The Casio SK sampler PHAT PHILTER BANK mod has arrived!!!!

[wax+wire]

i think this was the original circuit:: http://www.siliconchip.com.au/Shop/8/874

[Jaytee]

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!

[Jaytee]

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?

[Jaytee]

Whelp, one thing that's wrong is my cutoff pot value. Oooops.

[wax+wire]

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

[Jaytee]

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.

[gmeredith]

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

[Jaytee]

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.

[jepyang]

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.

[gmeredith]

Yes I got your email, and also a topic reply notification this time 

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

[jepyang]

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.

[gmeredith]

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

[jepyang]

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).

[gmeredith]

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

[jepyang]

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.