Wednesday, February 20, 2013

High Frequencies - Mono/Poly Filter Adjustments

Following from my previous post, where I re-calibrated the filters on my Korg Polysix, I wondered if re-calibrating the filter on my Korg Mono/Poly would add some of the high-frequency sizzle that I feel is missing.  So, I dived in, trying to follow the instructions in the Service Manual and started the adjustments.

Adjusting VR18 - the Trim Control for the Filer Cutoff

I found that I had a hard time following the instructions.  For example, they recommend tuning the response of the filter cutoff knob by setting the filter to self-oscillate and then tuning the resulting pitch to a particular frequency using VR18.  The target frequency value is somewhat low...140 usec, or about 7.1 kHz.  That implies that the filter will only open up to around 7.1 kHz, which is lower than I want.  Based on my experience with the Polysix, I think that I want up to 10 kHz or higher.

Filter Section of the Mono/Poly Schematic.  The arrows point to pots that I tweaked.  The biggest effect comes with the "VR18" adjustment at the top.  The lower two mainly affect the behavior of the resonance and have little to do with the high-frequencies produced by the synth.

So, I instead adjusted VR18 so that the filter would open up as much as possible  I turned VR18 until the filter opened up, but stopped turning as soon I perceived no more increase in the synth's high frequencies (via headphones).  It turns out that on the SSM2044, opening the filter requires more negative voltage to be applied to Pin 3.  Changing VR18 adjusts how much voltage gets applied to Pin 3 and, therefore, affects how widely the filter opens and closes.

After my adjustments, I'm reading around -95mV with the VCF cutoff knob turned all the way up and about +95 mV with the knob turned all the way down.  When I use the bend wheel (which the Mono/Poly can map to VCF cutoff), I can drive the Pin 3 voltage to -101 mV, but there is no difference in sound (on my synth) between -95 mV and -101 mV.  It turns out that -90 mV is also the value suggested by the SSM2044 datasheet.    So, I guess that I've got it all the way open.

Measuring the voltage at Pin 3 of the SSM 2044 filter IC.  Here, the VCF cutoff knob is turned to be all the way open.

As I mentioned, when first adjusting VR18, I did notice a small increase in the high-frequency response of the synth, but it was nothing too dramatic.  Quantitatively, how much improvement did I get?  Well, the graph below is what I measured using the technique that I described in an earlier post  This graph does show a slight extension in the high-frequencies produced at the synth's output due to my filter tweaks.  I still have a way to go, though, before I make this graph flat, which would be the frequency content of an ideal sawtooth.

Frequency Response of Korg Mono/Poly After Adjusting the Filter Calibration

Let's get specific...previously, at 10 kHz, I was 8.1 dB down from the ideal sawtooth.  After these filter tweaks, I'm now down by only 5.6 dB.  So, through my filter adjustments, I boosted the response at 10 kHz by 1.5 dB.  As I said, this is a slight improvement that can be heard, but it is subtle.

My Polysix, which is quite sizzly, is actually a little above the zero line on these graphs, which might be *too* high.  So, I think that it is reasonable to see if I can get to the Mono/Poly up to the zero line.  This graph suggests that I've got another 5-6 dB to go.  The pursuit continues.

Next Step: Investigating internal signals around the Mono/Poly VCF and VCA.

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