JB, 1 vol, no tone!

[UCSDBoy]

Re: JB, 1 vol, no tone!

Right .....
First off unless you are using linear pots it won't do that ... an audio taper (commercial approximation-twoo segment) turned halfway down will be about 10% of the pot's total R/Z. Not sure what you are referring to (haven't checked the link), the reason for putting a resistor across the CW and CCW terminals of a volume pot is to load the system as if the pot's value was the paralleled value of the pot and resistor. That's a volume pot, it's not quite that same for a tone control. The reason that someone would put a resistor there would be to damp the resonant peak of the pup. Therefore the tone at ten will always be just what it is, the maximum allowed by that addition of R. If you planned on using the extra brightness at times it wouldn't make sense to do this, but if you didn't ... well it wouldn't make much sense to have you tone control sitting on 7.5 all the time would it? If you are referring to a lrager vaued cap, and limiting the maximum attenuation level of the tone control ... that's a whole different thing altogether.

Hey Kent S.,

thanks! You're right, I forgot audio pots are logarithmic instead of linear taper... but the question is still kind of the same - if I wanted to hear what a 250k pot sounded like I should just have to roll the 500k pot down (in this case, a small amount) to get 250k, correct? I understood that slapping a resistor across the terminals would create a parallel circuit and what this does, but it was interesting that someone would write an article on how to simulate a 250k pot with a 500k (for tone pots, I guess), when they could just go out and buy a 250k...

The capacitor only allows higher Fcs thru so when you turn down the tone control the lowest Fcs are blocked from ground, but the highs aren't ... hence they are removed from the signal ... That's a greatly overstated simplification, but it gets the point across. You take the cap away, and all you have is a variable load the ground, it will damp the resonant Fc of the pups (amplitude of it that is), but will not cause a low pass filter slope to form ( attenuating higher frequencies past a cut-off or corner Fc more and more the higher they go), the only thing that would happen without that cap is you pull down the peak, then eventually cause some roll off, but the whole time your level would be dropping as you'd be changing the ratios of the voltage divider ... You, might eventually get the roll-off of highs you want, but what good does it do you when your guitar level is sitting at about -40dB compared to where is was? The cap establishes a filter in conjunction with the source impedance and controlling resistance of the pot. It's a first order low pass filter with a limited attenuation control, I'm sure you covered those in electronics.

Uhh, circuits for computer guys isn't the same for real electronics guys... lol we get retard electronics - 101. So if I read you right, then the capacitor is what actually lets the higher frequencies go to ground? The variable resistor does block the higher frequencies (reducing the amplitude of the wave), but if not for the capacitor, the entire signal would go to ground, and not just the high frequencies? If the capacitor is ultimately what dams the river, so to speak, how does it work? I thought they just stored a charge and after it filled, let it go...

 

[UCSDBoy]

Re: JB, 1 vol, no tone!

Actually I just found this in Wikipedia:
http://en.wikipedia.org/wiki/Low_pass_filter
ehh, AC circuitry is not my thing. =)

 

[Marcel]

Re: JB, 1 vol, no tone!

compare the "I know this has been beat to death but.... "-topic
It let me just think 250 K and no tone is a good compromise
i will check this option out!!

 

[Marcel]

Re: JB, 1 vol, no tone!

btw i coded a php script which calculates the frequence response of a pickup (the electric not of himself) with different setups it's just a bit theoretic but i may help. I'll search it after work!

 

[Kent S.]

Re: JB, 1 vol, no tone!

Hey Kent S.,

thanks! You're right, I forgot audio pots are logarithmic instead of linear taper... but the question is still kind of the same - if I wanted to hear what a 250k pot sounded like I should just have to roll the 500k pot down (in this case, a small amount) to get 250k, correct? I understood that slapping a resistor across the terminals would create a parallel circuit and what this does, but it was interesting that someone would write an article on how to simulate a 250k pot with a 500k (for tone pots, I guess), when they could just go out and buy a 250k...

That's just it though, a lot of people wind up likeing the loading of say what a 370k pot wouls give ... well you won't find a 370k pot anywhere, unless it's special order and in quanity ($$$$). The resistor gives you the freedom to lower the shunt to ground and load the signal to what the individual wants tonally ... But yes, all things being equal (which they never are) a 500k-A pot turned down to read 250K to ground will sound the same as a 250k pot at ten (assuming it reads 250k) with the same vlaued cap.

Uhh, circuits for computer guys isn't the same for real electronics guys... lol we get retard electronics - 101.

The digital curse ... Analogue, the only way it was meant to be! :laugh2:

So if I read you right, then the capacitor is what actually lets the higher frequencies go to ground?

The higher the frequencies the less resistance they see, because the caps reactance is lower, so the easier the get thru ... correct ...

The variable resistor does block the higher frequencies (reducing the amplitude of the wave), but if not for the capacitor, the entire signal would go to ground, and not just the high frequencies? If the capacitor is ultimately what dams the river, so to speak, how does it work? I thought they just stored a charge and after it filled, let it go...

The variable resistor blocks ALL of the signal from ground, it doesn't have much to do with blocking any higher frequencies in and of itself. The load does effect the resonant peak though thru it's damping action (which is any decrease shunt to ground and/or increase in series resistance), not the same as a filter though. If you want technical answer for how a capacitor works you need to go read up on how it. I understand it, and it's kinda second nature , but to explain every detail of it's function ... Nah, I think I'll pass. You have to research that for yourself, ... after a length of study you either get it or you don't. Hard to believe that something can react to dc, and every single ac frequency at the same time, but true. The resistance does a couple of things as well, for that you'll have to do a search under first order passive RC filters, specifically look up the limited attenuation part, as that alters not only the cut-off Fc but the low shelf corner as well. Also the pup inductance also plays a major role in the filter as well ... so it's not quite a straight nice and clean low pass filter.
Caps only store a charge when DC is involved, and ac signal causes the cap to constantly store and release it's charge. They block DC (after fully charged) but allow ac to pass, the the higher the angular velocity or frequency the less reactance they have to it, in essence the less they resist it, less resistnce allows a greater portion of the voltage to pass ... but in proportion with those frequencies.

 

[Kent S.]

Re: JB, 1 vol, no tone!

btw i coded a php script which calculates the frequence response of a pickup (the electric not of himself) with different setups it's just a bit theoretic but i may help. I'll search it after work!

Note, if you are trying to use the first order equation, it won't be accurate as the inductance of the pup gets thrown into the mix as well, plus impedance is different than resistance ... The basic slope may be in the ball park, but the sonic effect may be perceieved differently. The best way to do it is to bulid a model of the pup using it's electronic qualities and give it a corresponding load and sweep it with a sine wave, then do a db voltage plot, and graph it, using something like a 25 step monte-carlo to account for variables in tolerances. At least that will get you a more accurate example of it.

 

[Marcel]

Re: JB, 1 vol, no tone!

Note, if you are trying to use the first order equation, it won't be accurate as the inductance of the pup gets thrown into the mix as well, plus impedance is different than resistance ... The basic slope may be in the ball park, but the sonic effect may be perceieved differently. The best way to do it is to bulid a model of the pup using it's electronic qualities and give it a corresponding load and sweep it with a sine wave, then do a db voltage plot, and graph it, using something like a 25 step monte-carlo to account for variables in tolerances. At least that will get you a more accurate example of it.

i used complex numbers no first order equation
that's the way i did it but i've made it teorethical!!
i built a pseudo pup (which is a resitor capacitor and an a "Spule")
the datas i have from the web, they should be from a '59 (can't find datas for the JB)
they are:
5 Henries
0.00000000012 F
7430 Ohm *edit not kOhm :smack: *

the result is then next:
first line is 500k no tone
second is 500k Vol 500k Tone (on ten) (blue) and 250k no tone (red) (you cant' see it it's below the blue)
third line 250K tone, 250 K Vol

 

[Marcel]

Re: JB, 1 vol, no tone!

ok i had to resize the pic:
x-axis is frequence in Hz
y-axis is amplitude in db

 

[Kent S.]

Re: JB, 1 vol, no tone!

i used complex numbers no first order equation
that's the way i did it but i've made it teorethical!!
i built a pseudo pup (which is a resitor capacitor and an a "Spule")
the datas i have from the web, they should be from a '59 (can't find datas for the JB)
they are:
5 Henries
0.00000000012 F
7430 Ohm *edit not kOhm :smack: *

the result is then next:
first line is 500k no tone
second is 500k Vol 500k Tone (on ten) (blue) and 250k no tone (red) (you cant' see it it's below the blue)
third line 250K tone, 250 K Vol

Into what load? What's the capactive value of the pots themselves?
120Pf for distributed capacitance? ,... Did you measure it? ( sounds hinky ).
What you have is a transfer characteristics curve. Where's the secondary peak of a humbucker? (I intentionally omit this at times).
I don't know why you would actually calculate the impedance ( referencing the complex numbers) per frequency and then convert the resulting voltage (again into what load) to a dB voltage referenced scale. The reason I say this is because most EE programs will do this for you. The monte-carlo will still give a better real world model, although I use the nice clean lines as well; sometimes it helps to see an idealized version. I treat the series elements of L and R as idealized as well as the parallel C.Then assign a tolerance to them.
Now work thru the tone control and volume control changes, and you see many different things appen to the curves. Subing different pups into the equation changes the whole set also. The EE I used allows for EMF files for the graphs, but those aren't an accepted format that the forum recognizes for posting ... Then there is that pesky resize issue also. The amplitude differences look a bit much to me, maybe it's just the way the graph looks though (optical illusion); again into what load? That matters as well.

 

[Marcel]

Re: JB, 1 vol, no tone!

ok it'S a reallly easy and idialized model you're right! Maybe there sitill something wrong, it's a long time ago i made this.
Ok the pots don't have a capacity.
Just the load of the guitar itself. I thgought the load of a tube amp is >> 500 KOhm.
Second line of the humbucker?
yeah the 250k 250K seems to low for me!

 

[UCSDBoy]

Re: JB, 1 vol, no tone!

Man you guys are flying way over my head... lol but thanks for all the info -

 

[Kent S.]

Re: JB, 1 vol, no tone!

ok it'S a reallly easy and idialized model you're right! Maybe there sitill something wrong, it's a long time ago i made this.
Ok the pots don't have a capacity.

Capacitance ... pet peeve ...

Just the load of the guitar itself. I thgought the load of a tube amp is >> 500 KOhm.

My point was that the load has to terminate, and that termination must be specified ... that's all I meant. I didn't specify, so I have to ask into what load?

Second line of the humbucker?
yeah the 250k 250K seems to low for me!

The secondary Res Fc peak of a humbucker hooked in series.