Baxandal tone control

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Baxandal tone control

Postby KB » 16 Jan 2013, 12:11

Hi

Hopefully some one with a little more knowledge about these things will be able to answer my question(s)

The image is from the appnote for the LM833 chip. It shows a variation of the Baxendal tone control and the design formulas. However when using the shown values with the equations as shown I do not get the same results.

When using formulas I had: C values are in Farads, Resistors in Kohms ie. C1 5E-8, C2 5E-9, R1 1.1E4, R2 1.0E5, R3 1.1E4, R5 3.6E3

results (stated results in brackets)
Fl 31.83099 Hz - close enough (32Hz)
FlB 289.3726 Hz - (320Hz)
Fh 8841.9413 Hz - not close (11kH)
fhB 869.69914 Hz - not close (1.1 kH)

Am I doing something wrong? Are there some series or parallel relationships to be taken into consideration? Or is there a little poetic license being used by the Appnote writer when stating the response values for the given component values?
Can someone point me to a web page that has better equations of a description of how to design a Baxendal tonestack from the ground up.

All I wanted to do was to change the response from the traditional HiFi values (1kHz cross over frequency) to guitar values bass frequency around 150Hz and Treble around 5kHz...
Yes I know about the tonestack calculator, but I wanted to learn a little about the circuit, not just plug in some values.

Any recommendations and suggestions (please keep it clean :D ) and advice would be gratefully recieved

Cheers

Kevin
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Re: Baxandal tone control

Postby jmraney317 » 22 Jan 2013, 23:40

If you're actually trying to figure out how the circuit works, as opposed to just copying it with a few tweaks, you're looking at some basic electronics engineering. If you're just trying to grasp the fundamentals, a great way to start is by redrawing the circuit with each tone control at '0' and '10' - some of the parts drop out and it's easier to figure out how everything works.

The version you have posted won't work with the Duncan Amps Tone Stack Calculator, either - TSC assumes the tone components are passively sitting between a generator impedance and a load, while your schematic has a Baxandall network used as it properly should be, as part of the feedback network of the op amp.

Hope that helps!
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Re: Baxandal tone control

Postby KB » 25 Jan 2013, 15:51

Hi jmraney317

Thanks for your reply. I realized that the TSC was not working with this circuit, thanks for explaining why. Hopefully this will stop others getting frustrated tying to figure out what is actually going on.

I was more interested why the posted values in the app-note do not give the specified results. To be really bad and quote myself.

KB wrote:Am I doing something wrong? Are there some series or parallel relationships to be taken into consideration? Or is there a little poetic license being used by the Appnote writer when stating the response values for the given component values?
Can someone point me to a web page that has better equations of a description of how to design a Baxendal tonestack from the ground up.


jmraney317 wrote: If you're just trying to grasp the fundamentals, a great way to start is by redrawing the circuit with each tone control at '0' and '10' - some of the parts drop out and it's easier to figure out how everything works.


Yes this is what I have started to do using LT spice and plotting the results. To be honest I wish I could remember a lot more of my maths courses - too many years...

This all started with just trying to shift the cross over frequencies for bass and treble. As shown it has the typical center band centered around 1kHz (typical for HiFi - but not so good for guitar).
I know I could shift the center frequency to around 500Hz by doubling the cap values, but I have specific values in mind and would like to put the treble to around 4.5kHz - 5kHz. This is why I was looking for specific formulas or a document that outlines the design procedure. Sometimes the simplest ideas become the most frustrating

Regards

Kevin
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