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DIY Onboard Preamp?

Discussion in 'Pickups & Electronics [BG]' started by l0calh05t, Jan 4, 2005.


  1. Anybody here tried to building (and designing) an onboard preamp him/herself? (with success?)

    I want to give it a try but I was wondering how EMG and others manage to keep the required currents so low because if what they say is true a whole btc system for example only uses 400microA. A single TL071 uses far more than that (around 3milliA iirc), and other (read better ;)) opamps use even more than that.

    Anybody have any resources about this kind of stuff? (building buffers, filters and other preamp components with low current draw [of course also low noise and thd...])

    If it helps, my plan was to design a preamp which incorporates svfs (state variable filters, probably lowpass only) as tone controls. But if you have any links to existing onboard preamp schematics I am interested in those as well because i could probably learn from those.
     
  2. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    Yes.
    Successful engineering is about balancing the compromises to get as much of the desired performance as possible within the limitations of what you have to work with. When I started hanging around here, another TBer gave me a circuit he'd designed which performed and sounded great, but was still a bit battery hungry for most people. Having a couple of decades experience in working in RF, instrumentation and especially hifi audio, I threw together a few designs based on previous experiences in other fields for noise, dynamic range (overload/clipping), drive capability and distortion, especially those that would work off low currents and voltages. To my ears, they all sucked. Bad. So, I went back to designing for best performance, ignoring the big limitation of current demands and got large improvements sonically, but batteries would only last a few hours. Now I use external supplies for the onboard pre's a-la Alembic. Great sound and no battery issues.
    See comments above.
    A pre with a single low pass SVF is going to take a half dozen or so opamps, and for a setup similar to an Alembic Series bass about 10. That's quite a bit of current draw.

    Manufacturers have the advantage that they can tailor a specific piece of silicon to their requirements and optimise it for current draw amongst other things.

    I haven't directly answered your questions, but that's my experience. I'd suggest the MC33178/33179 as somewhere to start, but still higher current draw than you're looking for. Discrete's are an alternative too, and will likely sound a whole lot better.

    SVF's are quite generic and easy to design. Lots of resources out there.
     
  3. Hm, 6 opamps for a single SVF? I thought only 3 were required...
    But two SVFs (which is what i was planning) would alread use up a battery in no time if each only used 3 opamps and with 6 each... ouch

    I already thought about using an external power supply because it makes the design easier anyways (especially because lots of opamps require dual-rail supply (I think thats how its called...)), but if its possible to build a preamp which doesn't require that much power i would favor that.

    Hm... wouldn't it be possible to use a stereo cable and have +30v on one of the cable's conductors by putting a dc adapter box between bass and amp? Or actually even implementing this as a true phantom power circuit? (I suspect that the cable's resistance might be too high)

    I thought about discrete design as well, but i wasn't able to find any good information.

    Could you please point me to some good ones?

    I was just checking the TI site and they do have a few low-current opamps but the really low current ones have tiny slew rates. The first few with a good slew rate (>2) all use at least 0.5 uA (which isn't that bad compared with most others i guess) but i'M not sure about distortion etc. (didn't have time to read all the specs)
     
  4. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    Three for the SVF, 4 if you want variable Q. Plus a buffer for each pickup and one for the output, that's seven.
    OTTOMH I'm not sure how you'd do it.
    Yes. Cable resistance is trivial.
    For the buffers and drivers at least, not the SVF's.
    I'll see what I can dig up, but I need some sleep first. I mainly use the old Nat Semi Audio design guides or Jung's Opamp cookbook.
    Distortion specs are irrelevant as they don't describe the spectrum of the distortion, which is what gives the characteristic tone. All opamps sound different.
     
  5. slinkp

    slinkp

    Aug 29, 2003
    brooklyn, NY, USA
    Craig Anderton's "Super Tone Control" (from the old EPFM book) is described as a "variation on the state variable filter". It uses a quad op-amp (4136). Might be a useful starting point. Requires a bipolar supply, but should work with a pair of 9V batteries... current draw is given as +/- 7 mA. I dunno what that translates into in terms of 9V battery life.
     
  6. Passinwind

    Passinwind I Know Nothing Supporting Member

    The Active Filter Cookbook, by Don Lancaster, is something of a classic. Easily found at Amazon or Lancaster's website.

    I also do the outboard supply routine. In the past, I've used a regular XLR, with the shell carrying ground, and a bipolar supply and the bass signal on a three conductor + shield cable. There are 4 or 5 pin connectors out there that'd be better though.

    I've built many SVF circuits. I've come to prefer the plain old Fender tone stack, but with some impedance scaling, sandwiched between two opamp gain stages. A simple two opamp fixed frequency mid filter is pretty hip too, for my taste. YMMV, but have fun whatever you try.

    BTW, my current onboard pre was built with NE5532, but the next one will probably use OPA 2604, since someone set me up with a bunch for a nice price. Both will suck batteries right quick though...
     
  7. 4 for variable Q? Isn't it possible to implement variable Q with a pot?
    Trivial? (the formals and stuff are trivial, but i'm not sure if that is what you mean
    No problem, and thanks in advance.
    TI usually has distortion/frequency graphs, but in general, i would go for as little distortion as possible i.e. no opamp "tone" (if its <0.05% over the whole spectrum it should be pretty inaudible)

    Don't have that book.

    A standard 9V battery has ~120mAh so you can calculate it...
     
  8. stroy05

    stroy05 Supporting Member

    Feb 21, 2001
    Houston, TX
    Endorsing Artist: Fodera Basses, Noble Amps, Aguilar Amps, JHAudio Inears
    I would also like to see what you have.
    I am also in the process of getting a preamp designed.
     
  9. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    A 4th allows independent adjustment of gain and Q.
    How long are you planning your cables to be? Kilometers? Cables of 10 - 20m or less will have a couple of ohms resistance, max, and some supply decoupling onboard will make even that small amount irrelevant.
    They're not what I'm talking about
    This is a huge subject and one that I don't really have time to get into here, but standard THD measurements have little to do with the actual audibility of distortion.
     
  10. Ah, ok.
    I just wasn't sure how much Ohms/Meter a normal cable has, and didn't know exactly what you meant by "trivial".
    What are you talking about?
    Hm, I can't see how this could be unless the full-power bandwith is exeeded by the audio spectrum, well, doesn't really matter I guess I will have to try a few preliminary designs anyway.