Resistors in line between pickups and blend pot

Discussion in 'Pickups & Electronics [BG]' started by Spfairchild, Apr 10, 2014.


  1. Spfairchild

    Spfairchild Supporting Member

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    I built a dual pickup buffer a while back, and through the process learned about using "mix" or "summing" resistors on a pickup's hot lead to keep two pickups from loading each other down in a way that made the blend function more like a 3-way switch. I've managed to get this working just fine in one bass.

    I recently put 47k resistors in line with two EMG 45TW pickups and a 250k MN blend pot to tame the blend curve, which has worked well. The pickups reportedly have a 10k impedance output (these are the older, non-X type. The X type have a 2k output). I have kept the blend pot ungrounded, which also helps to keep the blend more gradual and doesn't provide a ground at this stage that can bleed off high frequencies. The blend pot feeds into a 250k audio taper master volume pot. The volume feeds the input of an Aguilar OBP-3 preamp.

    As far as I'm aware, putting resistors in series between your pickups and blend pot like this should only cause a reduction in volume, which was fine because this bass is still far hotter than the rest of mine, even after the mod.

    But would there be any effect on frequency, connected this way? Have I lost some high end? I didn't think I did, but the bass is new to me and I may not have noticed a subtle change. Electronically speaking, have I done anything to attenuate a certain band of frequencies as opposed to just attenuating the overall output level of each pickup?

    Thanks for the help!
     
  2. line6man

    line6man

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    Aren't those pickups active? If so, they are buffered, so there is no need for summing resistors, because the coils will not load each other.
     
  3. Spfairchild

    Spfairchild Supporting Member

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    Hi line6man,

    That's what you'd expect, wouldn't you! Yes, they are active pickups. But when I tried them with the blend without the resistors, I got total 3-way switch action. No real blend at all. And in fact this was also the case with the buffer I built!

    Prior to that project, I had expected that anything active could be mixed nice and smoothly without issue, but agedhorse (from Genz Benz) and others helped me to see that this actually isn't the case. With my buffer, I had an output impedance in the hundreds of ohms, as opposed to 10k like these pickups, but I still had to use, I think, 6.7k resistors on the buffer's output channels to get the blend to work smoothly.

    I don't fully understand the mathematics of it, and I was kind of bummed when I made that buffer because the whole reason I did it was to allow for a smooth blend - I guess I could have just forgone the buffer, used resistors with the stock passive pickups, then used a single channel booster before or after the preamp!

    Anyway, the resistors still seem to be necessary. So back to my original question, would connecting resistors in this way (pickup hot lead -> resistor -> blend pot center lug with outer lugs ungrounded -> master volume -> preamp) cause anything other than just volume loss? Would this lessen the high frequency response at all?
     
  4. line6man

    line6man

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    Note also that 250k pots are not appropriate for low impedance active pickups. You want 25k, 50k, or perhaps 100k.
     
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  6. Spfairchild

    Spfairchild Supporting Member

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    I thought the same, until I realized that the output impedance of these pickups is much higher than what would come out of say an active preamp - 10k is not that small. And going be the rule of 10x the output impedance for the next stage, I figured that actually 250k wouldn't be that far off. 10k is pretty much the same or higher than a traditional passive pickup, isn't it? And those typically use 250k or 500k blend pots and vol pots.

    But thank you very much for your help, I'm not trying to shoot you down! I'd like to learn more and figure this out.
     
  7. khutch

    khutch Praise Harp Supporting Member

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    Connecting the outputs of two active devices together is generally problematic. Under the right circumstances it can do wonderful things but I don't think any of those circumstances apply to a bass guitar. If you can't hear any high frequency loss after listening carefully then you have none. The 10k "impedance" of a bass pickup generally is only the resistive portion of the impedance. Depending on the frequency the reactance of the coil can be 10 times that or more and it is equal to 10k at a few hundred Hertz. So the impedance of a passive pickup is a lot higher than the frequency of your active setup and its increase with frequency contributes greatly to the high frequency loss produced by loading. Your active output will be fairly constant impedance over frequency by contrast. You still have some loss from cable capacitance but even that is milder than what you would get with a passive pickup.
     
  8. Spfairchild

    Spfairchild Supporting Member

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    Thank you khutch! Great info. Yes, of course - I forgot about the tendency of impedance to change based on the frequency it's measured at with passive components like speakers, pickups, etc. It makes sense that an active output impedance will pretty much be static in comparison. I appreciate the info, thanks for dropping some knowledge.

    I don't think I actually perceive a difference in frequency response with the resistors. I thought I heard some dulling of the tone while recording yesterday, but late last night I was practicing through my amp with headphones and the sound was very brilliant, so much so that I had to reduce the treble level I normally use. When I was recording I went directly into my interface, not using the amp's preamp and EQ to get my tone closer to what I'm used to, and I think this was the difference I heard that prompted me to wonder about what I may have done with the resistors.

    However, I will say, mix resistors work great! Everyone should try them :) Especially if you have a preamp that can add gain with the EQ flat (unlike the OBP-3) or if you don't mind using a signal booster like EMG makes! But I didn't even have to settle for much gain reduction at all, in this case. When you get it right, the blend pot really becomes a very useful thing, just like panning from L to R on a stereo - nice and even.
     

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