active tonestack

[smashguitars]

I was wondering if anyone had expecience with using a cut/boost type EQ where lows/mids and highs are each driven by half of a 12x*7.

would the signal have to hit the 3 filters in parallel? or is it better the have a lows filter that feeds the mids filter that feeds the highs filter? are there any designs out there that do something like that?

thanks

[Steve Conner]

Here are my two attempts at active EQ:

A two-band Baxandall
http://scopeboy.com/baxeq.html

A two-band semi-parametric, like the Ampeg SVT midrange, but using op-amps instead of an actual wound inductor. This means the frequency can be varied smoothly by a pot, instead of only 5 steps like on the SVT. I later upgraded the midrange band to full parametric.
http://scopeboy.com/peq.html

Both work fine, though in practice, I hardly ever use the cut position of the Baxandall, because real guitar tone stacks don't cut, so guitar tones that use cut just sound wrong. Also, the parametric is a little noisy compared to a passive tube EQ. I always meant to redesign it to try and lower the noise. I think it's worthwhile, because it turned out great for bass guitar and to make Boogie-style modern tones with scooped mids (that grandpa calls "Bees in a can")

[smashguitars]

i was thinking along the lines of something like this

Its partially derived from the active bax thats in the handbook, but tweaked to operate in guitar range. Now looking at the freq response it works exactly as i would like it, however, the problem is how to insert this circuit into into an existing design that uses a standard TMB tonestack.

So assuming the amp where this will be used is a SLO, how do i need to scale this circuit to keep overall gain in control, yet preserve tonal characteristics of the original amp? How do i properly setup input and output impedances?

Do i still need to use a CF? or is there a better place in the signal chain where this circit could go?

sorry for the barage of questions

[Satamax]

http://www.soundcitysite.com/sc_webpages/sc_120_1.jpg

[Enzo]

A number of recent vintage PV amps have active EQ, check them out. Ultra, Ultra Plus, Triple XXX, etc. Three bands on a single triode.

[Satamax]

A number of recent vintage PV amps have active EQ, check them out. Ultra, Ultra Plus, Triple XXX, etc. Three bands on a single triode.

Hey Enzo, you mean this one?

http://www.schematicheaven.com/newamps/peavey_xxx.pdf

Just a daft question, if it's active cut, and no active boost, can it still be called active?

Myself i was trying to think along thoses lines. http://music-electronics-forum.com/s...ead.php?t=5033 Schemo is incomplete. I explain a bit how i think it should work, and Steve gave me the answer i was looking for. But i'm lacking soo much on the theoretical side!

Bye.

Max.

[Enzo]

Just a daft question, if it's active cut, and no active boost, can it still be called active?

Are you referring to a specific example?

To me the active part means there is a gain stage involved - well, a stage capable of gain. Passive means made of only resistors, caps and inductors. If you configured it for cut only, I don't know why it would cease to be active. On the other hand, if it were to be cut only what would be the point of using the gain stage? The PV examples are certainly capable of boost.

[Satamax]

Are you referring to a specific example?

To me the active part means there is a gain stage involved - well, a stage capable of gain. Passive means made of only resistors, caps and inductors. If you configured it for cut only, I don't know why it would cease to be active. On the other hand, if it were to be cut only what would be the point of using the gain stage? The PV examples are certainly capable of boost.

Sorry Enzo, that's me being daft. Theses are capable of boost at the frequencies which are not cut This shows where my idea of using positive feedback for boost isn't necesary. I'll have to look again at that XXX peavey.

Thought that idea of using a litle pos feedback make me smile. It isn't new, but i find it way cool. Not that i know how well it would work. I'm just keen on crazy ideas usualy.



Thanks.

Bye.

[Steve Conner]

To me, an active tone control means one that uses negative feedback to do its work. So the classic Fender/Marshall tone stack is not active, even though it has considerable gain because of the two triodes you normally see bolted onto the ends. And a Twin-T notch filter that used an op-amp with negative feedback to sharpen the notch would be active, even though it never has a gain greater than 1 at any frequency.

Smashguitars, I'm not too sure of the exact answers to your questions... I see you've used negative feedback with R20, R23 and R24, but I don't think that's the proper way of applying negative feedback in an active Bax-style circuit. I did it by feeding the input to one end of the stack, the NFB from the plate to the other end, and connecting the tone control wipers to the grid. What you did will make the tubes into anode followers, so the grids roughly look like virtual grounds and the stage operates as a current-to-voltage converter.

My Bax circuit gave +/-12dB of bass and treble adjustment, and a flat gain of 10 with the first stage set up as a cathode follower having gain of 1.

The classic tone stack has a lot to recommend it. If you turn all the knobs to 5 it delivers guitar tones that just "sound right". Every guitarist I've known starts by turning the tone knobs half way, presumably thinking that's somehow standard or flat, even though it delivers a considerable bass and treble boost. A classic tone stack is flat with the bass and treble on 0 and the mids on 10 if fitted.

I have no idea how that Sound City circuit works, will need to figure it out :-o

[Enzo]

I would suggest that the classic Fender stack has no gain and is passive. The gain stages before and after it are not part of the stack. An active stack needs an active element as part of it. Perhaps "gain" was the wrong term to use.

[Satamax]

Hi everybody!

Steve, the sound city is pretty simple, and not a real active tone stack in it's early versions.

Basicaly bass is fed directly by the summing resistors, at full bandwidth, and the high cut filter is made by the 500k pot and cap in parallel to ground. There's obviously a local nfb on the cathode, since there's no bypass cap. In later versions they've added a cap between plate and grid, making it effectively active at some frequencies. Then the mids are fed by a T and a Π filter sections of the ladder filter making a cut iirc of -6db at 159hz. And again a filter made of the pot and cap cutting -3db at 318hz iirc. Same applies here, they've used a litle cap for neg feedback between plate and grid on later versions. Then the trebles are also fed after that ladder, at 159hz, but at something like -30 db iirc. (Tand Π sections adding up if i've understood correctly) Again a -3db filter made up of the cap and pot in parallel cuting them 3db at 6366hz aproximately. In later versions or the 50+ there's again a local nfb cap between plate and grid. Therre's a section in rdh 4 about T and Π filters starting from page 180 and ladder atenuators starting page 795. All that, if you have a better technical knowlege than me will tell you far better than me how this filter work. I have a gal who can understand equations, but i can't. I'm too daft.

[smashguitars]

the TS in the XXX is pretty interesting. i ran a simulation of it and it looks like each control only has about 1dB of boost/cut in the freq. seems like it wouldnt be enough...

i do however recal trying this amp out and commenting that i liked the way the TS felt.

btw any comments on the questions in my previous post?

thanks

[Satamax]

the TS in the XXX is pretty interesting. i ran a simulation of it and it looks like each control only has about 1dB of boost/cut in the freq. seems like it wouldnt be enough...

i do however recal trying this amp out and commenting that i liked the way the TS felt.

btw any comments on the questions in my previous post?

thanks

You mean this one?

I was wondering if anyone had expecience with using a cut/boost type EQ where lows/mids and highs are each driven by half of a 12x*7.

would the signal have to hit the 3 filters in parallel? or is it better the have a lows filter that feeds the mids filter that feeds the highs filter? are there any designs out there that do something like that?

thanks

Well, on the SC it's in parallel but it isn't realy an active tone stack, it's passive filters followed by gain stages. In the later versions, the local NFB between plates and grids was just bandwidth limiting. Imho, the SC style controls sound good up to a point, but when cranked some frequencies overlap making weird dips and peaks in the frequency response. Like in the atached pic. I still think it would be the way, since i can't see how you could make the series arangement work. At some point i had imagined a sort of fender TMB with a cascode inserted in place of the three pots, and the input or output of thoses tubes controled by pots. But i have no idea if this is even possible.

Bye.

Max.

[smashguitars]

in "theory" the series tonestack works if looking at the 3 stages by themselves.

i attached the frequency response where lows and highs are varied at the same time.

i can tweak the gain of each stage by adjusting the feedback resistors. i guess thats what i need to figure out. Looking at these 3 stages as ONE BOX. what must be the overall gain so that the rest of the amp wouldnt be effected? And where in the circuit would be the best place to put this?

[Satamax]

in "theory" the series tonestack works if looking at the 3 stages by themselves.

i attached the frequency response where lows and highs are varied at the same time.

i can tweak the gain of each stage by adjusting the feedback resistors. i guess thats what i need to figure out. Looking at these 3 stages as ONE BOX. what must be the overall gain so that the rest of the amp wouldnt be effected? And where in the circuit would be the best place to put this?

Smash, your tone control isn't an active one imho. You don't have either local or global variable negative feedback. So it is not active. In an active tone stack you need a frequency controled variable negative feedback loop. With either fixed frequencies, or variable frequencies and usualy a variable resistor controling the amount of feedback, fed back to the grid. (or cathode iirc!)