Active 3-Band tone control.

[m4268588]

I noticed that Baxandall's low frequency control could be transformed like this.


2-Band of Hi and Lo.


3-Band

Hi + Mid	Hi + Lo	Mid + Lo

Since haven't done any mathematical analysis yet, used SPICE to adjust the components value. There may be redundant components that can be omitted.

Code Select Expand

Version 4
SHEET 1 800 600
WIRE 48 -1072 32 -1072
WIRE 144 -1072 128 -1072
WIRE 256 -1072 240 -1072
WIRE 352 -1072 336 -1072
WIRE 192 -992 192 -1008
WIRE 192 -912 192 -928
WIRE 192 -816 192 -832
WIRE 208 -816 192 -816
WIRE 32 -768 32 -1072
WIRE 48 -768 32 -768
WIRE 144 -768 128 -768
WIRE 256 -768 240 -768
WIRE 352 -768 352 -1072
WIRE 352 -768 336 -768
WIRE 192 -688 192 -704
WIRE 192 -592 192 -608
WIRE 192 -512 192 -528
WIRE 256 -512 192 -512
WIRE 288 -512 256 -512
WIRE 192 -496 192 -512
WIRE 256 -496 256 -512
WIRE 288 -496 288 -512
WIRE 192 -400 192 -416
WIRE 208 -400 192 -400
WIRE 256 -400 256 -432
WIRE 272 -400 256 -400
WIRE 288 -400 288 -416
WIRE 288 -400 272 -400
WIRE 272 -384 272 -400
WIRE 32 -352 32 -768
WIRE 48 -352 32 -352
WIRE 144 -352 128 -352
WIRE 256 -352 240 -352
WIRE 352 -352 352 -768
WIRE 352 -352 336 -352
WIRE 192 -272 192 -288
WIRE 240 -272 192 -272
WIRE 240 -256 240 -272
WIRE 192 -240 192 -272
WIRE 32 -144 32 -352
WIRE 80 -144 32 -144
WIRE 192 -144 192 -160
WIRE 192 -144 160 -144
WIRE 240 -144 192 -144
WIRE 352 -144 352 -352
WIRE 352 -144 320 -144
WIRE 352 -112 352 -144
WIRE 352 -112 288 -112
WIRE 368 -112 352 -112
WIRE 32 -96 32 -144
WIRE 288 -96 288 -112
WIRE 192 -80 192 -144
WIRE 192 -80 176 -80
WIRE 240 -80 192 -80
WIRE 240 -32 224 -32
WIRE 32 0 32 -16
WIRE 224 0 224 -32
WIRE 224 0 32 0
WIRE 288 0 288 -16
WIRE 288 0 224 0
WIRE 32 16 32 0
FLAG 32 16 0
FLAG 32 -352 Sig
FLAG 368 -112 OUT
IOPIN 368 -112 Out
FLAG 176 -80 Inv
IOPIN 176 -80 In
FLAG 144 -352 L_3
FLAG 192 -288 L_2
FLAG 240 -352 L_1
FLAG 240 -192 0
FLAG 144 -768 M_3
FLAG 192 -704 M_2
FLAG 240 -768 M_1
FLAG 272 -384 0
FLAG 208 -400 Inv
IOPIN 208 -400 Out
FLAG 144 -1072 H_3
FLAG 192 -1008 H_2
FLAG 240 -1072 H_1
FLAG 208 -816 Inv
IOPIN 208 -816 Out
SYMBOL signal 32 -112 R0
SYMATTR InstName V1
SYMBOL e2 288 -112 R0
SYMATTR InstName E1
SYMATTR Value 100k
SYMBOL res 176 -160 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R1
SYMATTR Value 1e6
SYMBOL res 336 -160 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R2
SYMATTR Value 1e6
SYMBOL res 144 -368 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R3
SYMATTR Value 3k3
SYMBOL res 352 -368 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R4
SYMATTR Value 3k3
SYMBOL cap 224 -256 R0
SYMATTR InstName C1
SYMATTR Value 1u
SYMBOL res 176 -256 R0
WINDOW 0 6 40 Right 2
WINDOW 3 6 56 Right 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL res 144 -784 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R6
SYMATTR Value 2k2
SYMBOL res 352 -784 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R7
SYMATTR Value 2k2
SYMBOL res 176 -704 R0
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL cap 176 -592 R0
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL cap 240 -496 R0
WINDOW 0 6 24 Right 2
WINDOW 3 6 40 Right 2
SYMATTR InstName C3
SYMATTR Value 330n
SYMBOL res 272 -512 R0
SYMATTR InstName R9
SYMATTR Value 100k
SYMBOL res 176 -512 R0
WINDOW 0 6 40 Right 2
WINDOW 3 6 56 Right 2
SYMATTR InstName R10
SYMATTR Value 1k
SYMBOL res 144 -1088 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R11
SYMATTR Value 3k3
SYMBOL res 352 -1088 R90
WINDOW 0 -16 50 VRight 2
WINDOW 3 0 62 VLeft 2
SYMATTR InstName R12
SYMATTR Value 3k3
SYMBOL cap 176 -992 R0
SYMATTR InstName C4
SYMATTR Value 1n
SYMBOL res 176 -928 R0
SYMATTR InstName R13
SYMATTR Value 10k
TEXT 0 40 Left 2 !.AC Oct 10 10Hz 100kHz\n \n.Param Lo=0.5 Mid=0.5 Hi=0.5\n.STEP Param Mid 0.0 1.1 0.1\n+ Param Lo 0.0 1.0 0.1
TEXT 160 -368 Left 2 ;10k-Lin.
TEXT 160 -784 Left 2 ;10k-Lin.
TEXT 160 -1088 Left 2 ;10k-Lin.
TEXT 240 24 Left 1 !XU_L L_1 L_2 L_3 Tone_Pot Rt=10K set={Lo}\nXU_M M_1 M_2 M_3 Tone_Pot Rt=10K set={Mid}\nXU_H H_1 H_2 H_3 Tone_Pot Rt=10K set={Hi}\n \n.SUBCKT Tone_Pot 1 2 3\n.Param P_T 0.000001\nR1 3 2 { Rt * (1-P_T-max(min(Set,1),0)*(1-P_T*2))}\nR2 2 1 { Rt * (max(min(Set,1),0)*(1-P_T*2)+P_T)}\n.ENDS
LINE Normal 168 -352 144 -352 2
LINE Normal 172 -344 168 -352 2
LINE Normal 180 -360 172 -344 2
LINE Normal 188 -344 180 -360 2
LINE Normal 196 -360 188 -344 2
LINE Normal 240 -352 216 -352 2
LINE Normal 204 -344 196 -360 2
LINE Normal 212 -360 204 -344 2
LINE Normal 216 -352 212 -360 2
LINE Normal 192 -344 186 -332 2
LINE Normal 198 -332 192 -344 2
LINE Normal 192 -288 192 -344 2
LINE Normal 198 -344 202 -344 2
LINE Normal 198 -341 194 -344 2
LINE Normal 198 -344 198 -341 2
LINE Normal 198 -347 194 -344 2
LINE Normal 198 -344 198 -347 2
LINE Normal 168 -768 144 -768 2
LINE Normal 172 -760 168 -768 2
LINE Normal 180 -776 172 -760 2
LINE Normal 188 -760 180 -776 2
LINE Normal 196 -776 188 -760 2
LINE Normal 240 -768 216 -768 2
LINE Normal 204 -760 196 -776 2
LINE Normal 212 -776 204 -760 2
LINE Normal 216 -768 212 -776 2
LINE Normal 192 -760 186 -748 2
LINE Normal 198 -748 192 -760 2
LINE Normal 192 -704 192 -760 2
LINE Normal 198 -760 202 -760 2
LINE Normal 198 -757 194 -760 2
LINE Normal 198 -760 198 -757 2
LINE Normal 198 -763 194 -760 2
LINE Normal 198 -760 198 -763 2
LINE Normal 168 -1072 144 -1072 2
LINE Normal 172 -1064 168 -1072 2
LINE Normal 180 -1080 172 -1064 2
LINE Normal 188 -1064 180 -1080 2
LINE Normal 196 -1080 188 -1064 2
LINE Normal 240 -1072 216 -1072 2
LINE Normal 204 -1064 196 -1080 2
LINE Normal 212 -1080 204 -1064 2
LINE Normal 216 -1072 212 -1080 2
LINE Normal 192 -1064 186 -1052 2
LINE Normal 198 -1052 192 -1064 2
LINE Normal 192 -1008 192 -1064 2
LINE Normal 198 -1064 202 -1064 2
LINE Normal 198 -1061 194 -1064 2
LINE Normal 198 -1064 198 -1061 2
LINE Normal 198 -1067 194 -1064 2
LINE Normal 198 -1064 198 -1067 2

[ElectricDruid]

Both the low and high filters look a little bit "peaky" to me - e.g. the resonance is a littl bit higher than you'd usually see in a tone control.

Of course, it's your design, so you can do what you like!

Also, have you seen the National Semiconductor Audo/Radio Handbook 1980 that's flying around the web in PDF format? It's the only place I've seen all the equations for the Baxandall tone control. There's some useful stuff in there.

[Rob Strand]

I know I've tried the cap to ground.  At the time I rejected it over other methods.   I think the problem I saw is when you connect any components to ground on a band, when that band is boosted or cut it causes the other bands to become "active".  If you take your LO band you can see the mid and highs have some change.  I'm pretty sure you would have seen that effect when tweaking values.

Nonetheless, you have done a good job tweaking the parts to reduce that effect on your example.

The bass peak can be tamed by adding resistor in series with the bass cap.  However, only doing that causes other issues.  Probably need another cap to tame the peaking on the treble.

For the three band the way the mids and treble have nicer looking slopes than the National Semiconductor 3-band.   The National 3-band has a very low treble slope, low Q on the mids and a heap of interaction between the bands.

[fryingpan]

The peaking may be inconsequential in a guitar pedal. Possibly even pleasing.

[PRR]

National Semiconductor Audo/Radio Handbook

PDF downloadable 1980:
https://archive.org/details/bitsavers_nationaldaAudioRadioHandbook_17034677/page/n57/   
10 pages on Tone Controls and also other stuff (including "fuzz").

1976 edition as PDF
https://bitsavers.trailing-edge.com/components/national/_dataBooks/1976_National_Audio_Handbook.pdf
Note Dennis Bohn's name at top of Contributors page in 1976, another name (who??) in 1980.

1976-1980 paper copies were very cheap paper and are falling apart. The 2000(?) AA reprint is still usable but is the wrong size! (Won't fit right with my genuine databooks.)

[ElectricDruid]

The peaking may be inconsequential in a guitar pedal. Possibly even pleasing.

Yes, absolutely.

[antonis]

A 50 years-old relevant

[m4268588]

From 1976 edition
https://mrotqch.web.fc2.com/stomp/Other/BAX_3band.png
Hi + Mid, Hi + Lo, Mid + Lo

[m4268588]

It has been adjusted to suppress peaks at corner frequencies as much as possible.

Hi + Mid	Hi + Lo	Mid + Lo

It was difficult to find the mid-band component values in algebra, so I used a simulator.

Edit:Fixed drawing errors
I don't like it a little yet.