Thursday, July 18, 2013

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!

3 comments:

  1. Tempting...
    I'm personally not too keen on the raspy, but the reduced noise and sharper enveloppe seem interesting. So that might be the solution!

    BTW, great analysis!
    I wish I had your skills and knowledge in electronics. I have a few good ideas but I have no idea how to implement them yet...
    Slowly learning! And following your posts really helps me, so thanks again for sharing!
    -SOS

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  2. Hello and thank you for this information! Wonderfull analysis and documentation, btw. I will likely attempt these modifications, including the reduction of the high-freqeuncy compensation (removal) to tame the high frequencies.

    In addition, I purchased many audio-grade electrolytic capacitors (for the signal path of course) through the effects PCB. I also purchased high performance op-amps with lower noise floor, and wide-bandtwitdh (used for scientific instrumentation amplification where accuracy is needed), so as to make the sound more transparent and reduce slewing. I suspect therefore--I will want to make sure no other high-freq. compensation is still intact.

    I look forward to implementing this and will of course report back with some results!

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    Replies
    1. Thanks for reading!

      Note that the Polysix has numerous spots where it boosts the high frequencies. I only address one of the spots in this post. There are several others, mostly on the KLM-368 Effects PCB. IMO, they are mostly aimed at overcoming the treble loss with each pass through the LM13600 OTAs.

      IMO, it's the heavy use of LM13600 stages is the real culprit for loss in audio fidelity and noisiness (oops...I mean "vintage flavor) in the Polysix. If you really want to clean up the sound, do a quick trial and take out the audio signal prior to the Effects PCB. It's amazing how much cleaner and quieter it is.

      Chip

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