help me understand why a low-value capacitor changes how my volume affects tone?

[Dave Locher]

So I built myself a simple box with a rotary switch that allows me to put different low-value capacitors into the signal. The switch determines which capacitor is grounded to the guitar signal. I really like the .001 and .0015 to knock off the really high frequencies of my guitar. (I am not a fan of high frequencies!)

It all works great, but I have noticed this: when the box is in my signal turning down the volume knob on my guitar acts almost like a tone control. Lowering volume from 10 to 8 or 9 chops off a fair amount of high end from the signal and turning it down to around 5 or 6 really cuts most of the highs. (Bill & Becky L500XL wired with 500k linear volume and tone pots running through a 10-foot and 4-foot cable into a 1978 Randall RG100ES, which is a solid-state amp.) It is the capacitors causing this, not the box itself - I have a true-bypass switch that allows me to bypass the rotary switch completely and the rotary switch has a no-capacitor position on it so it is very easy for me to switch the caps in and out of the signal.

This does not bother me at all, and in fact I am finding it quite useful. But I am hoping someone can explain to me why having a tiny-value capacitor in the signal would have such a noticeable effect on the sound/volume relationship? Or, I guess, why adding such a small cap makes a much bigger difference when the guitar volume is turned down than when it is dimed?
Something a person with minimal electrical expertise can understand.

 

[gregory]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

1000 - 1500 pF is not a tiny amount.

What you're doing is simulating a ~40' cable in line with what you already have running from your guitar to the first buffered item in your signal chain.

You've created a low pass filter where the corner frequency goes down as you turn your volume down. That you have an inductive source (a pickup) cannot help matters.

It has been a very long while since I've done the math on this, so I won't be able to tell you what the influence of the inductive source has.

 

[AlexR]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

Is this box merely choosing what value cap is attached to a tone control.....or is that cap permanently fully grounded like a cocked wah filter effect?

 

[gregory]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

Fully grounded from the looks of it...

The switch determines which capacitor is grounded to the guitar signal.

 

[Dave Locher]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

Yep, fully grounded. The .001 and .0015 make a very, very subtle difference. The bigger values get into cocked wah territory.
Gregory, I follow you on the capacitance ~ long cable. That is exactly what I wanted. But I don't know what corner frequency means or why the high frequencies drop off so much with relatively small volume drops?
Like I said, it does not bother me. It's an interesting opposite effect of a fuzz where less volume = brighter. But I would like to understand it.

 

[AlexR]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

Where do you have the caps connected to?? If you have it modern style where the cap is to the pickup hot connection on the vol pot then that might be affecting things with volume shifts.

 

[Dave Locher]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

No, the caps are wired to a rotary switch in a pedal enclosure. Guitar>capacitor box>amp. It was easy to build and I can use it with all of my guitars vs modding any one guitar.

 

[gregory]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

That's the way I understood it.

A series resistor connected to a shunt capacitor forms a 1st order low-pass filter. The source impedance of the pickup combined with the resistance of the top portion of the voltage divider formed by the volume pot as it is decreased is the series portion and the cap in the external box in combination with the capacitance of the cable forms the shunt portion. These two elements form a voltage divider for high frequencies. What frequency where the highs start to get reduced is inversely proportional to the product of the series resistance with the parallel (shunt) capacitance. But again, to put it simply, it is like a volume control that only attenuates high frequencies. Instead of a voltage divider formed by two resistors, it is a voltage divider formed by a resistor and a capacitor.

 

[beaubrummels]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

You'd have to tap the project box off the pickup side of the wiring to avoid influence of the volume control. By putting it between the guitar and amp, it's behaving like 50's tone wiring.

 

[gregory]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

'50 wiring with a relatively small cap and the tone control on zero.

I assume there is still a modern-wired tone control.

Dave likes it which is all that maters.

 

[Dave Locher]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

'50 wiring with a relatively small cap and the tone control on zero. *Exactly.

I assume there is still a modern-wired tone control. *Yes, although it is mostly unnecessary at this point.

Dave likes it which is all that maters.

Yes I do, although I was not expecting this particular effect.
Thank you for explaining what is happening. I appreciate it.

 

[PicoRiveraTele]

Re: help me understand why a low-value capacitor changes how my volume affects tone?

It all works great, but I have noticed this: when the box is in my signal turning down the volume knob on my guitar acts almost like a tone control. Lowering volume from 10 to 8 or 9 chops off a fair amount of high end from the signal and turning it down to around 5 or 6 really cuts most of the highs.

(Bill & Becky L500XL wired with 500k linear volume and tone pots running through a 10-foot and 4-foot cable into a 1978 Randall RG100ES, which is a solid-state amp.) It is the capacitors causing this, not the box itself - I have a true-bypass switch that allows me to bypass the rotary switch completely and the rotary switch has a no-capacitor position on it so it is very easy for me to switch the caps in and out of the signal.

This does not bother me at all, and in fact I am finding it quite useful. But I am hoping someone can explain to me why having a tiny-value capacitor in the signal would have such a noticeable effect on the sound/volume relationship? Or, I guess, why adding such a small cap makes a much bigger difference when the guitar volume is turned down than when it is dimed?
Something a person with minimal electrical expertise can understand.

It's because of LC resonance. When the volume is at 10, the pickups inductance resonates strongly with the capacitance, giving rise to a treble boost right at the cut off frequency. When you turn the volume down, the pickup and the capacitors no longer resonate due to the resistance placed between them, so the treble drops off abruptly. If the cap selector box is in between the volume control and the pickup, you won't have as much of a drop in treble when you turn down the volume, because you will not be placing that volume control resistance between the pickup and the caps. Therefore it's best to put a rotary switch with caps directly in the guitar, sort of like a Gibson Veritone, and treat it similar to a tone control.