Outboard passive circuit

Discussion in 'Pickups & Electronics [BG]' started by wunnorf, May 22, 2018.

  1. I'm putting together an 'outboard' passive tone and volume control unit. The initial purpose for this is to assess the sound of two active basses that I have, with passive tone controls. Beyond that I may decide I prefer the passive tone for recording, or to use live, with the unit either as a stomp box, or attached to a modified music stand.

    The two basses are a Washburn Status, and a Squier Deluxe Jazz active V. The Washburn especially has a very 80's vibe to its sound (which I like) but I wondered if its passive tone would be anything remotely like a generic Jazz bass. What I have done is installed an output jack on the back cover plate, connected to the output from the pan pot. The switch on the jack disconnects the input to the active circuit as well so it is just the two pickups and the pan pot. I only got the Squier (used) yesterday, but based on what I have read and seen so far I should be able to do exactly the same to that.

    The outboard unit I am making has a standard P bass type volume and tone control circuit in it except that I have a 4 position rotary switch with four different value capacitors connected. I have also put in a Crate tuner module that I had left over from another project. The whole unit will look something like the picture below when it is finished. At the moment it is just the plastic panel which is a Strat spring cover. The tuner occupies most of the space where the string access holes were and the push switch to power up the tuner and mute the output occupies the remaining hole. The strap buttons are mostly for visual effect, but might allow it to be worn. All a bit experimental, but electronically simple - or so I thought....

    The problem I have is that as far as I can hear, with a bass as input I can hear no difference between the 4 rotary switch settings. The capacitors are 22nF, 33nF, 47nF and 68nF and the tone pot is 250k audio taper. I also tried putting a guitar through it and there is no discernible difference between the four settings. The tone control definitely works and rolls off the highs as you turn the pot, but it seems to sound exactly the same regardless of the switch position. There is of course a chance of course that I have a wiring error, so that will be the thing to check tomorrow, but I was wanting to ask the experts here how subtle would the difference between these four cap values be? Do I need to use more extreme values either side of the 47nF to hear a significant difference, or a different value pot? Do I just need my hearing checked? Another thought was is the length of cable between the bass and the unit affecting how it behaves? I have been using a cable that is only about 18" long for testing, so would not have thought that would be significantly different to the circuit being on board the bass.

    Your thoughts?
    Warren Passivator.jpg
  2. SteveCS


    Nov 19, 2014
    Hampshire, UK
    IME, the change from 0.022uF to 0.033uF is subtle, but you should easily hear the difference between .022uF, .047uF and .068uF.
  3. Try switching through your different caps with the tone pot turned all the way down. Switching caps makes little difference until the pot is turned down to about the last 1/4 of its rotation. If you still can't hear much difference with the tone pot all the way down, then I'd suspect the wiring or perhaps the diagram you used.
    bigtone23, sikamikanico and SteveCS like this.
  4. Answering my own question here just in case anyone else has this problem (unlikely).

    I pored over the wiring for a long time and couldn't anything amiss. I plugged the unit into my audio interface and looked at the output on a frequency analyser vst plugin. I couldn't see any significant difference between the switch positions. I plugged back into an amp and thought maybe position four was a bit darker, but then next position was brighter again.

    That was when the penny dropped. There shouldn't have been a next position. I was going for four options, not five. The four position switch has a little washer with a tab on it that limits the rotation of the switch. It fell off when I was shortening the shaft on the switch and I put it back on and checked out the operation of the switch - all good. Sometime between then and fitting it to the panel it must have moved one notch, so I now had a three pole four position switch with five positions. I'm not sure what that meant was happening inside the switch but by repositioning the switch I now have four audibly different tone settings (and a visible difference on the frequency analyser). The 0.022uF is probably as ineffective as I will ever need, but the difference between the 0.047uF and the 0.068uF is pretty subtle, so I might increase the value of the biggest cap a bit to get a significantly darker option.

  5. SteveCS


    Nov 19, 2014
    Hampshire, UK
    Or you could, if it has more than one wiper, add links to the switch to link up two or more in parallel...
  6. bigtone23


    Dec 10, 2014
    Denver, CO
    As mentioned above, the cap's tone doesn't really come into play until the very bottom range of the pot.
    If I had 4 switchable caps to use for bass, it would be .022, .047 .068 and .1uF. These would run the gamut from high end roll off to full on dubby boom. (FWIW, I tend to use .047 and .068uF on my basses as .022 is often too middy and .1 is often too muddy)
    Having less than about .020uF increments on cap values gets into subtle changes and overlap from the typical +/- 10-20% tolerance in caps. For example, an extreme example would be a +20% variance on a .047uF (.056uf) and -20% on a .068uF (.054uF) would net the same rolloff! I would get caps with +/- 5% tolerance for accurate assement in this experiment.
    SteveCS likes this.
  7. Thanks for the suggestions. I measured the caps at work today and they are actually pretty good. The 0.022 and the 0.033 are spot on. The 0.047 is about 0.049 and the 0.068 is about 0.07. I think I will change the 0.068 for a 0.1.
    I am wondering about making another change and that is to have no capacitor in the first position and change the pot for a linear one of a much lower value to try to get 100% of the rotation to be useful. You'd need the empty spot then to give a wide open option. I'm not sure I'd do that inside a bass, but for this little box it might make things a bit more tweak-able. Has anyone done that? Any idea of a good value for the pot? I'm thinking a linear one would better represent the bottom part of the audio taper curve.
  8. fermata

    fermata Guest

    Nov 10, 2015
    Audio taper is the only way to go for tone controls. 250K, to my ear, gives the smoothest range. Linear taper is nice for volume, but when used for tone, it squishes the range into a very small part of the rotation.

    I've played around quite a lot with cap values, and I find .1 to be a bit dull sounding, compared to .068 or .047. (But that also depends a lot on the pickups and strings.) I also think small value caps can be very interesting. I have a .015 uF cap on my bass (along with .033 uF, .047 uF and bypass; I don't have a tone pot). Some of the lower values can impart a honky mid-bump, so a resistor in series (around 6-10K Ohms, between the cap and ground) tames that. Very small values below .01 get pretty interesting, too. .0047 and .0068 uF impart a woodiness that's different from a larger cap turned down partway, so if you have an extra slot, a super low value cap could be interesting.
    sikamikanico and SteveCS like this.
  9. ofajen


    Apr 12, 2007
    92.4W 38.9N
    This. Linear pots don’t work out well for tone controls. All the action is packed into a very narrow region near full off.

    SteveCS likes this.
  10. I see why you are thinking this. A 250k audio taper pot is probably around 20K at 1/4 rotation. But think about it. If you limit your overall pot sweep to say a 25K pot (fwiw, still go with a log pot if you try this), yes, you'll hear a difference between the caps, no matter where the pot is, but you'll be limiting yourself to always having a standard tone pot rolled at least 3/4 off. And having a switch setting that has no cap will just give you full brightness. You'll still be missing everything in between.

    Perhaps another approach for you to explore is a rotary switch that goes between fixed tone settings. I have a 12 position rotary in my P bass that gradually rolls off tone, but with a small amount of resonance and a sharper roll-off in each setting. IMHO, this is far more useful and musical.

    Last edited: May 24, 2018
    okabass likes this.
  11. Thanks for the helpful replies. A this stage I think I'll just go with my original plan and have four cap values and the 250k audio taper pot. There's nothing stopping me changing it later. I like the way the caps are arranged on the rotary switch. I hadn't thought of adding a ground band like that - very tidy.
  12. okabass


    Mar 19, 2005
    My intention has also been making same kind of passive eq in a pedal.

    You could add a bass control like G&L basses: like 2,2 nF cap over 1 MΩ pot.
    Bass-cut and the G&L PTB system | GuitarNutz 2
    Or a bass cut like Rics: 4,7 nF in series.

    If you add a coil and cap you get mid control.
    passive mid control

    Or even a whole Pultec-type eq. You have active basses? Then this may work.
    Pultec Schematic discrepancies - Gearslutz Pro Audio Community
    I have buid an active Pultec type eq, and it works great with a bass.
    Last edited: May 25, 2018