Polysix - Permanently Removing the Post-Effects VCF

After living for a while with my non-destructive version of bypassing the post-effects VCF, I've decided that I really like it and that it is probably something that I want to keep.  Because the clip lead that I had been using is not a robust long-term solution, I decided that I would make this modification more permanent.  So, I replaced the clip leads and soldered in a single wire in its place.  It's pretty straight-forward...

I soldered a jumper wire from J28 to R168, thereby bypassing the now-empty socket for IC15.

The sound is the same as before, but the synth will now be more robust as I travel with it.  If I ever want to un-do this mod, I simply un-solder the single wire and put the LM13600 back into the empty IC socket.  Easy!

Polysix - Modifying the MG Delay Circuit

As I've been discussing in the last few posts, the MG Delay circuit on my Korg Polysix doesn't work quite right -- even with an MG Delay of zero, it still suppresses my MG signal at the start of each new note.  In my most recent post, I found that my MG Delay control voltage (CV) from the Polysix DAC is still a tiny bit too high, even when set to zero.  Since I can't do much about the DAC, I chose instead to modify the keypress signal that is compared to the MG Delay. The result is that my MG Delay now works as I think that it should -- when set to zero, the MG signal is applied smoothly across all notes, with no MG transients at the start of the notes.  Below is an audio demo with the MG Delay modified (top) compared to how it sounded on my Polysix prior to modification (bottom).  In addition to the MG Delay being set to zero in both cases, all of the other settings are the same, too.



Find the MG Delay Circuit:  A schematic of the relevant part of the Polysix circuitry is shown below.  The MG Delay functionality is effected by IC14, which compares the MG Delay CV produced by the DAC sample-and-hold (arriving at Pin 2 of IC14) to a voltage signal that pulses which each new keypress (arriving at Pin 3 of IC 14).  Whenever Pin 3 goes lower than Pin 2, the MG is suppressed.  I'd like modify the circuit so that, when the MG Delay is set to zero, the voltage at Pin 3 does not drop below whatever voltage is being delivered by the DAC to Pin 2.

Swap R98 to Control the Voltage Drop at Pin 3:  The voltage at Pin 3 drops with each new keypress because a keypress causes Q5 to conduct, which allows the charge stored in C33 to drain out via R98. Because Q5 only conducts for a short amount of time, we can limit how low C33 gets by constricting the flow of charge out of C33.  The easiest way to do that is to simply swap R98 from its default 4.7K value to a higher value.  At first, I tried 20K, but decided that 10K was better.

Removing R98.

New R98.  I first tried 20K.  I settled on 10K.

Viewing the Impact of R98:  To visually confirming that I correctly affected the voltage at Pin 3, I used my oscilloscope to view the voltage at Pin 3 (as altered by my modification of R98) and the voltage at Pin 2 (the MG Delay CV).  As shown in the picture below, changing R98 now keeps the voltage at Pin 3 from dropping below the MG Delay CV that is being applied to Pin 2.  As a result, the output of IC14 now stays high, which means that the MG signal is not suppressed by the start of the note.  Success!

Ensuring I Did Not Over-Correct:  It would be very easy to use too large a value for R98.  How do you know that you have an OK value?  If the resistor were too large, then the MG Delay functionality would be defeated for values other than zero.  I'm not trying to completely defeat the MG Delay, I just wanted zero to be zero.  So, to make sure that my MG Delay still worked well, I turned the MG Delay knob a little bit until the DAC put out a higher MG Delay CV.  It turns out that there is a pretty big dead zone on my MG Delay knob (that is unrelated at all to changes to R98)...I had to turn my knob all the way up to "1" in order to see the DAC output tick up one notch.  Once I saw it tick up, I retested the voltage at Pin 3 to ensure that it dropped low enough to trigger the MG suppression.  With my initial change to an R98 of 20K, Pin 3 did not drop low enough.  By swapping R98 to 10K, I got the result shown in the picture below.  As you can see, Pin 3 does indeed drop briefly below Pin 2, as desired.  Success!

So, with this modification, I have achieved my goal of making the MG Delay work as I want.  When turned to zero, the MG Delay is defeated.  When turned slightly above zero, the MG Delay works as before.  I'm pleased.

Polysix - Exploring the MG Delay Circuit

As discussed in my previous post, the "MG Delay" on my Korg Polysix seems to have a problem.  On my Polysix, there is always a delay between pressing a note and getting the MG effect, even with the MG Delay at zero.  I still don't know whether all Polysix's do this, or just mine.  Either way, I'd like to fix it. In this post, I dive in to try to see why my Polysix does this.  My goal is to find spots where I could modify the circuit to make it perform the way that I want.

First, let's look at the part of the schematic that controls the modulation generator (MG).  It's on KLM-367 and I've excerpted the relevant bits in the figure above.  I've highlighted four sections of the circuit: (1) DAC Sample-and-Hold, (2) "MG Delay" Comparator,  (3) "MG Amount" VCA, and (4) the MG Oscillator.  Let's work backwards from the end to see where the MG behavior deviates from what I'd expect.

View the Overall MG Output:  I start by looking at the overall output of this collection of circuits.  The overall output is on the bottom right at point "J".  This point yields the oscillating MG signal itself, including any modulation of the MG signal due to pressing a key on the keyboard.  You can easily access this point in the Polysix by clipping an oscilloscope probe to TP5, which stands proudly on the left side of KLM-367.

Clipping into TP5 to see the MG Signal

I then used the Polysix's knobs to configure the MG with frequency turned really high, with the delay set to zero, and with the level set to maximum.  I set the MG slider switch set to send the MG to the VCA.  When you set the MG in this way, and if you look at the signal at TP5, you usually see an oscillating triangle wave.  This is normal MG signal.  On my synth, when you press a key on the keyboard, you see the that the MG signal becomes suppressed for about 100 ms.  While this is exactly the effect expected when employing the "MG Delay", I think that it is wrong that it occurs even when my MG Delay set to zero.  In my opinion, when set to zero, there should be no gap at all.  So, while I'm not pleased that we are seeing a gap, I am pleased that I have confirmed that it is occurring at TP5 -- it means that I can continue to chase it back through the circuit.

Signal measured at TP5, Where a gap in the MG signal is seen even with MG Delay set at zero.

"MG Amount" VCA:  Looking at the MG signal shown above, we see that the amplitude of the MG signal is being reduced (attenuated) due to the keypress.  If we look at the circuit schematic, we see that the last block of circuitry prior to point J is a voltage-controlled amplifier (VCA) based around IC21, which is a classic LM13600 trans-conductance amplifier.  I've highlighted this part of the circuit in yellow.  We see that the VCA is being given the basic oscillating MG signal from the circuitry in blue.  Controlling the gain of the VCA is a combination of two signals: (1) the "MG Amount" voltage output by the DAC sample-and-hold and (2) the "MG Delay" Comparator voltage output from IC14.  Since its the influence of the "MG Delay" that I'm trying to explore, let's look at the "MG Delay" comparator (IC 14) in more detail.

"MG Delay" Comparator:  IC14 is an operational-amplifier that appears to be configured as a comparator.  Its output will be high if the voltage at pin 3 is greater than at pin 2.  Its voltage will be low if the voltage at pin 3 drops below pin 2.  If we consider the "output" to be the voltage at R79, the diode (D12) and the cap (C34) will slow the transition from low-to-high, but the overall idea of the comparator is the same.  Let's probe it to see...

Measuring Pin 3 on IC 14 using the red clip on R86.  At the bottom, probing R79 directly.

The figure below shows the signals around IC14 to see what happens when I press a key on the keyboard.  Again, this figure is with the "MG Delay" set to zero.

As you can see, the voltage at pin 3 drops from high to low when there is a keypress.  If I were to zoom out, you would see that the voltage at pin 3 slowly recovers back to its high state over a couple of seconds as C33 is charged up from +15V via R86.  Because of the slow recovery at Pin 3, the voltage at Pin 3 is, in effect, a measure of time since the last new keypress.  Looking at the yellow trace in the picture above, we see that the output of the comparator (as measured at R79) is normally high and then drops as soon as the key is pressed.  In this case, the output then smoothly recovers back to its high value after a short passage of time.  Because the voltage at R79 controls the gain of the "MG Amount" VCA, this drop at R79 is causing the suppression of the MG signal that we saw earlier.  With the "MG Delay" set to zero, I would not expect to see any suppression of the MG signal, which means that I would not expect to see the voltage at R79 drop.  But we do see it drop.  Why is it dropping?

Comparing Pin 2 to Pin 3:  The voltage of R79 is driven by the output of IC14.  The output of IC14 is driven by a comparison of the voltage at Pin 3 to Pin 2.  Let's probe these these two signals.  

Measuring Around IC14.  Pin 3 is via the red clip at R86.  Pin 2 is probed directly.

In the o-scope picture below, note that I've zoomed out the time axis relative to the previous o-scope picture.  As you can see, the red line is the voltage at pin 3, which shows the quick drop from high-to-low as the key is pressed and it shows the slow recovery in voltage as C33 is charged back up.  The yellow trace shows the voltage at Pin 2, which is the voltage produced by the DAC sample-and-hold to represent the "MG Delay" setting.  For clarity, I've turned the "MG Delay" knob up to a value of "2", which raises the voltage at Pin 2.

You can see how the voltage at Pin 3 is normally higher than the voltage at Pin 2.  You can see that, when the key is pressed, the voltage at Pin 3 drops below the voltage at Pin 2 for a brief period.  That drop of Pin 3 below Pin 2 is what causes the output of IC14 to drop, which is what causes the voltage at R79 to drop, which is what causes the gain of IC21 to drop, which is what causes the MG signal to be suppressed.  It's like dominoes falling in a line.  Great!  This behavior makes total sense when the "MG Delay" is set to "2".   But I'm seeing MG suppression even with "MG Delay" set to zero.  Let's look to see what happens when we drop the "MG Delay" back to zero...

So, as expected, the voltage at Pin 2 is lower because I turned the "MG Delay" knob from "2" down to "0".  In this view, it is unclear whether Pin 3 drops below Pin 2.  Let's zoom in...

Pin 3 Drops Below Pin 2:  Now we can see that, even with "MG Delay" set to zero, the voltage at Pin 3 does indeed drop below the voltage at Pin 2.  It is brief (~5ms), but it happens.  This would cause the output of IC14 (ie, Pin 1) to pulse low for a similar time period.  But our MG is suppressed for 100ms, not 5ms.  Well, the diode D12 allows even that short downward pulse at Pin 1 to discharge the cap C34.  The diode then prevents IC14 from charging C34 back up.  Instead, current must leak through both R78 and R79 to charge C34.  This takes time.  As a result, even a short 5ms pulse from IC14 causes the voltage at R79 to drop quickly but to stay low for a while as it is slowly charged.  Since R79 controls the gain of IC21, even this slight difference between Pin 3 and Pin 2 results in a noticeably long (100 ms) suppression of the MG signal.

What to do about it?  If the picture above were seen for an "MG Delay" setting other than zero, everything would be fine.  The problem is that, for an "MG Delay" of zero, the voltage at Pin 2 should be low enough that it is always below Pin 3.  If that were the case, the output of IC14 would always stay high, which means that the MG signal would never be suppressed.  This is not the case in my Polysix.  It appears that my "MG Delay" CV does not go low enough.  Unfortunately, even after readjusting my DAC (using the brief instructions in the Polysix Service Manual), the "MG Delay" voltage is still a bit too high and my MG still gets briefly suppressed with each new keypress.  My alternative, therefore, is to adjust the behavior of the voltage at Pin 3.  If the voltage at Pin 3 did not drop quite as low, it would stay above the voltage at Pin 2, thereby avoiding the suppression of the MG signal.  The voltage at Pin 3 could be adjusted in a number of ways.  I'll look at these possible modifications in my next post.

Update: I've modified the circuit so that MG Delay of zero works as desired.

Polysix - MG Delay of Zero is not Zero

Earlier this week, when playing my Korg Polysix, I noticed that the LFO ("MG") was not having the effect that I expected.  I often like to set the MG to sweep the VCF very slowly, especially when playing the arpeggiator.  Based on my experience with the Korg Mono/Poly, I expect that the slow MG sweep would cause the arp notes to smoothly change their brightness from note to note.  Unfortunately, that's not what my Polysix is doing.  As you can hear in the soundcloud sample below, there is a clear transition at the start of each note where the VCF goes from its default value (as set by the VCF Cutoff knob) to the current value of the MG.  If you have a Polysix, does yours do this?


The settings for this sound are shown in the pictures below (MG Freq = 2, Delay = 0, Amount = 6, VCF Cutoff = 5, Resonance = 0, EG Intensity = 0, KBD Track = 0).

To show visually illustrate this unexpected audio behavior, check out the screen shots below.  These are screen shots of the audio in the sound cloud sample above.  Each segment shows four notes from the arpeggiation.  The top figure is when the MG is at the high end of its cycle, which means that the MG is opening the VCF beyond the setting from the VCF Cutoff knob.  As you can see in the screen shot, the note clearly starts at a lower VCF setting and then, after ~100 milliseconds, the filter opens up to the value defined by the MG.  This is with the MG Delay at zero!  It should not be like this.  The bottom figure shows the same thing, except where the MG is at the low end of its cycle where the MG is closing the VCF to a value lower than that set by the VCF cutoff knob.  Again, there is ~100 ms delay before it transitions to the MG's value.  In my mind, it should not be this way...the beginning of each note should be no different than the middle of the note.

Even with MG Delay set to Zero, There is Still ~100ms Before the MG Affects the Sound

So, I don't think that it is supposed to work this way.  Unfortunately, I don't know whether this is a new behavior of my Polysix (ie, it has become broken) or if it has always been this way (ie, it is a "feature" of the Polysix design).  If you have a Polysix, does it respond like this?

Update: I explored the MG circuit to find the cause of this behavior

Mono/Poly - Replacing a Trimmer Pot on KLM-398

Sadness strikes!  I went to turn on my Korg Mono/Poly the other day only to find that it was totally dead.  No sound and no LED lights.  The fact there were no lights was a pretty strong clue that it had no power.  So, opening up the Mono/Poly, I found that the fuses were blown.  When I replaced the fuse, I saw smoke pour out of one of the internal trimmer potentiometers.  Death spreads!  This post talks about the process of replacing a trimmer pot in a Mono/Poly.  Just so that you know that this has a happy ending, here are some clips of me playing my beloved Mono/Poly once I got it working again.


Replacing the Fuse:  The story starts with finding that the fuses were blown.  When I first opened my newly-dead synth, I probed around with my digital multi-meter to see if it had power.  I found that I had power on both sides of my transformer.  On the power supply PCB, however, I did not have voltage downstream of the two fuses that separate the transformer from the diodes and power caps.  So, I pulled the two fuses and visually saw that they were indeed blown.  So, I went to Digikey and found some fuses rated for 1.6A as stated on the schematic (Digikey P/N: 486-1882-ND).  When they arrived, I found that they were not transparent as suggested by the Digikey product page, but were opaque as shown in the picture below.  Oh, well.

Old (dead) fuse on the left.  My replacement fuse is on the right.

Burning a Potentiometer:  When I put the new fuse into the synth and turned it on, it only took a couple of seconds until I saw smoke issuing from one of the trimmer pots on KLM-398.  Smoke is never a good thing when working with electronics.  Usually, if you let out the magic smoke, it stops working.  So, I quickly turned off the synth, removed KLM-398, and took a close look.  The smoke came from VR2 (see photo below).  Very close inspection (2nd photo below) does indeed show charring (carbon) on the pot.  It needs to be replaced.

VR2 let out the magic smoke.  It is probably dead.

Close Inspection Reveals Some Charring (Black Carbon)

Removing the Dead Pot:  To install a new pot, you must first remove the dead pot.  That's a simple process of de-soldering the pot, pulling it from the board, and cleaning out the solder holes of any residual solder.  The pictures below show how I needed to use a pliers plus my soldering iron to remove the pot.  Once it was off the PCB, you could see a small burn mark on the PCB confirming that I removed the correct one.  To remove the residual solder, I chose to use a solder pump (see this post for more discussion of solder wick and solder pumps).  With the solder holes clean, I'm ready to install the new trimmer pot.

Removing VR2 by applying heat to the solder while pulling on the body of VR2 with pliers.

After removing VR2.  you can see burn marks on the PCB.

After removing VR2, you have to clear the holes of solder.  I use a spring-loaded solder pump.

Holes for VR2 after removing the solder.

Buying a Replacement Pot:  Of course, to install the pot, you must first buy the pot.  The schematic calls out a 100 kOhm pot, but gives little other information.  Digikey has tons of options for 100K pots, so I wasn't quite sure what to get.  Looking at the PCB, I clearly needed a pot with 3 legs in a triangular configuration.  Also, given that I smoked the original pot, I wanted a new pot that had more than the absolute minimum power rating.  I found a bunch that seemed reasonable and bought a few different models.  In the end, I decided to use the pot shown in the pictures below.  It is Digikey P/N 3362F-1-104LF-ND.  The only undesirable aspect of this pot is that the legs are not wide enough.  You need to use pliers to bend them outward, as shown in the right-hand picture below.  When you do that, the legs are now barely long enough to poke through the holes in the PCB.  Since they're so short, you have to be careful when soldering that you are able to heat the legs before applying the solder.

Replacement 100 kOhm trimer potentiometer.  The pot is small, so you have to bend the outward.

Soldering the Pot:  After bending the legs and inserting the pot's legs in the holes in the PCB, you are ready to solder it into place.  The soldering process is pretty straight-forward, as long as you can actually tough the pot's legs with the soldering iron.  If so, apply the heat, apply the solder, and you're done!  Since the new pot is not the same as the original pots, it does look a little funny on the PCB (see picture below), but that's OK with me as long as it works!

Soldering the new potentiometer onto the KLM-398 PCB.

Re-installing the PCB and Tuning:  With the PCB complete, simply screw it back into its spot inside the Mono/Poly and reconnect the wiring harness (see photo below).  Now it is time to cross your fingers and turn on the synth.  I got lucky -- there was no more smoke.  It seemed to run OK!

Mounting KLM-398 back into my Mono/Poly

Re-Calibrating the Mono/Poly:  Now that it's powered, I can go through the calibration procedure in the Mono/Poly Service Manual so that I can tweak the new pot so that it is properly doing its job.  I chose to start from the beginning of the Service Manual's calibration procedure, which begins with checking the power supply voltages (always start by checking these voltages!).  I then checked and corrected the voltages on the Key Assigner PCB.  After that, you reach the part of the procedure where you check the voltages produced by the KLM-398 PCB.  In my version of the manual, it says to check the voltage on the red, yellow, blue, and gray wires.  I disagree.  In my opinion, they are wrong about the Red wire...it should be the black wire.  Note that the pots are in a funny order on KLM-398.  I just replaced VR2, which is not associated with VCO2 (gray wire), but is instead associated with VCO1 (black wire).  Once I got all of that straightened out, I used my digital multi-meter set on mV mode and adjusted my new VR2 until the black wire showed 0.000 V +/- 1 mV (see picture below).

Adjusting the new pot to achieve the desired voltage on VCO1.

Playing the Synth:  After confirming the voltages produced by KLM-398 for all four VCOs, I closed up the synth, plugged into my amp, and started playing.  It works!  Oh the joy!

It's good to have the old girl back working again.

Polysix - Adding Single-Trigger Mode

Returning to my long string of modifications (most recently adding a sustain pedal), I'd like to talk about a new trigger mode that I added to my Korg Polysix.  As you likely know, the Polysix can be played as a 6-voice polyphonic synth or, using the "Unison" or "Chord Memory" buttons, can be played as a monophonic synth.  For basses and leads, monophone mode is often more fun for me.  The problem for me comes with how the Polysix retriggers its envelopes.  Whether you're playing legato (notes are connected) or staccato (space between the notes), the Polysix always re-triggers its note envelope with each key press.  This behavior is called "multiple trigger" mode and it really interferes with making swooping/soaring legato lines.  Instead, I'd like the option of "single trigger" mode, which prevents the envelopes from re-triggering until all keys have been released.  Because the envelope triggering is handled by the Key Assigner, and because I've replaced the Key Assigner with an Arduino, it means I can do my triggering however I'd like.  So, I added a "single trigger" mode in addition to the default "multiple trigger" mode.  Here's a demo.

Triggering an Envelope with a Single Note:  The Polysix, and most other synthesizers, have their filter (VCF) or their volume (VCA) controllable by an envelope.  Usually, it's an an envelope with four parameters that can be controlled -- attack, decay, sustain, and release -- which is why they are called ADSR envelope.  The envelope starts (is "triggered") whenever you press a key (as shown in the figure below).  It runs through the attack and decay phases and sits in the sustain phase until the key is released.  Then it completes the release phase and is done.  In a polyphonic synth like the Polysix (when in "Poly" mode), each voice has its own envelope, so whenever a key is pressed, a voice is allocated, and its envelope is triggered.  That that key is released, that voice enters its release phase and is done.  Easy.

Triggering with Multiple Notes:  For monophonic synthesizers (and for the Polysix when in "Unison" or "Chord Memory" modes), the synth has to decide what to do if the player has two keys depressed.  This happens all the time during single-note runs when the player might press the next key before he has had time to release the previous key.  Many synths behave like the stock Polysix does, which is each new keypress retriggers the envelope.  This is shown in the figure below where the 2nd key press (which preceeds the first key release) causes the trigger to re-start its attack phase.   This is called "multiple trigger" mode because multiple notes cause multiple triggers.  Notice that the first key release has absolutely no effect.  It's only the last key release that finally initiates the release phase.  In the early days of synths, this "multiple trigger" behavior was one of the most famous differences between old Arp synths (which are "multiple trigger") and old Moog synths (which were "single trigger").  The benefit of the "multiple trigger" mode is that it makes a run of fast notes sound very articulate and punchy because the envelopes adds dynamics to every note, regardless of whether you're articulate and playing staccato or you're a bit sloppy and playing legato.  As a result, players across a range of skill levels (or sobriety levels) really sound like they're shredding when using "multiple trigger" mode.

"Single Trigger" Mode:  The alternative to "multiple trigger" mode is "single trigger" mode.  Here, only the first key press causes the envelope to trigger.  The envelope will not be retriggered until all notes have been released.  As a result, multiple notes will only cause that single trigger event (hence the name).  This is shown in the figure below.  The benefit of this mode is that the attack and decay can be set to long values so that a filter sweeps in a slow soaring fashion while the players musical phrase plays underneath.  It can be a very nice effect.  If the player wants to retrigger with every note, he must use his skill to be sure to release his key before pressing the next one.  So, with multiple triggers, good keyboard skills allows you to have it both ways.

Implementing on the Polysix:  To be clear, I do not have good keyboard playing skills.  But, I do like to occasionally play those kind of soaring "single trigger" kind of synth lines.  So, I modified the software on my Arduino Key Assigner to let me toggle between the two modes.  When I press either the "Unison" or "Chord Memory" buttons, my Polysix defaults to "Multiple Trigger" mode, like normal.  If I press the button again, however, the LED dims and I know that I've switched to "Single Trigger" mode.  If I want to switch back to "Multiple Trigger", I simply press the button again and the LED brightens to indicate the return to normal operation.  As you can see in the video at the top, I enjoy having this flexibility.

Are there any other triggering modes out there that I should consider?

Update: I shared my Arduino code here

Polysix - Removing Korg's Treble Boost

Following from this post, which discussed mods to bypass or remove the post-effects VCF, some folks listening to by demos have commented that the modified sound is perhaps too bright or too raspy.  That's a fair criticism.  Because we've removed the post-effects VCF, perhaps it's also time to consider removing circuit modifications introduced by Korg itself into the Polysix design to try to boost the high-frequencies that had been lost in that filter.  If we eliminate the "illness" (ie, the overly-mellow sound caused by the post-effects VCF), maybe we should eliminate the "cure", too.  That might remove the excess raspiness.

First, let's start with the quote from the Service Manual (see item 4):

Excerpt from Last Page of Polysix Service Manual

Intrigued by item (4), I went to the schematic and found that all of these components are around the last VCA at the end of the KLM-368 "Effects" PCB.  An excerpt of the schematic is below.

Looking at this circuit, what immediately caught my eye were the elements that I circled in red.  Normally, an LM13600-based VCA would be fed the input signal via a simple resistor like the R168 (22K).  Note that Korg has added a parallel path through the C76 cap followed by the pair of resistors.  What this allows is for high-frequencies (which will pass through the cap) to go around the relatively large 22K resistor and get to the LM13600 VCA via 1K resistor.  This boosts the treble.  And given how small that C76 cap is, it's going to boost only the highest of frequencies (the raspy ones).

Opening up 5Spice Analysis, I modeled this little bit of the circuit to estimate the corner frequency and to see what would happen if we modified this part of the circuit to try to eliminate the raspiness.  First, I modeled the circuit as drawn.  Then, I modeled the circuit as if I removed the 1K resistor that is in parallel with the 33K resistor.  Here's what I found:

Modeled Response of the Treble Boosting Elements on KLM-398

The red line is the circuit as it appears in the schematic.  Above 3.5 kHz, the treble response just goes higher and higher.  The black line is when I remove the upper 1K resistor.  Because the capacitor is still in the circuit (because of the 33K resistor), there is still 3 dB of boost to the treble.  But, the boost to the highest, raspiest frequencies is eliminated.  At 10 kHz, for example, removing that 1K resistor drops the response by 6 dB.  That could be just the thing to take the raspy edge off the sound.

Because I happen to like the very bright sound of my Polysix, I did not snip out the 1K resistor to try to tame these frequencies.  Therefore, I have no audio comparison to demonstrate the effect.  Sorry.  If you try this mod, be sure to let me know how it goes!