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Thread: Logarithmic potentiometer tapers: type A vs type B

  1. #1
    Member Synaptic Amps's Avatar
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    Logarithmic potentiometer tapers: type A vs type B

    Playing around with the tone stack calculator from Dave Cinga I learned that log pots come in two types: type A which is at 30% at the center point and type B which is at 10% at the center point. Fender and Vox style tone stacks call for type A logarithmic pots to achieve a balanced sound when all the controls are set to their mid points.

    I've measured all my log pots of various sources and brands including current production CTS, Mojo, and Alpha and of what I have on hand they're all type B measuring 10% of their full resistance with the wiper set half way. In addition to the consequences on tone circuit balance I've never completely liked the feel of these pots as volume controls: they stay low too far into their travel and then come on too abruptly in the last partial turn. I'm aware that most low cost "log pots" aren't truly logarithmic at all but just approximate the curve using two regions of different but constant resistance. Perhaps this underlies the abruptness in the onset of volume that I'm describing.

    At any rate, I'm wondering if anyone knows of a source for type A log pots. Alternatively, maybe someone might recommend a tested method to employ a resistor in conjunction with a linear or type B log pot to improve the balance and feel.
    Last edited by Synaptic Amps; 11-30-2007 at 04:14 AM.

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    Senior Member Old Tele man's Avatar
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    ...that's the "difference" between a LOGARITHMIC curve (30%R at 50% rotation) and a AUDIO curve (10%R at 50% rotation).
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    Senior Member chipaudette's Avatar
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    An Audio taper vs Log taper...is that really the difference? For real? Do you have any references that confirm this?

    Although I knew that there were type A and type B pots...I've always assumed that "Audio" and "Log" were synonomous.

    Cool question and cool reply.

    Thanks,

    Chip

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    Senior Member Enzo's Avatar
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    And let me throw this wrench into your works. The letter B is usually used to indicate linear. A 50kB pot would be a linear one.

    I think RG has some taper info on his Geofex site, plus also just google "potentiomenter taper" and you will find some resources.

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    Senior Member Old Tele man's Avatar
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    ...there are "different" amounts/rates of 'taper' within the general classification of "logarithmic"--only linear is (pardon the pun) 'straight-forward.'

    ...go search the CTS "potentiometers" data...it's posted there somewhere.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    I'm far from the expert on this topic, but I always thought that A-taper was according to audio logarithm, B-taper was the reverse of that (humped in the middle instead of dipped) and "L" designated linear. I think I've also seen somewhere "U" designating linear, but I could definitely be mistaken about that.

    I've seen lots of schematics that say "B" taper to indicate linear....always thought this was because some weren't familiar what B taper actually was and just assumed there was only "log" and "linear" (A and B). But then again, maybe I'm not familiar with what B-taper actually is

    I also wonder how much of it varies from one manufacturer to the next.

  7. #7
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    Quote Originally Posted by Synaptic Amps View Post
    At any rate, I'm wondering if anyone knows of a source for type A log pots.
    I've spent hours and hours recently shopping for pots and unless I really missed
    something somewhere I can say that you won't find new 30% taper log pots
    anywhere. The big companies will make some for you, but I wouldn't want to know
    how many you'd have to buy.

    The only thing I know of that might be useful is that Angela has a large lot
    of NOS pots amongst which you might find something useful.

    Here are some simple taper-modifying circuits :

    http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

    Paul P

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    Yea pots are definetly a royal pain! I dont think the A an B designations for different slopes of audio taper pots have been used for a loooong time! Most if not all of the major mfg's at least seem to have agreed upon A being log taper and B being linear. If you were to spec a custom pot from pretty much any of them, your choices seem to always be either A or B and that's it....unless of course you want a custom taper. And btw, dont even think about doing this as the minimum quantities are just obscene!

    Now having said that, there are a few things I've noticed over the years. First is that the clean channel on amps DO tend to appreciate a somewhat faster pot in the 30% range. However, almost any high gain design will almost demand a 10% pot...possibly even slower. I say this because with a 30% pot you'll be "all in" wayy too fast to make the control usable in any real way. At least for the gain control. I almost always throw a large value shunt resistor from the sweeper to ground on most of my amps to slow them down even more.

    Solution? If you really really want control of how fast the pot reacts you can always build stepped attenuators. Actually this would probably provide you with better overall performance in every area but DAYUM....if you're building more than a handfull of them, I bet this would turn into a huge headache fast!

    -Carl

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    Senior Member Enzo's Avatar
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    SInce pots are almost always custom made for the OEMs, you can always order a Fender or Peavey or whoever stock part of the desired taper.

    Reverse audio taper is usually called just that, and indicated RA, as in 50kRA.

    I haven't been there is a while, but I recall there was a list of available tapers on the Alps web site. And don't forget there are some that taper from the center out towards the ends for like EQs and such.

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    Just to add to the confusion: Some European mfgs (e.g.Piher) call their linear pots "A" and the logarithmic ones "B"...



    Cheers,
    Albert
    www.albertkreuzer.com

  11. #11
    Senior Member Enzo's Avatar
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    Oh great, that's just what we need.

  12. #12
    Noodle of Reality Steve Conner's Avatar
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    Yes, it's true, the letters are the other way round in Europe.

    Is the difference between a 30% taper and a linear taper (which would be 50%) really worth worrying about? There doesn't seem to be any agreement over whether to use log or linear pots in guitar amp tone stacks anyway. You can easily bend a linear pot to 30% taper by connecting a resistor between the bottom and the wiper. But that won't work when it's used as a rheostat, such as the bass knob in a tone stack.

    It's not like there is anything special about the middle of the travel on the tone pots, either. Hi-fi tone controls have a flat frequency response when set to the middle, but a guitar amp tone stack is closest to flat when the bass and treble are at zero. (and the midrange is at 10, if you have the midrange knob)

    I guess people have always just assumed that setting all the knobs to 5 gives a "balanced sound". So now every guitarist's idea of a "balanced sound" is at least 8dB of bass and treble boost with a kind of lower-mid-crevasse in between, and all superimposed on the 6dB/octave treble hillside that you get from the tiny coupling capacitor on a Fender "normal channel", to boot. Any guitar amp that doesn't do this sounds flat, honky and dull.
    Last edited by Steve Conner; 11-30-2007 at 11:53 AM.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  13. #13
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    I thought Ted Weber's audio pots were a 30% taper? I remember hearing something about that, though I could be wrong.

    Greg

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