[sdiy] Korg Poly61 a peculiar design!!

[Dan Snazelle]

I actually like the sound of the poly61 for SOMEthings!!!




Sent from my iPhone

On Apr 6, 2011, at 5:46 AM, Tom Wiltshire <tom at electricdruid.net> wrote:

> Have you got a link to some decent schematics for the Poly 61, Karl?
> 
> I've got a service manual PDF for it, but it's more-or-less unreadable.
> 
> Thanks,
> Tom
> 
> On 6 Apr 2011, at 01:44, karl dalen wrote:
> 
>> 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
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