How Tone controls work

[sikamikanico]

You'll only get resonant peaks this big when the pickup is unloaded, though. Put even a 500k pot across and the amplitude of the peaks will be dampened considerably.

Yeah, and that's the zoomed in pic too. Below is zoomed out. Both show they're (alnico magnets) more similar than different…

 

[iqi616]

I'm glad this conversation came back - I'd not read it before. It explains why I've been getting better results listening for the effect of the filter peak rather than the cutoff.

Generally, I have more neck pickup than bridge (bridge dialled back till the tone changes) but my recent discovery is that additionally dialling the neck pickup back a fraction gives me the best tone - it doesn't sound muffled but is nice and rich. I'd always had one of the pickups full-on.

I only thought of trying it after hearing that reggae bassists use just the neck pickup but dialled back a touch.

 

[Forrest27]

On a lot of threads, I end up trying to explain to people how passive tone controls on basses work (spoiler - they don't work like you think they do). Though they are simple circuits, the reactive nature of the pickups feeding them results in behavior you wouldn't really expect - they don't just "turn down the treble". Anyway, having designed circuits and other pro audio stuff for a living, Spice simulations (they way we predict circuit behavior before building prototypes) are in my wheelhouse. I won't go into tall the details of this (you need to characterize the pickups, which is a non- trivial task, but still not rocket science). Anyway, the point is, we'll be looking at the electrical behavior of passive tone circuits, which are part of the sound of a bass (pickup location is move complex, but also something you can simulate well - that's for another day).

So, We're starting with a bass that has 2 single coil (jazz) pickups - the ones used here are Nordstrands. The output here is what ends up at the amplifier input after loading by the volume control, the tone circuit, a cable (550 pF in this case - a 15 foot DiMarzio cable), and using 1 MegOhm for the amp's input impedance. The tone circuit is a 250 K pot, and a .047uF capacitor. Whew!

View attachment 5389970

OK, so we see a flat curve in the bass end and low mids (which is true for all magnetic pickups - there is no rolloff or scoop in your pickup (a topic for another day). In the upper mids, there is a peak - the pickups's inductance resonating with the cable and interwinding capacitances. So far, simple. Now, a question - what effect does the value of your tone cap have on your tone at max tone? OK, here's what you get with 3 different values of tone capacitors - the .047 we started with, a .022, and a .10 uF:

View attachment 5389971

Only see one curve? That's because the frequency responses using those 3 different tone caps are right on top of each other at max tone. If you buy a different value tone capacitor (or a "better" one), and hear a difference with your tone control dimed, well, that's confirmation bias.

OK, so let's see what happens when you turn down the tone control, starting with the .047 uF in all cases:

View attachment 5389972

What you' re seeing is the response at "quarters" - full tone, 3/4 tone, 1/2 tone, 1/4 tone, and minimum tone. Note that at 3/4 and 1/2 tone, what has happened is that the upper mid peak has been damped - the treble part of the curve (up at 10kHz) hasn't really moved much, even at half tone. This isn't what most people think is happening - what is actually happening is definitely counterintuitive.

OK, so let's see if different capacitor values affect tone when things are turned down. First off, 3/4 tone:

View attachment 5389977

OK, still right on top of each other - no difference in the sound when you change the capacitor up or down by a factor of 2. OK, what about at half tone?:

View attachment 5389980

OK, so there's a tiny bit of difference, but it isn't at high frequencies - it's much lower. Hmm

On to 1/4 tone:

View attachment 5389985

There is a difference - here is where the capacitor value you use starts to actually make a difference. But.. even at "1/4 tone" the difference is in the mids - the capacitor value you use does not affect what's happening at the upper end at all - where the treble is is dependent on the resistance in the circuit at this point.

OK, on to min tone (aka "tone off"):

View attachment 5389990

OK, finally - here and only here, with the tone control off, does it really all line up like you'd expect - the bigger capacitor value finally has less treble! There is a sort of abrupt change in character of things as you approach min tone, which you can hear if you pay attention.

One other note - what people think a tone control does is turn down the treble first, then a bit of the slight lower treble, etc. To get that effect, what you can do is add loading capacitance. here's a curve with added loading capacitance (1500 pF in this case), which shifts the resonant peak lower in frequency, though the height is still about the same:

View attachment 5390002

If you add capacitance and turn down the tone control a bit you get the red curve - what people think is happening when they turn down their tone control is achievable, but you don't do it by just turning down your tone control - it's a bit more complicated.

... thanks so much for this extremely informative post . Electronics is not my strong point but I managed to understand everything you wrote.

I just went thru a crash course on tone pots as I had to change out the circuit on my P. It now has the vintage orange drop cap and plenty of BASS

Giles
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[gwangi]

I'm planning on getting a Fender American Professional ii Precision bass and should I just keep the original factory capacitors in it or completely change them out? Recommendations ?

 

[micguy]

I'm planning on getting a Fender American Professional ii Precision bass and should I just keep the original factory capacitors in it or completely change them out? Recommendations ?

Start with what's there. Changing capacitors will not change the sound you get with the tone control full up at all; it only changes what you hear with the control dialed back. Basses don't put demands on capacitors to where the type of capacitor matters - the value is the important thing, and you won't know if you want to change the value until you've played it for a while.

 

[bpabon]

That said, and you may already be aware of this, you could try the OMG mode (from G&L L1000), with the two coils in series, and a capacitor in parallel with one coil, giving the "single coil + bass boost" sound.

In fact, I'm in the midst of a similar mod to my L-1500 (I might also do the kiloton...)
I swap the pickup selector to 3 way on/on/on
And swap the bass control with a push/pull so when the coils are in series I can pull to select OMB and adjust in either case.
Is G&L wiring deliberately sloppy?

I appreciate the comment from @micguy about the G&L community because when got my first G&L I felt like I stumbled into the next level of bass experience.