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[gmeredith]

Hi Everybody!!

Well, here it is - the magnificent PHAT PHILTER BANK mod!

This mod truly turns the humble SK5/8 into an analog monster machine!

It is also suitable for other SK keyboards, such as the SK1, SK10/100/200 etc, though the hooking up connections may be slightly different for them.

This true polyphonic 4-voice VCF filter has frequency and resonance controls, and CV input.

You can choose manual control cutoff frequency, or select "envelope" control, where the filter follows the attack/decay envelope of the sound selected. You can also select LFO control of the filter if you have the LFO mod installed.

For example, selecting a preset sound like the violin preset will give a "wwhhaahwww" type filter sound, if set to “Envelope” mode. Choose a fast attack/slow delay sound like piano or guitar will give an "aaaoowww" type sound!! Using the sample envelopes will give even more variety.

The fully illustrated, step-by step guide is in the FILES section onthe Yahoo groups Casio SK site:

http://launch.groups.yahoo.com/group/CasioSK/

It's in the FILES section on the site (you will need to join the group to access the file, though)

I will put some mp3's up there shortly, to give examples of the amazing sounds this thing produces.

Cheers, Graham

[gmeredith]

Hi all!

I've just uploaded an mp3 example into the FILES section on the Yahoo forum,
in the PHAT PHILTER folder, for you to all have a listen. When I get
time, I'll upload many more examples of different settings of the Phat
Philter Bank mod.

The one I just put up is the SK-8 trumpet preset
sound, with the Phat Philter Bank set at medium cutoff and fairly high
resonance. The sound also has my Slo-A/D Mod applied to it, giving a
nice slow attack to the notes!

Cheers, Graham

[Oceanus - XD515]

Hi Graham,

Do you have a schematic available, as I would like to lay out a PCB for this excellent piece of work.. I would also plan to use Xicor digital pots  for the Freq / Res controls to get the 4-gang effect.. If you don't mind I would like to have a crack at it :-)

Cheers

Paul

[gmeredith]

HI Paul, Sorry I didn't see your post in here first, I answered you via email and then realised you posted here. For those reading this and wanting to know the same question, the circuit is basically Tom Escobedo's Q&D VCF. I simply multiplied the circuit x4, and added some input and output resistors and capacitors to match the SK's circuit level gain. The Q&D VCF schematic is below.

Cheers,  Graham

[Dj Task Manager]

I built up one of the modules from the excellent v.clear and thorough pdf instructions the LED does light with the res knob and env input but no sound out    the only substitution i made was the 0.1uf cap was another green cap not a box type one.  Ive checked EVERYTHING and replaced the chip!!! Is it the 0.1 cap thats frickin'me up?!

[gmeredith]

That module is hard to build - i had problems myself bulding it, it's so small. That was my attempt to make it small enough to fit inside an SK without having to remove the speaker. I'm in the process of redesigning it so that it's larger and have less chance of making  a mistake.

Check to see if you put the cuts in the tracks in the right places - that's what stuffed me up the first time.

Cheers and good luck!

Graham

[Dj Task Manager]

ive decided to go back to the tim escobedo Q&D design.  I just had a thought (to spare a few components) for the Vref for each module is it acceptable to make a Vref rail (with two 100k resistors and a cap) for all of them rather than do it 4 times?  another simplification would be to use 2 dual or even a quad amp chip. Any recommendations for that?  

ALSO...

I spotted an error with the hooking up instructions, the voice 1 and 2 outputs are not where the instruction says they are, the one marked 2 on the diagram is where the voices are summed.  If you listen you can hear them all from this point.  The actual one for "voice 2" is labled "voice 1" and the real voice 1 is in a different place...



theres also a 'invert cv response mod' on the diagram for the Q & D that im going to have a pop at!

[gmeredith]

The 4 separate CV inputs will give true filter polyphony. If you bridge them, you will hit a note on the sk and the filter will only operate as a blanket filter, rather than respond to the other note triggers - in essence, like the filter on a Poly 800. But this will only happen once every 4 key hits, since the sk cycles through the 4 CV lines as more notes are held down. It certainly won't damage anything to do it that way, though.

Dual/quad op-amp - I looked into this but found that it didn't simplify things much at all - you still needed the same amount of external components, so it didn't reduce the complexity much. Plus it would make troubleshooting harder.

Error in hookup - thanks, I'll check that one. Is it an SK8 you have? On the last page of the Phat Philter bank guide, there is a photo of my board actually hooked up, working, with the wires attached to the same points but on the component side of the board.

Did you end up getting it going properly??

Cheers, Graham

[Dj Task Manager]

So you think doing the 'Vr' bit of the circuit* to a rail for all the modules wont work then?

(there is a 1m resistor between the Vr bit and the input audio signal so that should stop the audio summing right?...)


Im in the process of doing the 4 modules dead fly style, 2 down 2 to go.

Ive tried out the invert cv response mod by re-orientating the LED; it works nicely but there is a low hum... hmmm :-(

Thanks for your help,  I'll keep you posted!


*as in on the Q & D diagram

[gmeredith]

Well, it will work, "but not as we know it, Jim" 

It won't behave in the manner that the circuit was intended - but that doesn't mean that the fun stops! What i mean is that the filter won't behave truly polyphonic - ie. each note played will have the filter operating that note and that note only. What will happen is that if you play 2 notes together, the first note played will trigger the filter to open, but as it's closing, the 2nd note struck will cause it to re-open before it has completely decayed, and you won't get the full filter effect. Again, there are several older synths out there that had this type of filter - the Korg Poly800 is one example.

I guess the point is here, that if you're not going to have it operating in full polyphonic filter capacity, then you only need 1 filter module to do it - no need for 4. You just sum all the SK audio inputs together and run it into one module audio input, sum all the SK CV lines together and run them into one CV input of the filter. That's what will happen if you tie all the 4 audio and CV inputs together like you were saying - all 4 units will respond exactly the same and it will sound like there is only 1 filter.

Please try this out for the sake of the experiment - it wil be interesting to see and won't damage anything.
You might find though that tying the CV lines together may also affect the SK polyphony note behaviour - each CV line will influence the other one, when they were meant to be isolated in the SK. You might find that notes are cut off, or don't decay properly etc. Again, no damage, but it may not work as expected.

Cheers, Graham

[Dj Task Manager]

I think we are talking at slightly cross porpoises.  I was considering doing a switchable option with a network of diodes so that each CV ONLY influences the other voices but not the intended voice, for a sort of opposite CV effect.  However i might not bother now as thinking about it, though it sounds wizzy in concept it probably isnt worth the wiring and head scratching for what it will probably actually sound like.

The rail I suggest in the previous post was just for scrimping on the cap and couple of resistors that make up the voltage divider thing: (the grey bit)


I was imagining that the 1m resistor that links each module to the rail (in grey) would protect the audio signals from mixing, This may well not be any good as I really dont know very much about electronics, so appologies if your previous responses do already cover why this wont work!!

P.S. the saving overall on parts is only 6 resistors and 3 capacitors so im not really sure why I am so concerned about it!

[Gordonjcp]

Surely the DC bias alone should go to the non-inverting input and the audio should go to the inverting input?

[Dj Task Manager]

yup it does look odd but it seems to work like this. 
I'll give my crazy ass DC bias rail idea a go and report back...

[gmeredith]

I think we are talking at slightly cross porpoises

Ahh yes I get it now. This is the cool thing about it, you can do all these tricks with such a simple circuit.

Please post up some mp3s when you get it going!

Cheers, Graham

[Dj Task Manager]

...more money/component saving suggestions!...

I think you can get away with just a single gang pot for the cut off freq knob, just + to one side - to the other and all the cut off input from the centre  

...Infact in the wiring diagram I think the all the gangs of the pots are in parallel so that the entire range of them is 25k.

In the configuration I am doing i am using 2 twin 100k sliders (so that i can have alternate notes with different cut off frequencies (and because they are £1 each from maplin!)) im hooking up the tops and bottoms via diodes to + and - to stop them acting in parallel.