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What do want from an onboard preamp?

Discussion in 'Pickups & Electronics [BG]' started by xyllion, Jul 23, 2004.


  1. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    I'm considering the possibility of designing an onboard preamp and then publishing the schematic on the Internet. Included in this project would be ordering a run of printed circuit boards and then making the bare boards available to everyone at my cost. Depending on how many people want these boards, cost would likely be anywhere from $3 per board up to $33 per board if it were just me. The actual cost would depend on how many we order.

    I actually started this project in the past, but then shelved for lack of time. Now I'm thinking of reviving it. Before I do, I''m asking for input. My goals are a very simple preamp perhaps without any tone shaping, but I know others might have different opinions. If you are interested building your own preamp from a bare pc board, what would your requirements be? I can likely make the circuit modular so that you can use as much or as little as you require.

    If this project interests you, let me know.
     
  2. RyanHelms

    RyanHelms

    Sep 20, 2003
    Cleveland, OH
    OMG, everything every market preamp has to offer ;)
    Seriously, an elegant, well voiced 3 band EQ would probably suite everyone's needs. Would sweepable mid be too much? I'ld be happy just to get this mag/piezo blend figured out!

    BTW, passive would even be cool, since I've always been told it's better to cut than boost...
     
  3. Jazz Ad

    Jazz Ad Mi la ré sol Gold Supporting Member Supporting Member

    Yeah a buffered blend is a must have, so you can mix any kind of pickups including piezos.
     
  4. keb

    keb

    Mar 30, 2004
    Another vote for sweepable mids and buffered blend.
     
  5. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    I'm doing this myself at the moment too. My design criteria are,
    - quality, low noise, stable buffered inputs for each pickup
    - gain and blend
    - high overload ability
    - low impedance, high drive ability output circuit.
    - minimum gain, ie only what's needed
    - proven good sounding topologies, ie no TL072's.
    - possibilty to add an input shunt resistor/cap for each pickup on the PCB to simulate the load that passive circuits provide. Some people really like this effect.
    - I'd rather EQ at the amp, but depending on the number of knobs people want, a 4 band would be nice too, with sensible amounts of cut/boost, say 8-12dB. This also give people the advantage of building something that's not commercially available. Size constraints might make this impractical with components most people can actually solder.

    I think what you're suggesting is a great idea, and similar projects I've seen and/or been involved in in other audio fora have been great successes.
     
  6. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Doing this yourself for yourself or do you plan to share the result? If you are planning to share the result, I'll probably just back off and let you run with your design.

    Yep, that is definitely a constraining factor. If I were to design it to be professionally assembled, I would pick the smallest parts available, but that simply will not be practical in this case. I'm not sure how feasible a 4-band eq will given the size constraints.

    If stacked knobs are required, then we will have to live with what is commercially available. I want to avoid custom ordered pots since they get very expensive and minimum orders are not something that I want to deal with. In the end, I would like the PCB to be the only custom part and the other components should be easily ordered by anyone that ones to build one of these.

    Thanks. We'll see how it all pans out. If I can make the design modular enough, perhaps we can more or less satisfy the desires of everyone involved.
     
  7. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    I'm doing it for myself, because I don't like most of what I've seen used out there, but I have a small manufacturer interested, based solely on them trying out some of the tube amp designs I've built in the past. If it were to become commercial, possible on a small scale, pricing would be in the John East design ranges.

    Please don't back off, I think it's a great idea, both for the ability of some players to add something to their instruments, as well as some learning of basic theory that can help people understand what they're actually buying/using. I'd be glad to contribute to the project in any way I can, but I don't have the PCB angle sorted for myself, and not being in the US would make distribution of the PCBs much more difficult.

    Maybe with two small circuit boards it might be feasible laid out like the Ken Smith preamps. Three or four band EQ using gyrator or baxandall topology was my first idea, maybe using TLE2024's or something similar.

    Definitely.

    Yes. And try to get people to commit to an order before you actually have them made.

    That's also why I suggested something similar to the Smith designs. Two small PCBs can still be made on the same board and snapped apart for little difference in price if they're designed well.
     
  8. RyanHelms

    RyanHelms

    Sep 20, 2003
    Cleveland, OH
    A "catch-all" PCB using easy to find parts is a great idea. (a far cry from my the end results of my perfboard nightmares...) From what I'm seeing, between xyllion and Dharmabass designing and TB'ers chiming in with wants/needs, this ought to turn into something everyone can use.

    Me too. A transparent gain stage can do wonders, even w/o built in EQ'ing. Perhaps a switchable scoop that's adjustable at a trim pot on the board would still allow someone with a two knob layout to use the circuit.

    What are some good chips to work with guys? I'ld like to forge ahead with my own tinkering since I feel like I'm learning something.

    Designing around the typical Radio Shack stock may prove to be the real challenge. There weren't any dual opamps at the local store....

    Did I mention the only good place to get parts within 40 miles of me closed up shop?
     
  9. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    I highly recommend doing your shopping online. http://www.digikey.com and http://www.mouser.com are great sources for parts. I also like tapping into the free sample resource. Designing PC boards professionally has also made it easy for me to get samples for my home brew projects.
     
  10. Funky Doctor

    Funky Doctor

    Aug 28, 2003
    Australia
    Some really good sounding dual opamps I have had amazing results with are the Burr-Brown OPA2134's. They are one of the best sounding opamps ever made, and arguably the best. Although you may have some oscilating problems if you have long leads and stuf.... bah. Enough of that. That's my reccomendation.
     
  11. A9X

    A9X

    Dec 27, 2003
    Sinny, Oztraya
    I agree with the 2143/4134 recommendation sonically, though it is a bit current hungry for battery operation. But any sensible player should have a small battery charger and a few NiMH or LiON rechargables with a fresh set switched in right before a gig.

    For me, that's not an issue as <i>all</i> my basses will soon be powered remotely similar to my Alembics.

    Brings an important question for all those interested in this project;
    <b>What should the average battery life for this preamp be?</b>
     
  12. me im in preferance of an:
    active/passive switch, so if the batt runs out its switchable to passive, or if you're playing a song taht souits an active tone then straight after it the song suits a pasisve tone, there ya go

    2 band eq, not big on mid controls on my intruments althogh i can live with them

    i prefer humbuckers, single or dual, but a coil switch always comes in handy, so does a slap contour switch, i like stacked controls as well

    in reguards to battery life..nothing ive seen beats a kubicki so far, 18v preamp last 1000+ hours
     
  13. David Wilson

    David Wilson Administrator Staff Member Administrator Supporting Member

    Oct 14, 2002
    Lower Westchester, NY
    Not something you'd really do at the preamp level, IMO. As well as adding to complexity/cost, it'd be of no use to people using single coil pickups or humbuckers with no coil switching available (Bart #9j's, Sadowsky etc).

    The modular approach makes a great deal of sense, could let people choose from 2 band / 3 band / 4 band(?) versions.

    Not sure how much hassle this would be, but I would like to have dip switches or something to select different frequency centres - i.e. high of 4.1 khz or 6.5 khz, low mid of 250/400
     
  14. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Lot's of good ideas here. I need to actually start thinking this out because it turns out that I might have the opportunity to save some money by combining these boards with another PC board order that I'll be doing soon.

    As for op amp choices, we may be able to allow each person to choose their own if we pick from a group of parts that have common pinouts. Let me look at this as well. I have no love for the TLO72, but we do need to consider the tradeoffs of battery life vs. audio quality.
     
  15. chucko58

    chucko58

    Jan 17, 2002
    Silicon Valley, CA, USA
    I paid for all my gear myself. Well, me and MasterCard.
    Hi Bob. I've studied this topic for a few years, and I have some definite feelings on this.

    First, KISS (keep it simple sweetheart). A multi-band parametric EQ belongs in a rack, not on a bass. 3-band cut/boost EQ with semi-para mids is about the most sophisticated on-board tone control I could ever want.

    Second, battery life should be in the multiple hundreds of hours if at all possible. That way even a player who's gigging every night only has to change batteries about once a month. Most of us "weekend warriors" could go a year between battery changes. Make sure it can be bypassed in an instant if the battery fails! See also rule #1.

    Third, noise levels should be kept as low as practical. Bass pickups combine a moderate DC resistance with a large inductance, so input noise current (especially above 1 KHz) is critical, noise voltage somewhat less so. Discrete FETs for the input stage can be a win, but so can some bipolar op amps. See also rule #1.

    Fourth, the preamp will typically drive a load of at least 500 pF and usually 2 nF or more. The resistive component is usually in excess of 100K ohms. Designing an output stage that handles capacitative loading gracefully without losing high frequencies is tricky - and if you put the volume control after the output, you lose highs anywhere below max volume anyway.

    Fifth, gain and output level requirements are modest at best. Many bass pickups can be persuaded to generate several volts with aggressive playing, and many bass amps and rack-mount pres will clip when driven with more than 2 V peak-to-peak. So tell me why you need an 18V supply again? Unless of course you have piezos (which need buttloads of headroom and extremely hi-Z inputs), or you're driving a power amp or a long run of balanced cable directly. In which case you should probably phantom-power the thing through an XLR cable and be done with it!

    Sixth, conversely, there's no point in throwing away dynamic range if you don't have to. Many FET input op amps will be hard pressed to swing more than a volt or two when powered by a single 9V battery, mostly because of input common mode range limits. That and the current draw make me think FET op amps are a bad idea for on-board work.

    I'm not one of the golden-eared who can hear a difference between most common op amps when they're used within their limits. Having said that, the On Semiconductor MC33178 used by Carvin is not a bad device for the input stage - the noise specs are right for the application, it has very modest power requirements, and the frequency response and slew rate are adequate for low-gain audio use. Better still, it's cheap!

    Bob, phone or email me if you want to hear more of my biases. Or try some of my collection of low-power op amp samples. ;)
     
  16. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Will do Chuck. Haven't decided how or when I'll approach this. I was going to piggyback this on another PCB order that I needed to make, but it turns out that I won't have time to design the preamp before I need to get the other order out. I have to order by the end of this week to meet project deadlines.
     
  17. Moo

    Moo Banned

    Dec 14, 2002
    Oakland, CA
    Wal clone! :hyper:
     
  18. Funky Doctor

    Funky Doctor

    Aug 28, 2003
    Australia
    I know I'm resurecting an old thread, but what ever happened to this preamp? did it every go ahead? I was excited to see some results. Did it just die?