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Blend Pot from 250k AC to 500k MN - WOW!

Discussion in 'Pickups & Electronics [BG]' started by pogipoints, Jul 17, 2012.

  1. pogipoints

    pogipoints Custom User Title Holder Supporting Member

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    I finally got around to changing out the blend pot on my Elrick Gold NJS - Bart X4 pickups, NTMB918 preamp. The original pot was 250K AC, and there was quite a volume drop when at 50/50.

    I'd been reading through some of the older threads and walterw and SGDLutherie (sp?) recommended 500k pots, ungrounded. So I followed their recommendations and bought the Bourns 500K MN pot.

    As expected, being MN, the volume drop at 50/50 is now practically gone, and there's no noticeable difference in vol between neck, bridge and 50/50.

    What I did not expect was the VERY noticeable increase in clarity, I assume due to the pot being 500k vs. the original 250k. This saved me quite a bit of $$ since I had been contemplating changing out the preamp for something "brighter". Now I don't need to change my Bartolini preamp, and it's giving me a much clearer tone.

    Thanks guys! :bassist:

    Added: Link to referenced thread: http://www.talkbass.com/forum/f38/blend-pot-250k-vs-500k-issue-731261/
  2. SGD Lutherie

    SGD Lutherie Supporting Member

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    Owner, SGD Music Products
    M/N taper gives you full volume at the center. A/C does not.

    Even grounded M/N blends work great.
  3. pogipoints

    pogipoints Custom User Title Holder Supporting Member

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    Thanks David, your posts on the other thread gave me the idea to change the pot years ago, I just never got around to doing it till now :rollno:

    I'm still surprised by the difference in tone between 250k and 500k blend pots. That was shocking! The difference was almost as drastic as changing pickups.

    BTW, do you make pickups in the Bart X4 size? Seems nobody has this size except Bartolini and Delano (with different screws).
  4. Jim Baritone

    Jim Baritone

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    David,
    On my current EB0 project, with a Gibson humbucker at the neck and a Fender JB pickup at the bridge, would using the Bourns M/N blend pot with master volume, rather than a 3-Way switch and independent volume for each pot, be a better overall layout?

    I realize that the tone cap & coil series/parallel switches would need to go in ahead of a blend-balance pot. I'm just wondering if you think it would be a workable layout, with the two different pickups? Your advice would be appreciated.

    Thanks,
    Jim Bari
  5. LukeFRC

    LukeFRC

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    Interesting. I have Barts in my warwick and need a new blend pot too. Warwick recommend 220k MN leading into a 500k volume pot.
  6. pogipoints

    pogipoints Custom User Title Holder Supporting Member

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    If you already like the current tone, I would recommend staying with the same value blend pot, but MN if the original was AC (as on mine).

    I was looking for a brighter tone (my Barts are the J1s, not Classic Bass) and was actually considering a new preamp (Demeter, Audere), but decided to try 500k pots first.
  7. SGD Lutherie

    SGD Lutherie Supporting Member

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    I think the difference was more about going from A/C taper to M/N. A 250k M/N will sound pretty much like a 500k M/N.

    No, I don't, sorry.
  8. SGD Lutherie

    SGD Lutherie Supporting Member

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    It should work fine. The two pickups might not be at equal volume when in the center, but they wouldn't be at equal volume with two volume pots on 10 either. But you can vary the mix as you like with the blend.
  9. LukeFRC

    LukeFRC

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    at the moment it has no preamp and no pots! I got new pups and block preamp- hated the preamp so sold it. The old one would go in but needs a new blend pot and the old one was 25k for the active pups. So need a new blend pot.... or try a new preamp. Erring towards just a new blend pot- they aren't as easy to track down over here though- a MEC replacement would cost about £40 ($60+) and the cheaper replacement often don't seem to be sure if it's an MN one or not. :)
  10. pogipoints

    pogipoints Custom User Title Holder Supporting Member

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    Really? That's good to know. Now that I think about it, they're both at 100% and not grounded so there's no loading at all when at 50/50. And thus would not matter whether they're 250 or 500k.

    The tone difference does seem similar to running a passive V/V at both 70% or something, which was effectively what was happening with the A/C, am I correct?

    Sorry for the many questions, I'm the type who needs to understand exactly why these things work the way they do :bag:

    To LukeFRC, strike my last post, seems blend pot value doesn't have much to do with tone.
  11. walterw

    walterw Supportive Fender Gold Supporting Member

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    Only if they're run ungrounded like you did; otherwise they affect the sound like any other pot.
  12. pogipoints

    pogipoints Custom User Title Holder Supporting Member

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    Awesome, thanks for the info! This'll definitely be put to good use when I rewire another bass to passive V/B/T.
  13. LukeFRC

    LukeFRC

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    why would you or wouldn't you run it ungrounded? it does something to how they load the pickups doesn't it?

    anyone recommend a good place to get a good blend pot too? (that would post to the UK)


    (as in someplace that actually know what they are selling)
  14. vaesto

    vaesto

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    Hi,

    Wonder if there is any reason to leave 25k M/N blend pot (for the active EMG) ungrounded? Or should I ground it and don't borther?

    Regards,
    Vaesto
  15. walterw

    walterw Supportive Fender Gold Supporting Member

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    try it and see.

    ungrounded you'll get no loss, and more gradual shading.

    the drawback is, you might also get insufficient "throw" over to one pickup, as the other may not be sufficiently removed from the sound.
  16. chapito

    chapito Supporting Member

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    Hi i’ve been reading along in prep for my first mod experiment and I am trying to grasp the ungrounded pot idea.

    So if you wanted the same load as three 250k pots (V/V/T), i would use a MN500k blend pot grounded or a MN250k ungrounded?

    and using a blend ungrounded won't affect tone?

    if you unground a MN500k do you have to use a a500k vol to give the same (VVT) load?

    It seems like a MN 500k blend pot is a very flexible solution(grounded or ungrounded).
  17. walterw

    walterw Supportive Fender Gold Supporting Member

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    Ungrounded, the M/N presents no load, so you only have the volume and tone pots in the equation.

    Grounded, 2 500k pots should equal one 250k, so if you wanted the same sound as stock V/V/T jazz bass wiring, 500k blend grounded should do it.
  18. zamsden

    zamsden

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    So I'm doing the same blend thing with guitar pickups. Seeing as guitarists don't do this sort of thing often, but lots of basses do offer blend pots, I'm hoping I can get advice here.

    I was going to wire it up grounded, but then I saw in the schematic the ground problem.. 500K blend pots won't work grounded, even with 1M control pots you are still loading the pickups. Seeing as this is for passive humbuckers where 250k loading is the norm, I thought I was out of luck... nobody seems to make a 1M blend pot (see sidebar below). Then I saw this thread, and decided to give the ungrounded approach to blending a try.

    I actually modelled the thing in circuit lab (link: https://www.circuitlab.com/circuit/9qm9e8/guitar-custom-wiring-blend-pot-mod/) and tried setting both pots to 500k to see how series resistance affected output. It didn't show a significant DB drop!!! I had to up the series resistance up to significantly more than 1M to get a significant DB drop.

    Maybe I'm modeling the pickups as being too hot (quite possible), but I was wondering if you really can hear a far right and far left pan and kill the sound of the other pickup with the blend pot, or if you think I might expect this to work differently on a guitar (with a really hot bridge pickup).

    Does the series resistance add any hum or other unpleasantness to your signal?

    *Sidebar on 1M pots: got some custom pots from Alpha today that I was intending to use as push / pull control pots in this guitar. I was sadly disappointed to measure the maximum resistance at 846 and 906 kOhms. At least they are 10% audio taper, but otherwise not very pleased with Alpha pots. Unfortunately, Bourns doesn't make a 1M M/N taper pot, and I don't think they would build a single pot for me..

    So I am thinking of ditching this control mechanism and going for a dual volume setup instead, where I control each pickup volume independently, this way I can get the full range of in-between tones without signal loss. However I would really prefer to dial in the blend I want, then adjust master volume and trim highs as needed, rather than try to mess around with multiple volume knobs.

    Any advice appreciated.

    P.S. I also own a bass, this makes it ok for me to post here right? :bassist:
  19. walterw

    walterw Supportive Fender Gold Supporting Member

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    who says 500k blends won't work grounded?

    also, something must not have been added up right, because even 250k M/N pots ungrounded will significantly reduce the volume of a pickup when turned one way or the other. heck, even ungrounded 25k M/Ns will work to a large extent, though they still leave a good amount of the "off" pickup in the signal path.

    the next thing i wanna try is a 25k M/N run ungrounded, but modified by cutting the traces to make it "no load" at each extreme, so it disconnects the "off" pickup entirely.

    i think that will give me even better shading between the two in the narrow range where that matters, while still shutting one pickup off 100% like a switch.

    get out!
  20. zamsden

    zamsden

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    Math. Initially I thought, oh one 500k blend pot, 1M tone, 1M volume, I'm still at 250k ohms above ground.

    Even if I upped my tone and volume pots to 1M (which I can't, mine don't spec out to 1M), the actual resistance is going to be 1 / (1/500.0 + 1/500.0 + 1/860.0 + 1/906.0) = 159K Ohms. That's a lot less than the 250K I was at before, and I'm pretty sure that is going to cause significant volume loss. Simulating that circuit shows quite a bit of dB loss. I even noticed a minor tone change with the push/pull pots I installed before which came in slightly below 500k (which the stock pots were closer to). The Bourns blend pot I have is almost 500k dead-on on both wipers!

    I was thinking about doing a partial ground on the blend pot, maybe to totally kill the treble pickup - it is very hot, I'm not sure 500k resistance is going to kill it completely. This would get me back up to 234K ohms, which is probably acceptable.

    Would still love a way to completely isolate one pickup, but all the dual volume approaches I have considered still have issues with treble roll off when turning down the volume, and the standard way to compensate for that (bypass cap on the master volume) doesn't work the same on a dual volume.

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