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Potentiometers for a P/J Project…

Discussion in 'Pickups & Electronics [BG]' started by Malak the Mad, Jul 11, 2014.

  1. Fernando Costa

    Fernando Costa

    Aug 4, 2013
    Thank you Arvin for the clarification. This "thing" about pots always confuses me.
  2. adi77

    adi77 Banned

    Mar 15, 2007
  3. WillieB

    WillieB Battling Bass Guitar Bulimia since 1975 Supporting Member

    Apr 26, 2010
    Columbus Ohio
    I also have been checking out http://www.guitarelectronics.com/

    I had audio and linear reversed in my head as well.

    Potentiometer Tapers
    The taper (also called "law") of a pot is important. We need not worry with trimpots, since they are almost always linear, and I do not know of a supplier of anything other than linear trimpots. For all panel pots, we must be aware of the use the pot will have, and select the correct type accordingly.

    The most common use of a pot in audio is as a volume control. Since our hearing has a logarithmic response to sound pressure, it is important that the volume control should provide a smooth variation from soft to loud, such that a given change in position of the pot causes the same sensation of volume change at all levels.

    Figure 4 - Potentiometer Tapers
    First, the term "taper" needs some explanation. In the early days, when an audio taper (logarithmic, or just log) was needed, the resistance element was indeed tapered, so that it provided a different resistivity at different settings. By changing the physical taper, it was possible to make a pot provide the exact gradient of resistance needed. By definition, a linear pot has no taper as such (the resistance element is parallel sided), but the term has stuck, so we might as well get used to it.

    The violet curve in Figure 4 shows an antilog or reverse audio taper pot. These are quite uncommon, but used to be used for balance controls using a log/antilog dual section (commonly called dual gang) pot. It is shown on the graph mainly for its interest value, but they are generally an historical component now.

    All this tapering proved a rather expensive exercise, so manufacturers economised ("they" won't notice the difference!), and worked out a method of using two resistance elements of differing resistivity, and joining them to create what I referred to as the "Commercial log" taper. In short, it doesn't work (not properly, anyway), and the discontinuity where the two sections join is almost always audible with cheap "log" pots.

    Project 01 showed how this can be fixed, and I will explain the logic a little more as we progress. In the meantime, I suggest that you get an old pot and dismantle it so that you can see exactly what is inside. I could show you some photos, but there is nothing like doing it yourself to really get to know the subject.

    Pot Markings
    Now, this should be dead easy - a simple code to indicate the resistance and law of a pot should cause no grief to anyone, right? Wrong! It wouldn't have been so bad if someone hadn't decided to change it, and even then, it wouldn't have been so bad if there was no overlap between the "old" and "new" "standards" ... I think you can see where this is headed by now.

    TaperOld CodeNew CodeAlternate
    Log (Audio)CALOG
    Wasn't that a nice thing to do? It is obviously important to check before you make assumptions, or you can easily get the wrong type - especially if working on older equipment.

    At least the resistance marking is usually sensible, so a 100k pot will be marked as 100K - but not always. The coding system used for capacitors is sometimes used as well (especially on small trimpots), so a 100k pot could also be marked as 104 - 10, followed by 4 zeros, or 100000 (100k) ohms.

    Because they are variable, there is a much smaller range of potentiometer values, almost always in a 1, 2.5, 5 sequence. Common values for panel pots are 1k, 5k, 10k, 25k, 50k, 100k, 500k and 1M - 2.5k and 250k went missing along the way, and these are not stocked by very many distributors. 25k pots are becoming harder to get as well. Not all values are available in log and linear, and in some cases you may even find that for a particular type, you can get them in any value you want, as long as it's 100k (for example).

    Trimpots suffer a similar fate. The only way to know what you can get from your local supplier is to check their catalogue. In reality, everything is available, but you may have to go a very long way to get it.

    from here: http://sound.westhost.com/pots.htm
    Fernando Costa likes this.
  4. Malak the Mad

    Malak the Mad Over the River and through the Looking Glass Supporting Member


    SunnBass likes this.
  5. Malak the Mad

    Malak the Mad Over the River and through the Looking Glass Supporting Member

    Last edited: Jul 15, 2014
  6. WillieB

    WillieB Battling Bass Guitar Bulimia since 1975 Supporting Member

    Apr 26, 2010
    Columbus Ohio
    lol awesome gif there malak the mad ! might have to lift that one.

    My luthier told me that the soldier-less was an inferior to soldiered connections ...
    can't say. I've used the EMG connections that were pin based and it sounded fine to me.
  7. LoveThatBass


    Jun 28, 2004
    I am with you...I had same results. The ear doesn't work in linear fashion. It is that reason they made Logarithmic (audio taper) pots. I have heard you can use Linear on the tone control pot.
  8. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    sorry, that's exactly all backwards.

    the ear is indeed log, not linear, but you don't plug electric basses into your ear!

    if all the transition happened at the very top of the sweep, that's what audio pots do, not linear.

    and linear tone pots have the opposite problem, nothing changes until the tone knob is almost all the way off.

    it's linear volumes and audio tones for smooth, even control on basses.
    etherealme likes this.
  9. iiipopes

    iiipopes Supporting Member

    May 4, 2009
    jI would encourage the OP to not feel bad about the tech stuff being baffling. We all start somewhere. I started as a boy when my folks' office was next door to a TV repairman (back when TV's had tubes, so they were actually reparable), and I would bug him with questions. Then Radio Shack had these hobby kits that you could make various toys, from beepers to small radio receivers, all for an incredibly low price for the time, and all mounted on a plastic frame so you could see how everything fit together. Even with the internet having so much available information, it has come at a cost of the hands-on opportunities to learn bassics (pun intended) are less than they used to be, especially since most consumer electronics are of a unitary construction, so you don't repair them, you toss them and get a new one.

    Several places on the internet have pictoral diagrams to help lay out and solder the components together. When all else fails, start with the standard 250 kohm audio taper CTS pots (in my 40 years of experience of soldering, they can take the abuse of multiple solderings and unsolderings better), a couple of .047 microfarad capacitors, and a small spool of stranded wire to practice removing insulation and soldering a joint with a good iron, solder and flux. Then as you want to experiment with different tonal variations on the theme, you can substitute a component at a time until you tailor the circuit to the tone you desire. That's what I did to come up with my favorite circuit for the way I play, over the course of several years.

    EDIT: I just now looked more closely at the OP's picture of his bass. Being it is a P/J, but mounted on a J-bass body, the P pickup is a little farther upstream than on a P bass. This will give more low end and a little bit more output than a pickup in the standard P position, due to wider string excursion. This may be one reason the pickup seems to have more output than the J-bridge pickup. Since it is a quarter pounder, with the larger magnets, it can probably stand being lowered slightly to even the output of the pickups without losing character of tone, and/or raise the bridge pickup slightly, or a bit of both.

    I also agree that 25 kohm and 250 kohm pots seem to be only available where electric and electronic instruments are a primary item of an establishment's business.

    Oh - and just to confuse the issue - RIC now uses 330 kohm pots for just about everything except the 5th knob on the 300 series instruments.
    Last edited: Jul 16, 2014
  10. Arvin

    Arvin Underwound Supporting Member

    Sep 26, 2008
    On the bench
    ^ This

    Here's another way to look at it:

    On a V/V/T bass, e.g. a Jazz, the two volume controls are essentially used to create blends of the two pickups. With audio taper pots, all that blending happens at the top of the sweep, and the subtle nuances are harder to dial in. With linear taper pots, the blending is spread out over a greater arc of sweep of the volume knobs, and it becomes much easier to create the blend you want. The first time I re-wired a Jazz with linear taper volume pots, I was floored by how much more intuitive it was to use, and what a big improvement it was over audio.

    On a Master Volume/Master Tone bass, e.g. a Precision, there's no blending happening -- with only one volume control, there's nothing to blend. Most of us probably run the volume at full up 95% of the time, then turn it all the way down when we take a break. Occasionally, in the middle of a song, we might turn down a bit, but probably not all that often (gotta keep up with those drummers...). Many players prefer to leave the volume full up, and manage volume dynamics through playing technique. For this reason, I find an audio taper volume pot to be acceptable on a V/T setup, though I still prefer linear -- a small tweak of the control results in a small tweak in volume, not a big one.
  11. Malak the Mad

    Malak the Mad Over the River and through the Looking Glass Supporting Member

    I've sent out an email to Larry at Elek-Trix. I'll let y'all know if he has anything interesting to add to the discussion.

    Thanks again for all the technical know-how, everyone. Please, keep it coming…exploding heads and all.
  12. Malak the Mad

    Malak the Mad Over the River and through the Looking Glass Supporting Member

    I got a reply a mere twelve hours after I sent an email to Larry at Elek-Trix. :thumbsup: Rather than trying to elaborate, I'll just post the salient points of our exchange.

    The main question; Is the series/parallel the best choice for what I've put together? To elaborate a bit, I'm trying to achieve a somewhat aggressive tone but without sacrificing the deeper fundamentals. Also, I want to try to keep the Quarter Pounder from completely overwhelming the bridge pickup. If the Jazz pup isn't contributing to the sound, I may as well just have the single neck pup, y'know what I mean?

    I saw on an earlier blog post of yours that you allow for a choice between 250k and 500k volume pots, but there doesn't appear to be a choice given on the purchase page. Is that option still available, and if so, which values do you recommend for these pickups and what I'm trying to get out of them?

    I think it would be good for that aggressive sound you are looking for. It still provides the traditional vvt setup with the switch down. The 250k to 500k option is for the non bass kits. But for the nextgen bass products I think I will include this option. You can play around with cap values to determine the best sounding one which should help you out. Also if you do order it I will send you my version of a varitone to try out for 14 days. No cost to you unless you decide to keep it

    What's really cool is that I never asked about the Varitone. This was completely his idea. So for now, I'm gonna give their series/parallel Jazz circuit a try. I've heard a lot of good things about such wiring jobs 'round these parts. This looks like a nice and simple way for me to give it a go. I will, of course, report back on it once I've given it a proper shakedown trial.

    Thanks again for any and all feedback, info and tips! Y'all rock! And if you have an idea or some unique experience, feel free to continue posting to this thread. :thumbsup:
  13. wild4oldcars


    Jan 22, 2012
    Garner, NC
    I like to use log tone pots, but on a vvt setup, the volumes should be linear because you are hearing a blend of the signal, and want that blend to actually represent the percentage you yield on the knob. A P bass, however, I would want a log volume, because it is only the one pickup.
  14. Volume pots on basses should indeed be linear. There seems to be a lot of confusion, with people continually parroting what they have heard about the ear's logarithmic perceptions of volume. While the ear perceives changes in a logarithmic fashion, you're not actually dealing with longitudinal pressure waves. Just the voltages of signals feeding a preamp gain stage. You want the pots to be linear, so as to spread out the range of control. Logarithmic tapers just cause massive jumps in volume, inconveniently placed right in the area you want to be adjustable.
  15. Malak the Mad

    Malak the Mad Over the River and through the Looking Glass Supporting Member

    Yay! The Elek-Trix wiring harness showed up! I'll see if I can carve out some time tonight to install it and give it a proper shakedown. Reports and reviews will follow. :hyper:
    Last edited: Jul 25, 2014
  16. Malak the Mad

    Malak the Mad Over the River and through the Looking Glass Supporting Member

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