4 band Baxandall tone control

[rankot]

I am looking for 4-band Baxandall tone control, with Bass, Lo Mid, High Mid and Treble controls. Does anyone have a schematics for this (passive only), or I must try to figure out how to modify standard 3-band to 4?

[noisette]

passive will only give you grieve: the idea sounds nice, but...
rather look up the eq from the soundcraft 200b desk it´s very simple and is 4band semiparametric

[Derringer]

see if you can find some Gallien Krueger amp schems
they did a passive 4-band tonestack I believe

[GibsonGM]

Seems like you might get more control and more opportunity to mod (easier) if you were to do 4, or 5, EQ stages.  Just my 2 cents.

[rankot]

I have an old amp and I want to upgrade it's preamp section. It already has 4-band EQ, but it is transistor based and I want a tube preamp, and I need some simple 4-band tone stack (not parametric) so I can replace it without changing the front panel.

[GibsonGM]

How about doing a 3 band stack, and use  'pre' and 'post' gain controls for your preamp? 

I include them in my tube preamps (mostly), and like their functionality. You can really dial in some sweet tones that way, a lot like having a gain control and master volume....

[Rob Strand]

t already has 4-band EQ, but it is transistor based and I want a tube preamp, and I need some simple 4-band tone stack (not parametric) so I can replace it without changing the front panel.

Maybe an inductor based EQ like the Mesa-Boogie.

[Rob Strand]

Here's another way to approach it, split into multiple bands then add back together.

This one needs second order active filters:
http://el34world.com/charts/Schematics/files/sunn/sunn_coliseum_bass.pdf
This one is the poor mans passive version:
http://www.next.gr/uploads/52/5-channel-graphic-equalizer-by-bc548-transistor.jpg

I'm not endorsing the second one.  I suspect you will need to modify it so ensure the output is flat when the bands are set flat.  The second issue it to have equal boost cut on all bands.  It might even work better with a log or audio taper pot; more boost.

[teemuk]

What OP is searching for might not exist.

Three controls (B/M/T) is just about the maximum one can cram into "Baxandall" circuit without controls starting to interact with each other too much.

Common scheme is therefore simply to use two Baxandall circuits, one configured for usual bass & treble, second one configured for low and higher mid-range. Sometimes you see three "Baxandall" stacks in series. I think some of those Gallien-Krueger amps should indeed be very good examples.

Passive configuration seems senseless to me. You can't fit any more controls to it than you could to active version.

"Baxandall" circuit (though Baxandall really refers to active version of the circuit) is very lossy in passive form. In addition, the more range you need for the controls the greater the insertion losses are. When you start to cascade the circuits in order to implement more controls the losses of each circuit add up.

Even if you build a passive circuit you will need to introduce gain anyway, basically just to recover the huge signal attenuation. At that point SNR has already degraded. So why not implement that gain to the tone control stages? That's the whole ingeniosity behind the Baxandall circuit: No considerable signal losses to recover and noise levels kept in a leash. Why purposefully downgrade the design by going passive and then amplifying to recover immense signal losses?

[ElectricDruid]

+1 Agree with Teemuk.

For 4 bands, you really need to be looking at an active circuit for exactly the reasons given.

I hadn't thought of cascading two two-band baxandall circuits, but that seems like a simple way to go, and you could do the whole thing with just one dual op-amp. Otherwise, you could do some 4-band graphic EQ based on gyrators or whatever.

If you want it all tube-based, that's not my area of knowledge, but I'm sure there's stuff out there.

Tom

[roseblood11]

Why does it have to be tubes? For an active EQ, you need noiseless, linear amplification, that doesn't"color" the tone. Modern opamps are the obvious choice. The Carvin X100B is a very interesting solution.

[Derringer]

I have an old amp and I want to upgrade it's preamp section. It already has 4-band EQ, but it is transistor based and I want a tube preamp, and I need some simple 4-band tone stack (not parametric) so I can replace it without changing the front panel.

what's the old amp?

[rankot]

what's the old amp?

Čajavec GA185B. Made in ex-Yougoslavia. 

[Derringer]

Any chance you can post a schematic for it? I'm wondering if you could copy its tonestack design and implement it with tubes.

Otherwise though, I mis-spoke about the GK tonestacks. They are 4-band, but they are active.
like this one http://gallienkrueger.mivamerchant.net/wp-content/uploads/400rb.pdf

A cool tube-driven tonestack that's worth checking out it the one found in Ampeg V-4s and their like (v-2, vt-40 etc.)
It's a passive James stack with an active mid circuit that allows the user to boost or cut at 400hz, 1000hz, or 3000 hz.
Although you'd need to hunt down or wind your own inductor (there are directions somewhere online where someone did wind their own inductor for one of these)

If I were you, I'd just do a passive 3-band stack or a passive James-stack with a mid shift and build in an effects loop where I could run a 2 band parametric EQ. 

[Kennt82]

EQD Talons is a Guv'nor with a 4 band EQ.  First is a Rat-like "presence" control leading into an active 3 band Baxandall section.

[Rob Strand]

Čajavec GA185B. Made in ex-Yougoslavia.

So it was Bass and Treble then HI and LO ?
I was thinking you were replacing a graphic EQ.

From what I can see the HI and LO filters don't match-up to give a flat response.  The LO is lower than the HI and this leaves a gap in the mid to give the amp a scooped voicing.  The Fender tone stack does this.  If you replace it with a flat response EQ you know it will sound midrangy?

For Bass & Treble, why not use a Fender type Bass/Treble or a 2-band passive Baxandall (which you could match the old circuit or do your own).

For Hi & LO  maybe use a simpler passive version of what was there.  Go here and about 1/2 way down there is a muff with a tone control that has separated low and high frequencies:
https://www.premierguitar.com/articles/Electro_Harmonix_Russian_Big_Muff_Pi_Pedal_Mods

Another way to approach is to use a 2-band passive Baxandall tone control  for Bass/Treble but change the meaning of HI and LO.   You could for example make LO a variable frequency high-pass filter that cuts the bass, and HI a variable frequency low-pass filter that cuts the treble.   It usually works best with second order filters but you could do it with a first order.    A fender tone stack will give you the scooped mids but the bass control on that works kind of like a variable low-cut so having two low-cuts will be redundant.

Ask yourself you do what some mid-scooping and what would you like the controls to do.  Something that would be useful to the sound you want.

[rankot]

Thanks a lot, Rob, this post explains many things! I didn't like it's original preamp because it was too noisy, and too loud - Volume pot was unusable after 9 o'clock. So I built a FET preamp based on a schematic I found somewhere here (Alembic Ripthorn), and added op amp based parametric 4-band EQ. It works fine, but sound is a kind of boring.

Fortunately, while changing the preamp, I also added a jack for external preamp input, so I can experiment. I also built a pedal using Acoustic 220 preamp schematic, and it works really nice, has a real bass sound, but it has 3-way tone stack done with inductors. So I wanted to try something completely different, and built Alembic F2-B with 12AX7 and it really kicks ass, but it also has only 3 pots for tone control. Which brings us to this discussion - I wanted to make a simple one-tube-preamp with 4 pots for tone (to fill all the holes). I will loosely keep this Alembic design, but I will choose another tube so I can run it on 40V (which is available in amp's power supply).

I will try with the third schematic from BMP page, to see what happens if I add this to 2-band passive Baxandall.

Original preamp schematic can be found here:

[rankot]

Maybe I could try this combination - Baxandall bass/treble and two mid notch filters taken from Marshall JCM 800 preamp?

[Rob Strand]

There's a lot of options!

Honestly, you don't want to go backwards.  If you think the F-2B was the best sound you shouldn't compromise from that just because of a few pot labels.

I've been trying to think of a way to stay with the F-2B and to use the extra pot to *add* more flexibility.   The trick is how to "redefine" the meaning of the amp controls in a way that still makes sense.

If it were mine I would try this.   Keep the F-2B but make one mod.
- Bass   = F-2B Bass control
- Treble  = F-2B Treble control
- LO       = F-2B Mid control;   re-interpret "LO" as LO-MIDs
- HI        = added  Upper Mids control;  re-interpret "HI"  as HI-MIDs

Now,  the implementation of the Upper mids control mod is very simple:
- Connect the "Upper Mids" pot maybe 1MEG in series with a capacitor say 470pF.
- Then connect that combination in parallel with the 250pF treble cap on the F-2B.
- When you vary the pot it kind of varies the value of the treble pot adding more upper mids as the pot resistance is reduced.

You might have to play with the HI MID pot value and taper to make the control feel natural to use.

The downside is the effect of that pot will be quite subtle.  Is that a problem if it lets you keep the good F-2B sound?

[The other way around is to put a pot in there for the 100k slope resistor and call this LO and use HI for the mid control].
--------
[Edit: adding whole mid ckt a bit like you suggested but before the tone control
https://images.talkbass.com/attachments/traynor-bass-ts50b-amplifier-schematic-png.2385807/
]

[rankot]

I will try it, seems to be quite simple addition so I can use existing PCB for that. I will need to check how does it perform with available 50V B+, because I don't want to add another transformer or PS. Thanks!