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A preamp for each pickup and a stereo out

Discussion in 'Pickups & Electronics [BG]' started by Caridorc, Mar 3, 2021.

  1. Caridorc


    Jan 16, 2020
    Hi all! Watching a video on youtube about the bass made by Dan from Guns&Guitars for the Great Guitar Buildoff 2020 I had an idea.
    He made his electronics so that with a DPDT switch he can split the signal in 2, one for PU, so that using a stereo cable he can record the sound of each PU indipendantly.
    His configuration has passive PU and electronics, so I took his idea and thought about adding also a preamp for each PU so that each PU could be EQed indipendantly. This adds a lot of tone and also opens the possibility to plug "each PU" to a different amp, EQed specifically for that amp or for other reasons...the possibilities are infinite!

    QUICK DISCLAIMER: Dan's wirings for his build are for sale through his patreon and his website. I DIDN'T buy his schematics. I did like reverse engeneering with the info he gives in his video. So what you see here are schematics made entirely by me and are different from what he sells.

    So...I started from 2 schematics: the classic preamp wiring for one PU, and the "stereo output" wiring with 2 passive PU. And then mixed togheter.

    1PU, Classic preamp:
    preamp wiring.png

    2PU, Stereo out:
    2PU stereo passive wiring.png
    For this I have one doubt: what happens if I use a classic mono cable and set the switch to stereo? With my schematics the PU2 hot signal goes with grounds. Is that a problem or simply I hear only PU1?

    And for the whole project:
    wiring project.png
    As you can see the two ground wires coming from the batteries goes to nothing. That because there is a problem, and I would like some opinions.

    The problem is this: as you already know when using a preamp, you use a TRS output jack so that without the cable connected, the battery ground is separeted from the others. When the classic mono cable is inserted, due to not having the ring and the sleeve, but only the sleeve, you bridge togheter all the ground wires and give power to the preamp. This way when you unplug the cable the battery does not drain.
    With the stereo out wiring, both the ring and the sleeve are needed for the 2 signals, so the batteries ground wires cannot be soldered separately using only the TRS output jack.

    My first thought went to the output jacks that are used in smartphones. Because the headsets have also the mic, they are TRRS. But I couldn't find a 6.35mm TRRS outputjack. Only 3.5mm.
    My second thought went to an on/off switch (maybe with also a LED indicator) to trigger the preamp, but if you forget to switch it off when you put away the bass then the battery drains.
    My third thought went to a momentary push switch to mount inside in the output jack area to be pushed by the jack cable. But I don't know if it is possible to mount that efficently.
    My fourth and last thought went to mod the classic TRS output jack to basically make it a TRRS.

    Now it's your turn...

    Do you like my idea or think that it's stupid? What solutions you suggest?
    lucas303 and MovinTarget like this.
  2. LetItGrowTone


    Apr 2, 2019
    Many (most? all?) active basses much be switched off and/or even when switched off must have the cable pulled out, so this alone isn't fatal to your idea.
    MovinTarget likes this.
  3. MovinTarget

    MovinTarget Supporting Member

    Jan 30, 2018
    Maryland, USA
    Right, they usually sidestep the need for a dedicated on/off switch (to save the battery life, this is diff than an active/passive switch) by using that stereo jack with the mono cables. As long as you build in your own kill switches or accept the batteries will bleed out even when not in use you can reclaim that channel.
  4. Crater


    Oct 12, 2011
    Dallas, TX area
    There are specialized switching jacks that would allow you to use a stereo output, and have an independent set of contacts that close when a plug is inserted into the output jack. Stew-Mac sells them.

    Some Music Man basses use this type of jack, because those models switch the positive side of the battery instead of the negative, ground side. These only come in the 'box' style, so if your bass has an edge mounted barrel type output jack, you'll have to figure out something else.

    Both preamps can be powered by a single battery, by the way.
    MovinTarget likes this.
  5. bpc


    Mar 29, 2016
    Central Scotland
    This is a totally mad idea. Wish I'd thought of it. :thumbsup:

    How about two output jacks like a stereo Ric? You could then use a combiner if you want to connect both together. You'll need to do some sort of conversion for a stereo jack anyway but two jacks would solve the battery problem.
    Last edited: Mar 3, 2021
    James Collins and MovinTarget like this.
  6. How about using a 3-pin XLR socket instead of a TRS stereo jack socket? You can use pin 1 for battery ground, pins 2 and 3 for signal and XLR ground for common ground. Connect pin 1 to common ground in the XLR connector on the cable. On the other end of the cable you could use a TRS jack, with a splitter for two TS jacks. Just an idea. It would involve modifying the bass with a XLR socket. Maybe a problem there.
  7. bpc


    Mar 29, 2016
    Central Scotland
    The two output jacks would allow you to use the bass, albeit one pickup at a time, with any regular cable. Using a stereo jack or an XLR would require you to use a special cable just for that bass in order to use it at all. Not saying which is a better solution FOR YOU, just making that point.
    MovinTarget likes this.
  8. dwizum

    dwizum Supporting Member

    Dec 21, 2018
    I love creative ideas. One thing I don't like, personally, is instruments that "need" special cables/boxes/external stuff in order to function. You may feel differently though.

    Because of that, for me, I think the clear winner would be two stereo jacks. One for each preamp/pickup, wired "as usual" with the ring switching the battery. You can run each pickup to it's own downstream effects/amp/recording gear/whatever that way, using a plain mono cable. Or you can use a stereo splitter to combine them into a single stereo cable. And as an emergency backup, if you find yourself on a desert island without your bag of special cables, you can just use a plain old mono cable to play one pickup or the other.
    MovinTarget and bpc like this.
  9. MovinTarget

    MovinTarget Supporting Member

    Jan 30, 2018
    Maryland, USA
    So this would end up being a turbocharged Billy Sheehan bass? :D

    Bassiclees, mikewalker and dwizum like this.
  10. grouse789

    grouse789 Supporting Member

    Jun 13, 2004
    Westchester NY
    What kind of preamps are you looking for?

    why not both pickups to one stereo out jack, male stereo 1/4” to 2 female mono. Now you can use any tone hammer pedal, VT, EQ pedal on both lines.
  11. Alembic Series basses can do this already. Although the preamp is in the shell of each pickup, each has its' own tone network (their array of filters) that can be applied singly to each. They're phantom-powered and down at the power box there are three outputs: Both, neck, and bridge. Alembic even includes a master volume on the bass to where you can lower/raise both once you're blend is set. (Stanley Clarke runs this type of setup with Ampeg components, even a 'Y' to a little guitar amp when he wants some distortion so he can shred !)

    Series 2 controls.jpg

    The idea was this would output the two channels to their two channel F2B rack tube preamp, each channel with its own tone controls, and then that would feed a two-channel commercial power amp, with different cabinets if desired for the neck pickup and the bridge pickup. Want bigger? Then you simply daisy-chain as many amps and cabinets as required after the F2B. Just keep stacking those MacIntoshes, baby, like Phil did.

    Grateful Phil.jpg
    NBD ! And he used to play through that stack of MacIntoshes:


    They were once asked to build a Series with both their usual filter setup, and switchable bass and treble cut/boost. They did it, but even for them, it was mind-boggling and they swear they'll NEVER do it again.
    lucas303 and MovinTarget like this.
  12. On the other hand, while it's a great 'what-if' exercise, first of all, only you would hear it, and unless you were dealing with exceptional FOH and a great house mixer, chances of the crowd hearing it are slim and none. And in a world with fewer amps and more IEM's, this may be pointless.
  13. EdwardofHuncote

    EdwardofHuncote I Still Dream of Jeannie Supporting Member

    Aug 21, 2013
    Southwest Virginia
    A few of the first generation Tobias 'Growler' basses did something like what you are describing.

    Though they had only one pickup, it was a Bartolini Quad-coil, a big MM style, split in two with each half being powered by it's own preamp controlled with separate volume and tone stack, and with a mid-contour switch. The stereo output was switched by a push-pull pot on Vol-1.

    Controls.jpeg Electronics.jpeg

    I had #4743, and like an IDIOT, sold it.

    Yours sounds like a cool project... go for it!
    MovinTarget likes this.
  14. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    I tend to like non-standard implementations, including external power. My luthier friend "gave" me this bass to brainstorm on preamp ideas with, and I immediately thought back to the 70s and 80s when I used external power and various more or less Alembic inspired homemade preamps. I still haven't plumbed in a second channel yet, two years down the road, but it will be trivially easy at this point:


    The connectors are 4 pin XLRs on standard star quad cable (4 conductor plus ground), I wanted to avoid possible confusion with standard 3 pin or 5 pin ones. There are two 1/4" jacks on the right side of the box, and my intention is to use a rechargeable 12 or 13.2 volt battery set, although for the moment I've been using a single 9V battery quite happily since the preamps I've been using fo far all have fairly low current draw. I could also go with +/- 15V power though, as I did for years with a regular XLR cable along wth a regular 1/4" one, which was a bit of a PITA. I'm retired from gigging but if I ever change my mind carrying a spare cable would be no big deal to me at all. I don't have a bypass sowtch for passive operation and I don't care at all about ever using a standard mono 1/4" cable, I'd just bring a different bass if I needed those things for whatever reason. ;)

    My basic concept is one channel with both pickups mixed into a clean bass amp (i.e. what I have now), and a separate feed for just the bridge pickup into a tube noise toy and guitar-ish cab for slide bass. I'm hoping to finally finish up the tube amp build in the next few weeks, its taken the best part of a year at this point due to many 2020 related obstacles.
  15. Caridorc


    Jan 16, 2020
    Thank you all for the support! I appreciate a lot.

    Like some of you I prefer not to be in the need of special equipment. "That" has to be only inside the bass. So I want that if I find myself only with a mono cable because the stereo broke or is lost or whatever, I can still have a functional bass (meaning that both PU and EQs have to work).

    Now that's interesting! I didn't know about its existence...that could do the trick!

    Yes and no: I would like to have the possibility to use a [stereo to stereo], [stereo to 2x mono] or a [mono to mono mixed] cable. As much possibilities as I can with the least amount of gear

    At this stage (experimental) I'm starting with cheapest one that i find. Artec maybe. Stereo to 2x mono is one of the possibilities, but i thought about that to use the bass with 2 amp and/or different effect chains. For the EQ part I wanted to be included inside, but the possibility to use an external one remains.


    Crowd only "hears" the fender logo on the headstock, and for the guy at the recording studio the only bass that matters is the PBass.
    MovinTarget and grouse789 like this.
  16. Matthijs

    Matthijs Supporting Member

    Jul 3, 2006
    Well Bootsy’s bass might be something you want to have a look into. I think the latest starbass incarnation by Warwick had both 4 mono jack outs and some kind of multiple pin din connection.

    it probably also had external power, with all the lights and stuff going on. That might be a good idea anyway to solve your power-switching problem. If you already have to use a special cable, why not make use of it to feed power?
    MovinTarget likes this.
  17. Caridorc


    Jan 16, 2020
    MovinTarget likes this.
  18. grouse789

    grouse789 Supporting Member

    Jun 13, 2004
    Westchester NY
    [QUOTE="Caridorc, post: 25032267, member: 353334"

    Crowd only "hears" the fender logo on the headstock, and for the guy at the recording studio the only bass that matters is the PBass.[/QUOTE]

    No truer words “usually”.

    “do you mind trying a P bass?”
    MovinTarget likes this.
  19. dwizum

    dwizum Supporting Member

    Dec 21, 2018
    Those basically have the same function. Inserting a cable throws the switches, and the switches are independent of the actual signal circuit. You could use those to allow you to switch the battery without having to depend on the ring grounding through a mono cable.

    The trick will be, if you are ever stuck with a mono cable (or no way to get your stereo signal downstream from a stereo cable), you will lose one output, because you'll short the ring to ground when you plug a mono cable in. That's the compromise of using a stereo jack for an actual stereo signal - it half breaks your "no special equipment" rule, because only one output works with a mono cable.

    One way to get around that would be to put a "stereo mono" mini switch on the front of the bass. You could wire it between the preamps and the jack, so that throwing the switch one way kept the signals separate, but throwing it the other way combined both signals on the tip of the jack (and broke the second signal's connection to the ring). Basically, the switch would swap your second pickup from the ring, by itself, to the tip, with the first signal. That way, if you were ever "stuck" with a mono cable, or for any other reason you wanted the bass to behave like a normal bass, you could throw the switch to mono and have a fully functioning bass. Zero special equipment, and you achieve your desire to be able to split the signals when you want to.
    MovinTarget likes this.
  20. A9X


    Dec 27, 2003
    As a;ready stated, Alembic have been doing this since the 70's.
    No it's not.
    MovinTarget likes this.
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