Author Topic: Understanding adjusting potentiometer values  (Read 3582 times)

DuctTapeRiot

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Understanding adjusting potentiometer values
« on: December 11, 2011, 10:56:57 PM »
Ok, I have been trying to wrap my brain around adjusting pot values in my builds and I think I have some of it figured out and would like peer review so to speak, and some help with other bits.

So start with the most straight forward stuff that I think i have figured out:

Decreasing linear (B) pot values:  If I have a control (Fuzz say) on a linear pot where only the first part of the sweep of the pot is useful (say the first 1/2 of the rotation), and the last part is not (noise or too over the top say) I can just decrease the value of the pot (eg from 100K to 50K)

Decreasing audio (A) pot values:  Same scenario as above where only the first portion of the sweep is usable.  I can do the same thing, decrease the value, but keeping in mind the log taper will mean that the first part of the sweep is increasing resistance much more slowly than the last part. Here I would try to get the knob to the end of the useable part of the sweep then measure the resistance and replace with another alpha pot that maxes out near that value.

Ok, on to more complicated stuff:
Switching from audio to Linear:  If when using an audio pot the usable range is confined to a small section of the sweep (that is probably not at the beginning) I could try switching to a linear to "stretch out" the usable range so to speak.

Switching from Linear to audio :  If when using a linear pot, there was lots of change in the first part of the sweep and "slower" change in the last part I could switch to an audio pot, probably with a larger overall resistance.

Ok, and this is where I am lost
I have a one knob booster that was using a 100K audio pot.  It hits unity volume almost exactly at 12 o'clock (I measured this at about 15K).  The booster is pretty much totally useless as it goes from unit to dead quiet in less than 10 degrees counter clockwise, and from unit volume to way to @#$ loud in 2-3 degrees clockwise.  I am thinking that I should be switching from an audio to a linear pot, perhaps a 25kB??

Any help with my above question, and any thoughts or other scenarios, issues, etc would be most appreciated.
« Last Edit: December 12, 2011, 06:33:17 AM by DuctTapeRiot »

gtr2

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Re: Understanding adjusting potentiometer values
« Reply #1 on: December 12, 2011, 05:31:11 AM »
Alpha is a brand not a taper type, it's a company that makes pots.  :) 

(A) is logarithmic taper pot, commonly referred to as an audio taper.
« Last Edit: December 12, 2011, 05:32:52 AM by gtr2 »
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DuctTapeRiot

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Re: Understanding adjusting potentiometer values
« Reply #2 on: December 12, 2011, 07:09:01 AM »
Alpha is a brand not a taper type, it's a company that makes pots.  :) 

Ha, man that shows how tired I was when I made the post.  :P  I think when I measured the value of the A100k i read Alpha off the back of the pot and my brain just short circuited.  Edited the original post for clarity.

bigmufffuzzwizz

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Re: Understanding adjusting potentiometer values
« Reply #3 on: December 15, 2011, 05:05:07 PM »
This too is something I would like to know more of, there really is no general sticky I've come across that explains what your after. It seems like your on the right track though...
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JakeFuzz

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Re: Understanding adjusting potentiometer values
« Reply #4 on: December 15, 2011, 05:54:27 PM »
I don't think there is really a general rule of thumb for choosing pot tapers. You would really have to look at the desired electrical response as a result of changing resistance. I think our ears like to hear a close to linear response as we turn the knobs. Some of the parameters we are trying to control have a logarithmic response to changing resistance and require a non-linear taper to make it feel more linear. There are several ways you can go about finding the correct tapers. You can do very simple circuit modeling of similar circuits and plot the response as a function of a range of resistance (use Simulink in Matlab), you could actually do the math and look at the function and choose an appropriate function type for resistance to minimize non-linearities in the output or you could just build the circuit and try to choose the taper that sounds the best.

As for the booster with a really short range away from 12 o clock, I would use a 50K or 25K with identical resistors on the outer lugs to make up the resistance (to 100K). Remember pots also sometimes act as fixed resistors in addition to variable resistors, if you swap out a with a 25K you need to make up the lost resistance to make sure nothing is going to change. This just lowers the spread from 1/2 voltage on the voltage divider your pot is creating; making this range smaller will make it feel like you have smoother control with the same degree of motion.