Analyzing frequency response of tonestack

[Craftyjam]

Hello,

I'm looking to calculate the frequency response of the bass control on the matchless Clubman and other matchless amps.

I'm sorry if this seems tedious to the trained engineers here but this is what I have so far.

So when the bass control potentiometer is turned all the way up so that the wiper is in series with the 22n coupling capacitor, very little current flows through the potentiometer, and it also does not load down the approximate 12k source impedence of parallel V1a/V1b. The signal then passes through the 2.2n highpass filter, with the resistor leg being made up of the series combination of the 10k resistor and 100k resistor connected to the 2.2n output node (110k), and then this resistor is in parallel with the 100k resistor on the bass potentiometer end to ground, resulting in a 52,830ohm resistor making up the resistance of the RC filter.

The cutoff frequency of the filter is then:

1/(2π*52380*2.2*10^-9) = 1381hz.

When the bass control is turned in the opposite direction, the cutoff frequency of the filter is:

1/(2π*100000*22*10^-9) = 72hz

This is then attenuated by (12k//100000) +10k and 100k voltage divider.

Am I correct in my calculations?

I'm not the best at thevenin equivalent circuits, so any corrections would be much appreciated.

Thank you.

[66Strat]

Look at the Dr. Z tab of the attached Tone Stack Calculator. You can input resistor and capacitor values and the play with the controls to see the effect that the controls have.

https://www.guitarscience.net/tsc/drz.htm#RIN=38k&R1=330k&R2=330k&RT=1M&RL=1M&C1=10n&C2=120p&C3=4700p&RT_pot=LogA

[Craftyjam]

Thanks for replying.

I'm familiar with the TSC, but the Dr. Z tonestack is not really the same arrangement as the Clubman bass control.

Maybe I should just learn how to use spice and simulate the frequency response that way.

[66Strat]

Sorry. I was thinking that there may be a way to fudge the values for R1 and R3 to approximate the functionality of your tone stack. Alas, a swing and a miss, no joy in Mudville.

[Craftyjam]

They do look superficially similar but the extra cap to ground on the Dr. Z TS give the response a mid scoop, and the pot labeled "T" makes it act as a tilt control rather than a simple bass control.

[PRR]

What does it sound like it is doing?
Is that transcription even right?
I see it doing very little.

[Craftyjam]

What does it sound like it is doing?
Is that transcription even right?
I see it doing very little.

I'm a bit confused by your simulation PRR.

Shouldn't the 22n cap be between the source impedence and the bass pot, and then the 2.2n cap be connected between the bass pot and output node?

Also I failed to show it in my picture, but I calculated the source impedence to be around 12k with the low anode resistor and the lower parallel plate resistance in parallel.

The attached pic is what I got with my simulation, not sure if it's correct and I don't know how to label cutoff (-3dB) point.

[pdf64]

Shouldn't the 22n cap be between the source impedence and the bass pot, and then the 2.2n cap be connected between the bass pot and output node?

I agree.

Also I failed to show it in my picture, but I calculated the source impedence to be around 12k with the low anode resistor and the lower parallel plate resistance in parallel.

That seems rather low, it may be beneficial to ‘show your working’.
eg the cathode resistor looks to be unbypassed?

[nandrewjackson]

I would ask*why* you want to analyze it. Maybe you can get a better understanding of the circuit by establishing a goal.


Ignoring the source impedance and the treble control afterwards, it seems practically straightforward.


More signal through the 10K that bypasses the 0.002, more low end.


Less signal through the 10K bypass, less low end.


The (2) 100K R to ground almost seem irrelevant .


Are you looking to modify the bass control, or is it a genuine curiosity?

[Craftyjam]

I like the frequency response of the filter but I feel it could be implemented in a better way.

It would be nice if I knew if I was calculating the cutoff frequency correctly so I could transpose that frequency to a different high pass filter.

[nandrewjackson]

I see.




Without running a bunch of numbers it looks like the (2) 100K R to ground prevent too much audible volume change from minimum and maximum of the bass pot.


I'd suspect that changing the 0.002 cap to slightly higher or lower values won't require changing anything else.  Bigger changes in that cap would eventually lead to needing to change some or all of the other surrounding components to keep the functionality.

[Craftyjam]

After I did the spice simulation and found a different cutoff frequency (around 1340hz, which is quite high,) I recalculated the thevenin equivalent resistance and my new calculation agreed with the simulation so I guess I figure it out. Life is like a voltage divider sometimes. 

[PRR]

Shouldn't the 22n cap be .....

Yes. I was looking cross-eyed.

I calculated the source impedence to be around 12k

I did not see that; or the load.

Now it looks like a tone control. Over most of the range, a shelf control.

[Craftyjam]

.

Now it looks like a tone control. Over most of the range, a shelf control.

I see now. Would you care showing how you set the source voltage and how you got all the different traces from the potentiometer? Also, is there a way for the sim to output the -3dB frequency automatically?

Thank you for humoring me so far.

[PRR]

I would not say it has a -3dB frequency.

In .AC analysis everything scales so it is convenient to set the source at "1V" and for audio we like the output in dB.

The PARAM technique to sweep a value in (ancient) PSpice is called something else in LTspice. Do you want a point to a LTspice group?