[sdiy] Korg Poly61 a peculiar design!!

[karl dalen]

VCF:
12db LP LM13600 using a MS20 VCA with limiting diodes in resonance
feedback but before VCA also the feedback not only goes to the input
of the filter but to first cap like half way S/K and no local OTA
gain loops but one fixed over both. 

I have checked trough all other analog Korg products and this VCF
are unique to the P61.It should have a peculiar Bw hump because of
this.

VCA:
MS20 final VCA. One SSM2056 EG split use VCA and VCF.

DCO1:
It's a complete saw VCO core, a regular Op amp integrator/comparator
design. The ramp reset comparator also feeds another comparator that
sends this signal back to *both* 1/3 of DCO1 and DCO2 8253 timers gate
input, then one of the 8253 timers sends a pulse back to the integrator
NPN ramp reset transistor in the VCO!! Odd!

Could this perhaps mean that half of timers are actually used to measure
the frequency of the VCO and if wrong in realtime restart it on a per
cycle basis?! It could work if the value are compared to a predefined
LUT. Or is it just a bogus thing to distract anyone viewing the schematic?

The output of the ramp integrator are sent to a comparator to create
PW wave.

DCO2:
DCO2 are made out of 1/3 8253 driving a 4520 divider who's output's
are selected by a switches and summed by a weighted resistor net
acting like a DAC to create saw. A redundant signal goes to divider
reset to actually turn off the chip..

Apparently resolution of wave are likely dependent of octave selected
otherwise only one switch would be needed.   

VCO ramp integrator note CV are a 7bit R2R generated by the same CPU
that scan's KBD that then feed a common expo who are muxed out to
each VCO. (Polysix/Trident) 

The driver for the timer DCO's are a VC RF multi vibrator who in turn
are driven by bend and vib voltages, the CLK signal are then driven
into a prog divider to create semitone offsets for DCO2, finer detuning
presumably by 8253 timer itself.

There are no regular muxed DAC for the parameter CV's instead each
has its own DAC trough a bag of 8255 driving a even larger bag of
weighted resistor net's.

DAC for Cf of VCF are only 6 bits. However PWM, VCF, Vibrato modulations
are full resolution from the analog triangle MG.

In other words an awful sounding poly but a great, peculiar and funny
VCO/DCO design. 

Disclaimer: I might be utterly wrong in all that said here.

Thanks for your attention....G'day...

KD