I’ve made the mistake of not being a hater at all in this thread until I post build reports. I have builds ready to post but have been waiting to record some demos to include. I guess I take the “player” out of “Player Hater’s Ball”
And I just might be able to get to that after all. Recent health issues were making it seem highly unlikely, but I knocked out about half the build this afternoon, so hopefully I’ll just need one more good day to get it done. Just more of the usual boring clean preamp shizz, but hey…that’s what I do!
This build was a bit of an ordeal, mostly self inflicted since I stuffed the components by memory, effed up two resistance values, and ended up having to pull everything apart four or five times, destroying a couple of pots in the process. But the good news is that I finally worked up a tuning that I really like, and also came up with an SMD opamp pairing that works much better than the usual through hole suspects. This format was originally @Tassieviking’s baby, but I jumped in and created my own version a while back, was never satisfied, and moved back into my modular hybrid surface mount ones after half a dozen tries. I had one old unstuffed PCB left over, and recycled an unloved Tayda enclosure, two of the pots, and a lot of the offboard wiring.
I just did a quick listen with my acoustic guitar through a Spark 2 amp, which worked great. Looking forward to hearing it through my more uptown rigs later today, and seeing how it does with bass as well.
Schematic:
This is a fairly straight forward non-inverting opamp gain stage, Baxandall style Bass and Mid tone stack, inverter stage to keep input to output polarity the same, and then a second order Sallen-Key low pass filter, with swept frequency and variable resonance. Frequency sweep range is ~300-8.2kHz, resonance is variable from full cut mode to about a +12dB resonant peak. The inspiration for the treble section is mostly taken from Alembic and Wal onboard filter preamps. Although this build was a through hole board with SMD opamps on adapters, you could duplicate it functionally with two modular hybrid SMD boards available as a share from OSHpark via my GitHub repository here: https://github.com/Passinwind/PW3B-LPF/blob/main/V4 Schematics and BOMs.md
The repository also links to parts BOMs on Mouser for the boards. Pots come from Small Bear or Stompbox Parts; I can share graphics files too, but there's a new revision of the through hole one in the works, which will need a different drill pattern than the standard PPCB four pot one used above. This allows a considerably better looking layout, but there's no telling when I might get to ordering a test batch. Personally, I greatly prefer modularity, as I can do many mods without even pulling the boards out of an enclosure or instrument. But I know the prevailing culture here frowns on extensive offboard wiring and doesn't necessarily love using lots of SMD parts, which I also greatly prefer. But my preferences make for a build that takes up to 5 times as long, and doesn't make for great internal beauty shots, so there are definite tradeoffs, as usual.
LTspice sims for the resonant LPF response curves at max resonance, an intermediary resonance, and minimum (full counterclockwise):
That's my import myrtlewood Breedlove with a basic Fishman Presys through a DIY 500 watt rack mount amp and DIY 8" + horn cab, FWIW.
And for all the bass haters, suck on this: https://www.passinwind.com/tunes/Doggy_Snuggles.mp3 . Signal chain: Crystal Lettuce Silver Edition, PW3B-LPF, Passinwind Cabintech 3680 chorus/delay beta test pedal.
I added a new demo recording in my build post, I think my entry is complete now.
I also put up a revised schematic, as I did an opamp swap to see if it would improve the noise floor. Maybe a tiny improvement, but if nothing else now it’s two of the same ones. Because it’s a surface mount part I was able to do the swap in well under ten minutes, with no need to pull the PCB out of the enclosure…yay!
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