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 #123861  by Dozin
 Sat Jan 26, 2013 11:12 pm
I'm in a band with all engineers. It can be challenging for me because I'm not :? This is what happens when you give an engineer a Jerry guitar and he wants to know why the tone knobs don't sweep like a typical tone knob should. However, I get it and it makes perfect sense.

Image

Attached is a plot of the frequency response for the RC circuit we were studying.

The white dashed lines represent all frequencies delivered by every note between the Low E and the High E (12th fret). Note that the x-axis represents frequency on a Log-scale. One thing to remember is that pick-ups introduce non-linearities. So, for example, when the High E 660Hz) note is picked, the pick-up transduces that vibration into a voltage, but because the PU is nonlinear, it will also generate harmonics at 1320Hz, 1980Hz, etc. I'm inclined to say that only first few,say first three or four, harmonics matter, since their amplitude gets lower and lower.

Each curve in the plot represents a different resistor value (our tone POT). Looking at frequency response, beyond 660Hz, you can see the steep drop off in harmoniic amplitude by changing the POT resistance from 0ohms (red line) to 50Kohms (blue line). I think that this may represent the 'step' response when we turned the tone knob - it sounds like it's either OFF or ON. Beyone 50kohm there's not much change in relative amplitude so there doesn't sound like there's any difference. Although, as you pointed out, you may be better able to hear this difference at a very loud setting.

An interesting aspect of this plot is that note that when R = 0ohms, I should be able to get more punch out of my lower bass notes. That's because there's more gain to be had because the harmonic content in the low notes (low E, 82Hz) fall well below 660Hz. You can see that the filter passes more signal between 82Hz and ~400Hz (which is about where the red line intersects the blue line).

So, during a rhythm sequence, it's looking like I should set the POT too 0-ohms to get that fat bottom end. Then before I lead, I barely have to turn the tone knob (no guess work!) to have the highs break through. So, as Matt said, highs are either ON or OFF. I can't wait to get to the jam room to see if all this behaves as I think.

keep in mind that this is 'ballparked and not stated as fact' the source impedance is based on the input impedance I found for a PAF PU on Wiki. Regardless, the plot provides a conceptual visualization on how to shape the response.

Any feedback about this?
Last edited by Dozin on Mon Jan 28, 2013 5:41 am, edited 1 time in total.
 #123866  by Jon S.
 Sun Jan 27, 2013 7:43 am
I'm not sure I'm understanding this all now but thanks for sharing and I'll try read it again after my second cup of coffee. :P
Dozin wrote:I'm in a band with all engineers. It can be challenging for me because I'm not :?
Well, we have a lawyer, a pharmacist, a math teacher, and a lobbyist. So as for your challenges, you're not alone. :lol:
 #123882  by SarnoMusicSolutions
 Sun Jan 27, 2013 4:16 pm
The thing is, a guitar tone control is not simply an RC filter. It is also or moreso a LRC filter. The pot (R), the cap (C) and the pickup (L). The pickup is an inductor, and especially as R becomes very small, L becomes a major reactor. It actually gets pretty complex when L becomes a player in the equation, like that humped, horn-tone you get with the tone set real low or off (R=zero). Then it's an LC filter. But really, at any tone setting, L is part of it. So much of what's happening is that the pot setting changes the impedance load that the inductor/pickup sees, but the cap sets the frequency because of it's incorporated capacitive reactance. You can even test a tone control without the cap, just a LR filter. That's pretty cool too, to a point, and then the gain goes to zero because you've shorted the signal source to ground. But that demonstrates how the inductor's resonance amplitude will vary with difference impedance loads (R).

Brad
 #123891  by Dozin
 Mon Jan 28, 2013 5:40 am
Brad, I think the plots below do a good job capturing the resonant peaking, which are a results of reactive components L & C. The purpose of the R is to dampen (change the Q) of the resonance. More resistance yields more bandwidth, but less peaking. Less resistance does the opposite - more peaking, less bandwidth.

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Even if you switch the pot to a 250k, If you look at Fig. 1. It shows how the response changes when sweeping a 500K POT in 50K steps, which covers the range a 250K pot would give you. The difference you would feel is the sweep range in the tone know when you turn it. Most of the change in response occurs in the first 75K of sweep. So with a 500K POT, you only get 15% sweep. With a 250K POT, you get 30% sweep (ie. 75/250 *100). So it feels like you have more sweep range with a lower POT.

In theory, in terms of the sound that you hear - you get more brightness with a higher impedance POT (when it is pegged to the highest resistance). Changing to 250K in the Wolf would make a subtle difference, because there isn't much change in response between the 250K and 500K.
 #123896  by mgbills
 Mon Jan 28, 2013 9:12 am
Relevant to my question from Pete B. this weekend. I built his Tiger-ized Strat.

Pete called and asked why the tone doesn't roll off until well into the sweep. Now I've built (or rebuilt) five of these, and they react all the same. My assumption was that if you equipped the guitar with 250K pots (essentially tooling the beast for the split coil mode), that the highs in the humbucker mode would be rolled off.

But that is only my deductive process, as I lack the electrical engineering to really know the effects of potential changes.

Any way to shrink these images Dozin'. They're cut off on my work monitor.
Peace
M
 #123903  by mgbills
 Mon Jan 28, 2013 11:01 am
Thanks Jon. That helped.

So how will we, armed with this information, make the tone knobs more responsive across the broad spectrum of pickup outputs ?
In other words, how do we change the sweep of the pot to not loose the brights, but more logically roll off when appropriate.

Thoughts?
 #123908  by waldo041
 Mon Jan 28, 2013 12:38 pm
mgbills wrote:So how will we, armed with this information, make the tone knobs more responsive across the broad spectrum of pickup outputs ?
In other words, how do we change the sweep of the pot to not loose the brights, but more logically roll off when appropriate.

Thoughts?
Change pickups to a lower output reducing the capacitance and inductance reactance the Super 2/ Dual sound values have.

~waldo
 #123909  by TI4-1009
 Mon Jan 28, 2013 1:16 pm
There goes the baby with the bathwater....
 #123911  by Jon S.
 Mon Jan 28, 2013 2:01 pm
I'm lost (more than usual, I mean :oops: ). Where's the significant issue? If the tone doesn't start to roll off until well into the pot's sweep, why not simply set the pot for starters not on 10 but just above wherever it is the roll-off starts? Or leave it at 10 but just know, with this guitar, to roll off the tone pot more than most for starters/to get a response?
 #123913  by ringKing72
 Mon Jan 28, 2013 2:58 pm
mgbills wrote:Thanks Jon. That helped.

So how will we, armed with this information, make the tone knobs more responsive across the broad spectrum of pickup outputs ?
In other words, how do we change the sweep of the pot to not loose the brights, but more logically roll off when appropriate.

Thoughts?
A logarithmic 500K POT may do the trick. It will have less slope than a linear POT at the low end of the tuning range.

Image
 #123941  by jackevorkian
 Tue Jan 29, 2013 3:02 pm
So, what's the cause? The Super II, or having the buffer in the circuit?

I modified a Gibson ES336 to have a middle Super II, buffer, and OBEL. When I got the guitar back, the first thing I noticed was how bad the sweep of the tone pot had become. Up until now I had assumed that the tech had done something wrong with the wiring, since this was his first time wiring up a JG style schematic.

Before the guitar was modified, it had a pair of PAFs and the tone pot worked perfectly. Now the tone pot sweep is equally bad for the PAF's and the Super II. I didn't change the volume or tone pots...so it's not the pots themselves, but something else that's affecting the sweep.