A Mixer MIDI Control Hardware Hack!


by Sandy Sims

  I decided to take a break from a major project I've been working on (not music related) to build yet another project!
This is something I have wanted for a long time, and because it'd only take a couple days to build, I thought it'd be nice to have.

I presently have a Phonic Helix Board 18 channel Firewire mixer, and I love the thing...except the Firewire...but one thing that I'd really like is to be able to MIDI control some of the level controls.
The reasons behind this are:
- I don't have to remember to do them on the fly during a performance
- It's almost impossible to make adjustments in the middle of a song.
- It's really cool!


  Here's an example of the problems I've had:

 The Mic input is feeding into a vocoder that becomes active during some parts of a song. When the vocoder kicks in, the original Mic audio is still being fed into the mixer's bus, which sometimes overpowers the vocoder and ruins the intended effect. So as a stop-gap solution, I reduce the level some on that mixer channel for that song. Then I forget to turn it back up for the next song after my hands are full with the keyboard or a guitar!

  Here's another problem I've had:

  I want a vocal "chop" effect in one part of a song, and the level needs to be so high coming back from the computer that a feedback starts, so all you hear is the first one or two chops, then squeal!

  Well I've had it with all that. My requirements are light really. All I want is a Mic level down, Guitar level down, Mixer FX level up, and secondary mixer output level up/down. I had less requirements a couple days ago, as can be seen in the diagram to the right! A small PIC16F57 should be able to do the job.

The ideal thing to pull up/down slide or rotary pots is to use a CDS cell, or photo-resistor. This allows isolation, polarity doesn't matter, and they're a bit slow so no "clicks". Dimming an LED on it will change the resistance across the mixer control. I have used this method on other audio circuitry, and it's works great for a "quick 'n dirty, if you're not too picky about linearity and gain curves. Normally it'd be in a Wheatstone bridge, but I don't want to modify the expensive mixer too much!

  As long as it can pull down (or up) levels in rough proportion to an automated knob in FL Studio, it's good enough.



  Another issue is wasting a whole MIDI output on something so simple. I decided to split the MIDI that goes to the main keyboards at the source, inside a UC-33 MIDI control surface. I would suggest this to anyone, unless you have a surplus of MIDI outputs :P

This was a simple matter of amplifying the MIDI output and sending it to a new jack. This works better than just splitting it. As you can see, the UC-33 has no driver transistor for the MIDI out which is against MIDI rules, or so I thought!

An output into "static electricity world" straight from the processor seems a tad cheesy to me, but then hey what do I know right?
The 100k resistor is to stop any leakage to the output.  There, a new MIDI output.

The little board (picture at top of page) will only respond to MIDI Channel 16, which is ok as I never use that one anyway LOL!

OK enough goofing around! On to the circuit and programming.

   The schematic shows how simple the little thing is. I was going to photo-etch the board, but hand -drew it instead. A laundry pen works well in Sodium Persulphate.

  Now there's 2 new additions to the board as well. A fan speed control so it can be slowed to a crawl during recording in the studio, reducing mic noise.
I had to put a fan in the mixer because it gets so hot the caps evaporated!
  The other addition is a "silent metronome"  LED. (Orange) I know it's hard to crossfire the visual cortex with the rhythm center of the brain, but might as well give it a try. It's funny how well lights go with the music, but remove the sound....So far I'm useless at it...

  An opto-isolator, 6N137, is where the MIDI signal is fed in, and that is picked up by Port A,2 on the PIC. The  Main routine manages the PWM signals to the LEDs. The resolution of the PWM is only 64, but is plenty.

It's amazing how fast the 16 MHz crystal gets chewed up by PWM. First it's /4 for the PIC, then /48 by the main routine, then the PWM period is /64! So all that's left for PWM speed is about 1.3 kHz. The caps on the outputs smooth that off. I forgot to mention I put diodes off of each port RC,0-RC,4. So I didn't need to use big caps!

I wrote a rather complex MIDI RX routine because this PIC doesn't have a USART ( a challenge. again! ) but it seems to be stable.

The CTRL RM and FX pots on the mixer are stereo, so 2 CDS's will be needed on the FX pull-down, and 4 CDS's on the CTRL RM because I want to pull it up or down. This means I can't finish installing the unit inside the mixer until I can get more of those dollar store night-lights which have CDS cells in them :|

I'll post a couple of photos once that's done. This will be so sweet!  I can't wait!

BTW: the .asm / schematic/user guide/ etc files are in a .zip below, Cheers!


 Update March 19 2013:
Well I managed to get the board, power supply, and the fan installed, and I must say things are working better! With that fan off the mixers mains, there's only a light hiss in "dead recordings" and with a proper voltage driving it, the whole thing stays much cooler.

  As can be seen in the picture above, which I've marked out mostly for my benefit, a nice place was found for the board. It's very important to keep it away from any really sensitive circuitry, like the mic inputs, or a hum/tone may occur.  Happily, there is a nice metal ground plane meant to protect some of the FX circuitry, or the other circuitry from the FX module.

  The LED/CDS isolators shown are Channel1, Channel 3, and Group 1/2.  Yes, I changed the pull down/up stereo isolators over from CTRL RM to Group 1/2. This is because this can give me another layer of sound control of only the live elements...i.e. stuff that's coming into the mixer, not the PC interface. This will be more for effects in a build I think.

  The LED/CDS isolators not visible are the stereo FX pull-up, and the 1-side CTRL RM pull-down. Apart from the headphones amp, I only use the one output because the big Subs are just one channel. I chose left.

  The 500 or 600 ohm min resistance of those cheap CDS cells did better than I expected. The controls deviations average about 80% if they're within reasonable positions (1/4-3/4). The CDS on Channel 1 (my mic channel) is a better quality component that drops to only 50 ohms, because I wanted to be able to completely "kill" the mic input.
 I was also surprised that there is some linearity in the controls. I guess those white LEDs are just right. I *did* sandpaper  them though so their light would be more predictable.

 The MIDI plug is  not a MIDI plug, just a phono jack. I used the SPDIF socket for it as I never use SPDIF - ever. On the UC-33 MIDI repeater, I also used a phone jack, so there!  A matched set!

Here's a picture of the MIDI out control from FL Studio (left) The Groups are connected to a formula controller here. This allows one know to control the whole level, as one control pulls it up, the other down. It's convenient if I'm recording the controls to automate.

The MIC1 & GUIT controls are inverse, as they are turned up, they cut those inputs. The FX is not. I have found if I leave the main FX at about a quarter, and the channel FX's at 3/4, this control has a great range!

The CTRL RM is inverted, like an attenuator to to giant sub amps. I've already used this in a song. Then there's the tempo out which has enough juice to power a vibrator motor. Can't wait to test that out... ok ok enough with the jokes!!

  The fan speed is on a note on the same MIDI channel. I don't know how often I'll use that as the mic barely picks it up.
It was a really easy project, but well worth it! Reminds me of the old MIDI controlled motorized sliders (cool)  my Fostex mixer gear had, but  without all of the recorder issues. I like to be able to record into a PC. No matter how intuitive those HW recorders are, their still a pain to learn and remember...unless you use it every day in a professional capacity.

 I'm so happy I could share this project with you all, and I hope you've picked up some good ideas from it. I sure did!


- Mixer/MIDI PIC Files (zip) - Digikey


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