This time I decided to build something very simple, meant to be somewhat along the lines of an Ampeg and/or G-K vibe. It kind of morphed into something much different, but so goes DIY building. I started with some readily available project circuit boards from ESP and Welborne Labs. The ESP one is Project 94, the Universal Preamp/Mixer. The power supply kit from Welborne is this one, spec'ed for +/- 15 volts. I already had a nice toroidal transformer that I salvaged from an old BGW crossover. I went with OPA2132 opamps for the first two stages, and LM4562 for the mix bus. I wanted a transformer-balanced output, and settled on the JT-11-DMCF from Jensen. The pots are from Bourns and Noble. I found some really nice stepped Noble 50K audio pots on eBay that were NOS surplus from the Hafler factory. Finally, the case came from Par-Metal, and is fully RFI shielded except for the front panel. Front Panel Express will be engraving graphics for me in the next few days. Initially this build was meant as a simple preamp/effects mixer with just bass and treble controls in the Baxandall configuration. Before I did the effects mixer part I tried the basic preamp out on a few gigs and had some other players use it at some open mikes too. It sounded really good -- actually better than my Fender-based design from a few years back, which I've happily gigged with a ton. So I decided to eschew the effects mixer and try to come up with a workable active midrange control scheme with the second half of the ESP board. I lifted ideas from Ampeg and G-K schematics and came up with my own take, using two opamp sections. I started at ~800 Hz for the center point, which has often been a keystone frequency for fretless bass IME. I considered a swept mid or two, but that is really something I want to do as completely my own take, at a later time. I already own a great sounding parametric EQ if I feel the need for that anyway. I gigged that configuration for around 6 months pretty contentedly. But at a few GTGs we noticed that the mid control didn't always do all that much with other guy's basses. So a few weeks ago I took another stab at this, and moved the nominal center point to around 620 Hz while I was at it. I should mention that all this time the design was essentially optimized for one cab, my fEarful 12/6, and also mainly for my EUB. Now we're cookin'! Not one to leave well enough alone, I dive into the effects mixer idea some more. I get it working pretty well, but start seeing some polarity/phase issues. After a few days of head scratching I realize that I have one unused opamp section sitting there, so I re-do the whole gain structure yet again and insert the extra opamp inline. Much to my surprise, noise and distortion specs both improved, and the effects mixer section works phenomenally well with my Hafler T-2 tube distortion widget and Boss GX-700 modeler. My reference mixer is a Crest XR-20, and I'm easily matching the results I get using that now. Being able to blend in just a tiny bit of modeled signal is pretty cool in my book, much nicer for my purposes than using an inline approach. So, here are a few pics of the build: As I said, graphics are currently in process. Measured specs and basic frequency response curves of the tone controls: Some of the more unusual tone control response curves: Any questions?