Author Topic: Potentiometer selection  (Read 260 times)

CCS

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Potentiometer selection
« on: August 03, 2017, 10:58:46 PM »
Is there a method in selecting the optimal variable resistor (potentiometer) for your DIY pedal project?
Eg: Bazz Fuss: Volume, Tone and Fuzz control. At the moment I'm using what ever I have on hand, but I assume there is some sort of method. I tend to prefer Linear to Audio taper. Maybe Audio might sounder better for Tone and even Fuzz, haven't tried that combo yet.

somnif

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Re: Potentiometer selection
« Reply #1 on: August 04, 2017, 12:16:41 AM »
I assume you mean taper choice rather than value (which is electronically relevant and depends on what the resistor is doing in circuit).

Linear tapers are simple as it gets. 50% turn = 50% of the resistance, 0% = 0%, 100% = 100%. Nice straight line.

Audio/Log tapers are a bit different. At half way twisted, you're at 10% of max resistance. This is because of how the human ear reacts to changes in audio volume. We are better at detecting minor differences of low volume than high volume. So you want a lot of knob twist for those low levels for precision sake, whereas the high side of things our ears don't notice minor differences as profoundly.

So, Linear tapers are a good choice where you don't need extra precision on the high or low side of things, just a nice 1-10 rating.

Log tapers work best when you need fine precision on the lower settings. Volume is most common, but you sometimes find them used in things like rate knobs of delays or modulations where things get a bit weird.

Similarly, anti-log or reverse-log are where you want a lot of precision at the high side of things.

And the odd-ball W taper you see in Tube screamer type circuits is log in the first half, reverse log in the second half. So you get a lot of precision at the high AND low sides, but fairly low precision (aka a little twist = a lot of change) in the middle.

BrianS

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Re: Potentiometer selection
« Reply #2 on: August 04, 2017, 01:52:49 PM »
Very informing post Somnif. I knew the basic premise but you gave some really good info.  Short and concise, the way I like most things. Thanks. 

midwayfair

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Re: Potentiometer selection
« Reply #3 on: August 04, 2017, 10:08:22 PM »
For tapers, you need to know (a) the optimal resistance for the source impedance and load and (b) whether the function performed by the pot is best handled by a logarithmic ("audio"), reverse logarithmic, or linear taper.

There is absolutely no reason to assume that the name written on the pot's label gives you ANY information about its resistance value or optimal taper. You can't assume that a volume pot should be an audio taper, even when it's a basic output voltage divider type of volume control, even though that's probably most common. You have to test it empirically or trust the designer.

As far as the amount of resistance goes, you have to look at what the schematic is doing.

As an analogy, suppose someone asked you to give them the best number for something, but they don't tell you what they're using the number for. What could you possibly tell them? You have no context. The numbers on pots are the same way. There aren't infinite values, of course, but the values don't mean anything without context. I can think of a way to use the largest value pot in my parts bin in a way that turning it does nothing.
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nocentelli

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Re: Potentiometer selection
« Reply #4 on: August 05, 2017, 03:35:07 AM »
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« Last Edit: August 05, 2017, 03:47:51 AM by nocentelli »