How Tone controls work

[Kendescending]

Thanks for that micguy!

 

[micguy]

The general idea is that a larger pole has a "wider" magnetic field (more or less) and senses a larger portion of the string's vibration, theoretically leading to timbre differences. I don't know whether these differences are more down to geometry or due to the different electrical nature of the pickups.

I have done simulations of those effects - it's not terribly difficult to do that (just time consuming) and it's pretty subtle - small differences at the extreme treble end of things. People are able to see that a P pickup is "wider" than a J (the coil is but, the pole pieces aren't really bigger), and we like to think we know why things are the way they are, so there is a very strong tendency to assign the difference between what a J pickup sounds like and what a P pickup sounds like to things we can see.

Just between you and me (shhh - don't tell anybody), the fact that the inductance is typically 50% higher in the p pickup is most of the difference.

 

[micguy]

Thanks.

Point being that there is a whole tonal universe just tied up in the complex impedance of the pickups; which is generally somewhat ignored, but which has a profound effect on the character of the tone.

The complex impedance of the pickups is included in my models. In this case, the parasitics beyond the basic L, R and C are actually pretty subtle in their effects - J pickup only have magnets in the magnetic circuit, so things are pretty straightforward. When you get to things like Bartolini pickups, where there are magnets, pole pieces, and shorting structures (I don' t know the shapes of things they're using, but I'm familiar with such things, having designed them into phonograph cartridges very early in my career), the effects of those parasitics are more pronounced.

 

[micguy]

It's worth noting that tone pots are commonly audio taper, not linear. With the way the pot and cap are wired, at 50% mechanical rotation, the cap's path to ground is via only 10% of the pot's total resistance. This makes the transition between no effect and full effect a lot more gradual than it would be with a linear pot.

That is taken into account here - "half tone" is 10% of the total resistance at "full tone". "3/4 tone" is 33 percent, "1/4 tone" is 3.3%

Even with a log pot, the transition is still pretty abrupt. I often put stopper or shunt resistors in my tone circuits to tame some of the effects of that transition.

 

[tbz]

A+ post, doomed to be lost in the background noise of endless posts crying about bandmates.

 

[micguy]

A+ post, doomed to be lost in the background noise of endless posts crying about bandmates.

You may not like your pickups or how they sound, but they will never sleep with the lead singer or the wife of another bandmate. Or decide not to show up at a gig. I get it, this is the technical backwater portion of Talkbass. I'm as OK with that as I am that the lead singers get all the attention - if I wasn't OK with that, I've had over 40 years to realize I should switch instruments or learn to sing.

Don't like your bandmates?

Find other bandmates.

 

[Jerry Catanescu]

That is taken into account here - "half tone" is 10% of the total resistance at "full tone". "3/4 tone" is 33 percent, "1/4 tone" is 3.3%

Even with a log pot, the transition is still pretty abrupt. I often put stopper or shunt resistors in my tone circuits to tame some of the effects of that transition.

This is what I am getting when doing a log-domain sweep of the rotation angle of a linear pot (to simulate a log pot being turned by equidistant amounts). What am I doing wrong that makes it look so uniform? Perhaps the fact that I start at 1%, not 0%. I use TINA, and it crashes if I try to do a log sweep from 0%.

 

[micguy]

This is what I am getting when doing a log-domain sweep of the rotation angle of a linear pot (to simulate a log pot being turned by equidistant amounts). What am I doing wrong that makes it look so uniform? Perhaps the fact that I start at 1%, not 0%. I use TINA, and it crashes if I try to do a log sweep from 0%.

View attachment 5390820

I'd have to see the entire circuit to understand where the difference lies. And yes, the log of zero is negative infinity - there's likely a math error no doubt that's killing you on that detail.

 

[Jerry Catanescu]

This is what I used. The pickup modeling may or may not be anywhere close to what you may have used.

 

[PennyroyalWe]

Fascinating and well written, if a tad condescending.

 

[micguy]

This is what I used. The pickup modeling may or may not be anywhere close to what you may have used.

View attachment 5390825

OK, a few things:

You have no cable capacitance in your circuit. You are also missing the parasitics, but they're not going to change things all that much. Your values are in the ballpark - 2 Henries is a smaller than normal pickup inductance, but if it's 2 Jazz pickups in parallel, it's right on. 500 K's are not usually used for controls in basses, but not that far off.

200 pF for interwinding capacitance is in the range, though most pickups have more than that - the ones in my model have 550 pF and 620 pF respectively (1170 pF total). Between that and a cable at 550 pF (or more), you are missing a lot of capacitance, which will change things quite a bit - that's probably most of what we're seeing.

 

[SunByrne]

Nothing better than actual data. I will have to give a second more thorough read when I have a bit more time, but this is GREAT.

Seems like people’s intuition is that the tone control is essentially a HPF, and turning down the tone simply reduces the cutoff frequency. (This is, I believe, what the Wal-style controls actually do.) But that’s never what the tone control on a bass has ever sounded like to me, so I’m glad to see my ears aren’t wrong.

Thanks much, @micguy !

 

[SunByrne]

Just between you and me (shhh - don't tell anybody), the fact that the inductance is typically 50% higher in the p pickup is most of the difference.

I’m going to speculate that pickup location is also part of this story.

 

[micguy]

I’m going to speculate that pickup location is also part of this story.

Pickup location is a powerful force, but the E/A side of a P is fairly close to where the neck on a J lives.

 

[wellshuxley]

@micguy thank you for this - super interesting! I'd love to see your schematic or SPICE file if you're open to posting it. I'm curious to see your pickup simulation, since I've attempted to model a pickup in LTspice myself and haven't quite gotten it right.

 

[Boneless]

I have done simulations of those effects - it's not terribly difficult to do that (just time consuming) and it's pretty subtle - small differences at the extreme treble end of things. People are able to see that a P pickup is "wider" than a J (the coil is but, the pole pieces aren't really bigger), and we like to think we know why things are the way they are, so there is a very strong tendency to assign the difference between what a J pickup sounds like and what a P pickup sounds like to things we can see.

Just between you and me (shhh - don't tell anybody), the fact that the inductance is typically 50% higher in the p pickup is most of the difference.

I was more thinking of the difference between bar magnets and pole magnets, and to a lesser extent, large poles vs small poles, and their position. I imagine this will only affect the treble really, similarly to how woofer cone geometry dictates response in the treble range (especially off axis).

 

[BlueTalon]

Interesting and informative, thanks for taking the time to write this up!

And +1 on the stickie suggestion.

 

[micguy]

I was more thinking of the difference between bar magnets and pole magnets, and to a lesser extent, large poles vs small poles, and their position. I imagine this will only affect the treble really, similarly to how woofer cone geometry dictates response in the treble range (especially off axis).

The wavelength of the open G on your bass (which is 98 Hz) - we'll call it 100 for the sake of round numbers) is...68 inches (the string is vibrating at half a wavelength at the fundamental). For a wavelength of an inch, we're at 6800 Hz. A 3/8" pole piece (about the largest you see) will give you cancellation when half the wavelength of the frequency is about that length - the full wavelength would be 3/4". So that cancellation happens at 4/3 (6800), or.....9 KHz. That is well above the bandwidth that the rest of the system has. As you go down the strings, that cutoff moves down by a factor of 3/4 for each string - it's 3.8 KHz when you get to the E string. Some effect, but pretty small. The spacing of poles in a hum cancelling pickup with each string being "heard" at two points? - that can have some significant effects.

 

[J Wilson]

This is EXACTLY why I love TalkBass, the education that occurs for me from guys like @micguy, @ agedhorse, @brucejohnson and the others who graciously and patiently try to teach guys like me how the sausage is made.

I've always felt that 'behind the curtain's that engineers and technical people who can meld the technical with the musical are who make the magic happen as it's not magic to them.

Thank You very much for this.

All the Best,

JW

 

[stretch80]

Just "thank you!" Really fun to see what's really going on.