My new design, the slackfilter

[slacker]

After hanging out here for a couple of years and building a few pedals I decided it was time to give something back to the community. So here's my latest design, the Slackfilter.
It's an envelope filter/phaser/self oscillating noise maker based on Ken Stone's amazing Synthacon VCF clone. Check out http://www.cgs.synth.net/ for more details. Basically it's a VCF with seperate inputs for bandpass, highpass or lowpass rather than most similar filters that have one input  and seperate outputs. The cool thing is you can use any combination of the inputs at once. It will also self oscillate if the resonance control is cranked up!
I adapted his design so it runs on 9 volts and added an envelope follower  (from Tim Escobedo's phuncgnosis) and an LFO based on another of Ken's designs, the Phycho LFO.
It does autowah type effects using the envelope follower but the LFO adds a whole load of extra effects. The LFO has a triangle output which does synthy type sweeps and phaser type effects. It also has a pseudo random output that creates sample and hold type effects.

Here's the latest schematic
and original version

and some soundclips, the clips are guitar >> slackfilter >> condor >> soundcard. The dirty sounds are done using a BigMuff after the slackfilter.
Auto wah (bandpass)
Auto wah (lowpass)
Auto wah (highpass)
LFO driven effects
Random LFO effects
self oscillating weirdness

It's still a work in progress and some things need work, the envelope follower has a fairly limited sweep and the LFO can be a bit noisy.
If anyones got any comments or suggestions on how to improve it I'd be happy to hear them.
If anyone fancies building one then I've got some vero layouts that I'll be posting soon along with some build photos.

[MartyMart]

Hey, some great "noises" there Slacker ... many thanks and congrats !

MM.

[Mark Hammer]

Very very nice, and another reason for me to start looking into the Steiner filter in general.  I took the liberty of doctoring up the posted schem so that it makes for an easier view in black and white: http://img.photobucket.com/albums/v474/mhammer/SLACKFILTERBW.png

It's your baby so I won't leave this up for long, but I figured that it would be easier to look over and discuss in B&W for some folks.  If you're uncomfortable with that, just let me know and it will be deleted promptly.

[slacker]

cheers for the comments
Good idea Mark I've redone the schematic in black and white, does make it a bit easier on the eye.

[Eb7+9]

... good stuff !!

[christian]

Thats so cool  I love it.

btw, the vcf is not original work of Ken Stone, its originally from this synth

ch.

[slacker]

Thanks, yeah you're right it's based on the VCF from that synth, orignally designed by Nyle Steiner.
I should have said based on Ken Stone's amazing Synthacon VCF clone.

[birt]

sounds great

i love the random LFO, i was thinking of a way to make the LFO of a tremulus lune random.

[slacker]

check out this this page from GEO it's got some great stuff about random LFOs. I started from there and then used the Psycho LFO to get an idea of component values, even though I ended up with different ones because I thought it was too fast.
If you just took the top 3 schmitt triggers from my LFO and hooked them up via a switch to the depth pot of the tremulus lune that would probably work.

[slacker]

I've just updated the schematic, the random switch on the LFO wasn't how I'd done it on the final build. I've also added the power supply connections to the CD40106 used by the LFO.
I've used a DPDT for the LFO mode switch, which gives triangle or random output, but you could use 2 switches and have a square wave output as well.

[christian]

Oh yeah, heres one more modified "Steiner VCF".
The output stage is modified to make up the gain change with resonance. You might wanna try that

This inspired me to figure out that you can use digital noise generator as a sample-and-hold generator by itself !

ch.

[britt-stinker]

I cant wait until there will be a pcb layout for this thing!

[slacker]

Oh yeah, heres one more modified "Steiner VCF".
The output stage is modified to make up the gain change with resonance. You might wanna try that

Cheers I'll give that a try, those diodes look interesting as well

[slacker]

I've changed the design slightly to address a couple of problems with original.
Here's the revised schematic, the changes are shown in red.

On the original the frequency and depth pots were highly interactive. This was due the fact that adjusting the depth pots changed the loading on the base of the bias control transistor. I added diodes between the depth pots and the base of the transistor which has solved this problem. Now adjusting the depth pots doesn't affect the frequency, it also means that the LFO and envelope follower can be mixed with some nice results (sound clips to follow). Removing the 1Meg resistor from the envelope depth pot has also improved the envelope follower sweep.
I also added a capacitor to ground to the inputs of the LFO output buffers which has reduced LFO bleedthrough.

I got the idea for the diodes from this site which has a some alternative Synthacon filter designs. Including one using an opamp instead of transistors and an interesting looking guitar synth. It's also got a copy of Nyle Steiner's original article and design and loads of other good synth stuff.

If anyones interested here's a couple of photos

This is just the filter section of the pedal, the resistors connected with sockets are to adjust the sweep of the frequency pot.


Here's a guts shot, showing my messy wiring, like a said it's still a work in progress

[lovekraft0]

Brilliant!! I'm obviously gonna have to take a couple of weeks this summer and do nothing but filter projects, this one and Stephen Giles' multi- spacefilter idea in particular! Nice implementation on this one, slacker, especially getting that kind of feature set without letting the design become totally out of control. OK, who's workin' on the vero layout? 

[slacker]

Thanks, I was pleasantly surprised with how easily it all came together, that's probably more a testament to the original filter design rather than my skills. There's probably still a few things that could do with tweaking but I'm going to live with it for a while to get an idea of what it can do and what might need improving.
I've got vero layouts for how I built it, I'm tidying them up and adding the tweaks I've made then I'll post them. As you can see in the photo it's 3 separate boards, one for the filter, one for the IO and envelope follower and one for the LFO. I did it like this to make it easier to experiment with different ideas and because I already had the LFO board from another project that never quite worked out.
I'm hoping someone will take up the challenge of doing a PCB layout, because I don't have any experience in that area.

[moosapotamus]

This looks wicked cool, slacker!

I'm thinking that it might also be cool to mod the front-end so that you could actually mix three different input signals, one each to the LP, BP and HP inputs (ie: bass, drums, synth). I guess you'd repeat your input buffer two more times? Then you might also want three more switches to be able to select which input(s) will drive the envelope response.

I've only got about a dozen different projects in various states of completion. But, I'd love to tinker with this one. 

~ Charlie

[Peter Snowberg]

Very Cool!

Nice work!

[slacker]

I'm thinking that it might also be cool to mod the front-end so that you could actually mix three different input signals, one each to the LP, BP and HP inputs (ie: bass, drums, synth).

Thats a cool idea, I'd thought of having an FX loop but separate inputs would give even more possibilities. I quite like the idea of say running the wet signal from a delay into one input with dry going into another.
It would be great to see a moosapotamused version, I dread to think how many knobs and switches it would have

[Eb7+9]

thanx for the link to the original Steiner notes Slacker !!

http://yusynth.net/archives/ElectronicDesign/N-Steiner-VCF-1974.pdf

I notice in the conversion to Voltage-Control the op-amp in the first circuit is replaced by a two stage bipolar amplifier ... you can probably squeeze more Q out of your topology by changing back the two-device gain stage to a variable (higher gain) op-amp stage ...

or, for now ... in your improved schematic there's a non-inverting op-amp with a x5 gain at the output, to test this theory out you could take the resonnance feed after the op-amp instead of after that transistor stage and see how much Q increase you get ... you can rig a second pot at the output of the op-amp for the variability ... if that works then try replacing the 220k by 2meg pot in the feedback of that op-amp - at some point your filter should morph into an oscillator and give you a nice sine wave ... from your nice clips it sounds like you might still have some gain to go before you reach that point ...

also, I looked at the 47k input impedance thing - it's probably not crucial ... you could always buffer the input and stick a 100k pot in series to see what changing that value does to the response but it's likely minor ...

all in all, the VC conversion is a very neat trick - it's somewhat analogous to the way varistors work in parallel DC circuits in the Magnetone 480 pitch shifters ...

best regards ...

~JC