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.

Polysix OTA Overdrive

I still chase my dream -- to get a decent electric piano feeling from my Korg Polysix.  Adding velocity sensitivity got me a long way there, but the sound itself needs to be dirtier, with more compression, more grit.  I tried adding diodes to the signal path to generate some distortion, but it sounded bad.  Too fizzy.  Now, I've modified my Polysix to overdrive one of its OTA circuits.  Having a soft onset that eventually leads to a deep, warm saturation, I think that it worked out pretty well!

OTA Overdrive:  After the failure of my experiment with diodes, I looked for other places in the Polysix circuit where I could generate distortion in other ways.  I noticed that the Polysix has a number of LM13600 chips, which are operational transconductance amplifiers, or OTAs.  Seeing them there reminded that a number of newer synths (e.g. Moog Sub Phatty) say that they offer "OTA Overdrive" as a means of user-controllable distortion.   I don't know how these synths overdrive their OTAs, but I was certainly game to figure out how to overdrive mine.

Pre-OTA Attenuation:  A key requirement of the LM13600 is that the input signals have to be very small.  If the inputs are too strong, the out signal will become distorted.  Since I want distortion, this looks like a good place to make my modifications.  Looking at the Polysix schematic, I see voltage dividers in front of every LM13600 to cut the signal down.  One example is shown below, which shows the elements around IC20.  The signal comes in on the right and exits on the left.  The voltage divider in front of IC20 is formed by R155 (10 kOhm) and R163 (100 ohm).  This cuts the input signal voltage by a factor of 100.  That's 40 dB of pre-OTA attenuation!

Adding a resistor in parallel to R155 reduces the attenuation prior to IC20 enabling the LM13600 OTA to be overdriven

Reducting the Pre-OTA Attenuation:  If I modify the circuit to have less pre-OTA attenuation (ie, to allow the signal to be stronger), I will likely overdrive this OTA.  I can easily reduce the attenuation by adding a resistor in parallel with R155, which will reduce the effect of the voltage divider.  For example, putting a 1 kOhm resistor in parallel to R155 will result in the signal being cut down by only a factor of 10, instead of the factor of 100.  This means that the signal is attenuated by only 20 dB instead of the 40 dB.  In effect, I'm slamming the OTA with a 20 dB stronger signal.  This seems like an easy path to overdriving IC20.  That's what I want.

Connecting the Resistor:  Looking inside the synth, I first looked for IC20 on the KLM-368 PCB.  I found R155 nearby.  To easily try different resistors in parallel to R155, I attached clip leads on either side of R155.  See the pics below.  I also attached my oscilloscope to C75 so that I could visualize the output as well as hear it.

Clipping in on either side of R155.

Adding a resistor, via the clip leads, to be in parallel with R155.

Trying a Range of Resistors:  As you saw in the video at the top of the post, I tried a range of different resistor values.  For gritty electric piano sounds, I liked the 3.3K and 1.1K values the best because the effect was subtle...it simply warmed up the signal and only added some grit when I played hard.  At the other end of the spectrum of resistor values (ie, when I shorted across R155), I unexpectedly enjoyed the unpredictable chaos of the heavily saturated distortion.  It was really fun!

Seeing the Effect:  The video demonstrated what this modification sounds like.  The figure below shows what it *looked* like.  The figure shows the output of IC20 for different resistor values across R155.  As you can see, with no resistor (which is the same as a very large resistor), the output signal is a nice sawtooth, but it is relatively small.  Then, as I add a large resistor (3.3 kOhm) the signal gets quite a bit stronger due to the extra gain.  As I make the resistor smaller (1.1 kOhm), the signal gets stronger.  Continuing to make the resistor stronger (500 ohm and on), the shape of the waveform starts to deviate from a sawtooth -- the top and bottom are becoming rounded.  This is signature of the OTA being overdriven.  It's a softer, more rounded, distortion than seen in my diode mod from my last post.  Finally, when I short across R155, the distortion becomes so heavy that the signal is a square wave.  Under certain contditions, as seen in the video, that can be cool in its own way.

Output of IC20 recorded for different resistors placed in parallel with R155. Note that decreasing resistance results in increased gain, which eventually leads to overdriving IC20, causing the sawtooth to become distorted.  Note that, for these images, I played two notes at the same pitch and waited for the two voices to naturally phase into alignment.

More Than Just Gain:  Clearly, the main effect of changing the resistor is to boost the level of the signal going into the OTA, which then overloads the OTA and causes the output signal to be distorted,  But, gain and distortion is not the resistor's only effect.  Looking back at the Polysix schematic, I see that R156 and C80 are also in parallel with R155.  This resistor and capacitor act to boost the treble frequencies.  By adding my own resistor around R155, I will also be reducing the effect of this treble boost.  So, maybe the "warmth" that I felt was actually a result of changing the frequency response, and not necessarily due to overdriving the OTA.  To confirm this theory, let's analyze the circuit for different resistor values and see what happens.

Circuit Simulation:  To simulate this part of the circuit, I used 5Spice Analysis, which is a SPICE-based circuit simulation program with a nice graphical interface for Windows.  Like most graphical versions of SPICE, you start by drawing the schematic of the circuit that you want to simulate.  The screenshot below shows the schematic that I made to represent the elements leading into the IC20 OTA.  For this simulation, the input signal is generated by "SigIn" on the right.  "TPv1" on the left represents the output signal, which would normally go to IC20.  In between the elements of the Polysix's voltage divider, the R-C treble boost, and my added overdrive-inducing resistor.  Now I can run the simulation and see the expected frequency response.

Using "5Spice Analysis" to model the frequency response of the pre-OTA voltage divider network.

Expected Response, Unmodified Polysix:  The graph below shows the modeled response of the circuit.  On the bottom (in blue) is the the circuit response with no added resistor around R155.  As expected, the signal is 40 dB down, which is what we expect based on the 10 kOhm / 100 Ohm voltage divider.  Also note, however, that the treble frequencies are boosted due to the effect of R156 and C80.  At 5 kHz, the signal is boosted by about 5 dB.  While not a huge boost, this would definitely sharpen the sound.

Expected Response of the pre-OTA voltage divider network.  The different lines show the effect of different values for the resistor that I placed in parallel to R155.

Expected Response, Modified Polysix:  When the parallel resistor is added, I get the black traces shown in the figure above.  The primary effect is that the signal level goes up.  The 1.1 kOhm case, for example, shows that the signal is at -20 dB instead of at -40 dB.  This is the 20 dB gain that I mentioned earlier.  A secondary effect of adding the parallel resistor, however, is that the boost to treble frequencies becomes much less.  Perhaps this is the increased "warmth" that I perceived.  Hmm.  I think that this requires a little more exploration.  Maybe there are additional modifications that I can make to shape the frequency response to make the OTA overdrive sound even better!

Next Step:  I liked the sound and feel of this OTA overdrive much more than the diode distortion.  I think that, if I can figure out how to make this OTA overdrive controllable from the Polysix's front panel, I'll enjoy having this modification.  So, that's my next step: figure out how to have a controllable amount of overdrive for IC20.  Stay tuned!

Follow-Up:  How to make this mod controllable without clipping in new resistors?  In this follow-up post, I start designing this mod to use an Arduino and a digipot.  

Polysix Drive - Diode Distortion

I recently modified my Korg Polysix to have velocity sensitivity.  Now that it is responsive to the dynamics in my playing, my Polysix makes much more convincing electric piano and clav sounds.  The problem, however, is that those classic EP and clav riffs from the classic records from the 70s were often played through a guitar amp, which means that we're used to hearing them with the warm compression and light overdrive of tube guitar amps.  By comparison, the EP and clav sounds from my Polysix are too clean and too dynamic.  In this post, I start the process of finding the right "drive" mod to bring some of that compression and overdrive to my Polysix.  I'm going to start with some simple diode-based distortion.

A simple distortion generator: an LED, or a diode, or both!

Classic Diode Distortion:  Being a guitarist, I know that it is very difficult to simulate the sound and feel of a tube amp.  So, instead of heading down that challenging path, I'll look to the well-known overdrive and distortion tricks used in guitar pedals.  A very common technique used for guitar distortion is to use a pair of opposite-facing diodes in parallel.  This is how distortion pedals such as the Ibanez Tubescreamer and the ProCo Rat achieve their classic sounds.  Maybe it'll work well in my Polysix, too!

Putting Diodes in the Polysix:  Below is an excerpt from the Polysix schematic.  It shows the last VCA and the last amplifier prior to the audio signal being sent to the output jack.  In the blue region, it shows that the make-up gain after the VCA is simply an op-amp configured as an inverting amplifier.  Since the Tubescreamer also uses an inverting op-amp, I chose to follow the Tubescreamer's distortion approach of adding a pair of diodes into the op-amp's feedback loop.  I just need to clip in a pair of diodes?  That's it?  Easy.

Schematic of the last VCA and last amplifier in the Polysix.  I've chosen this location to add my "Diode Distortion Mod", which is simply a pair of clipping diodes in the feedback loop of the last amplifier.

In Parallel with R196:  In the modified schematic above, you can see that the pair of diodes are added in parallel with the other elements in the feedback loop.  To enable a quick test, I simply attached the diodes using clip leads on either end of R196.  The other ends of the clip leads (not shown) are clipped onto the diodes.  The only tricky part is to make sure that the diodes face in opposite direction.  I used my handheld multi-meter to determine which direction was "forward" for each diode, and then I swapped one of them around backwards.

Using small clip leads to attach my diodes (not shown) in parallel with R196.

Looking at the Output:  With the diodes hooked up, I used my oscilloscope to look at how the diodes affected the audio signal from the op-amp.  The pictures below show some screenshots from the oscilloscope.  To make a strong signal, I set the Polysix to play two voices at the same pitch.  Then, I set the Polysix's "Attenuator" dial to maximum volume (ie, "+10dB").  Now the signal is strong enough to really show the effect of the diodes.

Screenshots of my oscilloscope when recording the output of the op-amp.
I tried different combinations of diodes in the op-amp's feedback loop.

Clipping of the Waveform:  As can be seen in the top-left screenshot ("No Mods"), I've got a normal-looking sawtooth waveform.  When I add just the LED, I see that the top of each sawtooth has been clipped to a maximum voltage of 1.6V (ie, the LED's forward voltage drop).  When I swap out the LED for a backwards silicon diode, it's the bottom of the sawtooth that has been limited (clipped at 0.6V, which is the diode's forward voltage drop).  Finally, when I have both the red LED with the backwards silicon diode in circuit, both the top and bottom of the waveform is clipped.  So, the diodes act to distort the signal by limiting its dynamic range.  Pretty sensible.

Demo:  OK, these oscilloscope views are interesting and all...but how does it SOUND?!?  That's a good question.  Below is a short demo that I pulled together.  It shows the baseline Polysix with no distortion mod, the Polysix with LED distortion only, and the Polysix with the LED + diode.

Not What I Wanted:  By the end, my conclusion was that I didn't like the results.  My first reaction was that the distortion was too fizzy/fuzzy.  But, beyond just the fizzy sound, the diode distortion didn't have the right feel when playing it.  I had been hoping that the distortion would provide a sense of dynamic compression, like a guitar amp feels.  But, instead of feeling that kind of compression, I actually felt like the signal was being hard limited...which, of course, is exactly what diodes do.  I should not have been surprised at this disappointment.

Moving Forward:  Yes, I could have spent more time refining this diode distortion modification to try to address my criticisms.  For example, to reduce the fizzy/fuzzy sound, the Tubescreamer includes additional capacitors to reduce the high frequency sizzle.  I could have tried that.  But, in truth, this experiment reminded me of why I don't use diode-based distortion pedals with my guitar...I never end up liking the sound.  So, for my Polysix, I'm going to leave behind the diodes and try some other approaches.  Let the synth hacking continue!

Update: I've removed the diode distortion and, instead, modded the Polysix to overdrive one of its OTAs.  I like that sound much better!