(I posted this on vintagesynth.com)
I got the idea of Mono/Poly pre filter changes from this cool blog: http://synthhacker.blogspot.fr/
When I compared my upgraded P600 to my Mono/Poly, I thought: either I sell it, or I try to make it better.
I chose to change all the opamps for TL082, capacitors (using lower
value tantalum for envelopes), and added a pre-filter gain and treble
boost (using resistors and a cap).
The sound is now ridiculously
better, the filter starts to overdrive nicely past 1 osc at half level, I
have lots of treble, even past 20Khz, and while I can't do 20 seconds
envelopes anymore, they are way more percussive!
I changed all the 4558 opamps for TL082, because even if 4558 has a
nice vintage sound, it's awfully slow and not dynamic. The VCOs summing
amplifier (1/2 IC18 on KLM-354) would be the most important to change, I
think it's responsible for the so called useless VCO mixer on the MP4!
I changed R1 on KLM-355 for a lower value (I tried may values, 10k
seemed like the best compromise between sound fatness and mixer
- I added the treble boost from synthhacker, I had to choose different values to accomodate for R1.
- I changed C23 and C25 on KLM-355 for 2.2uf Tantalums, so envelopes are both faster and snappier (due to tantalums low esr).
A few demos I made after these changes : http://sfx.gligli.free.fr/zik/mp4_recap_reopamp_mods/