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Discussion in 'Pickups & Electronics [BG]' started by Tristan, Jul 5, 2002.

  1. Tristan


    Jan 28, 2001
    Ottawa, Canada

    I need a lil' engineering help. I want to build a simple active on-board preamp for my basses. I don't need tone controls or lots of gain, I just want something to make my basses' output low-impedance. I was thinking a low or unity gain opamp could do the job elegantly. Would this work? How would I set this up? CAn anyone name websites or books with more info?

    Thanks, Tristan

    P.S. I really appreciate everyone on here who gives good advice and puts up with people like me who ask way too many questions. Props.
  2. Tristan


    Jan 28, 2001
    Ottawa, Canada
    Okie, I wasn't sure where to put this...
  3. This looks like an appropriate forum for this thread...
  4. Tristan


    Jan 28, 2001
    Ottawa, Canada
    How about a TL071? It's a hifi audio opamp... I found the spec sheet on ti.com and it shows a way to wire it at unity gain... I may try this out soon.
  5. thrash_jazz


    Jan 11, 2002
    Ottawa, Ontario, Canada
    Artist: JAF Basses, Circle K Strings
    Sounds quite plausible, but, as AB said, you'd need the specs on the op-amp itself to figure out how much output impedance you'll end up with.

    Pretty much any basic electronics book should be able to help you figure out what you have to do.

    Anyway, if you already have the spec sheet and a wiring diagram, give it a shot. If you can, try it out on a breadboard using a scope first, so that if it doesn't work and you need to change something, you'll save a lot of time!

    Good luck :cool: fellow Ottawa bassist!
  6. Tristan


    Jan 28, 2001
    Ottawa, Canada
    tj, how old are you if you don't mind my asking...

    also, the specs don't say what the output impedance is... the input resistance is 10^12 Ohms, however...
  7. Munjibunga

    Munjibunga Retired Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Hmmm ... why not just buy one of those hi-z to lo-z thingies (Shure, Hosa, whatever), take out the guts and wire it into your output jack inside the cavity. They're about 12 bucks for a decent one. Or go all the way, and get a Countryman DI, and put the guts in your cavity. Ish, that sounds icky, doesn't it?
  8. Damian Coccio

    Damian Coccio Supporting Member

    Feb 7, 2002
    Endorsing Artist: Fodera Guitars
    Ive built my own actives for several basses I made. Use the TL062 or TL082 which are both good opamps, but the TL062 is lower power. Not sure about the TL071, but its probably OK. All of the the preamps I have put together have tone controls. If you just want to make a buffer thats real easy as you probably know. But my feeling is if your going to build a preamp, you might as well design in bass and treble controls. The nice part is that you can control the exact cut/boost point of the bass and treble controls and get a really nice custom sound. Whether you make your own tone control or just a preamp, your going to need to design in a rail splitter setup the opamp I/O for 1/2 Vsupply.

    I have a sketched schematic for my fretless preamp, but my scanner needs repair so I cant send it right now. I could fax it though.

    Any questions Ill be happy to help. Ive been designing these kind of circuits for about 15 years.
  9. john turner

    john turner You don't want to do that. Trust me. Staff Member

    Mar 14, 2000
    atlanta ga
    i think this would be more appropriate over here in pickups. if not, we can always send it somewhere else, in our quest to provide you, the talkbass reader and contributor, the most enlightening talkbass experience.

    thank you, drive through.
  10. jankjo


    May 22, 2002
    Hey nuwavedc,

    I'd love a copy of your preamp schematic! How do I get you a copy of my fax number with out posting it here?
  11. Tristan


    Jan 28, 2001
    Ottawa, Canada
    Thanks all...

    New Wave DC, If you get your scanner working please post the schematic. Also, I was just about to ask if i needed to split the power in two + and -... Is that hard?
  12. jankjo


    May 22, 2002
    I am not sure what you are asking, so forgive me if I am way off base here, but....If the chip you are using needs plus(+) and minus(-) power, you will probably need to use two batteries. Connect the + terminal of one to the - terminal of the other for ground. The remaining + terminal is your positive voltage supply and the remaining - terminal is your negative supply. Hope this helps.
  13. Tristan


    Jan 28, 2001
    Ottawa, Canada
    Is two batteries the only way? Do all opamps need +ve and -ve power?
  14. jankjo


    May 22, 2002
    No, not all opamps need +/- power. Look for one that only needs a single end supply. There are also +/- inputs on the opamp. These are for the signal and can be wired in a variety of ways. One common wiring is to put the signal into the - input and the + input to ground, but this isn't necessarily the best way to wire it. Hopefully nuwavedc will be able to post a schematic.
  15. gyancey

    gyancey Supporting Member

    Mar 25, 2002
    Austin, TX
    Opamps requiring a bipolar power supply can be run off one battery using a resistor divider to create a phantom ground supply. This will require an additional filter cap on the output to remove the DC bias. Basically use two resisitors of equal value (use large resistors to reduce excess shunt current) in series and use the midpoint for all ground connections, the ground of the battery for the - voltage supply and battery + for the positive voltage input. This will reduce the headroom of your design, however, since essentially you'll have a +-4.5VDC system instead of +-9V. Then again you could do the whole thing with one discreet BJT transistor in the emitter-follower amp configuration if you just want impedance transformation.

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