Active tone controls

[Xander8280]

Looking to put active tone controls in a build. A 3 band or 4 band would be nice. I have found some schematics like figure 18 on this link

http://www.angelfire.com/electronic/fun ... 7-A-A-back

Although this one looks promising, I see the graph and would like more boost out of the midrange control. I see that it holds the most spectrum real estate, so it doesn't get 10db+.
I'm wondering if anyone has build or used a tube active tone control?
How's it sound? The passive controls just are to subtle for my liking.
Also what's the formula/logic to the tuning for the bands?

Thanks guys! Looking forward to getting my build more versatile.

[Cliff Schecht]

You're overthinking the issue going with active tone controls. As the guy shows in sim, you don't get a lot of range out of each stage for the level of complexity and power consumption. The design quickly becomes needlessly complicated and it's not fun anymore (but sometimes cool). You could try to incorporate a few stages of active tone controls into a passive section but again, complicated if you aren't really good at designing filters. The formula for the active filter is given on that page, f = 1 / (6 Pi R C).

You're probably looking more for something that gives you parametric control. Find something that uses inductors, this will give you LC resonant tanks that will have an associated quality factor (Q). This allows passive circuits to give you a fairly substantial sweep range in both gain and frequency response. Ampeg is infamous for using these, a few Dumble circuits used inductors in the tonestack and you see them in a lot of bass amps where you want a lot of control over the entire range of frequencies.

The other trick you can do is build a tube gyrator circuit that simulates an inductor. They are pretty darn good at it too. I haven't googled around for tube-based gyrator circuits but I know they're out there. The added distortion from toobz might be a nice little bonus .

[roberto]

You can use LCs in a local feedback loop too.

[Xander8280]

Thanks guys! I'm used to a few MESA preamps and even without the graphic equalizer the 3band tone stacks had great sweeps. Could do some serious carving. My marshall build yields little change from zero to ten. Maybe I should change my tonestack values.

[Xander8280]

Is it the plate driven MESA tone stacks that differ from the marshall cathode follower?
Maybe if I move the tone stack I could get a result more suitable for my tastes.

[Cliff Schecht]

Which Mesa EQ are you referring to? Not the active ones right? Those are all transistor based in the MKIIC through MKIV amps.

But if you're just talking about normal tone controls, I agree that Mesa gets a lot of range out of their controls. You could look at implementing a mid-boost circuit that you can find in a lot of Mesa schems.

Also take a look at the Super Champ schematic (the old one), maybe you can cop their mid-boost circuit. These are one of my favorite amps and you get an insane amount of versatility from them.

http://www.freeinfosociety.com/electron ... php?id=917

[ampdoc1]

While we're on active EQ,......

I've got a Bell Carrillion 35 PA amp a buddy wants me to convert to a guitar amp. it's got three hi-Z mic inputs, each going to a 12AX7 gain stage. To achieve maximum signal, I intend to wire these in parallel.

At this point all the signal caps (one before and one after each 12AX7) are the same size, ...22 on the input to the tubes, and .05 on the outputs.

I'm thinking if I change the output caps to "average" tone control values, IE say .5, .05 and .005, wont' this give me a quasi active eq?

[TUBEDUDE]

The Bell could make a good guitar amp. Looks like there are enough tubes for a variety of designs.
For an active tone control, there needs to be a local feedback loop around a gain stage that incorporates the frequency selective circuit (caps, or coils and resistors). There is a good example in the Radiotron Designers Handbook 4th edition.

[Firestorm]

The Bell could make a good guitar amp. Looks like there are enough tubes for a variety of designs.
For an active tone control, there needs to be a local feedback loop around a gain stage that incorporates the frequency selective circuit (caps, or coils and resistors). There is a good example in the Radiotron Designers Handbook 4th edition.

That's what the actual Baxandall circuit is (as opposed to the James variant). Unity gain in the mid-band, boost and cut for treble and bass.

[TUBEDUDE]

All of the Baxandall implementations i have seen were between gain stages with no feedback loop. They were less than unity gain and although they had better range than simpler tone controls, and little interaction, they were very lossy.

[Firestorm]

All of the Baxandall implementations i have seen were between gain stages with no feedback loop. They were less than unity gain and although they had better range than simpler tone controls, and little interaction, they were very lossy.

http://home.comcast.net/~stphkeri/Negat ... ckTone.pdf
Here is Peter Baxandall's original 1952 article. His circuit uses negative feedback to create a virtual earth and then the filters set variable frequency-dependant impedances feeding that node to get boost and cut of the treble and bass bands. If you do it without the virtual earth, it's lossy, but as Baxandall did it, there's gain.

[Xander8280]

Which Mesa EQ are you referring to? Not the active ones right? Those are all transistor based in the MKIIC through MKIV amps.

But if you're just talking about normal tone controls, I agree that Mesa gets a lot of range out of their controls. You could look at implementing a mid-boost circuit that you can find in a lot of Mesa schems.

Also take a look at the Super Champ schematic (the old one), maybe you can cop their mid-boost circuit. These are one of my favorite amps and you get an insane amount of versatility from them.

http://www.freeinfosociety.com/electron ... php?id=917

I used a studio preamp into a simul-class 295, and a bass 400+. Both passive plate feed every responsive tone stacks

[Xander8280]

OP here,
Has anyone changed around their tonestack placement to plate feed(or to cathode follower) and got largely different sweep responces?

I know the sweep resistor needs to be changed for moving the tonestack.