Please identify this tonestack

[bushidov]

Hi All,

I know I've seen this Tonestack before, but I think it is not dawning on me because of how the schematic is laid out.



Its obviously an active Tonestack, but not sure what kind. It looks like a Baxandall in some way, but not one that I've ever seen.

[idy]

Maybe not a tone stack, two separate active mid(notch/peak) filters? Guessing they are an octave+5th apart.

[iainpunk]

looks to me like a modified baxandall, only the mid range section of the ´3 band baxandall´ doubled up. i suppose the bass and treble side were replaced by the two resistors

[marcelomd]

It's two one-band Baxandalls.

GK and Darkglass use this in their preamps for the two mid bands, plus a regular two band for bass and treble.

[Rob Strand]

It's a peaking boost/cut equalizer.  Q typically around 1.0.  Often used for mids but it's not limited to that.

Very much based on the Baxandall circuit.

The origins of the circuit are a little fuzzy but it was either one of these two sources:
- Dennis Bohn, "Analog Handbook", National semiconductors, 1976,
https://archive.org/details/bitsavers_nationaldaAudioHandbook_17103876
- Walt Jung, "IC Op Amp Cookbook"?, 197?
The tie breaker got down to which edition of Walt Jung's book it appeared in first

Also in, (some in errors the equations)
R. A. GREINER AND MICHAEL SCHOESSOW, "Design Aspects of Graphic Equalizers", 1983
http://leachlegacy.ece.gatech.edu/ece4435/f02/equalizer.pdf

There's few posts about it on this forum; one has the name of Walt Jung's book, posted by PRR.

[bushidov]

Thanks for the help on this. It looked like elements of a Baxandall, but doesnt line up schematically to any. Maybe it is because of how I drew it? Looking for a calculator to see how to change the frequency cut boost point.

[Rob Strand]

Thanks for the help on this. It looked like elements of a Baxandall, but doesnt line up schematically to any. Maybe it is because of how I drew it? Looking for a calculator to see how to change the frequency cut boost point.

Looks OK to me.  If you redraw it like the National Semiconductor circuit it will look more like Baxandall, just re-arrage the parts connecting around the pot.

The way it relates to the Baxandall circuit is:
- take the Bass and Treble Baxandall circuit
- Analyse high frequency equivalent of the bass circuit by shorting  the Bass caps.
   You end up with bass circuit reducing to three resistors in a T (or Y) pattern connecting to
   input node, output node and the opamp -pin.
- (Conceptually) convert the resistive T (or Y)  to a resistive Delta circuit using the Y to Delta transformation.
   The Y circuit and Delta circuit are *identical*.
- Throw the Delta resistor away which bridges the input and output, as this does not affect the response only the input impedance.
-  So up to this point the whole Baxandall bass circuit is reduced to the two 220k's in your circuit diagram, pretty much making part of the circuit look like an inverting opamp circuit.

The remaining part of the circuit is the Baxandall treble circuit, which has one mod: the addition of a cap across the pot.  The added cap rolls-off the high frequencies and changes the normal shelving treble response to a peaking response.

Now instead of thinking of treble imagine making the caps bigger and that peak and be move anywhere in the frequency range, bass, mids, treble.  So you have a peaking boost/cut.

[bushidov]

I am not sure I am understanding what you are saying Rob. Let me try a visual example as I am more a visual guy when learning new concepts.

- take the Bass and Treble Baxandall circuit

Okay. Done.

- Analyse high frequency equivalent of the bass circuit by shorting  the Bass caps.
   You end up with bass circuit reducing to three resistors in a T (or Y) pattern connecting to
   input node, output node and the opamp -pin.

I am counting 4 resistors (R6, R7, R8, and R9), not including the "double resistors" of the 100K Bass pot hard-set to noon.

- (Conceptually) convert the resistive T (or Y)  to a resistive Delta circuit using the Y to Delta transformation.
   The Y circuit and Delta circuit are *identical*.

I am lost at this point. Probably has something to do with what I did incorrectly or am not understanding in the previous step.

[Rob Strand]

am lost at this point. Probably has something to do with what I did incorrectly or am not understanding in the previous step.

You need to start with the variant of the Baxandall circuit that uses a single cap for the treble.
If you go to the first link I posted above (National Semi Audio Handbook)  on p2-45 and p2-46 (its about page 58 in the PDF) they show a suitable variant of the Baxandall circuit.     In fact they do some analysis there.  There's two variants of the bass circuit as well, two caps and one cap.  The bass variant doesn't affect the similarity to the circuit you posted since the idea is to remove the bass circuit.

I posted all the variants in reply 18 of this post *but* it looks like the site this forum uses has changed names or is purging files since all the images are missing, [not sure what's happening but they are back now]
https://www.diystompboxes.com/smfforum/index.php?topic=123231.0

Once you get the right treble circuit it might help.  Just in case here's the Y  to Delta transformation which is used to manipulate the circuit.
https://en.wikipedia.org/wiki/Y-%CE%94_transform

[PRR]

- take the Bass and Treble Baxandall circuit

Okay. Done.

No. Not drawn right. Where is the connection to the opamp output??

[DIY Bass]

The National Semiconductor manual has a chapter titled "Floobydust".  I think that is awesome. 

[ElectricDruid]

Looking for a calculator to see how to change the frequency cut boost point.

I use LTspice for stuff like this. Put the original part values in, check it works and gives the expected responses. Then start tweaking until you get it where you want it.

It's not very difficult - increase/reduce the caps to shift the frequency centre, but that will mess up some other stuff (Q and +/-dB range probably) so then you tweak the resistor values to get that back where you need it.

We can call it an "interactive design process based on simulated rapid prototyping" if it makes you feel better about it

[merlinb]

I know I've seen this Tonestack before, but I think it is not dawning on me because of how the schematic is laid out.
Its obviously an active Tonestack, but not sure what kind. It looks like a Baxandall in some way, but not one that I've ever seen.

You already asked this question back in June:
https://www.diystompboxes.com/smfforum/index.php?topic=122504.msg1155767#msg1155767