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Bypassing one EQ band control of a preamp?

Discussion in 'Pickups & Electronics [BG]' started by Lockout, Mar 6, 2005.


  1. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    I have a Spector NS-JH5 with a Spector TonePump preamp (2-band boost-only), and I was wondering if it would be possible to bypass the treble control of the pre so that the treble is always set flat. I've never really felt the need to boost the treble on any of the active basses I've owned, so I'm considering the possibilities of taking out the treble boost control and replacing it with a passive tone knob.

    Is something like this possible? How would I wire the preamp's treble wires to set the band flat without having the treble pot in the circuit? Am I making any sense? :smug:
     
  2. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    It would be possible to do it, simply replacing the variable resistor (the pot) with a resistor or two depending on how it's wired so that it was the same as the pot wired permanently in circuit in it's mid position. I dunno the Spector pre so I'd only be guessing on how to implement it. Then you'd need to add a series pot and cap combo to make the passive cut type tone control.
     
  3. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    it's possible, but could definitely affect the resale value.
    You might say "I'll never sell it", but you never know. Why not put the bass and treble on a stacked pot, and also add a passive tone control. That would probably increase the value of your bass, if not by much :)
     
  4. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Yes, it would affect the resale value if I sold it like that. It's easily reversible, though, so it doesn't really bother me.

    Since the TonePump is boost-only, I would need to wire it in a way that would cause it to act as if the pot was fully counterclockwise. What type/value of resistor would I need for this? Does it depend on the value of the treble pot?
     
  5. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    David has the fix I think. Dharma's no doubt right on but he's an engineer sooooooooo ............... unless you are to. Stacked pot, stick the treble where you want it and leave it. Don't know but push-pull may fit in well with the passive pot. At least it sounds the simplest and most practical route. And as you said, it'd be reversable.


    I'm not clear if you're wanting to use a passive tone with an active bass control or have an active/passive setup. I'm guessing you'd get some wierd stuff mixing them up.
     
  6. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    I'm not sure if I'm understanding your logic concerning the resale value. As I see it, selling it after modding it in either way would reduce the resale value of the bass. Both ways are also reversible, as long as I keep the parts.

    I'm not a big fan of stacked pots, and I never use the treble boost on the bass (in the rare case that I do want to boost the treble, I do it on my amp instead.) I'd like to keep the controls rather simple, which is why I'm thinking about this.

    Now, just to be sure that I'm being clear and making sense... I want to set the controls on the bass like this: Neck Volume - Bridge Volume - Passive tone - Bass. I want to wire the preamp's treble wires together in a way that would force the treble boost to remain at zero, flat. As for adding the passive tone control, I would be doing it like this diagram from Aguilar's site: http://www.aguilaramp.com/html/support/downloads/diagrams/diagrams/obp1passivetone.gif

    The other (small) reason why I'd rather do this instead of using a stacked pot is that using a stacked pot would be more confusing and would require more soldering... And with me, the less soldering, the better. ;)
     
  7. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    Yep, understand about the stacked pots. This may sound bourgeois but, you could leave the treble in the control bay, strap it down, and do the OB scheme on the passive tone. That would free up the pot hole. Below many players but it's probably what I'd do, at least to check it out. Not really that different than having batteries, dip switches, and trip pots under the hood. Plus you could reset the treble if you had the need.

    Another thing I'd consider would be a Bart NTBT that has boost/cut for both bass AND treble and more of it. Keep the OBP-1 and plunk it back in come sale time and sell the NTBT or stick it in something else. Personally, I found the OBP-1 to be an anemic pre and the NTBT was pretty toastie. Unless you just want a passive tone control cause that would put you back in a stacked knob situation.

    Anyway, that's about as sane as I get :)
     
  8. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Yeah, that's definitely a possibility... I just made this thread to see if it was possible to wire it without leaving the treble pot in there, and how complicated the wiring job would be if it was. Dharma kinda answered that... hopefully he'll check back here soon and help me with the more specific questions I had. :)

    It's actually a Spector preamp, not an OBP-1. They are similar, though, and the wiring for a passive tone knob is pretty much the same regardless of the preamp, I believe. I just took the image off Aguilar's site because it was the only one I could remember off-hand.

    As for using a boost/cut preamp instead of adding a passive tone control... I prefer the way a tone knob rolls off the highs compared to the treble cut on an active pre. I find this especially true for the Bart preamps I've used/owned... I don't particularly care for the sound I get from cutting the treble on them. Just my preference.

    Thanks for all the suggestions, though. :)
     
  9. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    right, as long as you reverse it it doesn't really matter.
    I didn't know if you were doing it yourself or getting a tech to do it.
     
  10. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW:

    yeh, value in general goes down with any mods that aren't reversed to stock condition. If tastefully done mods can make a bass go quicker but usually not for any more unless it's something that can be parted out for more. Players generally want something they're familiar with. Of course there's always exceptions.

    And yep, you'd said it was a Spec but I got hung up on the OB diagram and forgot about it.

    For the most part, I haven't found the preamps I've had to make much difference - except the OBP-1 which I felt was lacking. I wired it as an onboard and outboard and really couldn't tell it did much of anything. I ditched all the preamps about a year ago. But I thought maybe something with cut might get you more of what you may be looking for than a passive tone.

    Obviously you've had enough experience to make those assessments. Another thing about having the passive tone though is you can throw caps on and dramatically alter the tone to suit taste. Caps are cheap and readily available. You can also run passive bass, mid, and treble controls for that matter - which I can't but Dharma could probably explain. Like passive tone, cut only no boost. If your selective enough to prefer passive tone over treble cut, maybe passive bass is something worth exploring. You can boost with your amp as well as cut, depending on the amp. And that's basically why I ditched preamps.

    I wouldn't sweat the soldering. As long as you can see, have a low watt gun with a thin long tip, decent solder, and keep the gun out of the wires - short of a condition like Parkinson's - anybody can do it. It's a lot easier if you make it up outside the bass by making a cardboard pattern of your pot layout - then it's just a drop in, tack a few wires and it's done. Unless of course you have reversed the cardboard pattern from the way the pots drop in - which you have a 50/50 chance of doing if you weren't cognizant at the time.
     
  11. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Yeah, I'm actually not sure what type/value of cap I prefer, as I've never really experimented much with swapping them. To give you an idea of what I'm looking for, though... I'd like the tone knob to be able to roll off the highs along with that "upper upper midrange" area that seems to contribute to clankiness. I would not want it affect the rest of the midrange, though. Any ideas what sort of cap I might be looking for?

    I've thought about trying this before, but based on what I find myself doing with the mid and bass controls on active preamps, I generally either leave them flat or boost them slightly. I don't usually cut them, which is why I kinda ruled out the fully passive option. I like the Spector pre's bass boost, anyway. :)
     
  12. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    I'm aware of caps but I've only messed with them once. I run pups directly out to seperate jack with no onboard controls and work off the amp. Smash has a couple good threads on them and Dharma makes amps so I know those guys are up on them.

    I do know how dramatic the effects can be from the one bass I wired using two different size caps between volume and tone pots of each pup - ala '54 Les Paul Standard.

    The "clankiness" can be pulled out with eq'ing but you don't change one frequency without affecting the rest. Usually you can get want you want from twiddling enough. Caps are the same way - pop it in and see what you get, adjust accordingly.

    Standard bass is a .05 mfd (micro farads). To give some idea, Fender 6 string guitars use between .1 and .05. Gibson 6 string guitars commonly use .02. The higher the numeric value the more the treble cut (therefore bassier the tone). So .05 would be more treble cut and bassier than a .02. If you don't get exactly what you want, you've narrowed it down. They're only a few bucks a piece.
     
  13. Aram

    Aram

    Feb 2, 2003
    New York, NY
    Can anyone point me to a resource where I can learn more about this, or elaborate? I have a bass with a 3-band pre that I'd like to make all-passive. Sorry for the hijack...

    Thanks!
    aram
     
  14. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    Tone is personal, so you're going to need to swap in different values depending on what YOU actually expect to hear. Larger values mean the control begins to work from a lower frequency. Mylar caps, otherwise known as greencaps or MKT's would be what I'd suggest to experiment with as they're cheap and available. Sonically my preferences run to polypropylene usually Sprague 715P/716P 'orange drops' because they're very reasonably priced. But they are physically bigger and may not fit easily into some cavities. Differences between type and construction can be small sonically, and have a bigger effect in amps where the might be several in the signal path.
    Anything more complicated in 'passive' tone controls than a simple treble cut should remain in the amp itself for reasons I explained recently in another thread on this subject.
     
  15. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Ah... I'll have to read into that. I don't really plan on trying a passive system like that, but I am interested in learning more about how they work.

    Now, heading back to the original subject of this thread... I've got a few more quick electronics questions. (Please excuse my lack of knowledge on this subject.)

    1. When replacing the treble pot with a resistor, I should use a resistor with the same ohm value as the pot it's replacing, right? Do I need to be exact? What would happen if I didn't use the correct value? (Just out of curiosity)

    2. What value pot should I use for the tone control? This bass has active EMG's in it.
     
  16. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    Not having the circuit in front of me or being familiar with it at all, I don't know. The TP is boost only so I'd only be able to offer a wild assed guess which I'm not comfortable doing. My only suggestion is to do this: with the pot at 'zero' remove it from circuit and measure the values between the centre lug and each of the outside lugs with a DVM amd replace the pot with two resistors of the same value.
    Incorrect values would mean some sort of non-flat response.
    Wire it up with whatever EMG use in theor 'passive' arrangements that come with their pickups. Check out their site for diagrams and values.
    You're basically going to make a passive bass, VVT, then plug that into the preamp so the values would be the same.
     
  17. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Hmm...

    Here is how the treble pot is wired (I don't know if this will help at all, but I'll post it anyway, just incase it might clear something up)

    There are two wires from the preamp that connect to the treble pot. Imagine this is the treble pot...

    [​IMG]

    The yellow wire from the preamp is connected to both lugs 2 and 3. The black wire from the pre is connected to 1, which also grounds to the pot's casing.

    Does having the yellow wire connected to both of those lugs affect what you suggested at all? It seems like it'd be different than other preamps that have 3 wires running from the preamp to the pot, like Aguilar, etc. but I really have no idea. :confused:
     
  18. luknfur

    luknfur

    Jan 14, 2004
    DIXIE
    FWIW

    there was a post not long ago about that. You get the same results from the pot than if the yellow wire were only ran to the center lug. The conclusion was often they're bridged like that so that it doesn't appear a connection has been forgotten. I have seen basses wired with vacant lugs though.
     
  19. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    Assuming it is the way you described, with the pot fully counterclockwise (min treble), there will effectively be a short between the 1 and 2 pins, so you can connect the yellow and black wires together.
     
  20. Lockout

    Lockout

    Dec 24, 2002
    Illinois
    Okay, I'll give that a try.

    Would I then need to wire them to ground? Or does it not make any difference?