1 meg ohm, 500K, and 250K pot differences when all the way on...

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by EADGBE, May 12, 2006.


  1. EADGBE

    EADGBE Member

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    Are there any differences between different pot values when they're turned all the way up/on? I.e. does a 1 meg ohm pot have the same amount of frequency range as say a 250K pot when they're not turned down at all?
     
  2. justicetones

    justicetones Member

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    I have fiddled with this in a few of my guitars and the Higher the value of the pot it sounded like the signal contained more high frequencies and slightly more output.

    Someone else please chime in if I am wrong but from my understanding this is because there is less signal going to ground when on FULL. I stick to 250K pots for single coils and 500K for humbuckers after my experimentation.
     
  3. Clorenzo

    Clorenzo Member

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    http://www.thegearpage.net/board/showthread.php?t=118336

    Scroll down halfway to see some plots of the frequency response of a typical PAF with 250k and 500k pots (you can guess the effect of 1M pots), turned all the way up.

    Wow, it feels good doing a search and finding it all still there! :BEER
     
  4. EADGBE

    EADGBE Member

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  5. has-sound

    has-sound Member

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    The pot value is actually the amount of resistance between the input of the pot and ground. When the pot is full open, the lower that resistance (as in 250k pots) the more of the signal that is bled to ground, usually in the way of highs. The higher the value (as in 1 Meg) the more of your signal gets to the output of the guitar without going to ground. Single coil pickups are naturally bright so most companies compensate for this by recommending 250k pots. Humbuckers are darker sounding so 500k pots are the norm because they allow more highs to get through the circuit. Stan
     
  6. justicetones

    justicetones Member

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    Much simpler and better described than my attempt. Thanks........ :BEER
     
  7. AaeCee

    AaeCee Member

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    That's the first time I've read an explaination on this that I've truly understood. Thanks! AC
     
  8. billdurham

    billdurham Member

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    On top of has-sounds post. Think of the pot has having two functions. One is to provide a load to the pickup and the other is to allow a variable output of the guitar to the amp. By changing the value of the pot, you change the load on the pickup. This is why there is a little difference in the tone when the pot is wide open when different values of pots are used.

    BD
     
  9. 59 Deluxe

    59 Deluxe Member

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    i hate to hijack here, but i finally understand part of this and want to link it to the other half i don't. so when the pot is full open, all of its resistance is available forcing most of the signal through the tone cap which is in parallel.

    this is where i get lost. i know caps block dc and pass ac, but they also let different frequencies through depending on the farad rating? so how would different cap values make a difference here and why?

    thanks
     
  10. has-sound

    has-sound Member

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    Capacitors let highs pass. The larger the value of capacitor, the more highs pass through. A tone control has one end of the capacitor connected to ground and as you turn down the control it causes the highs to pass through the capacitor and to ground thereby removing them from the signal. A .1uf capacitor will affect a wider band of frequencies than say a .01uf, which will only take the top end off. I hope this wasn't too confusing...Stan
     
  11. Junior

    Junior Member

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    This isn't quite right...

    As billdurham said, in a sense the volume pot is used twice. Once as a voltage divider, and once as a load resistor on the pickup.

    If we put 500K pot in as a volume control, imagine we have 1 volt coming from the pickup to the top lug. We connect the bottom lug to ground, so we have 1 volt at one end of a 500K resistor, and 0 volts at the other. This will remain true no matter where we turn the knob on the pot. The knob moves the wiper connected to the middle lug along the resistor inside the pot and allows us to choose any voltage between 1 volt and 0 volts to send to the output jack. This is called a "voltage divider", perhaps for obvious reasons.

    No matter how we set the volume knob, we still have a 500K resistance loading the pickup. This load on the pickup determines the pickup's overall output voltage, and also dampens (reduces) the high end resonace (peak) that all pickups have (pretty much any coil - speakers have a resonance too, it's just in the bass end).

    [at this point, I need to mention there are two ways to connect the tone pot to the circuit - before the volume pot, or after it. It's called "fifties" and "modern" on the LPF, as Gibson put the tone control after the volume in the fifties, then changed, and has been putting it before the volume pot ever since. As the 'before' scenario is simpler (and more common), I'll stick with that for the moment.]

    The tone control pot is wired differently from the volume pot - it only uses two of the three lugs. It doesn't matter how it's wired, the cap can be before the pot or after the pot, you can connect the bottom lug to ground or the wiper, it's all the same. So, with that in mind, we continue...

    The tone pot is also a load on the pickup, and it's wired in parallel with the volume pot. As we turn the tone pot down, however, we do reduce the load on the pickup (an engineer would say we "increase" the load, but that's counter-intuitive, just keep it in mind if you're reading something else). This is because the tone pot's lower lug is connected to ground just as the volume pot is, but the wiper is connected to the pickup. As we turn the wiper down, the amount of resistance between the pickup and ground decreases, changing the combined load on the pickup, effectively having the same effect as using a lower value volume pot, except there's that pesky capacitor in the way. What's it doing? It's like a gate, saying 'yes' to high frequencies, and 'no' to the lows. So, actually, turning down the tone pot causes the pickup to see less load only for the highs. That is, with the standard 500K/500K/.022 setup, you'll get a 250K (two 500's in parallel) load for the lower frequencies, and from 250K to 0 for the highs. With a 0 load, the pickup puts out nothing.

    You'll notice I'm only talking about the load on the pickup, with no mention of the signal path, or things "bleeding to ground". That's also happening at the same time, but it's not affecting the output of the pickup the way changing the load does (for more on this, follow Clorenzo's link to the thread Scott started, then follow the link two posts above Colrenzo's charts to Terry Downs' PSPICE modeling).

    You know, it's a bit draining writing this stuff, and at this point I'm not sure what I've explained and what I haven't. I'm hoping this will help 59 Deluxe grasp this, but it's also for everyone that comes along later and pulls up this thread with a search. Feel free to ask more questions, or tell me to just go away.

    Junior :dude
     

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