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I order audio taper, and they keep sending me linear taper. Or is it me?

Discussion in 'Pickups & Electronics [BG]' started by jscomposer, Apr 15, 2010.

  1. jscomposer


    Nov 25, 2009
    This is the second 500k blend pot I've gotten. The first was Bournes from allparts, the second was Alpha from stewmac. In both, there's essentially zero change in the tone until you hit the center detent. So it's more like a switch. Both pots say MN500K.

    I should add that the pickups are Q-Tuner BS, neck and bridge. Is this a peculiarity of Q-tuners, that they don't blend smoothly?

    Or am I actually using audio taper blend pots, when what I really need is a linear taper blend pot? :confused:

    For reference, the blend pot that came with my bass says B250KX2 (no brand name anywhere), as part of an active Bartolini harness, and it did blend smoothly between 2 Duncan pickups. Figured I'd swap it for a 500K pot for a little more highs.
  2. B denotes a linear taper, unless you are in Europe, where for some bizarre reason, they use B for audio.
  3. jscomposer


    Nov 25, 2009
    Yeah, it just occurred to me, "B250KX2" probably means "linear taper, 250K, times 2 (for either side of the detent)!" :oops:

    So can someone please explain to me why a blend pot should be linear, while a volume or tone pot should be audio? Isn't blending just fancy volume control of two pickups?

    Still confused.
  4. jscomposer


    Nov 25, 2009
    Ah, wait a sec. I just read that audio taper can be approximated on a linear taper pot by adding a resistor.

    So, what kind of resistors do I add, and where? (I'm guessing I'd need 2, one for each side of the blend?)
  5. wow! the answer is obvious! it's your pickups! look at the name! "Q-Turner BS" ....duuhh! the answer was in the name of the pickups the whole time!

    i actually have no idea why you're not getting the results you want, and i have never heard of the pickups you have, but i'm sure they're fantastic. sorry if i offended you in anyway.
  6. jscomposer


    Nov 25, 2009
    Tell me about it. Every time I type out Q-Tuner BS, I find myself wishing they'd have come up with a better name. ;)

    So uh, anyways...

    I think I'm having this problem because the blend pots are linear taper. (I confirmed this with a multimeter.) But I can add resistors to approximate a log taper. I just need to know exactly what kinda resistors, and how to hook 'em up.
  7. gumtown


    May 7, 2007
    New Zealand
    a linear pot's resistance changes in equal proportions from one end to the other.
    A Log pot's resistance changes at a rate of x10 or in a logrythmic curve, so one end of it's travel there is a huge change in resistance, and the other end of the travel see's little change. This is because human hearing follows the log scale, and is not linear.
    If you look at a graph where one side is resistance (ohms) and the other axis is knob travel, a linear pot will give a straight diagonal line, and a log pot will sart off very flat then suddenly climb at a steep angle.
    Same thing said for watts, where 1000 watts is twice as loud as 100 watts, even though it burns 10 times as much power to add 6dB.
    Every 6dB increase in level is twice the level.

    sorry about the theory lesson, i though it might be useful to put it all into perspective.

    the pickup mix has to be a linear pot, or else you end up with all one pickup until almost the end of the knob travel, and won't be centre balanced.
    And if you have a linear one for a volume, it will be everything on at number 2, and little change after that.
  8. jscomposer


    Nov 25, 2009
    But that's what I'm getting now. From fully clockwise to just short of center detent, there's virtually no change in tone. Then all of a sudden you hit center detent and BAM, significant change in tone. Continue turning counterclockwise, and just a hair past the center detent you get another significant change in tone, and then virtually no change in tone the rest of the way counterclockwise.

    Here's what I got with a multimeter on the 500K blend pot (by Alpha): at the center detent, on the top row of lugs it reads 530K and 0K on lugs 1 and 3, respectively. On the bottom row it reads 70K and 540K on lugs 1 and 3. (So it seems reversed on the second row.) Turned fully clockwise it reads 530K and 0K on lug 1 and 3 on both rows. Turned fully counterclockwise it reads 0K and 530K on lug 1 and 3 on both rows.

    That tells me that a blend pot is basically a dual volume pot:

    Fully clockwise: bridge pickup 100%, neck pickup 0%.
    Center: both pickups 100%
    Fully counterclockwise: bridge pickup 0%, neck pickup 100%.

    So for example, from fully clockwise to center, the neck pickup should increase in volume from 0% to 100% in a log taper. But I confirmed with the multimeter that the taper is linear. So basically, the neck pickup is off and stays off until you hit the center, where it suddenly shoots to 100%.
  9. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    yep, that's how they tend to sound.

    try this:
    get a 250k m/n pot, install it as per normal, but leave all the ground connections off, including the jumper from one ground tab to the other.

    (ground the pickups themselves elsewhere, obviously).

    this pot will now have zero effect on tone when in the middle (just like a switch), and will only add series resistance to one pickup or the other when turned, "dimming" its volume without loading it to ground.

    i've had good luck with this trick, as it no longer loads the pickups with the extra ground path, and seems to pan a little more smoothly.
  10. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009

    try it.

    (it is true for guitar through distorted amps, but not for bass through a clean rig. linear volumes work perfectly even from "0" to "10" on basses.)
  11. I suppose going to a 500k may be goofing up the Bart circuit. I suggest trying the 250k.
  12. FunkMetalBass


    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    If you're using a 500k blend and a 250k volume, you aren't fixing anything. The blend is actually wired before the volume in the circuit, so you'd need to bump the volume to 500k as well, otherwise the 250k volume is just cutting off any of the signal that the 500k let through.
  13. jscomposer


    Nov 25, 2009
    I ditched the Bartolini harness for all-passive controls. I got a volume, tone, and blend, all 500K. I also threw in a 4-way rotary for choosing series/parallel combos. Everything works fine. I'm just not getting a smooth taper on the blend.

    Interesting. I'mma try adding resistors first. If that don't work, I'll try your suggestion. And if THAT don't work, I got an audio taper blend on the way from guitarpartsdepot. I had the guy on the phone confirm that it's audio--it says "A500K/C500K" on the pot casing. [​IMG]
  14. suraj


    Oct 1, 2008
    Mumbai, India
    Any update on this ??
  15. LowB-ing


    Aug 3, 2005
    EDIT: Removed, as I didn't read the quoted post closely enough
  16. JustForSport


    Nov 17, 2011
    I use audio taper vol and blend pots (which are already in most of what I have- there's a reason?) and have no sudden drop-off issues... everything is smooth and progressive.
    I really don't know why linear vol pots are recommended by some, and despite the explanations, there's a reason log taper pots are used to sound balanced to the human ear.
  17. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    try it :)

    with typical 250k audio volumes on passive pickups it's not as drastic, but it's still there, a quicker dropoff from "10".

    500k audio volumes really drop off fast, so 500k linear volumes are pretty much required if you want the brighter sound but still want a smooth sweep.

    (again, this is for typical clean bass; electric guitar through compression and overdrive is a whole different story, you do indeed want audio volumes there.)
  18. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    This was posted a good while ago but better to correct it late than never, I suppose. People do say that it takes 10 times as much power to sound twice as loud. However a 10 times increase in power is not 6dB, it is 10dB. An increase of 6dB is only four times the power, not ten. So every 10db increase in power level produces a doubling in the perceived loudness level.

    There are almost no true audio pots on the market so it is very doubtful that those in your basses or any other gear you own are true audio pots. Virtually all the "audio" pots you can buy are actually "bi-linear" pots. Typically they increase linearly in value up to 10% of the rated value at 50% rotation and then they increase linearly the remaining 90% of the rated value in the second 50% rotation. Over the whole range of rotation the "audio" pot does give you a smoother adjustment but near the top end of the adjustment range both a linear and an audio pot are linear and it is in fact that audio pot that drops off more quickly as it is turned down.

    Almost everyone finds that when used in a strictly volume control application the "audio" pot is preferred because the volume level of a linear pot increases too quickly at low volumes. There is less agreement when pots are used to blend pickups or adjust tone. One reasonable set of ears might prefer audio pots for these applications, another might prefer linear.
  19. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    maybe it's the definition of "volume control" that's different here.

    no argument about the pretty well-established logarithmic nature of power, dBs and human hearing, and audio volume pots do indeed sound the most even in many cases, like amplifier master volumes. (there's that ol' "music store amp" trick where the amp has a linear master volume, so the salesman can say "wow, listen to how loud it is and it's just on "3"!, while not mentioning that the amp doesn't get any louder past "5".)

    audio volumes are also the right thing for electric guitars through cranked-up amps, to put the biggest level change up where the amp is more compressed to make up for it. players who want to work the guitar volume for clean-to-crunchy transitions need audio pots.

    but for bass guitar through normal, clean bass rigs, audio volumes will have a noticeably fast dropoff from "10" and will almost disappear before you get to "1". this is especially true if you go to a higher-value pot to get more brightness.

    in this case linear volumes will have an even sweep from off to full, making them more useful for adjusting overall volume or the mix between two pickups from the bass.

    i've installed probably hundreds of these over the last few years, and it's not even a question, that is simply what actually happens.

    (oh, and linear passive tones are universally useless, they do nothing, then act like switches when you get down to almost "0".)
  20. gumtown


    May 7, 2007
    New Zealand
    My bad, You are absolutely correct !! :bag:
    Must have been late at night when I had posted that.
    What I meant was a 3dB increase will double your watts,
    and a 10dB increase will double your loudness at 10x more power (watts).

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