Tone control circuit help

[tonedawg]

I've been breadboarding simple tone controls I could add to a circuit and came up with this, its actually a mistake based on the SWTC 3, I accidentally connected the in to out then to pot instead of in to pot 1 and out pot 2. It actually works great but I'm wondering why it works? If I wire it like it should be, the bass end of the sweep doesn't sound nearly as good, more like a muffled attenuation.

[antonis]

Hi & Welcome..

but I'm wondering why it works?

R5 right leg / pot wiper node is the point of R5 & (pot 3-2 + C19) // (pot 2-1 + C20) voltage divider..

e.g.
Fot pot midle settiing, Vout = Vin x (50000 + 1/2π*f*0.000000001)//(50000 + 1/2π*f*0.000000033) / [4700 + (50000 + 1/2π*f*0.000000001)//(50000 + 1/2π*f*0.000000033)]

[GibsonGM]

Welcome to the forum, tonedog!

You're splitting the circuit with that pot and sending parts of the signal (highs) to either the left or right leg (a variable combination of both at some pot settings!).  Each cap has a different reactance* (AC resistance), and you are choosing how much signal each cap can 'see' (or a mix of both to varying degrees).  If you picture the 100k pot being on the TOP of those legs, it might make more sense to you. 

It's a lot like a guitar tone control, just with 2 options that you can 'slide' or 'morph' between.  The legs are passing highs to ground, in a 'shunt' configuration.

* re. reactance - read about it as you go along...for now, the smaller the cap value, the higher the frequencies it will pass (the lower the reactance the smaller the cap value!).  So the 1n passes higher frequencies to gnd than the 33n does.   The resistance of the pot plays a part in the equation (as Antonis shows)

[tonedawg]

Thanks for clarifying Gibson! So in this configuration, what's it doing differently than the traditional SWTC 3 tone control pictured below?

[GibsonGM]

"Your" version uses the 100k pot as the variable resistor, swtc is a 10k - that just changes the math, regarding that.

SWTC has a lowpass filter made of the 4.7k and 3.3n cap to ground...its 'cutoff frequency' is about 10kHz, so it rolls of at 10k (where a lot of high shrill un-needed stuff often is).  That part is fixed, it's always there - note that the 3.3n cap goes to ground.

NOW - the 10k pot works with the 22n like a guitar tone control, rolling off down to about 7kHz (and affecting the first filter too), but you can set it higher than that.

The 100k volume pot is just that, a volume.

In *Your* circuit, you have 2 interactive LPFs (lowpass filters, as far as the output sees them) shunting highs to ground using the math that Antonis provided. I don't like to go to that horrible extreme of math if I don't have to (ha ha) so I use simulation to find the cutoff frequencies. 
Anyway...in YOUR case, that pot can block signal from say the right leg, allowing the 1n to have 'dominance' over what is cut  (4.7k / 1n cap, 34kHz, too high to hear...as you roll the pot toward the middle, you add resistance, so the freq. of cutoff goes down.  With just 20k resistance, you're back in the ~7kHz range).  The R is blocking signal from the 33n cap in this case.

As you roll the pot to the middle resistance value (50k, but NOT the center on an audio pot!) you have both caps acting together thru 50k of resistance; then you start to work with the 33n cap and whatever resistance the pot is set to...say 20k again, that would give you a muffled-sounding ~2kHz cutoff, VERY low.  the higher resistance to the 'left' of this path is blocking the 1n cap...

This is a very basic 'descriptive' idea of what's happening - I hope it helps...there are entire books written on this stuff.  This one isn't that complicated, antonis' eqn's up there include the whole shebang. Most things can be broken down into an R and a C, following the equation:

    1 /  2pi R C  = cutoff frequency   
just like I did for SWTC's 4.7k and 3.3n cap

I use megohms for the R's and microfarads for capacitance to make it easier, and have a little VB program I wrote to do it like 20 yrs ago  

Introductory reading for you: https://www.homemade-circuits.com/how-rc-circuits-work/

[tonedawg]

wooooow. thanks so much for that!

[GibsonGM]

You're welcome, I"m sure there are even easier or more precise ways to explain but that's my take on it. 

Start simple and gradually you'll understand lots more, it just happens with time!  Asking questions is good.

[antonis]

This one isn't that complicated, antonis' eqn's up there include the whole shebang. Most things can be broken down into an R and a C,

That's exactly what Antonis tried to do.. 


For a given frequency (800Hz say), 1nF cap exhibits 198k75 "resistance" (impedance or capacitive reactance) and 33nF exhibits 6k22 one (33 times lower than of 1nF one)..
That said, both caps impendances are set in series with 100k pot respective wiper - lug3 or 1 resistances..
For pot FCW setting, 198k75 is placed in parallel with 100k+6k22 resulting into 69k equivalent resistance..
That resistance forms a voltage divider with 4k7 one, resulting into Vout = Vin X 0.94..
(less than 10% signal loss at this particular frequency..)
For pot FCCW setting, 6k22 is set in parallel with 100k+198k75 resulting into 6k1 equivalent resistance..
That resistance forms a voltage divider with 4k7 one, resulting into Vout = Vin X 0.56..
(almost 50% signal loss at this particular frequency..)

P.S.1
From the above we can safely sate that, despite filter type denomination (High pass or Low pass) CW pot turn cuts Bass..
(at least, much more than it cuts highs..)
P.S.2
Following stage impedance is considered "infinite" or else we should add it in parallel with C19/Pot/C22, further dominating Vout..
P.S.3
Cheers Sir Mike..
(:beer: :beer: :beer:)

[Vivek]

AMZ already invented all possible tone control combinations.

It's impossible to dream up something that he did not already document 15 years ago.

[tonedawg]

is there a link to this one on his site so I can read more about it? I only found the SWTC variants.

[GibsonGM]

Besides the link at the top of this page ( )   there is this!  http://www.muzique.com/lab/main.htm

Explore the site.  Jack has really come up with and modified/analyzed a whole lot that has helped out us DIYers for a long time.  Most of what you'll need in the way of tone stuff is in there!      Check out the mosfet boosts, too! 

Next you will probably want to mess around with tone stacks...  https://www.duncanamps.com/tsc/