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The Passinwind Open Source Preamp

#821
Passinwind said:
Unexpected update:

I ordered a test set of SMD boards for the standalone LPF on Tuesday. OSHpark comped me a service upgrade to their fast track and somehow the boards arrived in my mail box this morning. I went big on the tuning range at ~275Hz-10KHz for my first build, added a few dB of extra boost capability, and just finished initial bench testing. Looks great so far, totally stable and the dual audio taper pot in reverse trick seems to work well. I can't wait to have a listen tomorrow.

.................................... :bassist:

View attachment 4277684
Click to expand...

this looks awesome - adjustable LPF and resonance boost, right? How are you liking it?
 
#822
OJ Dorson said:
this looks awesome - adjustable LPF and resonance boost, right? How are you liking it?
Click to expand...

Yep.

So far I've only done bench testing to confirm correlation with my software model, but that looks spot on. I'll start extensive play testing on the new one tomorrow, as I just finished repairing and blueprinting a pedal build of the original one from a TB luthier to have on hand for a control. It's been quite a while since I played around with a stock one and it was cool to reacquaint myself with some of the design decisions I made way back when. I happen to have a pretty nice parts Jazz Bass in for a little work right now too, which will be good for a reality check as my personal basses are pretty quirky and don't even have preamp bypass capability.

I noticed that Tayda are carrying several values of reverse log taper dual pots from Alpha now, and very inexpensively at that. I'll be ordering some today; although a lot of the values are out of stock at the moment they do have the two I'm most interested in on hand, a few hundred units of each.
 
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#823
Passinwind said:
So far I've only done bench testing to confirm correlation with my software model, but that looks spot on.
Click to expand...

Gratifying to see the ball is still in play with this project. The first version I built in a box (and promptly had stolen :banghead:) was already impressive.

I think I need to do up a replacement. I recently managed to buy an ACG-01 so now i have two preamps to play with. And I finally managed to score the Alembic pickups I was looking for. I also still want to get a set of Rautia multicoil pickups.

I’ve had this build project on my drawing board for about 15 years now. And I figure that between these two top notch preamps and pickup sets, I should be able to get at least one active bass I can completely fall in love with.

Maybe 2021 will be the year it finally happens. Which would be cool considering I’m getting close to the price of a new MIA Fender just in the parts I now have sitting in the bin for it. :laugh:
 
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#824
Update:

Extensive listening tests with both new standalone LPF boards went really well yesterday, with both fretted and fretless passive and active basses. So I've built up one stomp box version and will do v2 today. I have a fairly long laundry list of experimental configs I want to try, concentrating on per-pickup mixing as so many people have requested, and/or integration with one or two "conventional" EQ bands as per the original design, and that process will be front and center this week. Progress...what a concept!
 
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#825
Passinwind said:
I noticed that Tayda are carrying several values of reverse log taper dual pots from Alpha now, and very inexpensively at that. I'll be ordering some today; although a lot of the values are out of stock at the moment they do have the two I'm most interested in on hand
Click to expand...

These have come in now and my new breakout boards for the pots should be in my mailbox this morning. As expected, section matching tolerances don't look great at first blush, but I'm still hopeful that I can get some useful results and better insight into what component specs I really need to keep things moving in the right direction. My wide sweep pedal that features up to +15dB boost capability has been rock steady and I'm looking forward to testing with a wider range of basses and oter instruments. I have my eye on a medium scale neck-through sixer that's been sitting unfinished in my friend's shop for several years and will pitch him on building it up with some custom low impedance pickups, because why not.

The first gain board went off to a TB luthier who's already installed it and reported very positive results. Now I just need to vet a few component subs based on the current long term stocking issues that are so common in the industry at the moment, but so far I haven't seen any deal breakers. My goal is to have default specs and BOMs done by month's end, but early adopters can PM me any time and we can work through specific application cases.
 
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#826
Here's yesterday's test build using the 80 cent Tayda/Alpha reverse taper pots with my new breakout boards:

PW23B_SMD2.JPG



Unfortunately I appear to have torched one film cap slightly, which in turn knocks available gain down at high resonance settings, but by now I've come to accept that hand soldering these things is always going to be a bit difficult and chancy, so off to rework it goes this morning. On the bright side, the control sweeps look immensely better than using linear pots and the interim 50K frequency pot value (what Tayda had in stock) still gives a sweep from ~350Hz - 8.5KHz.

So, my questions:

How low do you DIYers want and/or need this thing to go?

Would using all through-hole caps be worth the fairly small board size penalty that brings? It's less than a 1/8" hit and would make for a much easier build for many of us.
 
#827
Passinwind said:
Would using all through-hole caps be worth the fairly small board size penalty that brings? It's less than a 1/8" hit and would make for a much easier build for many of us.
Click to expand...

I'll field that now after a long day of chasing small circuit imbalances caused by hand soldering 2% tolerance film caps. Yes, it will be well worth going to the slightly larger board for many if not most DIYers. The current test set would absolutely require a well equipped test bench for results to match up with my design intent and one should expect board rework to be a fairly frequent necessity when doing a new build. Just one slightly burned cap can drop or add boost by 6dB or more, and also move the frequency sweep end points all over the place, which is completely no bueno. With polypro through hole caps this will never happen and if 5% tolerance becomes an issue we can always hand match to whatever standard is needed. So I'm going to order up a board set like that tomorrow and hold off on the OSHpark share until I vet that layout as the presumed default offering. It "should" go smoothly, but as usual it's always best to just wait and see. ;)

The Tayda dual reverse audio taper pots have actually turned out to work fine so far after all, but I'd defintely recommend buying a few sets and high grading via hand testing for this application. I found some that spec within around +/-3-5% from section to section, and others were more like 12-15%. I experienced no stability issues even with a rogue +20dB boost cause by a burned cap (AKA don't try this at home), and with further listening tests I should be able to throw out a ballpark minimum acceptable tolerance spec sooner than later.

For those of you with legit SMD soldering experience, especially via a proper reflow rig, feel free to PM me if you'd like to forge ahead with the current spec board. I wouldn't go there without a decent o-scope, sig generator, and DMM though. Better yet if you have access to SMD rework tools such as a hot air soldering rig and/or desoldering tweezers. I can still go smaller too, but that can wait for another time if and when it seems worthwhile. On to listening tests for the next couple of weeks...progress!
 
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#829
Jeff Siddall said:
A couple of through hole devices to greatly improve chances of success, with the only a few mm difference in dimension seems like the right way to go to me.
Click to expand...

Thanks for your feedback Jeff. No point in spec'ing 2% parts if we can end up at 50-100% off (or worse, total failure) after soldering, eh? I just ordered this trial set and expect the final release version to be very similar:

PW23B_V4_5.JPG


I ended up making all the caps through-hole parts so there's nothing smaller than 1206 parts as far as the SMDs. The resistors will tolerate quite a bit of abuse and still maintain their tolerance ratings (nominally 0.1% in this case), so the opamps will likely be the biggest potential roadblock for less experienced DIYers now. But for me SOIC-8 parts are pretty routine by now and they are definitely hand solderable with a smallish tip.
 
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#835
The new board worked right off with no rework necessary and tested out just as expected:

Sweep range: 800-8.5KHz.

Max resonant boost at any frequency within sweep range: +14dB, above +1.4dB native boost at minimum resonance setting (15.4dB total).

So I now have five different boards to work through auditioning before I settle on a default tuning. The board itself needs one very minor pad location adjustment for better wiring clearance and I think it’s good to go.
 
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#838
Passinwind said:
The new board worked right off with no rework necessary and tested out just as expected:

Sweep range: 800-8.5KHz.

Max resonant boost at any frequency within sweep range: +14dB, above +1.4dB native boost at minimum resonance setting (15.4dB total).

So I now have five different boards to work through auditioning before I settle on a default tuning. The board itself needs one very minor pad location adjustment for better wiring clearance and I think it’s good to go.
Click to expand...

I want one!
 
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#839
Freekmagnet said:
I want one!
Click to expand...

Your place or mine? :cool:

I put in a good bit of testing yesterday and have two really solid release candidate specs. I'll do an actual onboard install next, I'd like to see how the Q-Tuner pickups in my Crescent Moon fretless get on with a filter preamp and also try using the Q switch format in place of the resonance pot.

Anyhow, board shares are now up for the standalone LPF and the two breakout boards for the dual pots:

[Invalid or Expired Link Removed] (LPF board)

[Invalid or Expired Link Removed] (Frequency pot breakout)

[Invalid or Expired Link Removed] (Resonance pot breakout)

I won't have a bill of materials done until sometime next week, most likely. Due to parts supply issues I want to take a hard look at a few component subs and maybe an alternative vendor or two.

And then last night I started in on a pedal oriented variable HPF board share too:

PWHPF_v1.JPG


This is based on the ones in my DIY amps and rack preamps, it's not the variable resonance HPF design I've shelved for the moment.
 
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