Circuitbenders Forum
Circuitbenders Forum => Circuitbending discussion => Synths & Samplers => Topic started by: gmeredith on November 15, 2007, 01:12:43 AM
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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
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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
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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
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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
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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?!
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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
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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...
(http://2.bp.blogspot.com/-tuTFqHyetMA/Ub9BMTfNk1I/AAAAAAAAAVc/Y5qkRTPM_LQ/s1600/quad+vcf+hookup+correction.JPG)
theres also a 'invert cv response mod' on the diagram for the Q & D that im going to have a pop at!
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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
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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
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Well, it will work, "but not as we know it, Jim" :D
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
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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:(http://1.bp.blogspot.com/-Tx5ljWTEU7I/UcFUkia8M6I/AAAAAAAAAVs/mmTPnbCGzRo/s1600/RAIL.JPG) (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!
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Surely the DC bias alone should go to the non-inverting input and the audio should go to the inverting input?
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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...
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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
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...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 8)
...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.
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Well if that's true, that would make a big difference. I just got a handful of quad pots off ebay for my new board I'm drawing up - if you say I won't need them for that control, that is going to be a big expense cut!
I'll look into this.
Cheers, graham
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RE: 2 dual amp chips,
what would be awesome would be to use the spare bit of unused bit of board on the sk5 (just behind where it says CASIO on the back of the keyboard) for the quad filter. There are two spots that could just about accomodate 2 8pin chips then all other components would have to be carefully placed to best use the other pads and such. Probs wouldnt work out but would be a nice if it did!
Maybe some other smaller effect circuit would fit there better...
have a squizz see what you think...
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Could you just squeeze a quad opamp on, for a total of 14 pins and no gap between two chips?
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So you have an SK5 you're putting the filter into?? That must explain the audio output error you mentioned in my guide. I had assumed from other circuit bender's info that the sk5 and sk8 had nearly identical boards. Seems not...
Cheers, graham
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Could you just squeeze a quad opamp on, for a total of 14 pins and no gap between two chips?
the available pads that could potentially accomodate 2chips are not next to each other.
You could cut out the whole area and replace it with a custom circuit board or bit of strip board or something though...
I think thats what i'll do and put a delay circuit on it
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So you have an SK5 you're putting the filter into?? That must explain the audio output error you mentioned in my guide. I had assumed from other circuit bender's info that the sk5 and sk8 had nearly identical boards. Seems not...
Cheers, graham
yup into a sk5, maybe thats the discrepancy.
I fired up my squashed fly quad Q&D filter up and it works wonderfully! a real range of sounds from warm bubbling analoguey bass to the sound of stormy wind*. I need to try and squeeze it inside now. Video to follow soon when its installed... :)
The res pot does really crank up the wind actually, my sk is blowing a storm, anyone else get this? I do like it but I was wondering if it could be due to a fault? Anyone else get whooshing? I'll put a link to some audio tomorrow...
Another thought (once you get playing with this and making it modular it opens up so many possibilities!)... It would be neat to sum the envelopes (with diodes) then amplify it with a transistor to drive an LED/vactrol to modulate stuff (delay length etc) ARRGHH so many possibilities!!!
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some wild ideas out there.
I'm wondering how big the pay off is between 1 and 4 Filters. I understand the difference, but given my building quality, and difficulty with tight spaces, I'd probably only add a 4 chan filter if i put it out side the box. a single VCF might fit though....
basically, is it soooo much better to have four channel filter that i should dive into it?
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Just start with one if you're going to make an external box, then if you want to explore more, build extra modules. Tthe best of both worlds, and you have no risk!
Cheers, Graham
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Thanks for your work on this. did some more/started again on this today...
some component questions:
**does the diode need to be N4148 or would N4004 work - i only understand diodes in their most basic form??
**can the metalized polyester 0.1uF cap be a green cap instead?
also, any progress on the other ideas - i.e. using a single gang pot instead of quad?
hoping to build a mono one first to use as an effects pedal/other circuit bent instruments before attempting the quad version. fingers crossed
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I think the IN4004 is a power diode - this one needs a signal diode (IN4148) as I understand it - I could be wrong though. Best use the 4148 if you can get them.
Metallised polyester cap should be OK.
Single gang pot should be OK for the cutoff freq knob, but you'll need quad gang for the res. Here are some here:
http://www.ebay.com.au/itm/261215523097?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_2597wt_1074 (http://www.ebay.com.au/itm/261215523097?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_2597wt_1074)
These are really small, I bought a few of them a while back.
Cheers, Graham
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I finished my build (one unit). without any luck yet. a few questions to assist with troubleshooting.
1] The LED - what should it do (always on/flash with audio input??) mine does nothing.
2] the links on the back of the PCB, are they meant to be one point-to-one point (the black + green?) links...
Any other things to look for when troubleshooting. I've checked my PCB board trace cuts with a multimeter and they seem ok
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The LED is a power indicator, it should be on all the time. Check it is in the right way.
The green and black wires are point to point wiring.
Cheers, Graham
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arg. rookie error.
I flipped my LED, and did some more checking.
From my understanding of the circuit (its a bit hard with the front/back PCB) the 7.5+ flows like this (for test) -> to the cut-off pot leg A and to pin 7 of the IC?
*** Therefore, turning the pot should alter the LEDs brightness? (mine does). ***
it looks like it might be working - my next problem is i'm using stereo leads and mono sockets to test - which occasionally cause an issue depending on the sockets - so i'll try to fix that next
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i got it working *WOOT*
using the type of sockets i had is dodgy when using stereo 3.5mm plugs.
while i've only tested it using iTunes (ABBA's Dancing Queen) through one of those little 1W 4" marshall speaker/amps, I found the cut-off didn't go low enough. I kind of imagined it would roll all the way down to 20Hz LPF.
Is this normal? design feature/flaw?
I understand with the SK5 you probably would be fine with out rolling off all the way, but i plan to make both a stand alone filter+lfo, and a SK5 built in filter.
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That's great news! ;D
The cutoff can be adjusted to go lower or higher by changing I think some capacitor and/or resistor values in the filter network section, but at the expense of top range, if going lower. I think I settled on a value for me that worked in most situations for me. The circuit is very imperfect, it's rough and grungy and lo-fi (many would consider that an asset in this situation :D), don't expect too much out of it. But it's the only filter I ever found that was set up for 6-9V DC battery power - all the other designs out there were for split rail -15V-0V-+15V, which was no good for building into the SK and powering it as well from it. That's why I went with it for the SK.
LED changing - I don't recall mine doing that. I don't think you need the LED - it's just an indicator.
Cheers, Graham
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Pretty sure the LED is designed to match the cut-off amount, as it even matches my LFO.
I'd be keen to know if anyone has changed values for more extreme settings, including more resonance. I like the idea of it 'taking off'/self-feedbacking.
well, now i've built one, I'll house it as a standalone fx and start on the quad!
J
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What circuit did you use gareth for the LFO? I've used casper's 555 LFO circuit, though find the shape+depth knobs a little strange/fiddly
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Finally (been re housing things/repairing an MT-46 and rehearsing music) put my mono filter in a box and added a dedicated LFO (555 circuit).
Working nicely, though it is a lot greater range from the LFO than when just using the freq. and res. knobs. Maybe I need to alter some of the resistors that are trimming the range?
Also, I've used a real basic LFO circuit this time, as I just want one knob, rate and one switch, Triangle - Off - Square.
http://getlofi.com/control-voltage-output-from-555-timer-circuit/ (http://getlofi.com/control-voltage-output-from-555-timer-circuit/) -> is this the circuit you used Graham for that?
J
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So I dropped the 1K Resistor that is connected to the resonance pot A down to 500Ohms and the resonance is able to get really nasty (maybe too nasty for some people - maybe somewhere in between??)
Still no luck finding the best way to increase the cut-off frequency's upper and lower limit
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i'm finishing off my 4 channel version of the phat philter! (the mono version is working nicely - it even has CV in from my MT-60 casio).
however i forgot to pick up enough 10nF (103) green capacitors today :-(
Can i swap one green cap for a ceramic 10nF capacitor and not cause any issues?
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I finished!!! well not totally, still got to put it in side but i'm stoked it all works.
I was wondering would it have any advantage to make some sort of simple distortion circuit and make it 4 times before the filter, so each channel is distorted before being shaped???
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All working and housed.
initially forgot the comment about audio line 1/2 being wrong, but with that fixed it is much better.
main issue stopping it from being amazing is the LFO.
i use a basic 555 LFO (http://www.tradeofic.com/uploadfile/ic-circuit/2009624221917817.gif (http://www.tradeofic.com/uploadfile/ic-circuit/2009624221917817.gif)) but the triangle wave is way too weak.
should i work out a way to boost the triangle wave's amplitude, or build a better LFO all together?
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Hey sorry I haven't caught your updates - don't seem to be getting any reply notifications to this thread ???
That's great you got the quad working - again I severely regret designing the board layout so tiny - it has caused no end of problems building it. Maybe you could draw up a PCB diagram of the quad circuit for those who want to make up a proper board? I wish I had time to revisit the pat philter design but between family, band, music and work I just don't get long spans of time on my own to tinker with project reviews - plus there is the lure of new projects :D
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i did mine on a vero board.
I can confirm though you don't need a quad gang pot for the cut-off. (and you could probably remove the 100K resistor just after the pot's wiper and have just one straight off the pot.
strange thing is that the cut off is brightest at 2/3s full turn. maybe might pot is dirty/dodgy, or maybe it doesn't go beyond there in cutoff
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quad pot - that's great - that cuts down the cost greatly :)
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yeah, and now i have a spare quad pot.... thats why i'm thinking about some sort of quad distortion pre-filter????
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Try the DOD 250 overdrive circuit:
http://music-electronics-forum.com/t17025/ (http://music-electronics-forum.com/t17025/)
I've copied my mate's pedal and it's dead easy to make - and that link someone has already drawn up a circuit diagram. It's a good bitey distortion pedal - a bit tube screamer-ish in sound, not dull. It sounds good on bass synths sounds to squelch them up a bit. It has drive and volume controls. I made mine on cardboard and wired up the components with copper wire - easy as that! It uses a 741 IC - you could probably find a quad version of it and make up a quad board without too much trouble.
Cheers, Graham
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Thanks. I'll look into a distortion unit shortly
for now, i'm trying to improve the LFO section. I really am not liking 555 based LFOs. on mine the triangle is much quieter than the square, so it doesn't really drive the filer.
I've used this until now:
(http://www.seekic.com/uploadfile/ic-circuit/2009624221917817.gif)
but i've been looking at a LM324 based LFO over at electro-music:
(http://electro-music.com/forum/phpbb-files/lfo_229.jpg)
(i had to turn a single supply 9V power into dual supply 4.5+/- V
however the voltage output swings from -3 to +3v where i suspect the filter you've designed expects a + voltage only??
is that my problem?
or should i try a different 555 circuit like this:
(http://cdn.instructables.com/F8I/PM8H/GTIEW0SO/F8IPM8HGTIEW0SO.MEDIUM.jpg)
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going to finish off the 2nd LFO (sorry my pictures are so massive in the thread) and put it in today hopefully.
I've noticed the volume after the filter is considerably lower than before.
any thoughts on a simple circuit to boost the volume? or should i look at changing the input resistors?
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So today, after having gigged a number of times with my SK5 post-Phat Philter mod, and found the need for more gain needed, i opened it up to adjust the gain on the post-Phat Philter channels
1st up i tried swapping the 470K resistors that are set the input volume for something lower. I only had a pack of 100K, so I tried that, and while it was much lower, it was considerably more distorted.
so i went back to 470Ks and then put a transistor preamp i had built years ago at the output. much better. now the gain on the four voice channels is greater than the drum loops (but with a potentiometer, to pull it back it in)
much better.
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i think this was the original circuit:: http://www.siliconchip.com.au/Shop/8/874 (http://www.siliconchip.com.au/Shop/8/874)
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Been working on getting this circuit to work for awhile. Graham, I don't know if you remember this, but I emailed you about six or seven months ago having trouble building the first module. Never ended up getting it to work, set it aside for quite awhile. Lately, got the itch again. I tried breadboarding it up based on the strip board layout given...still no dice. I pretty much gave up and for the past week have been looking at alternatives (SSM2044? Space and power aren't a concern for the project I have in mind...)
Then, tonight, I stumbled on this thread and, encouraged by the success of others, decided to give it one last whirl. I decided to breadboard the Q&D circuit by itself to rule out the possibility of bad components... Success! Everything sounds all distorted to hell without any attenuation of the Casio's output, and I started to pick up a radio station(...?), but the important thing is that I was getting audio through the circuit and a clear cutoff/resonance response. Hot diggity!
So tomorrow I think I'll add back the input resistors/capacitors and work on a stripboard layout that isn't quite so crowded. no offense Graham, but that thing is freakin' tiny! It's pretty incredible that you got it to fit inside the SK case, but I plan on making mine external anyway. If I'm successful, I'll be sure to post my layout here so folks who don't need a tiny board don't have to.
Cheers! I owe you a few beers when this is complete, graham!
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Ok, so I got a (very messy) breadboard version of one module working mostly as expected. I built the breadboard from the original Q&D schematics before adding Graham's modifications and the levels sound about right (a little low actually, but I'm sure messing with resistor values at the output will solve that). Not sure if there's a mistake somewhere in the Filter Bank tutorial or if I just messed up the first two times I tried to build it, but who cares? It's working now!
Only hiccup is that the cutoff response when twisting the knob seems a little off. I used linear pots, as listed in the tutorial, but maybe audio taper would be better? (makes sense, I think, since you're controlling frequency?) I also saw Graham mention that he bought some audio taper pots elsewhere in this thread, so now i'm wondering...
Can anyone else say what they used?
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Whelp, one thing that's wrong is my cutoff pot value. Oooops.
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i find that the audio level is low too. i ended up sticking a little preamp circuit i had from a long time ago floating around inside the casio aswell [well, in my side enclosure]
the cut-off i find is full open at around 80% turn, rather than 100%, but i wonder if that is also my dirty pot
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I was finding I had the opposite problem: at about 20%, the filter would go from completely closed to partway open very suddenly. Maybe I was having similar behavior at the top, but I didn't notice. I ended up adding a 22k resistor going from the center lug to ground which seems to smoothed out the response a little.
I work a lot over the next week and I'm waiting on some larger stripboard (so I can do four of my larger layout on a single board), but as soon as the stars align, I'll be triple-checking my layout and building it. I may play with some resistor values to adjust the gain.
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Hey sorry for not seeing your replies - for some reason the auto topic notification doesn't send me a notice to my email....
Again yes, I apologise about the tininess of the board, I made lots of errors building it too. I managed to get it into my SK without removing the speaker but it really wasn't worth the troubleshooting it caused.
Best to go back, as you said, to the Q&D circuit layout and put your own attenuation into the inputs and outputs. After you've got it sorted, maybe someone could make up an etched PCB layout for the 4 channels in 1 sometime?? The main sources of error in my stripboard version is because to make it so small, I used cuts in the tracks etc and connected components through these cuts, and other things like that.
Cheers, Graham
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No apologies needed! Without your hard work blazing a trail for us, I'd never have gotten even this far.
Once I build and test the stripboard I posted above, I'll post a layout for all four voices together. I have zero experience with doing actual PCB layouts though, so that'll have to wait for a better informed person; hopefully the larger stripboard layouts are still helpful to someone.
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So close to having my first filter module done. I am having some trouble with the CV; I've emailed Graham to see if he has any ideas (since this thread doesn't update him). I am hoping it's just a matter of bad soldering with the 1uF capacitor. Fingers crossed.
I will post my updated layout and photos of my board when I am sure everything is working.
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Yes I got your email, and also a topic reply notification this time :D
The capacitor and diode can be removed just to test the circuit, they were added by me to optimise the generic circuit for the SK. Try it and see if you can get it to work without them
Cheers, Graham
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I removed the capacitor and had the same issue, only very slight response to CV. I am hopeful this means that the capacitor was never properly installed to begin with...
Do you happen to recall what the response to the SK CV should be without the capacitor in place? Like, what exactly is the capacitor doing to adapt the circuit to be better for the SK?
I may need to find another source of CV to see whether the signal or the circuit is the problem.
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The CV controls the openness of the filter, which is controlled by the cutoff pot. Try closing the filter with the cutoff pot and then play it with the CV hooked up
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Right, I understand that. The issue is that the cutoff is not responding very strongly to the Casio's CV.
My guess is that the filter is expecting CV in the range of 0-9v or so, while the Casio can only output about 2.3v max for its envelope CV, which would account for the decreased response. Is the presence of the capacitor before the diode meant to address this difference in signal strength?
I also wonder if I would see a greater response running the filter from the Casio's internal 7.5v instead of a 9v battery (what I've been using for testing).
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You have heard my examples - they were from 7.5V internal batteries exactly like yours- so it's not that - there must be some miswiring somewhere. Yes the CV rail is less than 5V but it still opens up well enough on mine. Does turning up the resonance make a difference?
Cheers, Graham
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Ok, more investigating. 7.5v vs 9v makes zero difference, fwiw, as does the presence of the CV capacitor or diode.
I am getting the correct CV out of the Casio..... but only when the filter is not hooked up. Once I hook it up, the voltage drops to almost nothing... I can still get sound out of the casio on channel 1, so the CV must still be there internally, but for some reason it's not getting into the filter circuit. I will have to do more fiddling around. Nope, I must have been measuring something wrong. The correct CV is definitely making it into the filter circuit...
Second, and unrelated to my CV problems... I am confused about the points for individual voice audio. Am I missing something or do the diagrams in the guide not match up when switching from component side to track side? attached is a photo to illustrate what I mean. It seems to me (and audio tests seem to confirm) that the points in the actual hookup photo are where the audio lines have already been summed. It's easiest to see with channel 1, since that goes straight into T15 (which goes into the internal filter), while the other traces go into the 4066 switch chip (and eventually to T15).
...but then, the photo of the actual hookups is from a confirmed working filter.... so I'm really confused here.
It may be that i've been staring at this circuit board too long. Sigh.
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Your photo didn't seem to attach - can you post it again?
Cheers, Graham
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sorry about that, attached now.
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building another one of these, cause the first one was so good [and it is now housed in an SK1].
the Jaycar electronics shop down the road from me doesn't have any 100nF/ 0.1uF green cap capacitors in stock. could i swap them for another type of capacitor?
also, has anyone tried a different input resistor instead of 470K. i feel like something more around 300k or something might be better. just my thoughts
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Yes, or another approximate value - it's not that critical.
Same goes with the 470k resistor - they were simply the value I used to get a level input/output response for my sk8, but are by no means critical.
Cheers, Graham
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So i've built my 2nd QUAD PHAT FILTER and installed it in a Casio SK5 (first one is an SK1)
A few things i did differently this time:
1. Swapped the audio input resistor to 370K. Probably still a little high, maybe 350K is ideal
2. Put a FILTER BYPASS switch in that switches between the pre-anti-aliasing filter output on the casio, and the post-PHAT FILTER output
3. Used just one single gang 100K pot for the filter cutoff. When I was testing it I used a 1M pot. I wonder if a 150K or 250K pot would be best, and drop the resistor on the 'B' lug of the resistor to only 70K or so.
4. Dropped the resonance input resistor to 820Ohms.
What I've noticed is:
1. Audio is still a little lower in volume than the non-PHAT Filter output
2. The Phat Filter output is less bright than the pre-Phat Fitler output. I really like the digital artefacts of the SK when you bypass the anti-aliasing.
3. The resonance is now too wild and crazy.
looking at the original - http://s1107.photobucket.com/user/joer0952/media/qd.gif.html (http://s1107.photobucket.com/user/joer0952/media/qd.gif.html) there is no input resistor on the resonance though. hm
I actually think ideally a few of the resistors should be soldered onto the lugs of the potentiometers, to make altering the circuit easier as you want to change up some of the resistance and get a better range.
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I think I added the resistor to control the resonance. Be aware that the different IC's you can use all behave radically different from each other - I designed this one for the NE5534AN op-amp. When I tried a 741 IC in it it squealed uncontrollably at a certain res position. A TL071 behaved differently again. So the values I finally used were for the NE5534AN
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Sorry for reviving the old thread, but....
I've been using QUAD PHAT Filter's in my SK5 & 8 and gigging with it and loving it ( https://spiritbunny.bandcamp.com/ ) but the filter has been a bit weird/wild/dodgy
So i had a repair day and discovered that the NE5534an chip on one of my channels was to blame. Simply swapped it and now filter is so much nicer - both with the LFO, by it self, and with the envelope control.
One query/thought - the frequency range of the cutoff still could be better. The cut off only responds to about the upper 1/2 of the turn on both instruments - and I don't get quite as big a sweep as I'd like. Ideally I'd like to be able sweep right down into those low frequencies for some rumble and back up again into shrill highs. Should I look at swapping IC, or is there another option to look into?
Cheers J
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Does the documentation for this mod exist somewhere? I tried following the link to the Yahoo group at the beginning of this thread, but it is a dead link. Would love to do this mod, but I really would like to read through a full walkthrough of the mod before trying it. Links appreciated! :)
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Hi all
Anyone interested in doing this, and/or the SLOW AD mod on an SK1 check out this.
(https://i.imgur.com/iWVKMcs.jpg)
There are other images around, but these are correct for the audio outs and the CV outs.
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oh, and the documentation is here:
http://warningwillrobinson.com.au/index_files/Professor.htm