4 X 12 Mixer Expansion
by Sandy Sims

The PCB above includes the amps and DC-DC supply for the Mixer Expansion.
Done in KiCAD it's going to be a first as a CNC with isolation routing was used
in place of the usual photo-sensitive PCB I generally use!


I have 18 channels of input from my Phonic Helix 18 Firewire mixer but lately it's not enough. After purchasing a VoiceLive Touch 2 and acquiring a Roland piano, plus more goodies coming, I find I'm doing a lot of plugging/unplugging on the mixer (I recently refurbished) so that's it! Time to expand it!


It's pretty simple really! 6 stereo channels, or 12 inputs, get mixed down to 2 stereo channels, or 4 outputs. Each input has a level control that covers 50% to 150%. I'll be using 4560 Op-Amps as they are low enough noise for industrial line level applications, which is what this is. The real trick was to get +/- 12 volts from batteries (4 x AA's) or a standard pedal 9V input.

A cheap DC-DC converter off of AliExpress will hopefully suffice. It's a Mornsun A0524S-2WR2 that is 5V to +/- 24V. I wanted 12V but they didn't have so because the current draw is so miniscule, I've added some basic regulators in the form of a 2n3906 & 2n3904 with a 12V zener on each. The voltage doesn't have to be precise but should be stable.

I've impedance matched the inputs with 10K load to match the standard so everything should be spectrally proper.

You may have noticed each channel has a peak indicator. The LED's aren't specifically for peak as they will flicker with higher levels, but brighten with peak. That's what I want as usually I run things pretty close with minimal headroom.

The pots are linear so will have equal gain at center (Alpha plastic pots). There's also a switch to switch the 6th pair to be a stereo input. This is mostly because my Yammy drums uses stereo jacks as they are too cheap to have anything more than a headphone output (WTF?!?)


The box will be 3-D Printed as I'm into that now! I've made tons of useful items like blind holders for some fancy bamboo blinds I bought second hand, boxes for myself and Gena's tractor, hooks, holders and covers for the boat, E-Cig holders for..well..everybody, personalized coasters for my parents... I could start a section of the site dedicated to 3-D printing alone!



Schematic - Click to zoom in

Anyway, this box is a special design to hold batteries and allow a pair of work clamps to be attached so it can be easily mounted to edges of things.

So the board is being CNC milled as I type this, wish me luck!


Well it didn't turn out half bad. Used the 15 milling bit at 50mm/minute so took a while.
I had an issue with drill bit sizes. I have none between .5mm and 1mm so I ended up using the dremel drill press to expand out the tiny holes for larger leads (headers, zeners etc) which can be seen in the photo right. CNC drilling is amazingly accurate..those are 1.6mm via's!

The edges need to be trimmed still even though I could have done the edge cut with the little cnc2417, it's just as easy to do it with our big bandsaw!

Well that's about it! April 6 2018  


Disclaimer: This is not an instructional page to build or manufacture the above project, nor are there any guarantees of accuracy herein.
This page is an "of interest" discussion, and the project is intended for my own personal use.
If you have any questions, or wish to pursue this project, you may contact me (Sandra) at fresh(at)freshnelly.com
Project Copyright : Fresh Nelly Musik