Guitar circuit design: how do you choose potentiometers?

[stevewf]

Earlier, I asked: "Guitar circuit design: what's your approach on switches?"  (https://guitarnuts2.proboards.com/post/105531/thread)

So, next: What about potentiometers? How do you decide on what resistance, what taper, and where in the circuit?

We know some popular rules of thumb: for a single coil, use a 250KΩ pot for volume and another 250KΩ pot for tone. For a humbucker, Vol=500KΩ, Tone=500KΩ. Reasons for this are all over the webbies, but most say it's about taming the output, mostly in the higher frequencies.

So, first:
Do you have a view on choosing pot resistances that doesn't match the popular take? Or anything to add, e.g. about two parallel coils, or out of phase coils?

Then, onward:
How do you go about choosing the taper? Includes when to choose reverse tapers.

Then,
50's vs Modern (definition). I've heard it's the placement of the tone pot in relation with the volume pot and the output; I've heard it's which lug of the volume pot goes to the jack. Boil it down for me! Or, if applicable, let's make a better naming system.

Finally:
Your favorite tweak? Could be playing with the TBX like phostenix. Or making normal pots into No-Load pots. Or scraping the trace to modify the taper. Or using a two-gang for treble bleed.

[newey]

My view on pot resistances is to go higher, you can always turn it down. I have a single HB guitar with a 1MΩ volume pot and a 500KΩ tone pot. The 1MΩ pot makes it quite bright, so if needed I turn the tone down to darken it a bit- but I have the brightness if I want it.

[Deleted]

ya High Resistance pots are way to go, low ones tends to kill the power to my Earth Shattering Laser ray
Must put a new one on Order

[Yogi B]

Oct 21, 2022 23:18:48 GMT -5 stevewf said:

Do you have a view on choosing pot resistances that doesn't match the popular take? Or anything to add, e.g. about two parallel coils, or out of phase coils?

My default position is to use 250k pots for everything: using regular pots for brighter pickups i.e. single coils, but a no-load tone pot for darker pickups i.e. humbuckers. However pickups are just one consideration, e.g. for volume pots I'll also base the choice on what kind of treble-bleed I'm adding (if any), as well as what is the long established norm for a specific model of instrument. For example I'd be very unlikely to deviate from the standard pot values on a Jaguar, if the guitar were equipped with the traditional pickups & wiring.

I'd only especially care about parallel or out of phase combinations if those were the main focus of a particular guitar, usually there's not enough spare switching capacity to think about doing something different for the special cases.

How do you go about choosing the taper? Includes when to choose reverse tapers.

Ultimately it depends upon the exact purpose of the pot, and also the value chosen. For example, when wired as a regular tone control, a 50k linear pot gives a reasonably even sweep across its range but is still quite dark at even its maximum setting (though within a Jag's rhythm circuit this is perfectly fine). For general use we require a much larger pot in order to retain more treble at max, but sticking to a linear taper does not work well (despite the web having plenty of advice to the contrary) and unfortunately I don't think there's a good way to intuit that. Usually it comes down to either past experience, some form of simulation/calculation (SPICE, Guitarfreak, etc.), or listening tests.

A standard volume pot is an easier case to consider because what we have there is predominantly a resistive voltage divider, so we can get a pretty good approximation by ignoring any effects due to frequency and account for DC only. If we further assume any following input impedance has negligible effect, then the output signal is exactly equal to the fraction of the resistance of the 'lower half' of a pot out of it's total resistance. That is, for a linear pot at halfway the output signal is reduced to one half (6dB volume reduction), whereas a standard 10% log pot reduces the signal to one tenth (20dB volume reduction). This does also assume that your signal remains perfectly uncompressed through the remainder the your signal chain, and that for me is the significant factor in what taper to choose: if you play through a relatively clean setup with little compression, a small reduction in volume will be accurately reflected at the output so a linear taper is likely most suitable; whereas, if you use a lot of 'gain' and thus the resultant signal is heavily clipped/compressed, a small reduction in volume will likely change the output very little thus you'll likely favour a log taper such that it's easier to 'clean up' the signal without having to turn the pot almost all the way off.

This decision can also be influenced by what treble bleed you plan on adding to a volume pot: the resistor in a parallel treble bleed has the effect of making a pot act in a less logarithmic (more reverse log) manner, e.g. a 500k log pot with standard 10% taper plus a 180k resistor gives an effective 28% taper, or about halfway between log & linear.

50's vs Modern (definition). I've heard it's the placement of the tone pot in relation with the volume pot and the output; ...

Correct: '50s has the volume control 'before' the tone control; whereas modern has the volume control 'after'.

... I've heard it's which lug of the volume pot goes to the jack.

That's incorrect, this is more commonly known as dependent wiring (wiper goes to output) versus independent wiring (input goes to wiper), also called forwards vs. backwards wiring (respectively). (Most often '50s wiring also implies dependent wiring, but they're separate concepts.)

Boil it down for me!

Through a non-straightforward interaction between a pickups impedance, the controls, and cable capacitance (when the tone is at maximum) rolling down the volume control preserves a little more treble with '50s wiring than with modern. The other subjective upside is that when used with LP wiring (as done originally), in the middle position the neck (for example) tone control has full effect upon the bridge pickup even if the neck volume is itself rolled back.

The downside is related to the problem that the cutoff frequency of the LPF formed with the tone cap is dependent upon the series impedance before it in the circuit. With modern wiring, where that is only the impedance of the pickups, which is fairly constant (on most guitars varying by at most around a factor of two, an octave, for single vs. parallel combinations). However, with '50s wiring this also includes the 'upper half' of the volume pot, thus when reducing the volume this adds a large amount of extra series resistance — assuming a typical Strat pickup + 250k linear volume or a somewhat overwound PAF + 500k linear volume pot, at halfway this is around an additional 25 times the DCR of the pickup (or 45 times at halfway on log pots). This causes much more significant reduction in the cutoff frequency (about 4.7 octaves with linear pots, 5.5 for log), meaning the tone control very noticeably cuts more than just treble — further encroaching into mid & bass frequencies as the volume control is lowered, effectively resulting in the tone control becoming a secondary volume control.

if applicable, let's make a better naming system.

I almost want to say: "the wiring employed by the vast majority of guitars" vs. "the wiring that Gibson used briefly in the '50s", but the truth is that any guitar with two (or more) volume controls & a single tone control is also almost undoubtedly wired '50s style because it's cheaper to use a single-gang pot wired after both volumes than it is to use a multi-gang pot where each gang is placed before one of the volumes. (I know there's a significant number of LP-esque guitars with three pots, but whether the majority are VVT or VTT, I know not.)

[unreg]

stevewf,

This post counters @angellahash and newey. Lower resistance is better for my humbucker guitar, bc one of the humbs has been assembled to sound LIKE a p90 (single coil). The treble was extreme under my previous 500K Tone pot.


Here is a quote from my repairing of installation of my 250K Tone pot thread:

Apr 25, 2022 22:38:41 GMT -5 unreg said:

250K Tone pot is lovely and amazing on my humbucker guitar!
That’s only bc my bridge humb is a DiMarzio Bluesbucker