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[djsynchro]

OK this is what I want to do: On the  Casio SK-1 in normal "Play" mode all 4 voices in the SK-1 are routed through the melody filter. Because the SK-1 samples at (only) 9.38 Khz This filter is set at something like 4Khz.
-->> Thus, resulting in a muffled sound.

I want to either bypass the filter completely, or modify it so it lets through more high frequencies, & deicious digital grunge. On the Linn drum for example te designers "cheated" with the filters and set the cutoff frequencies higher than is "proper" to let more high harmonics through.

I was thinking of connecting a wire to bypass the filter section to see if that would have the desired effect already.
(Red line I drew) Does anybody know if I can safely do that without breaking anything?

Does anybody know anything about analogue filters? I was also thinking of changing the value of some of the caps to  raise the cutoff frequency but which ones & what?

[gmeredith]

There is a lot that can be done regarding filters on the casio sk keyboards. Check this site out for some of the mods, especially regarding a post titled "Casio SK-8 AUDIO-OUT (by "plutoniq9") ":

http://www.jz-server.de/forum2/e107_plugins/forum/forum_viewforum.php?16

This is a direct audio out that is for the Casio SK8, but is very similar for the SK1. It bypasses all the anti-aliasing filters. pretty much in the same place as you marked.

Also check out in that same section for a post called ADSR envelopes, regarding VCA shaping.

Regarding your interests, there is the prospect of actuallly creating a VCF circuit and inserting it between the 2 points you marked, and then have controls on the sk casing for resonance and cutoff! You could also then connect the sk's envelope signal line for its VCA to the inserted VCF circuit, and the VCF filter would then track the envelope and get waaahhhaaaw type synth sounds for its presets. An example of a simple VCF circuit, controlled by a Control Voltage CV input is here:

http://electro-music.com/forum/topic-12875.html

I planned to build it, but haven't got around to it yet.

Let us know how you go with all this!!

Cheers, Graham

[djsynchro]

Hey thanks (applauded you) that's a good site some serious Casio info going on there.

My bro (electronix wiz) says halving the value of the caps in the filters raises the cutoff frequency.
I'm not really interested in a VCF because I have a small modular so I often patch bent stuff through there while sampling it, or else I just record it straight and then put plug ins on it. (Ableton Live has a really nice filter).

Whatever I come up with I will post here of course.

[djsynchro]

Soo... in the SK-8 the emitter of the LAST transistor before the melody filter is connected to the tip of an extra output jack. Sleeve to ground -->> totally unfiltered audio output.

Been staring at the Realistic service manual for the SK-1 but can't figure out which transistor is the last one before the filter. (T17 ) The manual is a bit of a lo-fi scan so that's a  problem (plus I'm not super experienced reading schematics... ahem!)

Anyone with da knowledge out there?

[djsynchro]

I think I found it, no extra output, just the bypass as originally planned but with a 1k resistor. Will post it with a picture when it's tested and found to be OK. had it going with the alligator clips -->> Sounded very nice! <<--

[djsynchro]

And... I got it! This is a seriously killer SK-1 modification!
Really easy one too.
Will post a diagram soon!

[gmeredith]

Looking forward to seeing it!!

Cheers' Graham