Friday, September 26, 2014

Anubis guitar preamp

So, on electric guitars there are usually 2 or 3 pickups placed along the string.  You use switches to select which ones to use and the resulting sound depends on the pickup placement relative to the nodes and anti-nodes of the strings.

Some people get adventurous and drill holes in the guitars to add switches or knobs to allow for more control than the stock wiring affords.  I decided to make a preamp that would allow any combination and polarity of the pickups (even time-varying ones) without requiring modifications to the guitar's body or pick guard.

And so, the Anubis preamp for Fender/Squier Stratocasters was born!  There's a digital potentiometer and polarity switch for each of the three pickups.  They're all controlled by a microcontroller.  The control software can be edited and uploaded to the Anubis with an Arduino UNO board.

The volume and tone potentiometers have been replaced with a program selection encoder and two program value potentiometers.  Here's a shot of the boards.  The minimum order from the board house was 10 units, so I have some extras :)

And here's the board populated and installed on my Strat's pickguard.  There's a single IDC breakout cable that goes from a header on the board to the switching jack, the pickups, the battery, and the programming cable.

I've made a mostly dry recording of the guitar (direct injected, a little compression and reverb added) to demonstrate a few of the programs I've written for it so far.  In the order of recording they are:

1. Sample and hold: abruptly switching between randomly selected pickups.
2. Fast chorus: The pickups are amplitude modulated with low frequency sine waves that are 120 degrees out of phase with each other.  Sounds kind of like a three tap chorus.
3. Slow chorus: same as above, but with a slower modulation frequency.
4. Some of the 13 different pickup combination and polarity options. 

If I have time to do it over, I may try nice VCAs (like the  SSM2164) instead of digital potentiometers to avoid the annoying stepping/zipper noise. 

Here's a link to the GitHub repository for the microcontroller's software:

As promised, here's the schematic:

And here's the bill of materials:

Description mouser part number unit price quantity line total number on board
Switching jack 502-113X 3.15 1 3.15
IDC plug 617-09-18-520-7803 2.06 1 2.06
IDC header 571-7-146256-0 3.81 1 3.81
Amphenol flat cables 523-135-2801-020FT 1.75 1 1.75
Potentiometer 311-1901F-10K 2.87 2 5.74
Rotary encoder 652-PEC11L4125KN0020 1.43 1 1.43
7805 regulator 595-UA7805CKTTR 0.767 1 0.767 U8
regulator capacitors 77-VJ1206Y334KXJTBC 0.08 4 0.32 C13, C14, C15, C16
decoupling capacitors 77-VJ1206Y104KXJPBC 0.033 5 0.165 C7, C8, C12, C17, C11
output capacitor 77-VJ1206Y332KXJPBC 0.06 1 0.06 C1
crystal capacitors 77-VJ1206A220KXACBC 0.05 2 0.1 C9, C10
pull down resistors 652-CRT1206BY1002ELF 0.31 2 0.62 R8, R9
diodes 696-SML-LX1206YC-TR1 0.1 2 0.2 D3, D4
16 MHz crystal 774-ATS160SM-1 0.24 1 0.24 Y2
power supply resistors 71-CRCW1206J-100-E3 0.03 2 0.06 R10, R11
AD5222 584-AD5222BRZ1M 2.02 2 4.04 U2, U3
DG403 968-DG403DYZ 1.43 3 4.29 U4, U5, U6
9V battery holder 12BH610-GR 1.05 1 1.05
Atmega328 556-ATMEGA328-AU 2.58 1 2.58 U1


  1. This is awesome! Will you be posting a file I can get fabricated anytime soon? Or will you be selling the boards?

  2. One of the neatest guitar mods I have seen, both physically neat, and well presented, but also neat in the sense of the effects it produces.

    My first thought was "wow!!".. followed by "I wonder if you could add bluetooth to the mix, so you have a Strat you can control from your phone."

    Good luck with this, I think you are on to a winner.

    1. Thanks! I like the idea of bluetooth control. That could also make MIDI control possible for use with MIDI foot controllers, sequencing software, etc. I'll have to get a bluetooth Arduino shield to play with.

  3. It would be nice if we could READ the schematic!

  4. Whoops! I had uploaded the wrong export. There's a higher resolution one up now.

  5. Hi

    THis link Dosent Work /Broken.