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!