Active/Passive blend?

Discussion in 'Pickups & Electronics [BG]' started by poit57, Jan 15, 2014.

  1. poit57


    Apr 30, 2010
    Oklahoma City, OK
    I am working on a bass that is routed for a P-bass pickup and a MM pickup. It was originally wired as a passive bass with a separate volume and tone control for each pickup. I am putting in some Seymour Duncan Basslines pickups and adding the 3 band preamp. The simplest setup would be just to follow this wiring diagram (I already have the harness that just needs the pickups connected).


    This might be a crazy thought, but would it be possible to wire the P-bass pickup as passive and connect only the MM pickup preamp without giving each their own volume control? I figured this way I could get either a passive P-bass sound or an active MM sound.

    If I can have the two pickups wired differently (one passive and one with active electronics), could the sounds be blended with a blend pot, or would a toggle switch work to select which pickup is in use?

    I am just curious is this is possible, and if so, how that would be accomplished. Otherwise, I'll just go with the diagram above to connect both pickups to the preamp.
  2. mmbongo

    mmbongo Chicken Pot Pie. My three favorite things!! Supporting Member

    You could do it, but not at the same time and you'd need separate jacks for each setup.
  3. Lo-E


    Dec 19, 2009
    Brooklyn, NY
    It's possible, I suppose, but I don't think it's likely it would work very well. The difference in impedance would be so great between the P pickup's output and the preamp's output that you'd need some sort of buffer for the P to match them...and then it's not really passive anymore.
  4. poit57


    Apr 30, 2010
    Oklahoma City, OK
    Thanks guys. I was sort of thinking that it wouldn't work so well, but I let my imagination run wild. I'll just stick with the original diagram.
  5. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    You could do that and it might sound pretty good. If you are going to run only the MM into the preamp then you can use the blend pot's spot for a volume control on the P. Yes, the output impedance of the preamp is low and if you do nothing it will takeover the output from the passive P. But you can put a resistor in series with the preamp output and then tie that signal to the wiper of the P volume pot which would also go to the output jack. You'd have to experiment with the resistor value, try some values between 470 Ohms and 22 kOhms. The output impedance of the preamp will be whatever value of resistor you put in series with it. The output impedance of the P varies from about 500 Ohms to 20k over the frequency range of 30 to 1000 Hz and the volume pot will add to that. So it isn't easy to say what value resistor would sound best or even to guarantee that the idea works. If you aren't up for experimenting then just wire the preamp as normal. If you do want to experiment I would not build it up in final form, I would hook it all up temporarily (using clip leads if you have them) and experiment with resistor values to see what, if anything, works best/at all.

    Another option that will work is to use an active passive switch that just connects the P to the output in passive mode. I am planning to do that with my active Reggie Hamilton P/J which does have an A/P switch, I just need to modify it to act on the P pickup only.
  6. tylerwylie


    Jan 5, 2008
    Dunwoody, GA
    Use a buffered blend potentiometer if you want to mix active/passive signals.
  7. RobbieK


    Jun 14, 2003
    Here's how I'd do it. Sorry about the hand-drawn diagram and dodgey phone-camera photo...


    The switch has 4 poles. One is to bypass the 3-band pre. The other 3 are to bypass the blend. In bypass, this wiring will solo the P and introduce the 220k load resistor and passive tone control. These are so your P, although buffered, will still sound pretty much like a passive in bypass mode. I like the idea of going from a modern sounding blend plus 3-band EQ to an old school P sound in one switch. But there's no reason you couldn't hook the load resistor and/or the passive tone pot permanently to earth. 4PDT toggles are prett easy to find, but I prefer using a rotary switch for this sort of thing. You'll end up with six knobs though. You could potentially use a push-pull and use stacked treble and bass pots, say, to keep it to 4 controls, but you'd need a separate analogue switch circuit which is a bit of a faff, plus it uses a bit of current...
  8. RobbieK


    Jun 14, 2003
    Actually, it's not a bad idea to put a resistor across the output of a pre like this...