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Separate TRS jack for internal/external power sources?

Discussion in 'Pickups & Electronics [BG]' started by Rusty the Scoob, Nov 11, 2010.

  1. I have a bass with two separate preamps whose voltages can't be mixed for complicated reasons not worth explaining here, and I need a good way to make one of them switchable. (the output jack is switching the other as normal)

    I thought of a toggle switch, but really don't trust myself to remember to turn it off and not bump it. So I'm thinking about a separate 1/4" TRS jack wired like this:


    If my thinking is correct I could insert a dummy mono 1/4" plug into it to power the preamp on the bass's internal 9V batteries, or use a stereo 1/4" cord for an external 18V source.

    Does this look like it would work?
  2. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Problem: you need the tip for your instrument's signal, separate from the + DC.
  3. Not a problem - this would be a separate 1/4" jack, right next to the output jack.
  4. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    So you'd be running TWO cables to the bass?
  5. mid_life_crisis


    Jul 8, 2010
    Much as you don't want to explain, we really need to know why the two devices need separate sources.

    Are you trying to turn on one or the other pre-amp at a time?

    Not all batteries appreciate being forcefed by an external power source, so you may want to consider that you'll need to isolate the batteries from the external source.

    If you are going to run just one at a time, I would consider one of those volume knobs with the push/pull switch built in. Set it up so in is one pre-amp and out is the other.

    I'd also consider using a normally closed relay to channel the power from the batteries, wired to disconnect them when the external source is connected.

    Am I even close to guessing what you are really trying to do?

    Edit: I almost forgot. Your diagram indicates that the sleeve is not used, but you have a wire coming from the internal battery box that indicates it is connected to a sleeve.
  6. Not necessarily. If your DC power is very clean, you might be able to phantom power it, provided you block the DC out on the signal path with coupling capacitors.

    FWIW, why not just add a voltage divider circuit in the bass so you can have both voltages?
    Or, failing that, just add another battery.
  7. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    True about the cap filtering... I was imagining the OP planned to just wire the jacks up and hope for the best. :p Seems to me another solution would be to wire the jack:
    Tip -- signal +
    Ring -- DC +
    Sleeve -- common ground
  8. That's the way Line6 does the Variax guitars.

    There is a DI box that you connect a stereo cable to,which is powered by a wall wart.
  9. The reason I need two separate voltages in the bass is complicated - there are 3 preamps in the bass:

    2 Bartolini EZQ semi-parametric filters, which I want to power via 18V

    1 D-TAR Eclipse - it's a blender/buffer preamp that's causing me all these headaches because it requires something called Swing Voltage that requires a power feed from the + terminal of one 9V, the - terminal of another, and the combined +/- feed. It's set up right now so that a switching output jack cuts this power feed off when a plug is inserted.

    I want to power the Barts with a separate pair of 9Vs for a total of 18V, but couldn't find a way for the single output jack to cut off both power feeds without mixing the feeds together.

    My first idea was just a toggle switch that cuts off the feed to the Barts when I'm not using the bass, but I don't trust myself to remember to deal with it. My second idea was to add the 2nd TRS jack and use the Ring and Sleeve as a switch when a mono plug is inserted, just like a standard switching output jack, but in this case the mono plug would be just a dummy plug that's hooked up to nothing. It'd be sturdier than a toggle switch and I'll remember to unplug the dummy plug since I'll be unplugging the cable too.

    But then I had the thought that this 2nd TRS jack could maybe also be wired to accept an external 18V input. A stereo 1/4" plug with the Sleeve not hooked up would leave the battery circuit open since it wouldn't connect the Ring and Sleeve. I could then use the Tip and Ring to power the Barts.

    But I may be crazy, nobody seems to have ever done this before, possibly since they never had my exact problem.
  10. Ok, so you need a +18V supply for two preamps and a +9V/-9V rail supply for the third?

    You can run that off of the same two batteries if you just add an on/off switch for the batteries.

    Why on earth do you have three preamps?
  11. FunkMetalBass


    Aug 5, 2005
    Phoenix, Arizona 85029
    Endorsing Artist: J.C. Basses
    So you want to phantom power the pickups and preamps, but be able to switch between the Bart preamp and the Eclipse at the flick of a switch?
  12. Nope, I want them all to be powered at once, but with more voltage (or maybe more current?) than a single pair of 9V batteries can provide.

    The bass was modded before I owned it with two Dark Stars, a parametric filter for each pickup, and then they're combined into the DTAR buffer. I like the way it sounds, if only it had more headroom. It's a Semi-hollow Starfire reissue so there's a ton of space inside it for all this stuff.

    Yep, your description of my power needs is accurate if Swing Voltage is the same as what you're describing, I honestly don't quite understand that part.

    The whole system is currently powered by just two 9V and I'm not happy with it... one of the batteries drains in a hurry and I get distortion on a really hard hit note once the voltage sags as Alkalines like to do.

    I don't get what you're saying about the on/off switch...
  13. Actually, nevermind on running all preamps from the same batteries.I wasn't thinking about the ground potentials.

    The +18V setup would require you to ground the negative terminal of one battery, but the rail supply for the +9V/-9V preamp would require you to ground the positive terminal of the "negative" battery, so that would just short one battery out.

    You could do it with three batteries though.
    Put them all in series. The negative from battery one goes to the V[SUP]-[/SUP] of the rail-supply preamp, the positive of battery one/negative of battery two is the ground, the positive of battery two/negative of battery three goes to the V[SUP]+[/SUP] of the rail-supply preamp, and the positive of battery three is the +18V supply for the 18V preamps.

    What I mean with the switch is that you can simply add a power switch to disconnect the batteries.
  14. I'm a genius, it works perfectly. :bassist:

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