dirt cheap/low parts filter?

[jmejia]

i'm working up a little series of cmos based module pcbs for a super cheap modular synth. i may make module kits if there's interest - the first board is at the pcb shop now - it's a voice board with a 40106, 4093, 4040, and 4030 - and a couple summing busses. (not too different from bbob's awesome looking board - but with a bit of a more single application purpose)

i'm looking for a super simple (but sonically interesting) filter to put on a filter board i'm starting to plan. i'm not really concerned w/ real CV - but some type of modulation from the square waves would be nice. my main design goal is low parts. i'd like to stick with cmos if possible - but I'm also considering something like this: http://thesquarewaveparade.com/images/srlpf.jpg

anyone have suggestions? i'm thinking since all my sources are squarewaves - i should put a little passive RC lpf at the modulation input of whatever filter circuit i choose - then i should be able to get smoother voltage sweeps from my square waves right? (transferrable into resistance sweeps with an LED+photocell if the filter circuit I pick has no CV input. . .)

am i on the right track here? anyone have favorite cmos filtery/wave shapey circuits?

[DGTom]

without op-amps right?

cheack out the BugBrand WOM for a simple boost / tone set-up.

I've had great success with the 4069 + vactrols in a crude LPG & VCF as well as some pretty good boosts, fuzzes & waveshaper type things.

[jmejia]

awesome! sounds like that chip might be the way to go! do you have any 4069 sound samples handy?

[ezekiel]

An XOR or NAND can function a little like a filter when you feed audio in one input and feedback the output to the other input, and either:

a. pass the feedback through a variable resistor with a capacitor to ground, or

b. pass the feedback through a capacitor with a variable resistor to ground (or to +V).

To make it voltage-controlled, replace the variable resistor with a vactrol.

A couple of these pseudo filters in a chain and you have a diverse-noise synthesizer. Best I found. Doesn't lose volume as bad as simple passive RC filters.

I tried the same with a shift register and various feedbacks and DAC output etc, but couldn't get much of a filter-like effect.

[DGTom]

+1 for logic filters;

http://www.youtube.com/watch?v=Jn3Uy2zbcLI

not always possible to get a trad. input -> output filter, but, there are some great sounds hidden away in those CMOS

thanks for reminding me, i really need to revisit this switched capacitor 4093 filter idea.

[Top Top]

I know it's not cmos, but last night I breadboarded the Tim Escobedo Q&D VCF, which is really simple (low parts count), and works well, with a nice squelchy sound at times. It can also take a straight voltage control signal (no vactrols needed).

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/simplevcf.gif

Though the "logic filter" sounded great in that clip too.

[jmejia]

thanks for the suggestions!

so sort of on the same topic... is there a nice easy way to get slow lfo-like swoops with cmos chips? would a lowpass filter on a square lfo get me anywhere near that at all or is there a better way?

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[monokinetic]

[DGTom]

I'll try to dig up my notes & get a rough schem together.

When I tried to transfer that idea from breadboard to module it was kind of an epic fail my building skills are much more refined now so I really should go back to it, I forgot how cool that video sounded, good thing I recorded it!!

But yeah, thats the basic idea, on the 4093 line up several caps thru the switches of the 4066, so that as more / each switch is opened the freq. of the 4093 osc. changes. If you did four you could get stepped filter sweeps using a binary counter to drive each switch in sequence. I think I was only using 2 switches / caps.

The whole thing uses 2 gates from the 4093 as oscillators. The vactrol is being used to get CV over the firsts frequency, the 2nd osc is having its caps switched as well as manual sweeping of its pot.

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I built this one last night. It's even cmos and only a handful of parts:

The voltage regulator can be ignored if you are running off a power supply/wallwart.

I changed the .0015uF cap to .01uF to get a deeper frequency sweep and used a 25k pot for the sweep. It's a nice squelchy/farty sounding filter.

[Ajax]

Came across this:http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/filcap2.html#c1

Seems easy as hell, low parts, low mantinance. I had one question though: What's the capacitor hooked up to on that bottom connection? From the output of the resistor, then to parallel with what?

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[Ajax]