The modification that I performed is located entirely on the KLM-368 PCB ("Effects"). An excerpt of the schematic is shown below. C61 is a 1 uF electroltyic capacitor that the uneffected must audio pass through. If you want to remove this cap, you could simply de-solder it and put a jumper in its place. If you merely want to try the mod without removing any components, you could instead use clip leads to jumper from one leg of C61 to the other leg. Unfortunately, my C61 was soldered too tightly to the PCB, so I chose to jumper between the easily-accessible points on R115 and Q15.
|On KLM-368, Bypass C61 by Using a Jumper Wire from R115 to Q15|
|The Green Jumper Connects the Right Side of R116 to the Left Leg of Q15|
|Recorded Output of the Lowest "C" with a Sawtooth Waveform (VCF at "5")|
If we're discussing frequency response, we really should be looking at the signals in the frequency domain. So, below, I do a frequency analysis of two 5 second audio samples of the output of the synth. The blue trace is the unmodified Polysix, which has the C61 in place. The red trace is the modified Polysix where C61 has been bypassed. Again, this if for the lowest "C" on the keyboard, which has a fundamental frequency of about 32.8 Hz. That is a *very* low frequency. Comparing the two traces, we see that bypassing C61 seems to increase the synth's response at this frequency by about 6 dB. That's a pretty big change!
|Comparing the Frequency Content of the Lowest "C" Using a Sawtooth Waveform (VCF at "5")|
In the real-world, 32.8 Hz is too low for our loudspeakers or headphones to reproduce accurately (especially for hobbyists like me). So I'm not sure that I'm able to hear the impact on these deepest bass frequencies with my equipment. But, you'll see that the next couple of harmonics (66 Hz, 99 Hz) are also slightly stronger after the modification. My system can easily reproduce these frequencies. So, when I'm playing my synth (or when I'm playing the Soundcloud demos at the top of this post), I do hear a difference between the unmodified and modified conditions, I'm just not sure its the change at 32.8 Hz that I'm detecting. That is some seriously deep bass..
Given that I do hear some difference in tone (whether at 66 Hz or at 32.8 Hz), which version do I prefer? Certainly, for raw visceral excitement, I like the added thickness and rumble of the full bass experience resulting from bypassing C61. But, the Polysix isn't intended to be a deep-bass rumble machine -- it isn't supposed to be a Minimoog, or even a Mono/Poly. Instead, it's a polysynth meant for chords and pads and strings and such. So, when used for these purposes, perhaps my modified Polysix now as too much bass. I think that it might sound too thick, too bloated.
It's interesting (to me) to note that the Polysix's main competition back in the day -- the Roland Juno 6/60 -- includes a high-pass filter as part of its architecture. One use for the HP filter is to cut the low-end rumble to purposely make the sound more skinny. For chord stabs, a skinnier sound can often sit better in the mix, especially when you've got other instruments providing a deep and punchy bass line (such as for dance music). Perhaps this ability to control the low end bloat to sit better in the mix is why the Juno's continue to be more popular than the Polysix.
But that's a digression...
Back on the topic of the increased deep bass on my Polysix, I'm still deciding whether or not I like the modification. I'm going to have to live with it for a while to see. Thoughts?
Update: Better frequency response graph here along with more discussion of how it's done.
Update: I decided to properly bypass C61 using a jumper wire instead of my clip leads. See here.