Trying to simulate a passive baxandall tone control with LTSpice...no luck

Thread Starter

Joppnl

Joined Dec 29, 2023
9
Hello,

this is driving me nuts...spending total evening on it but can't get the results out of it what should be expected...

I'm fairly new to LTSpice and doing some YT-courses and one of them had a Baxandall tone control.

I redraw it into my LTSpice and ran a simulation but no matter what I do...can't get a (almost) flat curve out of it.

What I would expect is that with the 2 potmeters half way there should be an attanuation (do not know how much exactly) but this attanuation should be constant with frequency.

My original LTSpicesetup had 2 'stepping' potentiometers, made of 2 seperate resistors. So pot1 = R2&R3, pot2 = R6+R7.

I learned the .step command in LTSpice but I deleted it all because I dould not get a flat curve with it so I decided to make it more simple by removing these commands and adjust the values manually and see what comes out.

I would expect a flat curve when R2=R3=R6=R7 and as the origanal schematic uses pots of 100K I made these resistors each 50K but the freq response is far from a straight line.

Also tried manually different values but no matter what...can not get a straight line out of it.

What am I doning wrong? Is it the schematics I am overlooking a fault or does it has to do with the instructions within LTSpice itself?

PS: did delete C1 as well, no difference. Original schematic had an output resistor of 50K but I deleted that, also no difference.

Thanks in advance for your support,
 

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Ian0

Joined Aug 7, 2020
9,822
A Baxendall tone control is an active circuit - it should have a gain element, usually an op-amp, but originally a small-signal pentode valve.
 

crutschow

Joined Mar 14, 2008
34,454
As Ian0 stated, it needs an active circuit for proper operation.
The reference you posted is in error as your sim shows.
You can see the added op amp in their Figure 4.2.6.
However, even with the added op amp, I could not get the desired output, so there appears to be a problem with some of the component values.

Below is the sim of a Baxandall circuit with a slightly different configuration for four settings of the tone control pots U2 and U3, with an op amp to provide the virtual ground and feedback.
This circuit has the advantage of requiring only two frequency determining capacitors, C3 and C4, rather than four.

1703896612310.png
 
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Thread Starter

Joppnl

Joined Dec 29, 2023
9
Thank you lan0 and crutschow.

I will build your schematic and see the results (it will be working for sure as you tested it) but just for my learning how to work with LTSpice.

For me it remains a little mystery why the schematic I posted does not work as I find this exact schematic on various places in various writings but well, my purpose was to learn how to work with LTSpice so any othe schematic is fine as well.

Thanks again!
 

Ian0

Joined Aug 7, 2020
9,822
Thank you lan0 and crutschow.

I will build your schematic and see the results (it will be working for sure as you tested it) but just for my learning how to work with LTSpice.

For me it remains a little mystery why the schematic I posted does not work as I find this exact schematic on various places in various writings but well, my purpose was to learn how to work with LTSpice so any othe schematic is fine as well.

Thanks again!
There no rule on Youtube that says you can't copy something that's wrong.
Baxendall's 1952 paper uses a virtual earth gain stage. If it doesn't have a virtual earth gain stage it isn't a Baxendall circuit. (It's probably the 1949 James circuit)
The James circuit lives on in guitars (because there isn't a convenient power source to run the op-amp), but the Baxendall circuit is ubiquitous everywhere else. I have never seen a passive tone control where there is a power source for an op-amp.
 

Thread Starter

Joppnl

Joined Dec 29, 2023
9
Ahaa...

I looked the "James tone circuit 1949" up and find different schematics including the original 1949 one and it looks like that the one I drew in LTSpice is indeed the James circuit!

Will dig into that and see how that ones should behave and see if my measurements do correspendent with that.
Thanks!
 

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