Trimmer pots for onboard guitar preamp tone controls?

Discussion in 'The Projects Forum' started by Geek, Sep 19, 2009.

  1. Geek

    Thread Starter New Member

    Sep 17, 2009
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    Can trimmer pots be used as tone/volume controls?

    I might use two or three electret capsules on an acoustic guitar with separate tone/volume controls.
    That would be too many pots to use normal sized ones.

    Anything I would need to know in order to do this?

    Oh and once the tone controls are set, they will rarely if ever be adjusted.




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    Last edited: Sep 19, 2009
  2. Audioguru

    New Member

    Dec 20, 2007
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    Most trimmer pots are adjusted with a small screwdriver. But I have seen small trimmer pots that have small knobs that you turn with your fingers.
     
  3. Geek

    Thread Starter New Member

    Sep 17, 2009
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    Ya, that's not a problem. The tonal balance between pickups will probably be set once and left alone after that.

    I just need to know if there's any physical characteristics of trimmer pots that are maybe different somehow.
    Like maybe not as clean of a signal for audio, maybe take too much power, not enough power, etc.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Not at all. You can make finer adjustments with 25 turns instead of 7/8's turn.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You will only know the function of the potmeters when you have the schematics.
    Turning pots without knowing the purpose of the pots can even lead to failure of the amplifier.

    Greetings,
    Bertus
     
  6. Geek

    Thread Starter New Member

    Sep 17, 2009
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    Well i'd probably use something with less turns than that, but it wouldn't really matter one way or another. Just as long as they work just as well as the normal audio pots would.

    Do you have a brand you could recommend? The higher quality the better, since these will likely sit untouched for long periods of time. I need something that wouldn't develop noise over time as a result of oxidation or whatever.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Bourns 3299W usually work fine. Didi-Key has them for about $3.14/ea. 3/8' trimmers are sealed, so they really don't deteriorate. Remember they are rated for 1/2 watt. Tone controls usually dissipate less, though.
     
  8. Geek

    Thread Starter New Member

    Sep 17, 2009
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    Thanks, i'll look those up for sure.

    That second part there brings up a question i've been wondering.

    Do the tone controls go in between the electret capsule and the preamp, or after the preamp?
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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  10. Geek

    Thread Starter New Member

    Sep 17, 2009
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    The thing is, I found a preamp design that I like, but it doesn't have tone controls.

    I guess it's one of the more popular preamps over the years, and it looks pretty easy to build.

    http://www.till.com/articles/GuitarPreamp/

    So i'm hoping to either be able to incorporate tone controls with that, or maybe one in series with it.


    [​IMG]
     
    Last edited: Sep 19, 2009
  11. Geek

    Thread Starter New Member

    Sep 17, 2009
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    Would this EQ be able to work with that?
    They both require voltage, so would there be any way to put them on the same circuit?


    [​IMG]

    [​IMG]
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    That circuit should work. Use a better op amp, like an OPA134, though.

    I can't quite make out -
    - who "they" are.

    By the way, the above circuit and my tone control example both require a dual power supply - positive and negative voltage.
     
  13. Geek

    Thread Starter New Member

    Sep 17, 2009
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    I meant the preamp I posted on the last post of the first page and the EQ from this page, both being powered.
    I was thinking about adapting the actual EQ circuitry into the preamp from the first page. It seems like that FET circuit is really ironed out.
    But I suppose it's pointless since the EQ is powered as is.


    So I wonder how hard it would be to add a variable Q to the midrange eq circuit?

    Oh, and the opa134 can also run all 3 midrange channels?


    Also, another option I ran across is using an EQ chip. I hadn't thought about it, but it makes sense that there would be such chips.
    Like the LMC835 mentioned in this pdf.

    http://www.google.com/url?sa=t&source=web&ct=res&cd=1&url=http%3A%2F%2Fwww.national.com%2Fan%2FAN%2FAN-435.pdf&ei=w3S2SrHNKovyMdLQ3NoO&rct=j&q=graphic+EQs+on+a+chip&usg=AFQjCNE95z-SebO8GNCrpXpaS8KnBHw9Nw

    A pair of those chips would be a seriously powerful combination.
    How much work would it be to use something like that?

    Complicated is ok, so long as it can work off of a 9v battery.

    But it would be great to have multiple bands of EQ available like that, one way or another.




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    Last edited: Sep 20, 2009
  14. Audioguru

    New Member

    Dec 20, 2007
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    An LM358 is horrible for audio.
    An OPA134 is an excellent single audio opamp. The OPA2134 is the dual and the OPA4134 is the quad.

    It is easy to bias the input of the EQ opamp at half the supply voltage so it can be powered from a single-polarity supply.

    They stopped making the LMC835 EQ IC about 10 years ago.
     
  15. Geek

    Thread Starter New Member

    Sep 17, 2009
    20
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    Well I see some available on Ebay right now. But if it's not so hot for audio, then i'm better off with something else.

    But I like the idea of multi band eq on this. Especially if it's something that gives me a fairly small circuit when i'm done. That's the main reason i'm wanting to use the trimmer pots. I could wind up with a lot of pots tied together in there.

    I'm sure I could make good use of 5 bands if they were at the right frequencies.
    That's 10 pots if I use two electret capsules.
    But I would most likely start out with just one electret at first until I see if it's something I want to stick with.
     
  16. Geek

    Thread Starter New Member

    Sep 17, 2009
    20
    0
    Nevermind on that stuff, folks.

    After poking around enough, i'm finding a ton of eq's for guitar stomp boxes and also car audio.
    So that's the route i'm going to take.
    Find something that already runs on lower voltage like a stomp box, disassemble it and use just the raw components.
    I think i'll most likely find some nice condensed circuit layouts.
    Using some trimmer pots, I should be able to stuff them in a fairly small area.
    Best part about it, is that stuff is really dirt cheap nowadays.

    [​IMG]






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    Last edited: Sep 21, 2009
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