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Proper taper for onboard preamp EQ pots

Discussion in 'Pickups & Electronics [BG]' started by ric426, Jan 26, 2014.


  1. ric426

    ric426 Supporting Member

    Joined:
    Mar 27, 2002
    Location:
    Southeast Michigan
    I bought some alpha pots from Mouser that are 50k linear taper with center detents and have them on a Nordy preamp. The pots I used previously worked as expected, but these pots don't give much EQ change as you rotate in either direction from center until they're at least half way through the range in that direction.

    I'm getting another Mouser order together and I want to get some different pots for the Nordy and a Bart preamp that will hopefully give smoother EQ change. Any suggestions? The center detent would be nice but I can do without it if necessary. Any diameter will work. They don't need to be the miniature ones.
     
  2. mjmeche

    mjmeche Supporting Member

    Joined:
    Jul 14, 2007
  3. RobbieK

    RobbieK

    Joined:
    Jun 14, 2003
    I'd check with nordstrand which pots their pre requires. Many of the preamps I've designed use log (audio) taper pots for the treble and bass, and will have a very strange response with a linear pot.

    I would test the old pots with a multimeter. (If they still work of course.) You have to remember that the A,B,C, etc that is stamped on pots can mean different things for different pots. There's no formal standardization unfortunately.
     
  4. ddnidd1

    ddnidd1 Supporting Member

    Joined:
    Mar 31, 2003
    Location:
    Southern California Coast
    You need to check with the manufacturer. The pot value and type (audio/linear) will be dictated by the circuit design of the preamp.
     
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  6. line6man

    line6man

    Joined:
    Jun 20, 2008
    Location:
    Close to Los Angeles, CA
    I've never heard of anyone using log taper pots for EQ controls, unless they were for boost-only controls. And in that case, it is usually because it is hard to find linear tapers. But does anyone even manufacture log taper pots with center detents? It really isn't a good idea to use them, because the level of the boost or cut will be greatly unbalanced, with the assymetric nature of log tapers.

    Perhaps you should consider different resistances, rather than different tapers.
     
  7. ric426

    ric426 Supporting Member

    Joined:
    Mar 27, 2002
    Location:
    Southeast Michigan
    That's what I thought too, especially with the center detents. I know the pots that were on the preamp when I got it were 50k center detent pots. They came in a bass I no longer have though. The original pots stayed with that bass, wired to a different preamp.

    I saw a reference to a W taper pot in the Mouser catalog, but no info on what it is. I'm wondering if it might be a center detent pot with log taper in each direction. That would explain the way the preamp behaves with these pots. I've got a stacked Noble 50k pot that I think I'll wire in temporarily to see how it works with the Nordy preamp.

    Here's a long shot. The Nordy preamp is currently wired without a volume or balance control. Is it possible that the circuit behaves differently without those on it's input?
     
  8. walterw

    walterw Supportive Fender Gold Supporting Member

    Joined:
    Feb 20, 2009
    this is the correct answer, anything else is really a waste of time.

    still, if a current linear center-detent pot does too little until you turn it all the way over, try one that's higher resistance.
     
  9. ric426

    ric426 Supporting Member

    Joined:
    Mar 27, 2002
    Location:
    Southeast Michigan
    Both Nordstrand and Bartolini spec 50k linear for all EQ pots. I guess what I was getting at is whether anyone has found that certain brands of pots (Alpha vs. Alps vs. Noble, etc.) seem to work better with these preamps than others. I would think a linear pot is a linear pot, but stranger things have happened.
     
  10. RobbieK

    RobbieK

    Joined:
    Jun 14, 2003
    Yeah, as I said, you need to check them with a multimeter. Something marked 50K will usually be anything from 45K-55K, and often even further off spec than that. There's not generally an indication of tolerance on the pot itself like there is with fixed resistors, and data sheets aren't especially helpful, especially for cheap alpha pots etc. In those type of circuits, the higher the pot value, the more cut and boost you'll get. The other reason you should check them is that you may find that the wiper moves a little way before it hits the resistive track, so the set resistance is actually squeezed into a slightly shorter rotation. This also means that one-quarter/three-quarter rotation resistances (where most of the adjustment usually happens of course) can be quite different from pot to pot, even of the same brand and spec. You may also find that pots of different brand don't actually rotate as far from end to end. This means of course, that the resistance changes more rapidly as you turn your knob.
     
  11. ric426

    ric426 Supporting Member

    Joined:
    Mar 27, 2002
    Location:
    Southeast Michigan
    Apparently, all linear pots are definitely not equal. Digging through some parts boxes I found a couple of 50k center detent pots and tried them in the preamp. Those are working as expected.
    I won't be using those Alpha pots any more. I wouldn't be surprised to find different log tapers between brands, but I assumed that a linear taper would be, well, linear. Such is not the case.
     
  12. RobbieK

    RobbieK

    Joined:
    Jun 14, 2003
    Yeah, they're not actually log unless you spend serious $$. They are 2 or 3 linear sections that sort of copy a log base 10 curve. But for all but the serious studio gear, it doesn't really matter. Actually a lot of hi end stereo gear doesn't even use pots for volume, but a fancy rotary switch with carefully matched pairs of resistors. It's called a stepped attenuator.


    And to be honest, if your new "linear" pots are all pretty close to 50K, even if they have a slightly different sweep, the pre will work fine, and you'll just get used to the different feel after a week or two. Alphas are generally a cheap and cheerful option, but I've used them for years, and as long as you have good soldering technique, they are fine.
     
  13. ric426

    ric426 Supporting Member

    Joined:
    Mar 27, 2002
    Location:
    Southeast Michigan
    I know what you mean about good soldering technique with the Alphas. I like to tinker with different configurations in different basses and have found that the Alphas don't hold up well to being soldered and unsoldered, even with good technique.

    I've finally made things easier by setting up several different preamps as outboard boxes with full bypass footswitches so I can select and compare different preamps at will. The three preamps I'm using at the moment all use 50k linear pots, so I should standardize on one brand and model of pot for all three circuits to remove that variable. This is all for my own curiosity and experimentation. I wouldn't bother doing it in a live situation because the differences are subtle and would get lost in a live mix.
     
  14. RobbieK

    RobbieK

    Joined:
    Jun 14, 2003
    I've been soldering since I was 10, but I still use aluminium clips on the rivets that hold the lugs on pots, and I generally use far finer wire than that typical guitar stuff. I also avoid soldering up to the pot casings. With active setups especially, you are better off running the earths to one place, say the big solder lug on the jack, anyway. And you can earth the pot casings for shielding via a strip of copper tape. The innards of cheap pots is simply abs plastic. It'll soften and distort at like 60 degrees or something. It's pretty much only guitar wiring diagrams that show the pot casings with wires soldered to them so unless a pot is specifically designed for guitars, they aren't manufactured with this in mind. If I really have to solder a pot casing, I polish the area with a little steel wool or emery paper first, then crank my iron up to max so I can get it over and done with as quickly as possible.
     

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