selecting pot value for tone control (output cap blend)

[m7b52000]

I wish to add a tone control to a pedal. This will consist of a linear pot sweeping between 2 output caps of 4n7 and 47n. How does one choose a pot value? 100k linear seems common but I not sure why.

[GibsonGM]

Could you give us more info about the pedal, M7? Post a schematic?  That would help us help you to pick a pot value  

[Rob Strand]

Yes, to ball-park the resistance you need to know what the circuit is.

A rough guide would be the pot is 10 times the resistance the caps form a filter with.   So what is that resistance? That depends on the structure of the circuit.

For example take this Timmy circuit, the resistance the caps form a filter with is 3k3 (R6).
http://drdfx.hu/wp-content/uploads/2016/05/PaulCochrane_Timmy_schematic-700x424.jpg

So 10 times that is 33k.   The circuit uses 50k which is 15 times the 3k3 value.  Which is fine.
You might even use 20 times but, depending on the circuit, if the pot resistance is too high
you end-up with half the rotation doing nothing.   That's where you need to try out a pot and
see if it works.    Using a tapered pot can help prevent wasting the rotation of the pot on nothing.

The reason 10 times (or whatever) works is because when the pot is maxed out it doesn't
affect the remaining impedance much.

Another thing to be aware of is to keep the cap ratio to a minimum.   Going form 1pF to 1uF is not going
to work properly.  It's not a really a variably capacitor circuit at all.   It can be made to sort of work like
variable capacitor *provided* you keep the range of adjustment fairly low.

[m7b52000]

Thanks, the circuit is a standard Vox ToneBender. The standard 4n7 output cap is very bright and a 47n cap is about right for my strat/vox set up. I have tried using a 100k linear pot in a simple blend control and it works ok... Is the output cap in this circuit forming a RC high pass filter with the volume pot?
I could of course simply add a switch to select either option...

[Dormammu]

m7b52000

You can try this.

R1, R4 can be excluded.
DC block cap 4n7 is better to increase to 47-100 nF.

[GibsonGM]

Is the output cap in this circuit forming a RC high pass filter with the volume pot?

Yes. The cutoff frequency changes depending on the setting of the volume pot.

[Dormammu]

Yes. The cutoff frequency changes depending on the setting of the volume pot.

But at such values — far beyond human perception.

[antonis]

Yes. The cutoff frequency changes depending on the setting of the volume pot.

But at such values — far beyond human perception.

Is 664 Hz frequency (counting 1k resistor added on 50k pot in RC time constant of transfer finction) far beyond human perception..??

[ElectricDruid]

Yes. The cutoff frequency changes depending on the setting of the volume pot.

Does it? How?

I see a fixed R to ground. Moving the wiper changes the amount of signal you get out, but I don't see how it affects the RC combination, unless the wiper was grounded by some following circuit (like plugging into an *extremely* low impedance!). Assuming the following impedance is 100s of K, we can pretty much ignore it.

4n7 / 50K gives a cutoff at about 700Hz, so I'm not surprised that 47n makes a significant difference. It should put much more of the bass back.

[antonis]

Actually, following effect input impedance affects the pot divider lower part..
(it's effectively set in parallel with wiper - lug1 part..)

[GibsonGM]

Yes. The cutoff frequency changes depending on the setting of the volume pot.

Does it? How?

Well, I didn't say it was 'consequential', LOL...and because it's a complex interaction (Antonis...) may not be worth thinking about!  But are there not times when this sort of situation ISN'T trivial, such as with a gain pot following a coupling cap between 2 triode stages?   (shrug)

[antonis]

Although R_s in schematic below represents signal source output impedance, we can (abusively) consider it as the pot lug3 - wiper part where R_p is considered as pot wiper - lug1 part..

Clearly what counts is the sum of R_s + R_p..
(and not the Rs/Rp ratio..)



Now, if we want to plot VOX TB output HPF frequency responce, Rs should be 1k (not 8k2) and Rp should be 50k (considering next effect infinite input impedance)..

[Dormammu]

Yes. The cutoff frequency changes depending on the setting of the volume pot.

But at such values — far beyond human perception.

Is 664 Hz frequency (counting 1k resistor added on 50k pot in RC time constant of transfer finction) far beyond human perception..??

I was referring specifically to the interaction between the 4n7 cap and the volume control.

[antonis]

I was referring specifically to the interaction between the 4n7 cap and the volume control.

To what kind of interaction, other than the formation of a voltage divider parts..??

[PRR]

...following effect input impedance affects the pot divider lower part..  (it's effectively set in parallel with wiper - lug1 part..)

There's no "following effect input impedance" in the original drawing.


I agree there always is, although user sanity suggests making it "insignificant"- i.e. does not change the response with knob change.

If you want to bring in more parts, introduce them to us by name value and location. "Hello 100k load! I didn't know you were out there."

[Rob Strand]

Is the output cap in this circuit forming a RC high pass filter with the volume pot?

Pretty much.

You can see from the response below:
- The adjustable resistance only looks like a cap only response at min and max settings.
   At intermediate settings there's a  step with a HF boost which originates from the smaller cap.
- When the adjustable is resistance equal the load the step is about 6dB.
- The step is largely removed when we get to 500k (10 times the load)
- The effect of the control is going to weaken around the 200k to 500k zone.

[antonis]

I agree there always is, although user sanity suggests making it "insignificant"-

Does that "user sanity" help to understand the reason for 2 cascaded ToneBenders been dead silent at low frequencies..??