It's been a few months since I wired mine, so I don't remember exactly what I had wrong in the pot wiring, but it didn't work right until Charlie provided me with the pot wiring diagrams. YMMV...
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The Passinwind Open Source Preamp
- Thread starter Passinwind
- Start date
I'll hold off. I won't have that enclosure for around eight more business days so I can wait for confirmation.
Thx!
- Dec 3, 2003
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- Designer/Toaster Tech @ Passinwind Electronics
I think I understand that there IS an error and I know what it is. But please feel free to correct me (Charlie especially) if I got this wrong.
When I wired the resonance control this is how I connected the Res1, Res2, Res3 and Res4 pads on the preamp circuit board with the schematic (the light blue labels are my additions):
I labeled these 4 points by simply taking an unpopulated board and finding that, for example, the point I labeled R1 in the diagram, which is Res1 on the board, is directly connected to one of the R14 pads. So I'm quite confident this is right. But here is the diagram on the wiki, which I think Charlie aka passinwind needs to revise:
This shows Res4 connected/jumpered to the upper wiper (middle pin), but the schematic shows that it is Res3 that connects to the wiper. I changed it so that R3 jumpers to the wiper and it works fine. Would it work the other way? Because of getting-started problems related to R11 and R14 I never actually found this out by trial. But I do have the conviction, subject to correction by the Bossman/OP, that the diagram is wrong and the schematic is right.
Caveat--Please don't rewire your work based on this post, until the master gives his ruling!
John
Hey John, glad to see you've made some real progress. And thank you for reviving this thread!
You don't really need those two jumpers on the Res pot at all, they are just a fail-safe in case the wipers open. It also doesn't really matter which is Res 1 or Res 2, just that those two pads tie in to the same pot section. Same for res 3 and Res 4. I'll have to double check the jumper locations I've used in my own builds in the morning, I am still in delirious mode from our brutal heatwave which has finally moderated just a bit today.
The last schematic I posted is missing the notes that R10 and R11 are mounted directly on the Bass and Mid pots, with an alt location for the mid cap being on the 3-way mid freq switch, as pictured. The pic on the first page of the Wiki shows those two caps mounted across the two outside lugs of the Bass and Mid pots, but that could and will be made clearer, for sure.
I'll also revisit those pesky Bass and Mid connections that I still get backwards a fair amount of the time. With the expensive Bourns pots rework is no problem, but with the $1 Alphas or whatever it's definitely prudent to get it right the first time.
Edit: Arggghhh! The Bass and Mid pot labels are still backwards for the 1 and 3 pins. I inadvertently shared the wrong board revision (the one I shared is the same as what I've actually been using myself), so I have revised the wiring diagram to reflect that for now. Whenever we get to the next revision I guess I can change the board labeling at the same time.
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- Dec 3, 2003
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- Designer/Toaster Tech @ Passinwind Electronics
Just wanted to mention that I finally had a chance to test out one of my floor boxes with my recently refretted guitar:
That worked exceptionally well and I think I'll be exploring some guitar oriented mods sooner than later.
That worked exceptionally well and I think I'll be exploring some guitar oriented mods sooner than later.
Fired up the preamp and it sounds great. I did noticed that with the Alumitone Bass Bar it seams to causes distortion. The Bass Bars are louder than normal a pickup. Is there any way to adjust the gain at the input?
Tedward
Tedward
- Dec 3, 2003
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Do you have a volume pot in front of the preamp? If so, try tacking a resistor in front of whichever pot lug the pickup wire is coming into. If not, you can build a simple resistive divider to attenuate the signal a bit. You can also change the feedback resistors (R3, R4) in the first stage to lower front end gain. Making R3 bigger or R4 smaller in value reduces gain. Edit: the second approach is likely to give the best signal/noise ratio, but that may or may not matter to you if the preamp is still sufficiently quiet, and the first way is a lot easier.
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The volume is in front of the preamp. What size resistors should I start with for the front end gain?
Tedward
Tedward
- Dec 3, 2003
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If you go with an equal value to the volume pot that should cut signal by around 6dB. If we need to get more precise you can try turning the Vol down to where the distortion goes away, then measure the two legs (wiper to each lug) out of circuit. Then we'll shoot for the same resistance ratio. That would be useful information if you decide to mod the resistors on the board as well.
So I feel like an idiot now. After checking to make sure that I had all the right parts in the right spot for the third time I saw that I used 50K audio taper (duh) instead of linear taper pots. Swapped them out and everything works great now. Thanks for a great design.
Tedward
Tedward
I just built the preamp and installed it in my vintage Ibanez and I have to say that this is one incredible preamp! I had to sell my Alembic a year or so ago to get my car fixed and I've been missing the filter preamp ever since. I love the marriage of a filter and resonance control on the top end and a tone stack on the mid/bottom. Being an electrical engineering student and gear head I'm inclined to tweak the preamp a bit to suit the bass it's in. I'm not too familiar with the ICs in the build so I've got a question for anyone who would know. Would replacing the frequency pot with another dual/linear-taper of a higher resistance, say 10k, give me a larger range of roll off? Thanks for the awesome design and making it open source!
- Dec 3, 2003
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You will want to use a reverse audio taper dual pot, if you can find one. That was my original plan. I settled on 5k linear as being the most tolerable compromise between availability and usability over the whole adjustment range. But by all means try a 10k lin pot, or maybe consider reverse wiring a 10k aud one and living with the sweep being backwards. And thanks for playing!
Thank you so much for helping me out with that. I'll see if I can't source the reverse taper pot and I'll report back when I have it installed. Do you have any plans in the works for moving on to pickups as well? You seem to be a fan of the more unconventional/musical designs and I'd love to see what you could do with pickup design.
- Dec 3, 2003
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De nada, this is pretty much the whole point of the exercise.
Well, I did build a few light beam pickups way back in the '70s, but my friends at Willcox are already doing a pretty nice job with that these days. But the luthier I am working with right now, Marco Cortes, has several cool things in the works and I intend to learn as much as I can by looking over his shoulder for a while.
It'll be very cool to see how those builds turn out. I saw in one of your demos that you were using a q tuner pickup and the clarity that thing had was impressive. What's your take on a bass with 2 q tuners in a Thumb-like position with one of your preamps on each pickup? I'm going for the Wal layout with a bit more clarity and flexibility. Beyond the fact that the control cavity for it may take up the whole bass and I've got next to no wood working tools, I'm considering doing a built like that.
- Dec 3, 2003
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- Designer/Toaster Tech @ Passinwind Electronics
I finally got to really check out a Wal at the last Seattle GTG. The owner offered to lend it to me long term, but I just couldn't swing that right then for a variety of reasons. But yeah, I think we could get pretty close to where you want to go.
For starters, maybe just having one filter section per pickup and then global bass and mids would be a little more "sensible."
Very cool! I'll keep an eye out an Namm this year. Just wanted to post an update about the frequency pot modification I did. I decided to go with a 10k audiotaper pot wired backwards and it came out perfectly; the extra cut range can really made the project come together although next time, I may go so far as to go up to 15k since I tend to like bright basses/rigs but time to time want that passive 70s tone as well. Also, I'm about to start the other project with one of your preamps. I found a Modulus body and neck that would a perfect host. I'm still torn between running 2 discrete preamp boards (with mids and bass being set via internal trip pots) or doing as you suggested and getting out the bread board and making the tone controls global. I'm still curious about the Qtuners as well but may not be able to afford them. Any other pickups that you've had great results marrying to your preamp?
Thanks again for the help!
Thanks again for the help!
- Dec 3, 2003
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- Designer/Toaster Tech @ Passinwind Electronics
I have one kicking around that I started doing some experimenting on for something similar. And I also have plenty more built up boards on hand. Let's go to PM for now and once we work it out we can post what we come up with in this thread, OK?
Sounds like a plan; I'm confident we can come up with an interesting and useful derivation on the preamp for others to try build as well
