Things have progressed.
The last image I posted was of a pretty boring looking mixer, long pot shafts without knobs and a front panel without any labeling. Things are looking a little more schmick now, though the labeling is still only done with an old school Dymo label maker.
The power supply was made up using a LM317HVT, the high voltage version of the basic LM317. The 40V supply required puts it just outside of the 37V range of the basic LM317. A 36V transformer is feeding this through oversized everything (the bridge rectifier is capable of 40A peak and the input filtering is a huge 5600uF 100V cap. There isn’t anything special about this supply other than being particularly sturdy, I wanted to be able to throw it into a bag without any worry it would be damaged. The die cast enclosure feels very sturdy and weighs in at 1.5KG all up.
PCB redesign is complete now, I have added a few more features to make the debugging easier should that be required. I also changed the footprint of the output transistors to accommodate EITHER the BD139/BD140 or TIP31/TIP32 devices. These both have the same pinout but with a slightly different package style (TO-126 vs TO-220). I’ll try both and post results on differences in the sound. As as aside, It seems that ST Micro and Fairchild (now part of ON Semiconductor) list different pinouts for the BD139/140 series transistors. Just a matter of turning the device around 180degrees but quite annoying:
Anyway, have also been shopping around for PCB suppliers who cater to slightly bigger runs than I normally get and am going to be supplying the PCBs for this project with EING (gold plated) finish. They will of course be ROHS too for everyone in Europe. I’ll be ordering enough sets for 10 mixers, and after the builds I’ll be doing for friends I think I’ll have a half dozen to sell to people. These will only be available with a 3mm aluminium front panel.
On to the sound, This has been lots of fun to test! I use a set of Genelec speakers at home and have certainly gotten things quite loud and shaking. The mixer is everything you would expect from the classic american discrete circuitry, smooth and creamy with a very solid bottom end. I won’t stray too far into this kind of writing as a). I don’t know how to do it and b). writing about sound is like dancing about architecture (Zappa I think?).
I also want to say thanks to everyone who has written in! I’m happy to answer any questions and always appreciate the kind words. I guess I’ll next be posting about his when I have the next lot of PCBs and front panels in my hands.
Attention Melbournians! I’ll have a unit to loan out for gigs (bigger things than your best mates house doof) in the local area. I find the social media thing a bit of a chore so will be offering the classic sound in exchange for a post about the mixer. Get in touch with me for more information.