Potentiometers:

[Micah]

This is probably a better place to ask this than somone elses' post about 250K fender pots:
Something Ive never really understood is: When we say a potentiometer is 1 meg, 500k, 300k, or 250k, that is the potential for resistance to ground right? If that is the case, when your volume or tone pot is "off" you have the maximum amount of resistance the poentiometer can produce right? so a 500k pot is resisting the current coming from the pickup to ground @ 500 ohms when its closed or "no sound coming out"-right?

If that is the case, Im not sure I understand why a different resistance potential ina potentiometer changes the tone going to the amplifier... heres why:

If it only takes 250k of resistance to stop the flow of current coming from a single coil pickup in a strat, then what is 500 ohms going to do after the current is stopped @ 250 ohms??

Some guys play JB's with 250k pots--wouldnt that mean the pickup would never be "off: if it takes 500 ohms to stop the current?

Im obviously off somewhere here because I know you can turn a JB "off" with a 250k pot! Is is that more resistance equals more output of the pickup because its resisting its connection to ground?

Can somone set me straight here??

 

[uOpt]

Re: Potentiometers:

Volume pot:

the way that the volume pot is wired in a passive guitar applies the terminal-to-terminal resistance as a load resistance (partial shortcut) over the output of the pickup. This resistance is added to the impedance of the low pass filter that a pickup is. This dampens the amplitude (not the frequency) of the resonance peak. So the lower Kohm value the poti has the less resonance peak you get and the mellower (or deader) the sound gets.

Tone pot:

the way this is wired in a passive guitar the capacitor is never out of the picture, it is always applied to the output of the pickup with the pot's value. A 500 Kohm poti at full open (or "10") means that the capacitor is "visible" with 500 Kohm before it. That's not much but it's there. It is a common misconception that a tone pot at "10" doesn't do anything just because the resistance of the slider to the open terminal is zero.

 

[Fikealox]

Re: Potentiometers:

I think...

Basically, you're looking at a voltage divider. The signal is trying to choose whether it wants to go to output or straight to ground. If the pot is fully "off" (full resistance), then the resistance to ground and to output are the same (probably slightly less resistance to earth, due to other pots etc in the output path).

So you can see if you roll the pot back a bit, you'll be making the path to ground less desirable because there's more resistance that way than there is to output. It's basically a ratio - the actual value of the pot doesn't make a difference.

Or to look at it another way: the "output signal" is the voltage between hot and earth. If you've got hot and earth connected (as they are when you've got the resistance turned to full - because you're making the split in the paths come "after" the resistor) there can be no resistance between them, and thus no voltage, and no output signal.

Does that help?

 

[P-Bass]

Re: Potentiometers:

I expect that there's a bit more to it than that. Although two pots (say 250k and 500k) both act as 50:50 voltage dividers when rotated to mid-position, the current flow through the two will not be the same. In the 250k case, the current flow (I=V/R or I=V/Z) will be higher than for the 500k pot. This difference in load should alter the total current traveling through the circuit including the pickup and should alter magnetic flux etc. So...there should be some effect.

All the best,

Mike

 

[uOpt]

Re: Potentiometers:

A passive guitar pickup is a second-order low pass filter with a resonance peak, made up of a capacitance, a resistance and an inductance.

Adding the potentiometer's base resistor value by "shortcutting" the pickup's output adds that resistance to that low pass filter. As with any resistance added, it dampens the resonance peak.

This effect is separate from the volume control, and partially an unwanted side effect, but also used to dampen the resonance peak e.g. in the Strat/Tele case.

But the really unwanted effect is that when you turn down the volume pot you add more load and the resonance peak's amplitude gets lower and lower. That is the reason why in a passive guitar the volume control changes sound when turning it down.

 

[Artie]

Re: Potentiometers:

Actually, as you lower the volume, you decrease the load. Consider a 500k volume pot on "10" going into an amplifier with a 1 meg input impedance. (Typical.) 500k and 1M in parallel is 333k. At the halfway point, you'ld have the "bottom" 250k in parallel with 1M, (200k), in series with the "top" 250k, for a total of 450k. As you lower the volume, the impedance the pickup "sees" slowly rises back up to 500k when the wiper is on ground and the amp load is eliminated.

A volume pot does two functions at once. First, it acts as the pickup "load". Second, it acts as a voltage divider. (As others have already said.) Its simply a vavle that controls how much of the pups voltage makes it out of the guitar.

 

[Micah]

Re: Potentiometers:

Great info everyone! Am I understanding this correctly? The volume pot is actually adding DC Resistance to the pickup which (like the SD Tone Chart describes) decreases the resonant peak of the sound of the pickup and makes it sound quieter? So if this is the case, I can see why changing the potential DC Resistance potential of a pot could change the tone of the pickup.

It sounds like the variable resistance which is controled by the potentiometer, is actually applying resistance to the path to ground and not to the signal going to the amplifier-correct? So @ "10" on the potentiometer you have the full resistance value of the pot (lets say 500K) on the path to ground whigh makes the path to the amplifier more desirable to the current (it takes the path of least resistance). Is that correct?

 

[uOpt]

Re: Potentiometers:

Thanks, Artie, I didn't think of that.

Micah, the volume pot messes with the amplitude of the resonance peak, not with the frequency of the resonance peak. To lower the frequency of the resonance peak you add a load capacitor. Load capacitance lowers the frequency of the peak, load resistance lowers the amplitude.

Lowering the amplitude makes a pickup sound less harsh, or more dead, depending on what you start with. Lowering the frequency make it sound darker, fuller but less clear and transparent.

 

[Micah]

Re: Potentiometers:

Thanks, Artie, I didn't think of that.

Micah, the volume pot messes with the amplitude of the resonance peak, not with the frequency of the resonance peak. To lower the frequency of the resonance peak you add a load capacitor. Load capacitance lowers the frequency of the peak, load resistance lowers the amplitude.

Lowering the amplitude makes a pickup sound less harsh, or more dead, depending on what you start with. Lowering the frequency make it sound darker, fuller but less clear and transparent.

Ok! Understood- How would you describe what amplitude sounds like in lamans' terms? Brightness? Power?

Also, Im still trying to relate this to the effects of adding or decreasing the resistance of the path to ground--how that potential changes the top-end response of the pickup sound to the amplifier. Is is because even with 500k of resistance when the pot is "open @ 10" there is still some amplitude bleed-off to ground? So the more resistance your pot can produce, the less amplitude bleed off to ground, which is translated into more top-end response to the amplifier?

 

[uOpt]

Re: Potentiometers:

Yes, both tone and volume pot as typically wired in a passive guitar have an effect at "10".

It's very hard to describe what lowering and amplitude and the frequency of the resonance peak does. I recommend crocodile clipping a couple of resistors and capacitors parallel to your pickup.

 

[Micah]

Re: Potentiometers:

Yes, both tone and volume pot as typically wired in a passive guitar have an effect at "10".

It's very hard to describe what lowering and amplitude and the frequency of the resonance peak does. I recommend crocodile clipping a couple of resistors and capacitors parallel to your pickup.

Completely understand man! I appreciate you taking the time to explain it to me! Was I tracking correctly on the second paragraph referring to the more functional side?

 

[Zhangliqun]

Re: Potentiometers:

Great info everyone! Am I understanding this correctly? The volume pot is actually adding DC Resistance to the pickup which (like the SD Tone Chart describes) decreases the resonant peak of the sound of the pickup and makes it sound quieter? So if this is the case, I can see why changing the potential DC Resistance potential of a pot could change the tone of the pickup.

The DCR of the pickup and the resistance of the pot are two different things. The resistance of the pickup coil tells us only roughly how much wire (how many turns of wire) there are on the coil. It isn't the resistance of the coil that makes the pickup "hot" or vintage, etc. (all other factors being equal), it is the number of turns of wire.

At every point on the coil wire, current (signal) is being generated due to the wire being in the presence of a fluctuating magnetic field (the magnetic field moves around when the steel strings vibrate), so the more turns of wire you have, the more current and thus signal is generated.

The resistance of the pot is a different animal in terms of effect. It tells you how much of a buffer you have between the signal going straight to the amp or straight to ground. The higher the resistance value of the pot, the brighter and clearer the signal. This is because even when the pot is on 10, some of the signal still leaks away to ground, and the first to go is the treble. This is why 250k pots sound a little duller/darker than 500k or 1,000k (1 meg) pots -- there is less resistance to keep the signal from going to ground.

 

[Micah]

Re: Potentiometers:

The DCR of the pickup and the resistance of the pot are two different things. The resistance of the pickup coil tells us only roughly how much wire (how many turns of wire) there are on the coil. It isn't the resistance of the coil that makes the pickup "hot" or vintage, etc. (all other factors being equal), it is the number of turns of wire.

At every point on the coil wire, current (signal) is being generated due to the wire being in the presence of a fluctuating magnetic field (the magnetic field moves around when the steel strings vibrate), so the more turns of wire you have, the more current and thus signal is generated.

The resistance of the pot is a different animal in terms of effect. It tells you how much of a buffer you have between the signal going straight to the amp or straight to ground. The higher the resistance value of the pot, the brighter and clearer the signal. This is because even when the pot is on 10, some of the signal still leaks away to ground, and the first to go is the treble. This is why 250k pots sound a little duller/darker than 500k or 1,000k (1 meg) pots -- there is less resistance to keep the signal from going to ground.

Absolutely man! Im tracking with you! That makes perdfect sense! Thanks for taking the time to explain-everyone on this thread! Thats how we learn!

 

[uOpt]

Re: Potentiometers:

This is because even when the pot is on 10, some of the signal still leaks away to ground, and the first to go is the treble.

I'm sorry but this is technically not correct for the volume pot. The volume pot lowers the resonance peak's amplitude but it does not generally affect high frequencies. High frequencies right below the resonance peak are unaffected.

A single resistor as in the volume pot is not a low pass filter.

The resistor in combination with the capacitor that the tone control is, that is a low pass filter and that one kills "highs", inside or anywhere close to the resonance peak, all highs are lowered. But the volume pot resistance affects the peak only.

The volume pot is it's single resistor isn't a filter on it's own. It becomes part of the second order LPF that the pickup is.

This is why both volume pot and tone pot do something to the highs but in an entirely different manner. If the volume pot was implementing a LPF we could just use the tone pot for everything and don't bother with 500 or 250 Kohm volume pots. But we can't because the effect, electronically and acoustically, is totally different.

 

[Fikealox]

Re: Potentiometers:

It sounds like the variable resistance which is controled by the potentiometer, is actually applying resistance to the path to ground and not to the signal going to the amplifier-correct? So @ "10" on the potentiometer you have the full resistance value of the pot (lets say 500K) on the path to ground whigh makes the path to the amplifier more desirable to the current (it takes the path of least resistance). Is that correct?

It's my understanding that the variable resistance is getting applied to the path to output, but that the path to ground always has the full resistance (as shown by soldering one leg of the pot to the chassis, as in some wiring diagrams). That also gels with the purely symbolic diagrams I've seen, that show a fixed resistance in the earth-path, and the wiper (hence, variable resistance) in the output path.

However, I guess the pots must be made so that a clockwise turning is actually decreasing resistance from source to output... otherwise turning the pot clockwise would decrease volume...

 

[Micah]

Re: Potentiometers:

I'm sorry but this is technically not correct for the volume pot. The volume pot lowers the resonance peak's amplitude but it does not generally affect high frequencies. High frequencies right below the resonance peak are unaffected.

A single resistor as in the volume pot is not a low pass filter.

The resistor in combination with the capacitor that the tone control is, that is a low pass filter and that one kills "highs", inside or anywhere close to the resonance peak, all highs are lowered. But the volume pot resistance affects the peak only.

The volume pot is it's single resistor isn't a filter on it's own. It becomes part of the second order LPF that the pickup is.

This is why both volume pot and tone pot do something to the highs but in an entirely different manner. If the volume pot was implementing a LPF we could just use the tone pot for everything and don't bother with 500 or 250 Kohm volume pots. But we can't because the effect, electronically and acoustically, is totally different.

Absolutely man--I was tracking with what you are saying here (maybe not as technically as your saying it). But the effect to our ears is that it sounds weaker in the high frequencies when using a 250k pot as opposed to a 500k pot right?? Sometimes referred to as "smoother"?

 

[uOpt]

Re: Potentiometers:

Absolutely man--I was tracking with what you are saying here (maybe not as technically as your saying it). But the effect to our ears is that it sounds weaker in the high frequencies when using a 250k pot as opposed to a 500k pot right?? Sometimes referred to as "smoother"?

Absolutely.

But the volume pot dampening (the one that is there even at full open or "10") leaves all highs intact except the ones directly on the resonance peak. That's why it just becomes smoother, less piercing or deader (depending on where you started) but not dull, dark or less transparent.

And that's why the tone pot cannot do that exact sound change that the volume pot causes.

 

[Shane28462]

Re: Potentiometers:

Thought this might help!

http://www.stewmac.com/cgi-bin/haze...G=3&s1=Free_info_sheets&item=freeinfo/fi.html

 

[Zhangliqun]

Re: Potentiometers:

I'm sorry but this is technically not correct for the volume pot. The volume pot lowers the resonance peak's amplitude but it does not generally affect high frequencies. High frequencies right below the resonance peak are unaffected.

It may not be technically correct, but that's how the ear hears it. Loss of treble on the resonant peak sounds like loss of treble, period.