Technical question: pots

[GuitaRasmus]

Hi there. I've just been wondering about something. Some people say that you're able to brighten up a dark sounding guitar, with say 250k or 500k pots, be replacing them with a higher value, for example 1m Ohm or 2 mOhm.

Now, as I understand guitar electronics, this would only have something to say if the pot isn't turned all the way up, right? I mean, I almost always run full volume on the guitar, and get levels from pedals etc., so in effect my pots have zero resistance, right?

 

[beej]

Nthis would only have something to say if the pot isn't turned all the way up, right? I mean, I almost always run full volume on the guitar, and get levels from pedals etc., so in effect my pots have zero resistance, right?

No. What you're forgetting is that though the variable part of the resistor (the "wiper", or middle lug) does go to the output jack, the outer lug goes to ground.

There is a fixed resistance between the outside lugs (the 250k or 500k you're talking about). So no matter how you turn the pot and feed some of the signal to the output jack, the pickups are still seeing the full resistance of the pot between them and ground, like so (excuse the lame-o drawing):

Note that this is the basic principle and applies for the volume pot. The tone pot works a little differently because it's impedance is frequency dependent, due to the capacitor. But wide open it's basically a resistor to ground.

Some people say that you're able to brighten up a dark sounding guitar, with say 250k or 500k pots, be replacing them with a higher value, for example 1m Ohm or 2 mOhm.

This is right. The guitar is an "LCR" circuit- it has the properties of Inductance (L, from the wound coils), Capacitance (C, from the cable) and Resistance (R, from the coil windings and pots). Changing any one of these properties changes the overall frequencies produced by the pickups.

These frequencies peak around a particular frequency, the "resonant peak". In a nutshell, when you increase the resistance in the circuit you shift that resonant peak upward in frequency space. And so more highs are passed, like so:

This makes the pickup sound brighter. Conversely, lowering your resistance will darken the sound somewhat.

 

[GuitaRasmus]

Wow, thanks Beej! Now I get it.

In laymans terms:

The pots have the signal running through them. When it is turned down (0 resistance), the whole signal is sent to ground. When it is turned all the way (500k ohms resistance) there is 500k resistance to ground - which means that no matter what position the pot is in, there will be some signal sent to ground. And hence, installing a higher value pot, less signal will be sent to ground and more to the jack.

Correct?

 

[beej]

It's a little more complicated, and affects both the pickup and the output jack. The pickups see 500k from the pot regardless of how the wiper is turned. (Assuming we're talking about one 500k pot.)

When the pot is "off", the signal from the pickups go through the 500k pot and to ground. But also, the signal going to the output jack is grounded. (The wiper is physically touching the grounded lug at that point.)

When the pot is "on", the signal from the pickups go through the 500k pot and to ground. Again, they are seeing a 500k load from the pot. But also, the signal going to the output jack is coming right from the pickups.

This part is a bit confusing, but basically the wiper is physically touching the pickup's output. This connects to the amplifier's input (usually 1MegOhm or so), so this part sees a high resistance. So what you do is add the two resistances to find the effect on the pickup. It theoretically goes like this:

1/TotalResistance = 1/500k + 1/AmpResistance.

When you do the math, you see that the effect from the amplifier isn't as significant, and so most of the resistance seen by the pickups is from the volume pot's resistance, in the order of 500k.

 

[GuitaRasmus]

Erm... But... Errr...

LOL. I just think I'll try putting a 1 meg pot in my axis ss - I just think it can sound a little muddy sometimes.

It's this:

Stock pickups, replaced the tone with a 3 way toggle, 5 way disconnected and has Petrucci wiring.

 

[beej]

I haven't tried a 1 Meg on the Axis ... but it will definitely brighten things up. There's usually a limit to how much/little resistance you can put in there, but that would probably work with the humbuckers. That said ... might be really bright with the single coils in the middle position.

Btw, sweet guitar. I loved the thread on that guitar.

 

[GuitaRasmus]

Thanks a lot! And thanks for taking your time to explain it. I'm not at all afraid of soldering*, but I wasn't really aware of the scientific side of things.

Yep, I really think I gave that axis new life - It looked really bad before, and now it's one of the favorites. There IS nothing quite like the EBMM necks - It is not flashy, ultra thin or anything - just one of the most comfortable necks there is.


* Just for kicks, check out this custom tele I made last summer (now sold off, I didn't have any use for it)

It can do ANY pickup combination possible, and has serial/parallel/split switches for both humbuckers, and a phase switch for the singlecoils. It took me 4 hours to wire that monster!

 

[beej]

Wow! Steve Morse would be proud of that one

 

[GuitaRasmus]

VERY inspired by his old tele. Guess what humbucker sits in the bridge position?

 

[beej]

Hey Sixtyfour-

1) What is resistance and how does it work?

It's basically a measure of the degree to which a substance impedes the flow of electric current induced by a voltage.

It's counterintuitive to me to that higher resistance values are brighter and louder than lower ones.

Not louder. But changing the resistance has an effect on the entire circuit comprised of you pickups, pots, cable, amp, etc. It shifts the frequency peak upward, allowing the pickup to pass more highs and fewer lows. Changing any one element will have an effect.

Here's a great site that describes these effects in more depth. (I stole the above graphs from there.)
The Secrets of Electric Guitar Pickups by Helmuth E. W. Lemme

2) Do other components (tone pot, switches) contribute to the overall resistance affecting a pickup's sound? If so, is there an easy way to calculate this?

The extra resistance from the switches and wires are low in comparison to the that of the pots, so it's the dominant factor. If you use really lossy cable that would make a difference.

For resistors in series (connected to one another), you just add their resistances. For resistors in parallel (connected across one another), you use the following formula:

1/Rtotal = 1/R1 + 1/R2 + 1/R3 ... etc.

3) Can you accurately simulate a lower-valued pot by turning down a higher-valued pot? For example, can you simulate a 250K pot by turning a 500K pot down a little?

Depends how it's wired. The outside lugs of the pot have a fixed resistance across them. But the resistance between the middle lug (wiper) and the outside ones changes as you turn the pot.

You can convert a higher pot into a lower one by adding a resistor in parallel using the above math, though. For example, you can turn a 500k pot into a 250k pot by putting a 500k resistor from lug 1 to lug 3. Though it will affect the taper of the wiper.

And one last observation: it seems pots don't affect frequencies evenly through their range of travel. Highs bleed off quicker than lows, which is why things muddy up when you turn the volume down. Also, 500K pots seem to have a much more drastic drop-off in highs and output when compared to a 250K pot. Would these things would affect the accuracy of the simulation in 250K?

Most audio equipment uses pots that have a logarithmic taper ("audio taper"). It's non-linear. You can use linear pots, but most guys don't like 'em.

As for the highs getting lost, yes that happens. What you can do to get around it is to wire a capacitor and resistor in parallel from lug 1 to lug 2 of the volume pot. Do a Google search for "treble bleed". That allows some of the treble to always bleed to the pot's output, regardless of how the pot is turned.

 

[Oldtoe]

Every now and then something like this comes along and reminds me why I got a degree in Spanish.

 

[Oldtoe]

Hope you like being broke, sixtyfour!

The Bachelor of Arts degree: helping add to college debt and little else since The Renaissance.

 

[beej]

Every now and then something like this comes along and reminds me why I got a degree in Spanish.

Ay caramba!

 

[GuitaRasmus]

I see your point, and it was the reason I sold off the tele - not practical for live use. (and it didn't really sound that great - more of a novelty than a great instrument) but notice, every switch on it is a 3 way toggle.

Dunno - I just can't get to grips with blade switches and 5 ways. I must say, that the Dimarzio 3 way toggle (as used on the Petrucci and the Morse) just feels great - really nice quality.

 

[GuitaRasmus]

Don't worry - you've got to do a lot more to piss me off. I didn't take anything in the wrong way at all.

I hear you about the orientation - the toggle switch on my axis is also at about 45 degrees angle - it really makes for a much more natural switching motion. I really like the switch position on my LP, but I can't bring myself to do that to the axis.

 

[whitestrat]

Wow, thanks Beej! Now I get it.

In laymans terms:

The pots have the signal running through them. When it is turned down (0 resistance), the whole signal is sent to ground. When it is turned all the way (500k ohms resistance) there is 500k resistance to ground - which means that no matter what position the pot is in, there will be some signal sent to ground. And hence, installing a higher value pot, less signal will be sent to ground and more to the jack.

Correct?

I'm not sure that when pots are turned down, it's 0 ohms resistance. It's more like infinite resistance, i.e. open circuit, hence the cut in sound.

 

[whitestrat]

Thanks a lot! And thanks for taking your time to explain it. I'm not at all afraid of soldering*, but I wasn't really aware of the scientific side of things.

Yep, I really think I gave that axis new life - It looked really bad before, and now it's one of the favorites. There IS nothing quite like the EBMM necks - It is not flashy, ultra thin or anything - just one of the most comfortable necks there is.


* Just for kicks, check out this custom tele I made last summer (now sold off, I didn't have any use for it)

It can do ANY pickup combination possible, and has serial/parallel/split switches for both humbuckers, and a phase switch for the singlecoils. It took me 4 hours to wire that monster!

Now THAT'S Cool!!!!!!

 

[beej]

I'm not sure that when pots are turned down, it's 0 ohms resistance. It's more like infinite resistance, i.e. open circuit, hence the cut in sound.

When the pot is turned down (lowest volume), the output jack is grounded as the wiper is at the ground side of the pot. But the pickups still see 500k- the signal goes from from one outside wiper of the pot to the other, which is always a 500k resistance.

 

[GuitaRasmus]

Well, I thought I'd give you guys my experience.

As said earlier in the thread, I think the pickups in the axis ss are great and punchy, but I find them a bit muddy on the lower strings - a bit muffled. Kinda wierd since the guitar is very bright and resonant played acoustically. Anyway, I tried putting in a 1meg Ohm pot - what a difference! It feels like another set of pickups, and theres more of everything, especially high mids and highs. Just to make sure, I temporarily put in a 1 meg resistor in parallel, making it act as 500 k pot - big difference. It still retains the punchy middly character, but the bass is'nt as overpowering, and as said, more defined.

I am definitely keeping it in. It makes the guitar sound much fuller and more defined. The only drawback is that the "steps" in volume are smaller - it is bit harder to find the different sounds in the travel, but in my mind it's a small price to pay for a much more defined sound. Great tip!