Arduino-Controlled OTA Overdrive

I'm really enjoying playing my OTA Overdrive Mod for my Polysix.  I also enjoy how simple it is -- I just added one resistor.  If I want a different amount of overdrive (or none), I swap in a different resistor.  So simple.  The question now, however, is how to make this mod be robust and usable day-to-day?  Well, today, I start that process.  My goals are to make it controllable from the front panel *and* to save and recall its setting with the Polysix's patch memory.  It's this second goal that makes this mod a little tricky.  Let's figure it out!  (Not surprisingly, my solution involves an Arduino...)

Controlling my OTA Overdrive with an Arduino and a Digipot.  

Why Not Use a Pot?  As I said, my OTA overdrive is controlled by changing resistors.  So, the easiest solution is to mount a potentiometer (a "pot") to the Polysix face, to wire it as a variable resistor, and to connect it to the correct spot in front of the OTA.  Fit with a nice knob, a pot is easy to use, simple to install, and inexpensive.  Sound great!  The main problems with using a traditional pot are that (a) I don't want to drill a hole in my Polysix to mount the pot and (b) a pot is not programmable, which means that its setting can't be saved and recalled with the overall synth patch.  I'd like to find a way to overcome both issues.

Using a Digipot as a Variable Resistor for my OTA Overdrive Mod

Use a Digital Potentiometer:  While a traditional pot isn't quite satisfactory, I think that a digital potentiometer (a "digipot") would work great.  I first used a digipot to add velocity sensitivity to my Polysix and I found it to be very useful.  A digipot is like a regular pot in that it can be used as a variable resistor, but a digipot has an advantage in that it can be controlled by digital messages sent from a microcontroller.  As a result, I can use buttons and knobs that already exist on the synth to control the overdrive.  And, in theory, I can even change the overdrive settings with each synth patch.

Control by an Arduino:  You don't get something for nothing, however.  To use a digipot, you need a microcontroller to drive it.  Microcontrollers are pretty easy to add...in fact, I've already added two to my Polysix: (1) an Arduino Mega acting as my key assigner and (2) a Teensy 3.1 acting as my velocity processor.  I could use either of these existing devices to control my digipot, but neither is conveniently located to the KLM-368 Effect PCB where the overdrive mod lives.  I hate running long wires carrying noise-spewing digital signals, so I think that I will add yet another microcontroller to my synth.  This time, it'll be something small like an Arduino Micro.

Block Diagram of my Solution

User Control:  While the Arduino can control the digipot, how will the user tell the Arduino what overdrive level is desired?  Since I don't want to drill a hole to add a new dedicated knob, I looked at my Polysix control panel with fresh eyes.  Which knobs ore buttons could I re-purpose?  My eyes fell upon the "Attenuator" knob.  For me, I never use the knob below 0 dB, which means that the whole left half of the knob is basically unused.  I could totally re-purpose part of that knob for adding overdrive.   No drilling necessary!  But how do I sense the Attenuator's setting?

Re-Using the "Attenuator" Knob to Be My Overdrive Control

Look for the Signal, not the Knob Itself:  The key innovation is that I should not try to sense the knob itself.  Instead, I should sense the signal that the Polysix CPU generates in response to the knob.  The reason to sense the signal instead of the knob is because of patch memory.  To enable patch memory (even when patch memory isn't being used), the Polysix CPU scans all of the knobs and switchs on the panel and then generates control signals in response to those settings.  Then, if a patch is recalled from patch memory, the CPU ignores the switches and knobs and generates the control signals based on memory.  If I am successful in having my Arduino listen to the signal generated by the Polysix CPU in response to the Attenuator knob, my overdrive setting will respond to both the knob and to the patch memroy system.  This is what I want!  But which signal in the Polysix corresponds to the Attenuator knob?

"P-Vol" is the Attenuator Knob:  Looking around the schematic, I found that the Attenuator setting corresponds to a signal called "P-Vol", which presumably stands for "programmable volume".  It turns out that P-Vol is a simple analog voltage signal.  Therefore, via its Analog Inputs, my Arduino can listen to this signal and engage (and increase) the overdrive whenever the Attenuator knob is turned below 0 dB.  Great.  The P-Vol signal is also available on the KLM-368 Effect PCB (at R188), which makes it convenient to the overdrive mod itself, which is likely where I'll mount the Arduino.

The "P-Vol" signal reflects the setting on the Attenuator knob.  I'll control the amount of overdrive by sensing the value of this signal.  I can read the P-Vol signal at R188 on the KLM-368 Effect PCB.

Interpreting P-Vol:  Attaching a wire to the upstream side of R188, I read the voltage using my digital multi-meter for various settings of the Attenuator knob.  As you can see in the graph below, it's a nice, simple, straight line.  I can use this data as part of a lookup table on my Arduino to determine whether to engage the overdrive (if knob is set to less than 0 dB) and how much overdrive to apply (a lower knob setting will engage more overdrive).  Via this lookup table, the Arduino will command the digipot to actually induce the overdrive that I want.  This plan is coming together nicely.

Data that I measured relating the "P-Vol" voltage at R188 to the Value of the Attenuator Knob

The Wiring Plan:  Now I have to figure out how to wire up all of the pieces to put this plan into action.  Luckily, I already figured out how to connect and use a AD5260 digipot.  This time, however, instead of using a full-size Arduino Uno,  I'll use an Arduino Micro.  The pin mapping is the same, so the connection plan is still valid.  The image below shows my plan for wiring it up.

Wiring the Arduino Micro to the AD5260 Digipot.  Lots of signals are needed from the Polysix, including +5V, -5V, Gnd, P-Vol.  These are in addition to the input and output audio signals that'll be manipulated by the digipot to overdrive the OTA.

Both +5V and -5V:  There is one important difference in this new plan: the digipot needs to be supplied with both +5V and -5V.  The -5V is new.  The reason that it needs a bi-polar power supply is that the audio signal that I'm attenuating is a bi-polar signal.  The audio signal from the synth could span +5V to -5V, therefore the power supplied of the digipot needs to span +5V to -5V.  Lucikly, the Polysix makes -5V, so I can grab that voltage rail from the synth.  I had to pull it from the Polysix's power supply PCB (not the KLM-368 Effects PCB), but that's not really a problem, just a bit messy.

Wiring it Up:  I wired up the Arduino and digipot onto a solderless breadboard, as shown in the picture at the top of this post.  I then tacked in a bunch of wires to the KLM-368 PCB and grabbed the +/-5V from the main power supply PCB.  As you can see below, it's quite a mess.  I usually find that messiness is part of the discovery and invention process.  Things always seem to get messier before they get simpler.

Trying new things is a messy, messy business.

Software:  After wiring up the hardware, I need to write software for the Arduino to listen to the P-Vol signal and to drive the digipot.  For the digipot, I simply re-used my previous software that I wrote to drive the AD5260.  Then, I added the logic to read the analog "P-Vol" signal and decide what resistance level to set in the digipot.  It was nothing complicated, though it did take some trial and error to find the right voltages at which to transition between resistance levels.  My code is shared here on Github.

Success!  After firing up the system, and after fixing a bunch of bugs in my software, I found that this Arduino-digipot solution does successfully control the amount of overdrive based on the Attenuator knob setting.  Success!  And, yes, it also correctly respond via patch memory changes.  Yay!  It feels so good when stuff works.

Issues:  This setup does have one small problem, however -- the lowest setting of the Attenuator knob (-10 dB) isn't detected by the Arduino.  The -8 dB setting and the -10 dB setting give the same behavior.  The problem should have been obvious earlier.  As shown earlier in my graph of voltage vs knob setting, note that both the -8 dB and -10 dB knob settings yield a P-Vol voltage that is negative.  The Arduino can't measure negative values.  To the Arduino, they both look like zero volts.  As a result, the Arduino can't tell the difference between the two knob settings.  I totally should have realized that earlier.  Bummer!

Next Step:  There are two easy solutions here: (1) shift the DC level of P-Vol with an op-amp prior to sending it to the Arduino's analog input, or (2) abandon P-Vol and, instead, sample the four digital signals that the Polysix uses to generate the P-Vol.  The latter approach definitely looks easier.  Next time, I'll give that a try instead.

Follow-Up:  I'm thinking about doing a custom PCB with the digipot and Arduino integrated together onto a single board.  To help me learn how to do that, I created my own breadboard Arduino from raw parts.  You can check it out here!

Follow-Up:  I've completed the PCB design.  It was actually pretty fun!  You can check out the story here.