I finally followed-up on Tony's suggestion that C61 on KLM-368 appears to have no purpose and that it could be removed.
When I look at C61, I also see no purpose. I see that it could possible attenuate the lowest bass frequencies. I was curious to see the effect of removing this component. Not wanting (yet) to permanently remove C61, I simply bypassed it with a single clippy jumper wire. Easy!
The result appears to be ~6 dB more of the deepest bass frequencies (down in the 33 Hz neighborhood). That's deep.
If you're interested in the mod, you can check out the details at:
http://synthhacker.blogspot.com/2013/08/polysix-deeper-bass-bypass-c61.htmlIf you've got good headphones or really good speakers, there's also an audio comparison.
My conclusion is that maybe the Polysix doesn't need these super-deep frequencies. It is a fun change, but I'm not sure it's how I want it to sound all the time. I'll have to live with it for a while before I make my decision.
Thoughts?
Chip
--- In PolySix@yahoogroups.com, Oakley Sound <oakleylist@...> wrote:
>
> I've been following Chip's progress with interest. And his recent post
> on the dynamic noise reduction filter on the effects board spurred me
> onto to investigate further. I too have noticed that the P6 is somewhat
> duller than other synths and I had always assumed that with the ensemble
> mode off the output was as bright as it could be.
>
> Chip's findings that all four ensemble modes pass through the DNR filter
> was something I hadn't really thought about before.
>
> http://synthhacker.blogspot.co.uk/2013/02/mystery-circuit-polysix-post-effects.html
>
> In summary: the P6 has a 12dB/octave low pass filter that is set to
> around 600Hz (the actual value depends on whether the BBDs are engaged)
> when no notes are playing. This filter opens up when it detects audio
> from the voice card. As Chip noticed it takes a finite time for the
> filter to open and the filter doesn't always open fully in all cases. I
> measured the filter took up to 100mS to open fully. That's a significant
> loss of high frequency sound at the start of each note.
>
> Yesterday I went a stage further than Chip. Chip's mod basically forces
> the filter open. I decided to remove the filter from the circuit
> altogether. This has the advantage of no dynamic filtering (which is of
> dubious amenity) and reduces the circuit pathway that the audio has to
> go through. The LM13600 isn't a hi-fi part and removing it improves the
> signal quality.
>
> The mod:
>
> Remove links J28 and J29. Remove R115 and R109 (which disables the
> envelope follower). Remove C73 and C61.
>
> Link out, with a piece of solid core wire, the two left hand holes of
> J28 and J29. Now connect with a piece of insulated wire the lower of
> these now filled solder pads to the lower pad of C73. The filter is now
> bypassed.
>
> Link out, with a piece of solid core wire, the two holes of C61. This
> capacitor serves no useful job that I can see.
>
> Result: the P6 can be as bright as any synth. Percussive sounds ring out
> where they didn't before. String sounds are really very majestic - you
> can really pull off sounding like a string machine now. The slight
> increase in background hiss is not a problem for me.
>
> Tony
>
> http://takla-makan.bandcamp.com/
> http://www.oakleysound.com
>