It applies for the schematic on top.
For the bottom schematic - do I simply calculate the cap value as the values of the two caps in series?

I have a tone control kit, claiming it controls the lows +-16db/30hz, but I want to adjust it to app. 50hz.
This is the reasonable low freq. for my sealed box speakers.

The frequencies below that freq. will be handled by a subwoofer, and the signal for the sub is not going through that tone control.

Regards :)

p.s. actually this is the circuit for the kit I have:
C3/4 is 220nf, Pot is 47k. Substituting into the formula and I get ~30hz
(series cap value - 110nF)
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2015-09-30 6:43 pm
The Baxandall tone control has a very smooth response, in fact talking about its "frequency" is somewhat arbitrary and it won't make a huge difference to move it from 30 to 50Hz. What I mean is that if it's specified for +/-16dB at 30Hz, it may do +/-14 or so at 50, which could be enough for you. In any case if you want to change it, as with almost any active filter, just decrease the caps in inverse proportion to how much you want to increase the turnover / cut-off / whatever frequency. In this case C = 220n * 30 / 50 = 132n. If you round it to a standard value like 150n you get ~45Hz, which will probably be close enough.


I used to try and do it with math, but now days I just throw an existing circuit into a circuit modeling program (SPICE, etc - there are free ones), and then scale the values of the key parts until I get what I want. That way it's verified before you build it. I also make sure it has a passive Rf filter at the input (1K in series and a 2.2nF to Gnd for [email protected]). Digital sources can have significant Rf energy. If you don't ask an opamp to do something it can't do well, it should work very well. The Rf filter helps that cause. I also make sure the circuit has a 100 - 200 ohm R in series at the output, so the opamps won't mind the reactance of whatever load it drives. And of coarse you want to have some .1uF caps within an inch of each chip from both power supplies to Gnd. Personally, I prefer a 4 section Baxandall; Lo, Lo-Mid, Hi-Mid and Hi. It allows you to dial in a much better loudness comp curve, and etc. Two controls is not enough.
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2010-09-28 1:19 am
Thanks for answers.
I am still slowly doing something on my project. A long way to finish it. Nothing high end, but I think it will work pretty good.

Is it possible to alter the baxandal, so I get a band pass filter for the low frequencies?
(bessel for more smooth Q)

The point is to keep a simple schematic. One opamp buffer, one opamp to operate the baxandal.

I have made sealed loudspeakers with a range down to 60 hz. (sealed boxes)
So I want to control the bass at that frequency (boosting).
Tried with EQ and +3 to 6 db is pretty fine, +9/12 would be rarely used.
Baxandal boosts all frequencies bellow the "knee" frequency (if we really have a fixed filter frequency there).
I don't want to push the lower frequencies into the speakers and I want to save the PS resources for the third chanel
- a subwoofer that takes over for the low freq.

Despite the sub I would like to control over the L+R bass output. To be able to get the most of the bass from them and play the subwoofer from 60hz bellow.
Baxandal for Highs is pretty nice. So I'd like to keep it :).
Need to experiment a bit in the room.
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