The Passinwind Open Source Preamp

Discussion in 'Pickups & Electronics [BG]' started by Passinwind, Dec 25, 2016.

  1. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Disclaimer: For a while I've been wanting to put one or another of my DIY onboard preamp designs into an open source DIY oriented format that other TB'ers can build and readily tweak to their particular wants and needs. I've finally settled on a specific concept and am well into testing a few different versions. I'm about to send out a build kit to my first beta tester, and I will also have a few builds mounted in stompboxes and a bass or two at NAMM in a few weeks. I'm hoping to have v1.0 wrapped up a few weeks after the show, at which point I will release a board share at, where anyone can order up three boards at a time at OSHpark's usual DIY-friendly rates. I'll have zero financial stake in that, and likewise for the BOMs for Mouser and Digikey that I will also provide. All of this has already been vetted by the TB Admins, and I recently ported my status to Commercial User in anticipation of eventually licensing this or some other of my designs to one or more OEMs. I think that puts everything up front, but please just ask if anything seems shady in any way. This is new territory for me and I want everything to be fun for all involved. Or_wink.gif

    Intro: OK, so what about the pie, already? The design features pretty standard bass and midrange control circuits, with a treble section using a swept frequency, variable resonance low pass filter. Here's a graphical representation of various treble control sweeps modeled in LTspice:


    And here's a look at the Resonance control function, with Frequency set at the lowest setting:


    These are just sample filter alignments and overall gain settings, there is a fair bit of adjustability available with a few component value tweaks.

    Here's the first gen circuit board, which has already been superseded by a new one of the same size, which is roughly 2.2" x 1.67":


    Current cost from OSHpark shows three boards at $18.30, shipping to nearly anywhere included. Things like board sharing or group buys on pots or other parts will be up to whatever community evolves from this thread. Open Source means that any or all of you are free to do your own perfboard or PCB layouts, mods, ruthless criticism, and so on -- as long as you share all that stuff here. I'll be happy to provide tech support until a mature build spec is reached, and hopefully we can then have a TB Wiki featuring collective wisdom on best building practices, wins and losses with specific pickups and instruments, sound samples, and of course build pics.

    As far as overall cost, I anticipate that $50-100 will cover most build specs. Shipping from at least two or three parts sources will likely suck up a fair amount of that at first, but hopefully we can eventually figure out a way to mitigate that some.

    I've gone out of my way to use simple "cookbook" type public domain circuits and to avoid using hard to source parts like reverse audio taper dual pots. I have spec'ed fairly expensive and somewhat obscure opamps, but many cheaper and more common subs will probably also work pretty well. If that proves to be true then paring the BOM down to a single source list will be a lot more feasible.

    There you have it, my Christmas present to TB, the PW3B-LPF. If any of you are not familiar with Creative Commons licensing, here's a link to the specific license. 88x31.png

    The license link will always give access to the current schematic version. Please use that link and do not post the schematic in this thread, since TB will cache it and it will inevitably become outdated.

    The TB Wiki for this project starts here.
    Last edited: Jan 1, 2017
    gtirard, T-34, Huw Phillips and 78 others like this.
  2. Alivefor5

    Alivefor5 Supporting Member

    Jul 17, 2006
    Really nice, thanks for this. I wanna play. Subbed.
  3. Subbed!
  4. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Last edited: Dec 25, 2016
    Bushmaster and Al Kraft like this.
  5. ctmullins

    ctmullins fueled by beer and coconut Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    Oooh, nice. Merry Christmas indeed!
  6. 4StringTheorist

    4StringTheorist Supporting Member

    So so subbed! This is exciting!
  7. MPU


    Sep 21, 2004
    Valkeala Finland
    Very much looking forward for this. I think this could fit nicely to my ongoing headless carbon fiber bass build.
  8. 40Hz

    40Hz Supporting Member

    May 24, 2006
    Astonishingly generous in this era of IP and patent madness and litigation.

    Thank you!
  9. wcriley


    Apr 5, 2010
    Western PA
    Damned hippy giving stuff away!
  10. Freekmagnet

    Freekmagnet Commercial User

    This is super cool. Sub'd!
  11. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    The first taste is always free... Or_wink.gif

    Just finishing up the first build with the second gen PCB. Bass and Mids are on a concentric pot, with separate Resonance and Frequency pots. I think the pots cost more than the rest of this build, but I've already used much cheaper ones in one stompbox mounted build perfectly happily. The trick is going to be finding linear dual pots with long enough bushings for basses with thicker tops. Not sure if my beta tester/luthier friend is going to use a mid shift switch or not, but fine tuning the mid frequency is really easy in any case since the primary adjustment cap is right on the mid pot. I've set it up so that the mid control can be omitted completely, and the resonance control could just be a 2 or 3 position toggle switch, or switched via a pull pot. I think a lot of people might do fine with just cut-only and one peaking function, such as +3dB or +6dB.

    Last edited: Dec 27, 2016
  12. pickles

    pickles Gold Supporting Member

    Mar 23, 2000
    Ventura, CA
    Very cool! Any reason you didn't do a resonant HPF for the bottom end? That would be amazing alembic style.
  13. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Heh, that thought didn't elude me. For many years that was out there as an eventual design goal. There are a few reasons that's probably not gonna happen now though. I'm really not into cloning other people's work too closely at this point in my journey, especially outfits I really respect like Alembic, Wal, ACG/East, etc. I've made a few exceptions for Fender/MM-ish things that seem to be regarded as public domain these days, but even then I've tended to lay on some pretty heavy tweaks of my own. But hey, if someone else spins off a resonant HPF section as a design fork, good on them. Yel_wink.gif

    This preamp design is strictly a product of "what if" being applied to an existing design, based on my rack preamps and bass amps. That design has a very unusual quasi-passive treble stack that people typically either love or don't get at all. But pretty much everybody has liked the bass and mid sections a lot, especially in my outboard builds that use a variable HPF that's somewhat similar to @fdeck's latest design. I tried modeling several alternative treble control formats and eventually revisited something @Spacecase had sent me a while ago, which I mashed up with some of @RobbieK 's and @khutch 's ideas and then added a few twists of my own to try to come up with something relatively simple and maybe even economical for DIY'ing. I just literally erased the board layout traces for the old treble stack and tried grafting this LPF one in its place. Much to my surprise, the overall board size got smaller rather than larger. So I ordered three prototype boards just on a flyer, never thinking that would amount to much. But the very first listen was an eye opener, and for the last few months I've been working on the refinements that led me to post this thread. Hopefully this is just the start though!
  14. 12bass


    Jan 2, 2003
    Victoria, Canada
    Hmmmm... new preamp build in 2017???? Rough idea of input impedance?
  15. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    ~400K ohms in my prototypes, but I might even want to go a little lower for some pickups. The idea is to neuter the pickup's passive LRC resonant peak in order to let the preamp's EQ do its thing as effectively as possible. One potential benefit of that approach is that you might not need especially uptown pickups to get good results. OTOH, a natural design fork might use a buffer section in front of this preamp for those who want to retain more of their pickups' inherent passive character. I've played around with that a bit and I felt that some tweaks to the frequency sweep range might be needed to avoid intermodulation distortion effects and "double dipping" when the sweep frequency coincides with the passive peak.
    Last edited by a moderator: Mar 30, 2021
  16. 12bass


    Jan 2, 2003
    Victoria, Canada

    Was thinking of possibly building one to replace the Bartolini NTBT in my Pedulla Rapture 5. Erno recommends 1M pots for Q-tuners, so I'd gather that he'd also suggest a choosing a preamp with a high input impedance (though he also recommends not using on-board preamps with his pickups, preferring passive-only). I'd guess that 130K would seriously tame the Q-tuner's resonant peak.
    Bushmaster likes this.
  17. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yeah, I don't think Erno would like my approach too much, but vive la difference.

    I have a different preamp in my bass that uses the BL-4s, but the input impedance is roughly the same. Works very well for my purposes...and the bass is actually much less dark than it used to be with various higher impedance commercial preamps in it. Different strings probably have a fair amount to do with that too though. Yel_wink.gif

    I'm a big fan of including some lossy elements in most of my designs: transformers, intentionally non-classical impedance loading, and a few other tricks I'd rather not get into. I don't use passive basses at all though, not even a bypass switch. Go figure!
    Last edited: Dec 27, 2016
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  18. Brad Johnson

    Brad Johnson Inactive

    Mar 8, 2000
    Gaithersburg, Md
    DR Strings
    You are truly a gentleman and a scholar.
  19. :bassist: Thanks Sandy "PW" Claws! :hyper:

    The Oshpark-order Trifecta: One for my Jaguar, one stuffed in a Hammond, and a spare for replacing whichever one of the first two I screw up.

    Firing up the soldering iron in anticipation...
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  20. rufus.K


    Oct 18, 2015
    Sub'd for sure
    Dean N likes this.

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