Discussion about the Korg PolySix synthesizer

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