Polysix - Flatten the Treble Response

Previously, I modified my Korg Polysix to remove the post-effects VCF.  To my ears, this mod nicely opened up the sound of the synth.  Others, though, thought that it now sounded too "raspy", which might be a way of saying that it now had too much treble.  That's a fair criticism.  In this post, I discussed how the Polysix has a built-in treble boosting circuit to partially compensate for its previous lack of treble.  Since my modifications to the post-effects VCF  restored some of the synth's treble deficiencies, the treble boost circuit might now be over-compensating.  Today, I discuss how I modified this circuit (in particular, around R168) to try to flatten its treble response.  As usual, let's first jump to the end of the story...here are some audio demos illustrating the effect of the mod to flatten the response.



Korg's built-in treble boost is on the KLM-368 Effects PCB.  An excerpt of this part of the schematic is shown below.  Looking at the schmatic, one can see the main part of the audio signal passes through R168, where it is attenuated as a voltage divider with the 1K resistor to ground.  In parallel with this main path, the elements circled in blue provide a second path for the higher frequencies, which means that the treble passes through with less attenuation than the lower frequencies.  By being attenuated less, the result is that the treble frequencies appear to be boosted relative to the lower frequencies.  In my most recent frequency response measurements of my Polysix, I saw that the boost starts around 2 kHz and peaks around 12 kHz.  At the peak, the boost is about 4-5 dB.  For a vintage synth, 12 kHz is very high pitched...it is basically just the "sizzle".  When boosting the sizzle by 4-5 dB, some ears might indeed find the new sound to be a bit obnoxious.

So, I looked at ways to flatten the treble response to reduce any obnoxiousness.  The most obvious modification would be to simply remove the elements circled in blue.  Unfortunately, that does not work well because some treble boost is needed to compensate for treble loss elsewhere in the system.  So, if we want to flatten the response, we need to adjust -- but not eliminate-- the treble boost of this circuit.

After some trial and error, I settled on the approach of adding a resistor in parallel with R168 (the 22K resistor).  The idea here is that, by adding a resistor in parallel with R168, I'm lowering the overall attneuation of the direct path while leaving the treble path unaffected.  Therefore, the relative boost of the treble via blue elements is lower when compared to the now-lessened attenuation of the direct path.  If the relative boost is lower, the overall frequency response will be more flat.  In the end, the best value for me was to add a 33K resistor in parallel with R168.

Implementing the modification is pretty straight-forward...just add a 33K resistor across the existing 22K R168 resistor.  Unfortunately, because of my modification to remove the post-Effects VCF, I already have a jumper wire flying into one leg of R168.  You can see what I did in the picture below.  The blue resistor is the newly-added 33K.  Below it, and slightly behind it, the tan resistor is R168.  You can see that I curved the leg of the blue resistor in a funny way so that it stuck out before looping back and connecting to the leg of R168.  Because of the perspective, you cannot see it loop back to connect to R168.  I then connected the yellow wire (my jumper wire going around the post-Effects VCF) to the looping leg of the blue wire.  Done.  Note that the green capacitor has not been touched in this modification.  The angle of my photograph may look like it is connected to my modification, but it is not...it is just unfortunately aligned in the background.

Adding the blue resistor (33K) in parallel with the R168 (the tan resistor underneath and behind the blue resistor).  The yellow wire is the audio input coming from my modification where I removed the Post-Effects VCF.  The green capacitor is not involved with this modification.

After doing this modification, I re-measured the frequency response of the synth (using the Maximum Length Sequence technique discussed here).  A comparison of the original and modified response is shown in the graph below.  As you can see, adding the 33K resistor did indeed boost the response through the low and mid frequencies.  The relative weighting of the high treble to the rest of the tone is now more balanced.

A side-effect of this mod is that the signal level is overall about 3-4 dB hotter going into the IC20.  For the loudest sounds, this might cause it to saturate a bit...for it to add a little compression or distortion.  I'll be keeping my ear tuned for that possibility.  Since Moog, with their Sub Phatty, has been extoling the virtues of adding a little OTA distortion, maybe any slight overdrive added here would be a good thing.  I'm not convinced either way, but I will keep my ear open for it.

So how does it sound?  Well, the sound samples at the top provide a comparison.  These are recordings straight from the synth to my M-Audio Microtrack.  Because the modified version was 3-4 dB louder, and since we humans are very sensitive to (and partial to) louder sounds, I cut the volume of the samples from the modified synth by 3.2 dB to equalize their RMS power.  So, even though I did not equalize their volumes on an A-weighted scale, hopefully I'm close enough that it is a fair comparison.

Do you think that the treble-flattening modification sounds better?  Or does it sound too dull?  I'm curious to hear your thoughts!

Follow-Up: I had this mod in my synth for a couple weeks.  I decided that I preferred the super-sizzly sound that I had before, so I removed the 33K resistor.

Polysix - Frequency Response with Deeper Bass

In my previous post, I modified my Korg Polysix to strengthen the deepest bass frequencies.  The key is to bypass (or remove) C61 on the KLM-368 Effects PCB.  In my previous post, I attempted to show the frequency response due to this modification, but the graph was pretty poor.  Today, I have taken new measurements and made a much better graph.  Now we can clearly see the effect of bypassing C61.

Lower Cutoff Frequency:  This graph clearly shows that the low-frequency cutoff for the synth drops substantially by bypassing C61.  As measured at the -3dB point, removing C61 drops the cutoff from about 62 Hz down to about 20 Hz.  This means that removing C61 extends the deepest bass frequencies.  Whether or not this is a good idea is up to you.  For me, after living with it for a few more days since my original post, I like it.  I think that I will keep it.

Let's talk about some details of the measurement technique...

Measurement Approach:  By treating the Polysix as a "black box" system, I evaluated the frequency response by measuring the transfer function of the "black box".  I did this using a standard technique -- I injected a known broadband signal into the system and I recorded the output signal that was generated by the system.  Comparing the output to the input yields the transfer function.  By looking at the transfer function in the frequency domain, you get the frequency response of the system.  In this case, of course, the "system" is my Polysix.

Injecting the Test Signal:  All of the circuits that interest me at the moment are on the KLM-368 Effects PCB.  To measure its frequency response, I need to inject my signal before the audio pathway gets to KLM-368.  I chose to inject my signal at the end of Voice 1 on KLM-366, just before it is mixed with the other voices and sent off to KLM-368.  As seen in the picture below, I injected my signal at R133.  To allow my signal to mix properly into the synth's audio path at this point, I used a 10K resistor in series between my computer (which is playing the signal) and the green clip lead shown in the picture.  For the "output" of KLM-368, I simply recorded the main output of the synth because there is very little circuitry after KLM-368.

Injecting my Signal on the Lower Leg of R133 (the Green Clip).  Not shown is the 10K Resistor Between my Signal Source and the Green Clip.

Processing with Matlab:  To produce the frequency response graph shown at the top, I processed the audio recording of the input signal and of the output signal using Matlab, which is unfortunately not cheap nor readily accessible.  It is a very good programming environment for doing this kind of signal processing, but there are other choices.  The Matlab functions that I needed are the FFT function (which converts time-domain signals into frequency-domain signals) and Matlab's plotting functions.  As an alternative to Matlab, I believe that this analysis could be easily done in Octave (which is free) because it has a perfectly fine FFT function, as well as, perfectly fine plotting functions.

Compute the Transfer Function:  Whatever computational tool you use, the core of the calculation is to take the FFT of the output audio and divide it by the FFT of the input audio.  This division operation in the frequency domain yields the output/input transfer function of the system being measured (in my case, KLM-368).  Take the magnitude of the transfer function, plot as "dB", and you've got the amplitude response as a function of frequency.  This is what I show in my graph.

Chosing the Input Signal:  For anyone who has made these kinds of measurements before, you know that there are several different choices for "broadband" input signals that one can use.   Ideally, the input signal is flat in the frequency domain, so that any deviation from a flat output is most easily assessed.  The typical choice is to use either a linear frequency sweep or some random white noise.  Personally, I like to use noise.

Maximum Length Sequence:  In the category of "random white noise", I chose to try something new...instead of traditional Gaussian white noise, today I tried using a Maximum Length Sequence.  Unlike traditional white noise, which is only truly flat in the frequency domain after lots and lots of averaging, an MLS sequence is designed to be perfectly flat within whatever fixed period of time that you'd like.  As a result, you get much smoother results in a much shorter recording.

Smooth MLS Results:  I generated a sample of MLS using the "MLS.m" routine downloaded from the Matlab File Exchange. I generated the sequence and saved it out as WAV file, just like I would do for any other noise sample.  After running it through the synth and processing the results, I get the very nice graph seen at the top of this post.  This is the first time that I've used MLS and, given the smoothness of the graph (copied again below, but with different annotations), I like how the results turned out.

One More Look at the Graph:  OK, sorry for the digression about transfer functions and maximum length sequences.  Let's get back to the results at hand.  However I got there, this new graph shows the frequency response of the synth much better than my old one.  It shows that the effect of bypassing C61 is substantial, but only at the deepest frequencies.  As a secondary result, I also see that the KLM-368 PCB (with or without C61) produces a sizable boost seen in the treble frequencies.  I believe that this is the effect of Korg's built-in treble boost that was discussed in this older post.  I'm going to address this "feature" in another post later.

Update: I decided to properly bypass C61 using a jumper wire instead of my clip leads. See here.

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 - 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!

Polysix - Bypassing the Post-Effects VCF

In a previous post, I discussed how the Korg Polysix has an amplitude-driven VCF that is located just after the synth's effects section.  I discussed how it fails to open all the way, which attenuates the sizzling high-frequencies produced by the synth and makes it sound muffled.  The VCF is also slow to respond to changes in amplitude, which softens the synth's attack.  At the end of that post, I mentioned a mod to defeat this VCF, which restores its sizzling response and fast attack.  This post adds more detail on the removal of this VCF.  To start with the good stuff, here's a very simple soundcloud demo:

Overall Circuit:  To get started, let's look at the schematic for this part of the Polysix.  All of the elements that I'm going to discuss are on the KLM-368 "Effects" PCB.  Below is an excerpt of this schematic with the different blocks labeled.  The most relevant blocks are the VCF circuit itself (based on an LM13600) marked in yellow.  What drives the cutoff frequency of the VCF is an envelope detector circuit marked in blue.  These two circuits work together to determine how much high frequency sound gets through the synth.

A picture of this part of the synth is shown in the photograph below.  The LM13600 at the heart of the VCF (ie, U15) is shown in the socket at the center of the photo.  This will be the area for our modifications.

Picture of the Unmodified Circuit Around the Post-Effects VCF.

Defeating the VCF:  The first modification is the mod that I discussed very briefly at the end of my post linked above.  I said that you could defeat the VCF by forcing it to be fully open all the time.  You can do this by applying +15V to Q14.  Q14 controls the current flowing through the filter, which controls the filter's cutoff.  By applying +15V, you force the filter open as far as it will go.  As shown in the schematic below, I do this mod non-destructively by using a clip lead to grab +15V from R125 and apply it to Q14 by clipping to R89.  If you do this, make sure you get the correct side of R125 and R89.  As shown in the picture below the schematic, you need to clip onto the bottom of R125 and of R89.

By Jumping from R125 to R89, +15V is Applied to Q14, which Forces the VCF Open.

Using a Clip-Lead to Defeat the Post-Effects VCF by Forcing it Open.

The Sound of Defeating the VCF:  After adding this one clip lead, how does it sound compared to the stock Polysix?  Well, in that soundcloud demo at the top, you clearly hear that muffled sound of the stock Polysix is removed and that the sizzling high-frequencies come through.  To my ears, it's fantastic.  If you like the more mellow sound of the stock Polysix, simply remove the clip lead.  No harm was done!

Permanently Removing the VCF:  On the Polysix Yahoo Groups, there was a post by Tony of Oakley Sound who suggested that the best course would be to simply remove the VCF entirely.  This would remove any noise contribution of the VCF and of its associated envelope follower.  This is a fantastic idea.  In his post, he discussed how to do the mod.  Because it involved soldering and de-soldering components, it can make people nervous.  So, instead, I propose a non-destructive version of his mod.  

Non-Destructive Removal of the VCF:  As you can see in the photogrpahs so far, U15 (the LM13600) is socketed.  This means that you can simply pop it out of the circuit without hurting anything (though do turn off the synth first).  Removing U15 removes the VCF from the synth.  Easy, eh?

Pop U15 (an LM13600) from its Socket, and You've Removed the VCF!

Reconnecting the Signal Path:  Unfortunately, removing U15 also breaks the audio signal path, which means that you'll get no sound.  That's not so nice.  To fix this problem, Tony says you'll need to find J28 and J29, which are jumpers (ie, zero ohm resistors) that are not on the paper schematic.  Once I found out where they were, I added them to my schematics, including the excerpts shown here.  As you can see, they bring the dry audio signal (J28) and the effected audio signal (J29) to the VCF (U15).  So, to reconnect the audio path, you can use a clip lead to connect J28 and J29 and then use a second clip lead to jump from either J28 or J29 (remember, they're now connected) all the way over to R168.  As you can see in the schematic below, this jump to R168 brings the dry and effected audio down to the final VCA, which is also the overall output point from this PCB.  As you can see in my picture below the schematic, be sure to connect to the left side of R168 (though it doesn't matter which side of J28 and J29 you clip to).

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To Non-Destructively Remove the VCF, Pop out U15 from its Socket, Clip J28 to J29, and Clip J28 to R168.

The Empty Socket was U15.  The Clip Lead Jumps J28 to R168.  I still need to Jump J28 to J29.

The Sound of the Removed VCF:  After removing the VCF in this way, how does this sound?  Well, in the simple demo at the top, it sounds much like my first mod -- it greatly increases high-frequency sizzling sound of the synth.  At first listen, removing the VCF doesn't really sound different from simply defeating the VCF.  But, I have yet to play the synth at any length now that I've removed the VCF, so perhaps this mod will show its differences under other types of playing.  I do like the idea of completely removing the VCF from the circuit.  So I think that I'll keep this version of the mod, for now.

Improved Attack Time:  Besides increasing the high-frequency sizzle of the synth, defeating/removing this post-effects VCF also improves the synth's attack time.  Sometimes the Polysix is criticized as having slow envelopes because the start of the notes can sound a bit soft.  Since the per-voice envelopes are very snappy (~1 ms attack time), a little investigation shows that the problem is the slow response of the post-effects VCF.  The graphs below show the start of a high-pitched note output by the Polysix when in its stock configuration (top graph), with the VCF defeated via +15V at Q14 (middle graph) and with the VCF removed via removal of U15 (bottom graph).  Along the bottom is the time in seconds.

Attack of a High-Pitched Note.  (Top) Stock Polysix.  (Middle) Defeated Post-Effects VCF.  (Bottom) Removed Post-Effects VCF. 

As you can see in the top graph, the stock Polysix can take about 10 ms for the sawtooth amplitude to reach maximum (though it does get within 3 dB within of 3 ms).  By contrast, defeating VCF (middle graph) or removing the VCF (bottom graph) allows the sawtooth to reach its maximum in less than 1 ms.  The snappy sound is back!  Furthermore, look at how sharp-edged the sawtooth is in the bottom two graphs versus how rounded it is in the top graph.  This is a visual illustration of how the VCF is muffling the very high frequencies of the Polysix.

Effect on Noise:  The assumed purpose of the post-effects VCF is to attenuate the noise generated by the Polysix effects circuits.  They are hissy.  By defeating the post-effects VCF, we are allowing all that noise to pass.  By removing the VCF, we might all the noise to pass, but we might eliminate the noise of the VCF itself.  To see if this was the case, I did some measurements of the noise produced by my Polysix.  The noise spectra are plotted below.  (Notice the logarithmic spacing on the frequency axis.  Sorry, but it's the only decent way to show such a wide range of frequencies.)

First, let's ignore all the spikes in these graphs.  Most seem to be due to line noise (60Hz) and its harmonics.  Different configurations look different at the different spikes, but none of them is clearly superior.  So, instead, let's ignore the spikes look more broadly.  What I see is that the stock Polysix (blue) has the worst noise of the three configurations for the frequencies from 100 Hz out to about 1000 Hz.  Defeating or removing the post-effects VCF seems to lower the noise level in these frequencies.  That's great!

Above about 2000 Hz, though, we see that the green line is the worst.  The green line is the case where I defeat the VCF by forcing it to be open all the time.  This condition is about 2-3 dB noisier than the other two cases.  Because this vintage synth only has a signal-to-noise ratio of ~45 dB to start with, loosing another 2-3 dB is definitely noticeable.

Conclusion:  Overall, I'd say that the red line is the best.  This is the case where the post-effects VCF is removed by pulling U15 and jumpering J28/J29 over to R168.  It shows the lowest noise in all frequency bands, with the exception of the spike at 120 Hz.  If you can live with that, you get decently low noise everywhere else, you get the full high-frequency sizzling sound available from the Polysix, and you get the fastest attack for each of your notes.  It's a winner.  And, it's fully-reversible if you decide you don't like it!

Update: Is this modification now too bright?  Try removing Korg's own attempt to brighten the synth!
Update: Want to make the mod more robust?  Try soldering a jumper wire instead of using the clip lead!

Mono/Poly - Audio Comparison of the Sizzle Mod

In this post, I showed how I modified my Korg Mono/Poly to boost the very highest frequencies to add a little more "sizzle" to its sound.  Because I'm lazy, I tried to get away without posting any sound clips of my modifications.  Well, I knew that I wouldn't really be able to get away with that kind of laziness.  I did have a couple of people ask me for some audio.  I've never posted a sound comparison before, so I'm hoping that this works.  Here's a comparison via YouTube....

Are you able to hear the difference due to the modification?  This was recorded directly out from the Mono/Poly into my M-Audio recording device.  VCO1 only, sawtooth waveform, filter wide open, no resonance.  To my ears, both the stock and the modified versions have lots of good "buzz", but the modified version has (to me) a little extra brain-tingling sizzle to it.  Hopefully, the YouTube audio compression still allows that extra little tingly sizzle to come through. (Note: when I play this video on YouTube, a weird hashy noise comes in around 0:45...that's YouTube, not my synth.)

Now, we should talk about the effect of subtle changes in sound.  In the demo above, maybe you'll hear the increased sizzle and buzz.  Or maybe you won't.  Or, maybe you'll say that the original version sounds better because it sounds more balanced, whereas the modified version has too much sizzle.  That's a fine opinion.  An important part of this equation is what sound reproduction equipment I use.  Recording straight from the keyboard into the computer (or M-Audio recorder, in this case) is not how I normally play, so the recordings in the video above are not representative of my everyday experience of how I hear my keyboard.

When I play, I play live through a keyboard amplifier with a big speaker (Roland KC-550).  Sure, there are higher-fidelity sound systems out there, but I just like that live feel where the bass from a big speaker really rumbles your guts.  The KC-550 has that.  Unfortunately, through this keyboard amp, the sizzle that you hear  in video for the stock Mono/Poly is often not apparent...the sizzle gets lost in the amp, or in the room, or in the off-axis orientation of my head relative to the amp.  With this modification, though, the extra sizzle makes it to my ears...and it makes me smile.  So, for me, the subtle 3-6 dB of added high frequencies makes a small but pleasurable difference.

Maybe six months from now, though, I'll be complaining about how I hate the sound.  I'll say that the extra high-treble is too fatiguing.  That would be so me.  I am a human being after all.  Very fickle.  :)

Thanks for checking it out!

Mono/Poly - Treble Boosting "Sizzle" Mod

After my recent successes with my mods to my Korg Polysix, I finally took some time to play my Mono/Poly again.  While I had a great time playing it again (such a big smile on my face!), the experience also reminded me how I wish that she had a bit more "sizzle".  Previously, I compared my Mono/Poly to my Polysix and confirmed that my Mono/Poly does indeed have less high frequency content than my Polysix.  So, my mission was clear...can I recover the sizzle in my Mono/Poly?  Well, after over-thinking this problem for way too long, I finally found a super-easy way of doing it.

My "Sizzle" Mod -- One Resistor and One Cap

The figure below was the key finding from by Mono/Poly vs Polysix comparison.  It clearly shows that the Mono/Poly is clearly missing some of the highest "sizzle" frequencies compared to the Polysix.  To put some numbers to this graph, the Mono/Poly is lower by 3dB at 4 kHz and it's lower by 10 dB at 10 kHz.  Now I have a quantitative target...boost the treble with a 3dB point at 4 kHz.  Let's go!

Measurements Comparing the Frequency Content Sawtooth Wave on My Polysix vs My Mono/Poly

Previously, I tried adjusting the Mono/Poly's VCF and achieved a slight improvement in the Mono/Poly's highest frequencies, but not enough.  I then dived into the internal signals isolated the high-frequency loss to somewhere in the VCF or VCA, but not in the VCOs and not in any of the circuitry that follows the VCA.  Having partly isolated its location, I assumed that I'd begin the detailed process of trying to find the broken component that might be causing the high-frequency roll-off.  While that's a noble goal, it then occurred to me that I could just take the easy way out and artificially boost the high frequencies in order to flatten out the synth's overall response.  That would be easy.

The easiest way would be to turn up the "Treble" knob on my keyboard amp.  That worked great until I got my Polysix and was running it through the same amp.  Cranking the Treble knob on the amp makes the Polysix way too hissy.  As a result, I now something that'll boost the treble on just the Mono/Poly.  Sure, I could add an EQ pedal, but that's more clutter.  It would be best if I could just add a bit of circuitry to the Mono/Poly and be done with it.

So, looked around the Mono/Poly's schematic and found a really nice-looking target for adding a little high-frequency emphasis.  Specifically, just before the VCF, there's a simple voltage divider that provides the massive amount of attenuation (50 dB!) necessary to get the signal level down to tiny level necessary for the input to the SSM 2044 filter IC.  As shown below, simply adding a resistor and a cap together around the 47K resistor in that voltage divider should result in a nice little treble boost.

Circuit Modification to the KLM-355 Board to Boost the Sizzle of my Mono/Poly

To get the specific values for theadd  resistor and cap, I used a circuit simulator (5Spice Analysis) to predict the effect of different component values.  As I said earlier, I was targeting a boost of 3 dB at 4kHz.  After a bunch of trial and error, I settled on the 5kOhm resistor and a cap that was in the neighborhood of 1nF.  The graph below is the output from 5Spice, which says that a 750 pF would be an even better choice because it yields +3.3 dB at 4kHz.  Unfortunately, I didn't have a 750 pF...I only had a 1000 pF (aka 1nF aka 0.001uF) on hand, so that's what I used.  Given the wide tolerances band on real-world caps (10-20%?), this analysis is all approximate anyway.

Expected Response of the Sizzle Mod Using Either a 1000pF or a 750pF Capacitor

After soldering in the resistor and cap (see picture at the top of this post...ugly!), I fired up the synth and measured the actual frequency response resulting from the modification.  The figure below shows that this mod did indeed boost the high frequencies quite nicely!

Measured Response of the "Sizzle" Mod to the Mono/Poly Relative to the Stock Mono/Poly

Now, to loop back around to the beginning, did I achieve my goal of giving the Mono/Poly a "sizzle" that is similar to my Polysix?  Well, if you care to believe in graphs (I do, obviously), the graph below compares the Mono/Poly's new response to my Polysix.  Check out those high frequencies...they're lined up stunningly well.  Mission accomplished!

Measured Response of the Modified Mono/Poly to my Polysix.  Mission Accomplished!

At this point, I'd love to present to you a sound sample comparing the modified to the unmodified Mono/Poly.  Unfortunately, I don't yet have a SoundCloud or anything for sharing audio.  So, while I could share it via YouTube, we all know that their audio compression can really mess with subtle and fine details of synth recordings.  So, sadly, I've got no sharing of sound right now.  Sorry!

Given how simple this mod is, though, maybe you should just give it a try yourself!  Smell the solder!

Update: I was guilted into posting some sound samples.  Check it out!

High Frequencies -- Signals Inside Mono/Poly

Continuing down the road of this post, I'm trying to put a little more sizzle into Mono/Poly.  I perceive that its highest frequencies aren't as present as they should be.  In previous posts, I showed how my Mono/Poly output started to gently roll-off starting around 4 kHz compared to an ideal sawtooth wave.  In this post, I open up the synth and start measuring the signal at various points in the synth's circuits.  I'm trying to isolate where the roll-off of the highest frequencies occur.

My Tools for the Job

To measure the signals at internal points in the circuit, I've chosen to use my trusty M-Audio handheld recorder.  It should be noted that the signals inside the synth can be very strong or very weak compared to the regular main output of the synth.  Therefore, one has to be careful to adjust the gain on the recorder so that it can properly handle the signal level at the given location in the synth.

The next issue is how to get the synth's internal signals out to the recorder.  Well, as you can see in the picture, I had a black coaxial cable with a BNC connector on one end and two clip leads on the other.  I then bought a BNC-to-Phono plug (1/4") so that I could plug this cable into my M-Audio recorder.  To record signals from within the synth, I just touch the point of interest with the clip leads (one to ground, the other to the point of interest).  Done.  If you don't have this kind of cable, you could take a guitar cable (1/4" on one end), cut it in the middle to expose the two internal conductors, and strip and tin the tips of the conductors.  Bingo!  Instant test cable.

Moving forward, I needed to decide where I was going to start measuring.  I already showed that the overall output was missing the highest frequencies.  So, I looked at the schematic (below) and chose to record the signal prior to the VCF at R1 (left side of the schematic) and I chose to record the signal after the VCA at R41 (ride side of the schematic).

I chose to record the signal prior to the VCF (left) and after the VCA (right).

I setup the synth to play one voice, sawtooth, with the filter wide open and no resonsnce.  I recorded a C1 (low) note at the R1 location.  Then I recorded a C1 (low) note at R141.  What did I see?  Well, I saw that I saturated my recorded because the signal was too strong, even with the M-Audio gain turned down to its lowest.  So, I turned down level of one oscillator using the knob on the front of the synth.  Then, I repeated my measurements.

What did I see this time?  In the time-domain (ie, like an oscilloscope would show), I got the tracings below. Notice that the blue trace (pre-VCF) shows those nice over-shoots at the vertical transition in the sawtooth. That means it'll have lots of sizzle.  The green race (post-VCA) lacks those overshoots.

Here is the Sawtooth Signal Inside the Mono/Poly from Before the VCF (blue) and from after the VCA (green)

When I measured the amplitude of all the harmonics, I got the frequency-domain plot below.  It clearly shows the roll-off of the highest frequencies in the post-VCA trace (green).  Notice that the signal before the VCF (blue) is totally flat.  It is as good a sawtooth wave as one could hope for.  Wow.  So, it appears that my sawtooth is loosing its edge somewhere in the VCF or in the VCA.

Comparison of the frequency content of the recorded signals to an ideal sawtooth.

What's the next step?  Well, there's a lot of circuitry between the two points that I measured.  Unfortunately, the signal levels are really low (20 mVpp) right after the filter (ie, the midpoint between the two points that I measured in this post), so it's really hard for my tools to measure the signal at this point.  My previous post discussed how I think that I've got the filter pushed open as much as it can be opened.  This suggests that the loss might be occurring in the VCA.  I guess that it's time to start probing the VCA.

Update: Here's my "Sizzle" Mod, where I solve my high-frequency problem!

High Frequencies -- Polysix Adjustments

Continuing from my last post, I've been exploring the high frequency performance of my Korg Polysix and Korg Mono/Poly.  I'm trying to add more sizzle to the Mono/Poly and I'm trying to reduce a bit of upper-treble harshness in the Polysix.  My latest attempt at improving the Polysix was to follow the re-calibration procedure from the service manual, particularly regarding the re-calibration of the filters.  Sadly, it didn't affect the high frequency performance of he synth.  It did, however, bring my resonance and filter frequency control into better consistency between the voices.  As a result, I had fun discovering the unique vibe that comes with actually trying to play the self-oscillating SSM2044 filters...

But, back to the beginning.  After my last post, I received an interesting comment by "terjewinther" from the Polysix Yahoo Group.  He suggested that the calibration can have a strong effect on the sound of the synth.  So, I opened her up again, brought out the multi-meter and oscilloscope, and started in on the calibration procedure as listed in the Service Manual.  I made it through the filter tuning including offset, filter frequency, resonance, and EG intensity.  What I found was that my DAC was a bit off and my EG intensity was way off.  My filter cutoff and resonance were pretty close, but there was some variation from voice to voice.  So, I'd say that the biggest impact of the calibration was to make the 6 voices more consistent with each other, especially at higher resonance settings.

After completing this portion of the calibration (I stopped just before the calibration of the Keyboard Tracking), I closed the lid, plugged in the audio recorder, and took some new measurements of the trusty sawtooth wave.  Did I smooth out the high end harshness??? Did I get rid of that weird bump that I was seeing around 7 kHz???  

Well, the graph below has the answer....and the answer is "no".  There appears to be no change in the frequency response.  I doubt, therefore, that the calibration got rid of the harshness.  (Sure, when I play with the synth over the next few days, my ears will tell me things that this graph can't...but you can't really trust your own initial impression because we humans are so easily prey to confirmation bias...hence, objective measures are best for immediate trouble-shooting and feedback).

Frequency Response After Following the Tune-Up Procedure in the Service Manual

So, calibration did not appear to affect the high frequencies.  I would not, though, consider my effort to be wasted on the calibration.  For example, the EG Intensity range on the filter is so much more usable now.  And, as I mentioned, the 6 voices are much more consistent at high resonance...and this has actually been a bit of an inspiration.  For the first time ever, I found myself actually trying to play the self-oscillating filters in a musical way.  The video at the top of this post shows some of my results.  Sure, the pitches from the self-oscillating filters are not perfectly in tune (that's really really hard to do), but their out-of-tuneness is what engaged me.  Their tone (a fairly pure sine wave) is also strangely engaging to me.  It's a whole type of sound that I didn't know was inside the Polysix.  Now I know.  Thank you calibration!

High Frequencies - Polysix vs Mono/Poly

As you know, I have both a Korg Mono/Poly and a Korg Polysix.  I've had the Mono/Poly for longer and my visceral response to its sound is what motivated my purchase of the Polysix.  Being from the same vintage, and having many of the same components (like the SSM filter chip), made me assume that they would sound as similar as two analog synths could.  Well, once I'm got the Polysix home, I found that it didn't sound the same as the Mono/Poly.  In some ways the Polysix was better and in some ways the Mono/Poly was better.

Recording Tones from my Korg Polysix (Left) for Comparison to my Korg Mono/Poly (Right)

Specifically, I felt that the Mono/Poly had better bass and that the Mono/Poly had a much more engaging (less harsh, more smooth) lead sound in the upper octaves.  The Polysix, on the other hand, had a bit more sizzle and, on chords, the upper mids / low treble felt more liquid and present.  Being a bit geeky, I immediately wondered if I could measure and quantify the difference.  If I could quantify it, then maybe I could understand it, which means that maybe I could control and command it at will.  And, therein, lies the power of synth hacking.

OK, let's do some recordings, analyze them, and see what we can find...

For recordings, I simply plugged the output of the Polysix into my trusty portable audio recorder (M-Audio Microtrak II, shown in pictures above).  I recorded raw WAV files at 44.1 kHz at 16 bits.  I recorded 4 seconds of C1, then four seconds of C2, and so on up through all the octaves.  The synth was configured to play one voice, it was set to sawtooth, and the filter was wide open with no resonance.  I repeated the same process for the Mono/Poly (again, just one voice).  I brought all the files into Matlab for some plotting and analysis.  Below is plot comparing the raw time-domain audio of a sawtooth waveform at the lowest note, C1.

Polysix and Mono/Poly when playing a sawtooth waveform down at C1.

Clearly, there is a difference in the shape of these two waveforms.  To those used to looking at oscilloscope traces, this plot will be familiar.  You'll note that neither plot is as much like a sawtooth as one might like -- in both cases, the "ramp" portion of the sawtooth is not as straight as one might expect.  This is due to some roll-off in the bass frequencies in the output.  The Polysix waveform (blue) is more rounded than the Mono/Poly (green), so it appears to have a little less bass.  As for high frequencies, note that the Polysix (blue) has a very sharp downward spike at each vertical transition in the sawtooth.  It actually appears to overshoot.  This kind of sharp, narrow spike requires very high frequency response, which suggests that the Polysix does indeed have more "sizzle" than the Mono/Poly.

Now that we've seen some interesting features, let's try to quantify them.  I've chosen to take the FFT of each recording in order to assess the frequency content of each note.  After a bit of normalization to equalize the volume of each recording, I would get plots like the one below.  Note that frequency is now on the horizontal axis instead of time.

Spectrum recorded from the Polysix and the Mono/Poly when playing the lowest C ("C1").

The first thing to notice is that the spectrum of the note is composed of a large number of "spikes" in the frequency domain.  This is what one should expect for a sawtooth wave.  If I had used a sine wave instead of a sawtooth, I would have just gotten one spike...the one at the fundamental pitch.  What differentiates a sine wave from a sawtooth wave from a square wave is the number of harmonics and their relative magnitude.  Therefore, this plot is very normal.

Looking now at the Polysix spectrum (blue) compared to the Mono/Poly spectrum green), we see that they are largely similar until we get to the highest frequencies.  Above ~3 kHz, we see that the Polysix has stronger high frequencies than the Mono/Poly.  This could be part of the difference in "sizzle" that I'm hearing.  Let's dig in a little deeper.

The plot above is confusing because it shows the signal energy at important frequencies (the fundamental and all the harmonics) and at unimportant frequencies (everything between the harmonics).  Let's extract just the energy at the fundamental and at each harmonic and plot just those values.  The resulting plot (below) is much simpler and easier to see what's going on.  For reference, I even include a line that shows the spectrum for the ideal sawtooth waveform.  You'll see that both the Polysix (blue) and Mono/Poly (green) have pretty good sawtooths.  Only above ~3 kHz do they begin to diverge in any significant way.

Spectrum Assessed at Just the Fundamental and Harmonic Frequencies.

Let's further simplify this plot by taking the difference of each spectrum relative to the ideal sawtooth spectrum.  The result is shown below.  Now we're getting somewhere.  

Spectrum Compared to the Spectrum of an Ideal Sawtooth Waveform

First, look at the left-most side of the graph. Here are the low frequencies. The fundamental at C1 is abut 33 Hz. That's where this graph starts. Note that it shows that the Polysix (blue) is a little below the Mono/Poly (green). At this very low frequency, the Polysix is showing 1.6 dB less bass than the Mono/Poly. That's not much of a difference. So, maybe my subjective assessment that the Mono/Poly has more bass than the Polysix is supported by this data, or maybe not.  Lots of other factors can affect the psychoacoustics of bass perception besides just the literal amount of bass in the air.  So, I'm going to refrain on making any conclusions about bass.

Looking at the right-most side of the graph, we see huge differences in the treble response.  Unlike the assessment of bass, this difference in treble is very clear.  Comparing the Polysix (blue) to the Mono/Poly (green) we see a 3 dB difference by 4 kHz.  That's definitely audible.  By the time you get out to 10 kHz, we've got a 10 dB difference.  That's a big difference in "sizzle".  This definitely confirms what I was hearing.  This doesn't tell us *why* they're different, but it's an objective measure that we can now use to probe within each synth to find where the difference occurs.  That's an exercise for later.

Another behavior that catches my eye in the figure above is that the Polysix diverges from the ideal sawtooth by first going *up* before going down.  It's as if there is a treble knob within the synth and that it is turned up a bit in the 3 kHz to 10 kHz region.  The peak response is at 6.8 kHz and is 2.2 dB above the ideal sawtooth.  What's the cause of this apparent enhancement of the treble?  Well, I'm not sure, but to my eye, it appears that the resonance of the VCF on the Polysix might be a bit active, even though I turned it down to "zero".  I'll have to open her up and check it out at a later time.  

The more important question is whether this boost in treble is the cause of the "upper mids / low treble feel more liquid and present" perception that I mentioned earlier.  Maybe.  If I'm able to tune the boost out of the system (through adjustments to the resonance or whatever), we'll see if the "liquid and present" feel goes away.  Given that I like the "liquid and present" feel, I might choose to keep it the way that it is.

A downside of this enhanced upper-mids is reflected in my original comment that I preferred the sound of the Mono/Poly for lead work in the upper octaves.  I felt that it was more engaging and less harsh.  Excessive upper-mids could be the source of "harshness".  When I examine the data for a high note (C6) many of the conclusions drawn from the graphs above still hold for C6...

For example, below, the time domain plot shows that the Polysix still has its over-shooting downward spike suggested lots of high-treble.

Time-Domain Plot of Raw Waveform of High Note, C6

The FFT output comparing the two C6 notes shows that the Polysix definitely has more more treble.

Frequency-Domain Plot of High Note, C6

And, comparison of the amplitude of the fundamental and harmonics to the ideal sawtooth waveform (below) shows the same high-frequency roll-off in the Mono/Poly and the same slight high-frequency boost in the Polysix.

Comparison of Harmonic Content of High Notes (C6) to Ideal Sawtooth

So, while "harshness" is also a complicated psychoacoustic phenomenon, these plots confirm that there is a substantial difference in the amount of treble above 3 kHz between the Polysix and the Monopoly.  This is true for both low notes (the C1 analyzed first) and for high notes (the C6 analyzed second).  In my opinion, this extra treble is at least one of the factors of the apparent harshness (for lead work) of the Polysix compared to the Mono/Poly.

Edit: Follow-up on the Polysix is here.
Edit: Follow-up on the Mono/Poly is here.
Edit: Modification of the Polysix to restore the deep bass is here.