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Linear Pot to "Quasi-Audio" Pot - A SIMPLE solution

Discussion in 'Pickups & Electronics [BG]' started by PilbaraBass, Mar 13, 2008.


  1. Do you have an cheapie passive bass that doesn't have audio taper volume pots?
    Are you tired of having to dial your pots 80% before you get any volume out of them?
    Or worse yet, you try to cut back on the volume and it drops off like a cliff?

    I have a very easy, very inexpensive solution instead of spending $$ and putting in audio taper pots.
    By attaching a properly-valued resistor to your volume pot (one leg goes on the wiper, the other to where your pickup is connected) you can quite closely simulate the value of an audio taper pot.
    I haven't tried it, yet (be sure, I'll be doing this one tomorrow)...but my calcs show that if you have a 250k linear pot, then you use a 33k resistor, and a 66k resisor for a 500k pot (0.132*pot value)...

    it's quick, inexpensive...and if it doesn't work, easily reversible...


    It can't be that simple...but I suspect it IS....has anyone done this yet?
     
  2. Yup... works OK in a pinch. In fact, it's probably better than cheap "pseudo-log" pots which are actually just two distinct regions of linear resistance track. The low down is here.
     
  3. funny...I've just stumbled across that page on my own, while searching for confirmation to my suspicion...

    I'll be putting a couple-o-resistors in my bass tomorrow or sunday..
     
  4. Basshole

    Basshole Banned

    Jan 28, 2005
    They say music aptitude goes hand-in-hand with mathematical aptitude. I'm clearly the exception to this rule. Color me clueless without a calculator.

    I have a Hot Rod Deluxe Fender amp, which I love, but in the typical "make it sound loud at 2, so customer thinks he's buying loud amp" methodology, my amp is virtually uncontrollable at lower volumes. To complicate remedying this, the pots are mounted on the PC board, and aren't easy to replace.

    This is a long shot, because I don't have the amp open, or the schematics handy...but the folks on here tend to be relatively sharp, and rather helpful.

    Would anyone know what value resistor I'd need to add, and to what pot legs, in order to make the volume control less-than-worthless at fine control over lower volumes on this amp?

    Sorry to boost your thread, but it's related...thought I'd ask.
     
  5. you really need to know the value of the pot you're trying to modify (again, use my 13% rule...)

    can you take photos of what you're talking about? it may help.
     
  6. DavidRavenMoon

    DavidRavenMoon Banned

    Oct 20, 2004
    I've seen this done from time to time... but it's not a big deal to go and get a real log taper pot if you need one.

    I mostly use linear taper pots.
     
  7. It's probably a case of the pot being too big a value, so it reaches max gain at 10% of rotation and the other 90% does nothing. Replacing it with a lower valued pot might also work, but again, it would require some investigation.
     
  8. Basshole

    Basshole Banned

    Jan 28, 2005
  9. not a big deal for log taper pots, no...BUT...

    $6.00 minimum for two pots vs. $0.12 for two 33k resistors...

    3 solders, per pot (including thos pesky solders to the pot body) and a bit of fussing with nuts and knobs vs. two quick solders and no knob fuss per pot...

    and the winner is?.... ;)
     
  10. tallboybass

    tallboybass Supporting Member

    Feb 25, 2003
    Tulsa, Oklahoma
    Being a lefty I have a problem finding reverse audio taper pots. Any way to turn an audio taper into a reverse audio taper?
     
  11. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    I've done it many times. But from the Geofex site linked above in post #2, here's the gotcha:

    Unfortunately there's a gotcha in there. It's true that the voltage division ratio of this rig is arbitrarily close to that of a log taper pot. However, neither the load seen by whatever drives Vin or the source resistance as seen by the input of whatever is connected to Vout is close to what would exist for a real log pot of value R. In fact, the load on Vin varies from 1/(1+1/b)*R up to R. That means that if we're trying to do a log taper with b = 1/4, the load on Vin will be as much as 0.2* R. This may be OK, but you have to keep it in mind.

    [Copyright 1999 R.G. Keen All rights reserved.]
     
  12. yep...I thought about this, too...and by the way I'm connecting it (across the wiper and the pickup "hot" lug on the pot, the resistance to earth (loading) will range from 250k (to 0k) just as they do now...

    If I were connecting the other way between the wiper and the ground lug on the pot (why I would, I don't know)...then I'd worry about the loading issue...

    so, actually...for this application "gotcha factor" = zero
    :)
     
  13. play the bass while standing on your head ;)

    seriously...
    no, but you can change a linear to a quasi-reverse audio...just connect the pot in reverse and connect the "taper resistor" (that's what we'll call it to be consistant with the geofex article) again between the pickup hot and the wiper...
     
  14. update...

    didn't touch the electronics today. even though I did get a pair of 33k resistors ($0.15 as dick smith electronics)...

    reason I didn't...I was changing strings on it this afternoon and my nut broke...:eek:....so I spent most of my free (me) time today fashioning a new nut out of PVC...good news is the nut came out quite good...

    church tomorrow, so I won't be touching it 'til tuesday night (next "me" time)...
     

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