Taper for tone pots

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KX36

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Hi, I've been reading a lot of contradictory information about tone pot taper.

Some people say a log (audio) pot will give a smooth "roll off" and a linear one will give a big difference in one end and not much difference for the rest. Other people say the reverse.

I know for volume, you use log pots for a smooth taper.

Which one gives a smooth taper for tone?

Cheers,
 
Re: Taper for tone pots

Carvin and Ibanez use Linear for Volume and Audio for Tone

their engineers and experience is what I default to
rather than what is written by enthusiasts on the INTERNET

jeremy ( one of the admins here ) prefers both Linear
I have guitars that are both Audio

most of mine use the conventions of my favorite Guitar makers
 
Re: Taper for tone pots

I use audio pots on my guitars, but I think the advice here has been audio for volume & linear for tone.
 
Re: Taper for tone pots

The traditional wisdom has always been audio taper pots for volume and tone, but i question that wisdom. Almost every electric guitar i have played has a 'lump' in the operation, where the volume control usually has most of the volume action happening between 7 and 10 on the dial, and most tone controls have a lump where all the action happens like a wahwah, often around the 3 or 4 setting. Very frustrating.

I've changed volume pots to the linear equivalent with great results, and am now thinking i need to do the same with the tone pots. I am completely at a loss to understand how people put up with the lumpy operation of audio taper pots, except for a few who might find the lump in the operation as 'useful' (e.g. for volume swells on a Strat). Personally i'd rather any control operate smoothly and evenly throughout the entire rotation of the pot. To my ears, audio taper pots do not achieve this in electric guitars.
 
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Re: Taper for tone pots

I've heard it said dozens upon dozens of times that linear pots can be used for tone, but honestly have to how people come to this conclusion. All I've been able to conclude is that it must be "I heard it somewhere on the internet". ;)

Audio or linear can be used for volume, and each works fine for different guitars and players. Long discussion could be had on the merits of each for volume pots, and probably over half the new guitars being sold today use linear volume pots. For tone pots however linear taper simply does not work. Try it. Put a linear pot in your tone control, and what you end up with is a tone on/off switch between 2 and 1. There is no taper, there is no fade, it is all or nothing right there at the bottom.

If anything, the standard audio taper available today is already too gradual for the ideal tone control, and switching to linear exacerbates this even further - much further. The only time I've seen linear pots used for tone controls was on a number of of PRS SE series about 8 years ago. All other import guitars use linear for volume and audio for tone. When I contacted PRS about this, they thanked me for letting them know about the factory error, and switched to audio for both volume and tone to prevent factory screw ups like this from happening again.

It seems that 10 years ago, no one in the service or replacement parts business had ever heard of linear pots being used for volume controls, as few seemed to have noticed that Gibson had been using them exclusively for this since '73. I bugged AllParts to start stocking 300k linear pots for direct replacement Gibson volumes for a few years, yet when they finally did start carrying them, they somehow got confused and listed them as tone pots, further confusing the matter.

Volume and tone pots function and react with the coil and input of the amp in very different ways, and the fact that the volume pot works from both sides at the same time is but one reason why an audio does not actually result in an audio curve in end results in the circuit. Tone pots however, which only work from one side, do not work with linear pots in a passive guitar circuit. They end up functioning as a tone on/off switch at the very bottom.

Of course with as much as I've played with this, and as certain as I am to state this as "fact" (I hold pretty high tolerances for use of that word), until you try it yourself it's still just "something you heard on the internet". Get yourself a linear pot, test in on a meter and chart it to make sure it's truly (or at least approximately) linear, and try it for yourself. I'm quite confident your conclusions will be the same as what I've said.
 
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Re: Taper for tone pots

So as helpful as you've all been, I still don't know what to do.

-------

I know volume should be a logarithmic function as decibels (phons really) are designed to increase linearly with hearing, i.e. doubling the dB feels like double the volume but decibels don't have a linear relationship with power (watts; or energy, joules). dB increases by 3 for every doubling of energy.

Power has a linear relationship with resistance (P=V^2/R).
So to drop in 3dB increments for illustration. (This is just an arbitrary example with made up numbers nothing like those that would physically represent a guitar)
Code: 
Knob	dB	R
------------------------
10	30	0.5
9	27	1
8	24	2
7	21	4
6	18	8
5	15	16
4	12	32
3	9	64
2	6	128
1	3	256
0	0	512

e.g if you want to use a volume pot to half the dB from 30dB to 15dB you actually have to give it 1/(2^5)= 1/32 the power.
If you notice in this example;
At 10, R=0.5; at 0 R=512.
If volume was linear; at 5, R would = 256. Actually; at 5, R=16.

Standard log taper may not be perfect for relating Watts to decibels as logs have different bases, but it's a much better approximation than linear.

-------

Tone controls I don't understand. I know it uses an RC circuit as a low-pass filter, but I don't know any of the maths behind those. Assuming the way they work is they completely cut off 100% of everything above a threshold and 0% of everything below the threshold and that the relationship doesn't change with volume (although hearing changes at different frequencies with volume anyway), then the main factor would be the formula between the Resistance, Capacitance and cut-off freqeuency, but I have no idea how it works and i doubt it's as simple as this anyway. I spent a while looking at Equal Loudness curves but this got me nowhere.

According to the print on the backs of the pots I have at the moment, the volume pots are log (Alpha A500K) and the tone pots are linear (Alpha B500K) and it probably doesn't sound like an even swell, but there is definiately a difference between 0 and 1 and still a difference between 9 and 10, so it seems like these are more or less right, otherwise there would be a big difference at one end and no difference at the other.
 
Re: Taper for tone pots

After spending some time researching low pass RC filters, I've found that the cut off frequency on a standard RC filter is inversely proportional to the resistance

fc = 1/(2.pi.R.C)
with C fixed,
fc α 1/R

(Apparently, although without an explanation of where the 2pi comes from I don't know if this is a simplification or what)

where fc = cutoff freq, R = resistance, C=capacitance. (and α means "proportional to")
It's complicated in an AC circuit such as in a guitar because the capacitor's internal resistance (capacitive reactance) comes into play, which varies depending on the frequency, but I've ignored that for now.

So I think what you really need is a pot with the taper y=1/x rather than linear or log

However, this is for a traditional RC filter, such as this:
fil5.gif


From looking at guitar schematics, the hot line coming out of the filter is not between the series resistor and capacitor, but before both of them with the earth after both of them. I don't understand how this works and I put it into a simulator which seems to think it doesn't work (the one in the picture above works as you'd expect, cutting higher freqencies more than lower ones).

I'm experimenting around with these things with maths, an electronics similator program and google/wikipedia and I'll probably get a breadboard and experiment with my old pots when I've disassembled the guitar, but I'll have to buy new stuff before I can try that.

Really doesn't help that I haven't done the smallest bit of maths or physics for the last 6 years.
 
Re: Taper for tone pots

Whatever you do on paper will be difficult to make accurate predictions in practice, unless of course you introduce a lot more variables in to the equation. You are not running a line level signal through one side of the pot on a volume control, but rather are connected to a high impedance coil which generates a low level signal on one side, and the input impedance of the amp in the middle, which varies in connection between the pickup coils and ground. As you turn the volume down you are slightly increasing the resistance of the load connected across the coil, increasing the impedance between the coil and the amp, and increasing, then decreasing the resistance of the load across the input of the amp. A lot of things can change in funny ways here, differently for different frequencies, and the net result of a logarithmic volume pot is not a logarithmic change in volume coming out of the speaker.

It's been discussed before, but the actual results in the end are typically this for volume pots. In generalized Real World results - Audio pots deliver a relatively rapid roll off from about 10-7, which is great for a quick clean up from overdrive. After that, they hit a fairly broad plateau of slower decrease from about 7-4. Then from 3 down to 0 they do a nice job of giving a smooth roll off at the bottom for fading in and out. Where they lack, is that if you play without much distortion and don't need that quick clean up, they can roll off too fast at the top for many, and seem too stagnant in the middle. The bottom fade out is still nice, but the top two thirds don't always respond as some players may desire. They also suffer a bit in the Gibson double-master volume setup. In this arrangment, as you roll back one pickup in the middle position, you have relatively short usefull range to adjust balance before the pickup you are dropping seems to cut out entirely, then not much change, then the overall volume can start to drop around 3 or 4.

Linear volume pots on the other hand, tend to offer a smoother volume change from about 10 to 2 (when you're playing clean), which though contrary to what we've all been told about linear vs audio pots, nonetheless proves true. If you want a quick clean up from overdrive however, it can seem to take forever to get there, sometimes not until you are down in the 4-5 range, which is one reason why they are not always the ideal choice. They do offer a wider range to balance pickups in an arrangement like the Gibson middle position. I've actually used reverse log pots for double independant volumes in some cases, where they are used more specifically to balance middle positions (and accompanied by a standard audio taper master volume), and had surprisingly useful results. One of their bigger drawbacks in my opinion, is that they do indeed have a more rapid drop off at the very bottom, which makes smooth swells and fade outs a bit more difficult.

One of my favorite volume pot arrangements has been with a stacked pot, linear on top and audio on bottom. On the top linear pot, I connect the upper lug to the pickup/switch, and the bottom lug of the bottom audio pot to ground. Then join the center lugs together to the output, and you get the best of both worlds (in my opinion). Smooth steady volume change for clean in the top half with the linear taper, then the smooth roll off on the bottom as the audio pot grounds the amp input out. I'm honestly not so picky as to do this often, and am generally quite happy with linear volume on some, or audio with some style of treble bleed on others. Different guitars and pickups respond differently though, and different players have quite a variety of preferences. No "perfect" solution.
 
Re: Taper for tone pots

As to tone pots, I can't explain how you're still hearing a difference between 9 and 10 if you currently have a linear tone pot. Even on an audio taper it's rare for there to be an audible difference in tone that high up at the top. Simplified, all a tone control does is ground out the signal through a capacitor, through which the higher frequencies pass more easily than the lower ones. Thing is, for most pickups it doesn't have a terribly noticeable effect until you get down toward the 70-100㏀ range. Sure, it will take off a little more at 400㏀ than at 500㏀, but probably not as much as it will between 40㏀ and 50㏀. Most of the audible range of the tone pot occurs below 70㏀ or so. If your current tone pot were indeed a linear one, I would be quite surprised if you could detect any difference between 9 and 10.

Wire up a linear tone pot outside of the guitar, and blindfolded or with your back turned, start playing and have a friend wait an undisclosed amount of time before gradually beginning to turn the tone pot down. Shout when you first hear a difference, and if its truly a linear tone pot I doubt you will detect one before they get to around 3 at the earliest.

Personally, I would like to see a tone pot with a more rapid taper even than a standard audio. One of my favorite ways to wire a tone pot is to cut the ring on a 100㏀ audio pot to make it a no-load, then wire it standard with a .022㎌-.015㎌ cap This spreads out the useful range for me over a wider area, and I've never heard any drastic sudden jump when the wiper actually engages the active area of the carbon ring. There's a jump, a little one, but nothing terribly significant.

In any case, for the most part I would set aside the math and theoretical formulas for this application. The pickup, pots, capacitors, and input impedance of the amp all work interdependently, and it would take a rather complex formula with a lot of variables to accurately predict results as they are truly heard. In a circuit so basic and simple to wire as this, it's much easier and more practical to simply get out your breadboard or alligator clips and start making real world observations. Every guitar and player is different, and the only way to know what works best for you is to try it.
 
Re: Taper for tone pots

Thanks David, that's been very helpful. After a few hours of messing around with formulae and simulations, I've come to a similar conclusion to you, there's too many confounding variables, I'm going to have to experiment.

I had been hoping that with a bit of logic I could work out just roughly what shape a curve would be, but the circuitry in a guitar totally defies logic (well, obviously it works, but its totally different to every scrap of information I can find about RC filters) In a standard low pass filter, you have a signal input to a resistor, then a capacitor in series, then the ground. The output comes from between the resistor and capacitor. (Reversing the resistor and capacitor for high pass filtering) All my maths is for this example, but on a guitar, the input and output are both at the same point before the resistor. I can't work out how that works at all and asking most guitarrists and techies HOW this works doesn't help because more often than not the answers i get are "it's a low pass filter" or "capacitors [magically] stop high frequencies".

guitarvslowpassfilter.png


I do know how a standard RC filter works on a sine signal (and a square signal), but I don't see how the circuitry in a guitar compares to the usual RC filter layout. There should be a component between the input and output. There must be more of the circuit involved than just the tone pot and cap. I get the feeling it's to do with altering the "load" on the pickup; something to do with the tone circuit in parallel with the pickup is actually using the pickup's resistance and capacitance to combine to a second order low pass filter where the tone circuitry on its own is just like a fine tuner for the filtering properties of the pickup. That might explain why the tone circuit is not a filter on its own, but just alters the resistance and capacitance, and therefore frequency, of the filtering property of the coil. This is all way over my head.

As long as it works, who cares, right?

The conclusion I came to yesterday before your reply was... screw it. I bought 20 crocodile leads, 4 log pots and 4 lin pots and I'll have a play around and see what I can do. It cost me as much as a pickup for all that, but nevermind.

I've tried your blindfold thing and I certainly hear a significant change a long time before the pot gets down to 3, where you said I should hear it if it's linear. Alpha's website says this pot's number should be linear; however this guy says the supposedly audio pot from Alpha in his epiphone was measured to be linear. I'll definately measure the resistance at fixed increments once I've desoldered it but until I do that, all I can do is assume it's labelled correctly. I'll be measuring the DC resistance of all my current and new pots and pickups on my multimeter when they're isolated anyway just for future reference.

Thanks a lot everyone.
 
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Re: Taper for tone pots

KX36
I too have OCD
but yours has amazed and captivated me
please post on

this information and the weeding process to get there is simply astonishing
 
Re: Taper for tone pots

ehdwuld said:
KX36
I too have OCD
but yours has amazed and captivated me
please post on

this information and the weeding process to get there is simply astonishing
Click to expand...

Haha, yes, I'm way up there on the autism spectrum. I have OCD and Aspergers. When I get into something, i REALLY get into it, spending 12h a day straight on it at the expense of food, drink and sleep.

My simuations seem to support this:
"something to do with the tone circuit in parallel with the pickup is actually using the pickup's resistance and capacitance to combine to a second order low pass filter where the tone circuitry on its own is just like a fine tuner for the filtering properties of the pickup. That might explain why the tone circuit is not a filter on its own, but just alters the resistance and capacitance, and therefore frequency, of the filtering property of the coil"

And now I've burnt myself out and will have to try to stop thinking about it and wait for my pots to arrive

Cheers All.
 
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