Yet Another Korg Poly800 Mod

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Mixolydian
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Yet Another Korg Poly800 Mod

Post by Mixolydian » Sat Dec 15, 2007 7:02 pm

The poor Korg Poly800: Not a bad synth, but it's the most hacked on analog synth out there with the Moog Slayer mod being the most common alteration. The Moog Slayer adds a filter cut-off and resonance control pot, bypassing the digital touch pad. But the biggest problem with the Poly800 is that all 8 DCOs share one filter.

I have the module version, the EX-800, and use it often. But I've been hesitant to mod it. So I thought I'd get another EX-800 or a Poly800 and design a new filter. My idea is to build a separate filter module with one SSM 2044 filter chip per DCO.

I haven't found a schematic yet, so I'm not sure if I can break in to the middle of the circuit board and rejoin it before the amp section.

Any thoughts?

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Post by solderguy » Sun Dec 16, 2007 12:19 am

The EX-800 is very similar to the Poly 800 internally, you can look at the Poly 800 schematic to get a good idea of what is going on.
2 problems:

a) how are you going to polyphonically generate the additional envelopes to feed each of the added VCF chips? Simplest would be to add another microcontroller to generate the extra envelopes.

b) I would love to see the datasheet for the OKI M5M5232 digital oscillator chip that is the heart of the Poly/EX, still haven't found the datasheet online. Looking at the schematic, it might not be straightforward to demultiplex the multiple oscillator outputs into eight separate lines, one to feed each VCF.

Much easier to get a DW6000/8000/DSS-1.

One mod which I found useful for the EX-800 was to add the Chord Hold function which is present on the Poly-800, need to get into the panel switch matrix of the EX-800 to do this.

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Post by Mixolydian » Sun Dec 16, 2007 2:12 am

Thanks for the responce. It's the kind of info that I was hoping for.
solderguy wrote:2 problems:
a) how are you going to polyphonically generate the additional envelopes to feed each of the added VCF chips? Simplest would be to add another micro controller to generate the extra envelopes.
There are 3 envelope generators on the EX-800: One for DCO 1, DCO2, and one for the amp section. Hopefully, the DCO envelopes can be used.
solderguy wrote: b) I would love to see the datasheet for the OKI M5M5232 digital oscillator chip that is the heart of the Poly/EX, still haven't found the datasheet online. Looking at the schematic, it might not be straightforward to demultiplex the multiple oscillator outputs into eight separate lines, one to feed each VCF.
Yes, this is the bigger problem. It's an unusual chip design. 2 sets of 4 DCOs that are all square and can be combined to modulate each other (sounds like FM.) It may not be doable

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Post by solderguy » Sun Dec 16, 2007 3:52 am

Right, the EX/Poly 800 actually is 8-note polyphonic if you ignore the VCF, each voice has 2 osc each with independent envelopes... but.... the amplitude control for each osc and voice appears to be internal to the MSM5232 (similar to the SID chip), the envelopes may also be internal, or it may be that the 80C85 microcontroller calculates each envelope and sends it as binary data to control the oscillator levels in the MSM5232... either way you have to intercept the digital envelope data, convert it to a voltage and send it to the appropriate VCF chip. Probably better off getting the datasheets for the MSM5232 and writing your own operating system firmware to control the MSM5232 and also generate separate envelopes for each VCF. If you like this kind of stuff and have the datasheets, probably straightforward (but not trivial). The MSM5232 has been used in arcade games, hopefully someone has an OKI databook.

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Post by otto » Sun Dec 16, 2007 4:26 am

You could always build external filter modules you could use with any of your gear.
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Post by Mixolydian » Sun Dec 16, 2007 6:57 am

solderguy wrote:... either way you have to intercept the digital envelope data, convert it to a voltage and send it to the appropriate VCF chip. Probably better off getting the datasheets for the MSM5232 and writing your own operating system firmware to control the MSM5232 and also generate separate envelopes for each VCF. If you like this kind of stuff and have the datasheets, probably straightforward (but not trivial). The MSM5232 has been used in arcade games, hopefully someone has an OKI databook.
.

Thanks again solderguy.

I didn't think that I'd be going through the DACs. I'm over my head here. I can design circuits, but I don't know how to write software.

It would probably be easier to build a module from scratch or a kit.

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Post by metrosonus » Sun Dec 16, 2007 5:30 pm

typicallyl that's what I do with mine, I run the filter wide open and run the audio through my modular gear. Im really wanting the polyvox filter as I think that would sound awesome with the 800.

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Post by SecretAsianMan » Sun Dec 16, 2007 6:52 pm

How much polyphony do you need? Perhaps you could buy multiple EX-800s and build a device to poly-chain them.

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