Monday, November 30, 2015

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!

3 comments:

  1. The SSM2044 in the Polysix is known to be a 'mellow' sounding filter, it may be a bit contradictory to try and push that into sounding like a different filter? Better to treat the sound after the fact, otherwise you're fighting -24dB roll off and going to lose those additional harmonics rather quickly? If you had a -6db rolloff, even -12db, then you have more of a rolloff to accommodate those higher harmonics... that would be my 'qualitative' take on the scenario. If you want to prove it out, take an SSM2044 chip and breadboard it with those 3 scenarios' (-6 vs -12 vs -24db rolloff) then see the results when you pump a harmonically rich (distortion etc.) through it. May also be why they use bandpass in a Wah since it is 'keeping' the frequencies of interest and you get two roll-offs to create phase shifts that further affect the sound... again, more of a qualitative take on the whole scenario...

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    1. To be clear, my attempts at adding overdrive/distortion are all after the Polysix's filters. So, I'm not trying to do anything to the filter. I'm merely adding grit after the voices have been generated but before the chorus/phase/ensemble effect has been applied. So, given this location in the circuit, I'm not sure what you mean by "pump a harmonically rich distortion through it [the filter]".

      But, I do think your comment about the Polysix having a 24 dB roll-off filter is an important consideration. I think that the auditory impact of having velocity mapped to filter cutoff (which is what I did) might be more pleasing, more EP-like, if the filter were not so sharp. I bet it would feel more natural if it were a 12 dB filter instead of a 24. I hadn't considered that. Thanks for bringing it up!

      Unfortunately, isn't the SSM2044 chip in the Polysix inherently a 24 dB filter? If so, I think that hacking a 12dB VCF into the Polysix is beyond my synth hacking abilities.

      Good discussion!

      Chip

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    2. Sorry, my mistake on the pre/post filter positioning... even with guitar, you can do it either way. Glad you thought there was some relevance to my comments, I must be learning something myself along the way afterall. ; )

      Unless the configuration of the filter is a cascade of -6db stages, then it's not likely you can just tap a point along the way for -6/-12/-18/-24, but that would be very cool, wouldn't it? Not to mention very versatile, sonically speaking.

      Or consider mapping velocity to modulation, per each note in a polyphonic manner, would be pretty wild and verging on CS80 territory, with its polyphonic aftertouch... imagine a sequence of notes, each note having a varying tremolo or whatever you've chosen - tempts the imagination if nothing else. ; )

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