[sdiy] Korg Poly61 a peculiar design!!

[Doug Terrebonne]

I think they sound great - 2 detuned saws into that MS20 style filter with the 
arpeggiator running... Damn nice for under $200. 


Doug


----- Original Message ----
> From: Dan Snazelle <subjectivity at hotmail.com>
> To: Tom Wiltshire <tom at electricdruid.net>
> Cc: "synth-diy at dropmix.xs4all.nl" <synth-diy at dropmix.xs4all.nl>
> Sent: Wed, April 6, 2011 3:02:33 PM
> Subject: Re: [sdiy] Korg Poly61 a peculiar design!!
> 
> 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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