Two Tone Pots in active circuit. (assistance needed)

Discussion in 'Pickups & Electronics [BG]' started by Ross Xod, Oct 14, 2016.


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  1. Ross Xod

    Ross Xod

    Oct 16, 2014
    Portland, OR
    Hi All,

    I'm curious if anyone has designed or worked with active circuitry that has an individual tone control for both pickups.

    Is this technically possible? I heard a luthier mention the possibility to rolloff the treble on one pickup and use the treble on the next pickup to create a new tone combination.

    I'm curious to create that sound, as I love the bridge pickup in a jazz bass configuration but I'd love to add the low end of the neck pickup and have the ability to cut off the treble on just that pickup.

    I'm planning to be working with a sadowsky pre and pups on a 4 string jazz.

    Thanks! - Ross
     
  2. testing1two

    testing1two Supporting Member

    Feb 25, 2009
    Southern California
    Yes this is possible but you would need to use the "parts only" version of the Sadowsky preamp and you would also need to order two 250k/250k stack pots to replace the Sadowsky's master volume, passive tone, and pan pots.

    You're basically doing early '60's stack knob jazz bass wiring scheme but instead of the output going to the jack, it's going to the preamp. There are potential loading effects with this type of wiring so you'll see some resistor/capacitor circuits wired across the pots to compensate. Do some homework on this as there are a couple of different methods.

    60s Stack knob Jazz Bass wiring?
     
  3. walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    it would really only work with the pickups in series, or with some sort of dual preamp setup.
    that specific trick actually works really well with the pickups in series.

    series creates a big low end boost, and also allows a tone control to only work on one pickup.

    combine these two things and you can indeed have one pickup fully bright, roll off the highs on the other pickup, and have the lows of both pickups combine to get the series bass boost while still hearing the highs of one pickup.

    it actually works both ways, you could add low end to the neck pickup the same way.

    i even have a diagram for it:

    here's the thread where we talk about this very thing.
     
    Will_White likes this.
  4. In the interest of practicality, I doubt that this would be very useful, despite sounding like a good idea, on paper.

    What you want is a dual input preamp. Or simply two preamps in parallel.
     
  5. Ross Xod

    Ross Xod

    Oct 16, 2014
    Portland, OR
    Thanks guys! I've been waiting for my guitar to come in before I start on this.

    So far I'm thinking to go with the two preamp route, although it will be twice as expensive as I planned.
    But I'm creating a versatile bass for myself, so might as well..

    My next questions is - If I run two pre's, how do I sum the signals? Can I have two outputs as well on a summing switch?
     
  6. There are a few ways to do this.

    The passive way is to sum the pickup outputs with a couple of resistors from your blend or two vols, before the preamp. Search around, this has been covered many times on this board. People think you have to go with the original stacked knob jazz circuit, but not so.

    You could also use a very simple circuit to buffer the pickups individually. This would also make blending them a little better. Plus you have the option of using a single gang pot as a blend. This means you can stack it with say your volume control. If you (or a friend) can read a schematic and make it, I could post one.

    You could use a commercial active twin buffer circuit, like emg ABC.

    I know you mentioned sadowsky passive pickups, but you could also simply use say, emg's instead. These already contain the buffers so, with the right blend pot, the tones will be isolated.

    Having said all this, I'm afraid it doesn't really work like that. When you combine two pickups, the signals are (more or less) summed. I doubt you'll be able to hear which pickup has had its treble rolled off. Especially a couple of almost identical jazz pickups.

    But I still say go for it. I think the idea works ok with a blend pot. For instance, you can have the treble rolled off the bridge pickup, but right up (full bright) on the neck pickup. Then you can use the blend almost as a mid-range control in reverse.
     
  7. Ross Xod

    Ross Xod

    Oct 16, 2014
    Portland, OR
    Robbie, I can kind of see what you mean, but I don't know if you should take time drawing up a passive circuit for me.

    So after doing my research on sadowsky pres, I'm confident in using that specific pre because it alters the tone of even stock jazz pickups to give it sadowsky character. That is specifically something I like in their guitars, the sound.

    So I think I'll keep it with the sadowsky pre.

    Now id love to go towards having the signals have their own eq and tone adjustment and then sum after the pre. Of course my knowledge of circuits is not too advanced so I'll need some help past the pre. I assume it can be possible to take two amplified signals and sum them into one, am I thinking correctly?

    I also don't mind saving almost 300 bucks and going with one pre if necessary or is an attractive option.

    I also would like to keep the passive function if I can. Just to run the signal without active amp. That may add some complexity. Maybe.

    Advice on this side of the pre? :)
     
  8. Well, if you use two whole preamps, then simply run each pickup to a 250K vol pot, then to each preamp, then sum the outputs with a couple of resistors. Without knowing the actual output z of the sadowsky pre, I'd think a couple of 22K to 47k resistors would do the trick. Bare in mind, you will lose a little output this way, but that is the nature of passive mixers. Use 1/4 watt or 1/8 watt metal film resistors. They'll only be a few cents each.

    If you want a blend control for the two-pre set up instead of two volumes, the pot would have to be after the preamp, and I'm guessing you'd use a dual gang 50K linear pot. This is not going to work with a passive bypass switch, so the two-pre with passive bypass setup would have to be with two vols, not single vol and blend.

    For the passive bypass, you'd need to either use a 4 pole switch (toggle or rotary) for true-bypass on both preamps. Or if you use a push-pull, you will have to settle for a non-true bypass on both preamps, but really, a fet preamp like the sadowsky should have a very high input z, plus you are only using one pickup per pre, so I doubt you'd hear a difference compared to true-bypass.



    If you decide to run one preamp, but have two individual (passive) tone controls. Simply sum the signals from your pickups with a couple of resistors. The tone pots, coming also form the individual pickups, will have a good degree of isolation with the right sized resistors. IME, with typical single coil jazz bass pickups, a couple of 15K resistors should do the trick. You can do this from the clockwise lugs of the two volume pots, or if you use a blend, where the hot comes off opposite lugs from the blend.

    For bypass, you are using just a single pre, so it is just standard.
     
  9. Ross Xod

    Ross Xod

    Oct 16, 2014
    Portland, OR
    Ok, so I like having 2 volume pots over a blend. So lets remove the blend from this equation. Which will be easier to wire? A push pull or a switch? I prefer switch for use, but don't mind a push pull wired with passive pulled out. If its not too much work, is it possible to draw up a schematic of this to visually see both options? Thanks!
     
  10. Doner Designs

    Doner Designs Steve Doner Gold Supporting Member

    Jun 2, 2012
    Metro Chicago Area
    Doner Designs is an alias for Steve Doner
    I once ordered a Siamese twin preamp from Audere as a custom. It used all stacked pots and each stack had one control for each pickup VV, MidMid, TT, BB.

    Normally I don't think that level of complexity is necessary, but it was ideal for the piezo/magnetic bass I used it on.
     
  11. Clark Dark

    Clark Dark

    Mar 3, 2005
    earth
    I bought this:
    ACG EQ-01-4K Preamp, 2-Pickup, 4-Knob without knobs - Best Bass Gear
     
  12. Well sure. But do you want two whole preamps? or one preamp and two passive tones?
     
  13. Primary

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    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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