So, I redid the tone stack for the Dr. Boogie....

[MicFarlow77]

Hi All,

I have been working on tone stack tweaks for the DRB for several weeks now. I was working with a couple of design restrictions, but the big one was finding a tone stack that used standard value pots. I know we can slip in tapering resistors, but I have been less than satisfied with the taper afterwards. I also wanted to work within the pots I can get from Steve at SmallBear...

After spending 3 or so weeks tweaking with the Duncan Tone Stack Calculator, I just wasn't happy with all the interaction between the controls... also, I am limited to the selection of pots that smallbear has... specifically the long pin ones.... BTW.. Have searched high and low and Steve has by far the largest selections of those things at a price that can't be beat....

So, I got frustrated yesterday and decided to ditch the Fender/Marshall type design all together.... here is what I finished with:



The initial circuit I found was an active 3 band EQ that was part of an op-amp circuit that was designed to be a stand alone deal. I took those pot and cap values and basically tweaked it to be passive. That link is below:

http://freecircuitdiagram.com/2008/11/25/3-band-graphic-equalizer-circuit/

I happened to have a tone stackless DRB on my breadboard... so, I put this in, both at the end of the last stage where the original TS is and also where I have it above. It sounded no different in either place, so I left it like above. Sounds great and the output impedance should be low too. There is tons of range too...

What I really like it that I can use off the shelf, readily available audio taper pots with no need to use tapering resistors and since the bands are roughly centered on 60Hz, 1kHz and 10kHz it offers a lot of flexibility. There also seems to be very little interaction between the bands too. They are also very easy to scale. I also suspect it will be easy to tweak each bands center point, though I have not messed with that yet.. seems to be working well for me now... (I will probably move up the low band to maybe something like 80 or 100-120.... maybe....)

I am interested in any comments.... I know so little about Fender/Marshall tone stacks and the math that goes into them. but this one seems to be pretty easy to tweak and it is easy to see what is happening... at least to me anyway... I wonder if there is much insertion loss and what the frequency curves look like....

In any event, I am interested in comments...

Thanks,

Mick

[Ripthorn]

My only question/comment is that the mid control doesn't look like a typical bandpass filter.  Does it shape the mids nicely?  It is an interesting idea, I might have to try it...

[MicFarlow77]

My only question/comment is that the mid control doesn't look like a typical bandpass filter.  Does it shape the mids nicely?  It is an interesting idea, I might have to try it...

Hey Brian,

It seems like it to me... There is a ton of flexibility for my ears... 60, 1K and 10k centers might be better suited to program type material, but as it is, tacked onto the end of the DRB, it does real well....

Thanks,

Mick

[Paul Marossy]

Interesting tone control. It looks to me like it the controls wouldn't be very interactive, which I like.

[nelson]

On the pot situation, did you consider just using thick guage solid core wire to connect to the normal pots?

It's a little bit more work, but it's pretty quick and easy to do, still allowing PCB mount.

That would up the range of values available.

If you plan on doing more pedals than this, being limited to the range in the long pin PCB mount alphas is a pretty annoying design limitation.

There's also the possibility to have a seperate board for the pots and just link the two PCB's with interconnects.

Personally, I prefer to use the 9mm alphas and have the board spanning the depth of the enclosure keeping PCB stress to a minimum using PCB interconnects for the other peripherals.

There's also the possibility of designing the layout to have the knobs protruding from the back and sides of the enclosure, removing the need for 90 degree PCB mount pots.

I'm sure you've considered all of that.

[jakehop]

You can also dissemble the pots and change the taper for another one of a similar kind. All Alpha 16mm's are interchangeable in my experience.

Kind regards Jake

[MicFarlow77]

Interesting tone control. It looks to me like it the controls wouldn't be very interactive, which I like.

Very true Paul... they are very independent of each other and offer lots of range as well... I wish I could "program" the Duncan Tone Stack Calculator with this version... would be cool to see it interactively...

On the pot situation, did you consider just using thick guage solid core wire to connect to the normal pots?

It's a little bit more work, but it's pretty quick and easy to do, still allowing PCB mount.

That would up the range of values available.

If you plan on doing more pedals than this, being limited to the range in the long pin PCB mount alphas is a pretty annoying design limitation.

There's also the possibility to have a seperate board for the pots and just link the two PCB's with interconnects.

Personally, I prefer to use the 9mm alphas and have the board spanning the depth of the enclosure keeping PCB stress to a minimum using PCB interconnects for the other peripherals.

There's also the possibility of designing the layout to have the knobs protruding from the back and sides of the enclosure, removing the need for 90 degree PCB mount pots.

I'm sure you've considered all of that.

True Nelson on the pot selection, but there is an issue with Pot selection and having to add in tapering resistors in most other tone stack designs out there....

I did think of pot extension wires and have actually done it on a few boxes... too much work in terms of time, though it did work acceptably....

Also did the separate board idea as well, again, too much work in terms of time...

I like this tone stack configuration well enough to substitute it in place of any existing arrangement on any designs that follow...

You can also dissemble the pots and change the taper for another one of a similar kind. All Alpha 16mm's are interchangeable in my experience.

Kind regards Jake

I haven't done that yet... but you have to buy twice as many pots as you need and have a 50% waste at the start... also seems like it would be a bit of an effort and time consuming....  still worth considering though.... it would all depend on what was needed at the time...

Thanks,

Mick

[ayayay!]

This looks very interesting.  I agree that the tonstack can be a balancing act.  Especially when the treble knob starts getting up there.  It makes everthing else interactive if you crank the treble or presence too much.

I have no plans at the moment of building another Dr. Boogey, but if I do I'll try this.  Also looks like it could easily be used on some other circuits. 

[MicFarlow77]

...Also looks like it could easily be used on some other circuits. 

Thanks my thinking.. it should be a straight-forward swap in general.....

What I need to find out next is the math behind figuring the center frequency and the width of the Q... it sounds pretty wide though....

Thanks,

Mick

[sean k]

Not to rain on your parade but just to understand things a little better.

My understanding of the active bax is that the cut is made by feeding back some of the output, inverted, back into the tone controls so the inverted signal phase cancels those specific frequencies. I'm saying this because the bottom legs of your tone controls go to earth which doesn't mean it won't work just differently... I'm supposing.

What if the drain and source had 4.7k resistors and the earth connections you have at the bottom end of your tone pots then went to the collector?, maybe through a 100k resistor. Would this help to make an active cut more apparent?

Like wise the ground connection of the presence control could go to the source?

[MicFarlow77]

Not to rain on your parade but just to understand things a little better.

My understanding of the active bax is that the cut is made by feeding back some of the output, inverted, back into the tone controls so the inverted signal phase cancels those specific frequencies. I'm saying this because the bottom legs of your tone controls go to earth which doesn't mean it won't work just differently... I'm supposing.

What if the drain and source had 4.7k resistors and the earth connections you have at the bottom end of your tone pots then went to the collector?, maybe through a 100k resistor. Would this help to make an active cut more apparent?

Like wise the ground connection of the presence control could go to the source?

Hey Sean,

What I know about tone control networks I could write on a sugar packet and still have room to write what I know about building enclosures... 

As I mentioned earlier, I just got plain frustrated with the Fender/Marshall style with all it's interaction between the bands... so I got to digging and found the active bax and thought to myself... "Self, you just might be dumb enough to make that work"... redid it as above and as it is it worked out ok... I am in the process of tweaking the center frequencies and am scaling the treble to be based off of a 100K pot as well...

All that being said, I am serious about trying anything to make it work better! I'll draw up your suggestion here shortly and see if I hit it right.... Then I'll breadboard it up and report back results....

Thanks,

Mick

EDIT... Had a chance to draw it up while I could still edit this post, so I decided to pop it in right here.....



Does this look like what you were thinking?

Thanks,

Mick

[MicFarlow77]

So I hooked it up this way and it was as if were not even there.... no effect on the circuit at all.... at least with the pot returns going to ground it worked as one would expect... course, I might have it wrong up above too.... lemme know if so.....

Thanks,

Mick

[dschwartz]

the dr boogies i make are very modified, almos unrecognizable..the first thing i ditched was the tonestack, the FMV style is too subtle if you want versatility and nice mid scoop..i changed the tonestack to something more efficcient and controllable..hint: check carvin amps tonestacks...

[MicFarlow77]

the dr boogies i make are very modified, almos unrecognizable..the first thing i ditched was the tonestack, the FMV style is too subtle if you want versatility and nice mid scoop..i changed the tonestack to something more efficcient and controllable..hint: check carvin amps tonestacks...

Will do Daniel! Thanks!

[sean k]

Nope, but it's interesting what you did do.

You've got the 100k going to the source so it's basically seeing the same signal, in phase, on the pot, 1 and 2, whereas I'm thinking that the 100k could go to the drain which would see the reverse phase going back to the pots at 1.


Not that I know much either but the presence has confused me (on the opamp you have isolation from the non-inverting and the output but with a single jfet it can't happen) so I'd leave it going to ground but a 100k, or a 1M pot to adjust and find the sweet point, going from the drain to the resistors coming off the 1 leg of the pots should throw enough negative feedback into the mix to allow good cuts.


As you can see in your original inspiration the output from the centre of the pots goes to the inverting input of the opamp and the bottom leg of the pots, or the cut end( 1 on your diagram) goes to the output of the opamp so the cut is acheived with negative feedback or phase reversal. Given the gate and the drain are out of phase then this is what I'm suggesting to further emulate the original.

But then again, if it aint broke...


   

[Auke Haarsma]

the dr boogies i make are very modified, almos unrecognizable..the first thing i ditched was the tonestack, the FMV style is too subtle if you want versatility and nice mid scoop..i changed the tonestack to something more efficcient and controllable..hint: check carvin amps tonestacks...

Something like can be seen in this thread?
http://www.carvinbbs.com/viewtopic.php?t=14736&sid=b1b4329257a6b6dd3dadc1c5e65a42c5


@Mick: Interesting thread! Often I like the 'magic' of interactive TS, but it can indeed be frustrating as well!

[WGTP]

http://ax84.com/static/corepreamps/Lead%20II/AX84_Lead_II_Preamp_Schematic.pdf

IIRC this is a passive bax type tonecontrol.  Didn't HiWatt or Orange use one???   

[Ripthorn]

The passive Bax and active bax are quite different if I understand correctly (in fact, the passive "bax" is not a bax at all).  The passive one bears no resemblance to what Mick is doing, but it is a good tone stack (one that I have in an amp I'm building right now).  Controls don't interact too much either.

[MicFarlow77]

Hey Sean,

I switchted the 100k over to the drain... still no effect on the circuit... so I am going to go back to the original way I had it drawn up. My original attempt was more about taking the active version and extracting the pieces necessary to make it passive.

Also, I have no clue about the presence control either... I'm not sure what it did or rather how it worked in the original. In my adaptation, it seemed to add a little extra "hair" to the top end and was rather musical... I think it does need to be tweaked though... will try some things out and report back....

@Mick: Interesting thread! Often I like the 'magic' of interactive TS, but it can indeed be frustrating as well!

Yeah, Auke, I love the interactivity when it can be implemented as designed, but when you have to scale it in different ways to account for available parts, it becomes not so magic....

The passive Bax and active bax are quite different if I understand correctly (in fact, the passive "bax" is not a bax at all).  The passive one bears no resemblance to what Mick is doing, but it is a good tone stack (one that I have in an amp I'm building right now).  Controls don't interact too much either.

Yeah, I have no clue what I am doing here or what it should be called (maybe a modified bax with a mid control)... I just saw what I thought was a pretty neat tone stack that would not be too interactive and would be easy to tweak....

Got some more tweaks to experiment with as the day goes on... will report back....

Thanks,

Mick