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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Simple active filters?
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corex



Joined: Mar 02, 2010
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Location: Las Vegas

PostPosted: Sun Sep 05, 2010 11:14 pm    Post subject: Simple active filters?
Subject description: Other than the WSG filter, what are some other resonant LP/HP filters?
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Filters come up pretty often on this board, but often it seems they're VCF designs and too complex for my purposes.

I've found the WSG filter (isolated here: http://www.fluxmonkey.com/electronoize/opampFilter.htm ) and it has the basic features that I'm interested in: the design is pretty straightforward, there's no VC section, and there are knobs for both resonance and cutoff. It's not clear how steep this filter is, though (6dB/oct? 12?).

I would like to find other filter designs like this (and high-pass as well). Googling for filter designs is bewildering and in most of the pages that I've found it's not clear whether the filter can have a resonance pot or even where the cutoff pot should be placed. However, some of the designs -- although abstract -- seem to require even fewer parts than the WSG filter, which gives me hope.

Found anything good in your own wanderings?
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tjookum



Joined: May 25, 2010
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PostPosted: Mon Sep 06, 2010 2:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

VCF? Filters?

It all depends on what you want, in the world of lunetta's it's really not that usefull to have a filter with cutoff and a nice resonance. In my opinion a lunetta is about patterns, variation, noise, simplicity and not about sound quality or nice filter sweeps. I haven't used one in mine and I get a new sound every time I turn it on and make a patch, that works for me.

Try this for quick and easy, 1 pot and a cap and voila, instant lowpass filter.
http://www.fluxmonkey.com/electronoize/passiveDividersFilters.htm

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MirlitronOne



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PostPosted: Mon Sep 06, 2010 4:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Tim Escobedo:

http://folkurban.com/Site/GuitarEffects-681.html

Explore and enjoy.

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corex



Joined: Mar 02, 2010
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PostPosted: Tue Sep 07, 2010 5:58 am    Post subject: Reply with quote  Mark this post and the followings unread

tjookum wrote:
Try this for quick and easy, 1 pot and a cap and voila, instant lowpass filter.
http://www.fluxmonkey.com/electronoize/passiveDividersFilters.htm


I slapped together a passive low-pass and high-pass filter a few days ago, and while the high-pass sounds kind of interesting, I found the passive low-pass to be really boring. I think it isn't steep enough, and without resonance it's actually sort of hard to hear the cut-off frequency.

This is why I inquire about active filters -- I think resonance and a steeper fall-off would sound more interesting.

As far as "the world of Lunetta" -- eh. *shrug* I'm just trying to make weird noises. If I can find a couple of filters that are simple enough for me to build and that sound interesting, I'll pile them into my first noisemaker just to broaden its palette.

This Tim Escobedo's guitar effects page looks interesting. His descriptions may be too terse for me to follow, though. I'll stare at these for a while and see if I can figure out how to build one of them.
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Draal



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PostPosted: Tue Sep 07, 2010 8:36 am    Post subject: Reply with quote  Mark this post and the followings unread

tjookum wrote:
VCF? Filters?

In my opinion a lunetta is about patterns, variation, noise, simplicity and not about sound quality or nice filter sweeps. I haven't used one in mine and I get a new sound every time I turn it on and make a patch, that works for me.


Tjookum, my experiences/views have been similar. Haven't found a need for the filters yet; I think they are more useful in modulars, but hey, that's MY set of ears! I'm also a guitar guy and I've had my share of wah wahs; got bored with em. But again, others think differently than me.

Part of the fun is experimenting; so get to it corex! Smile

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Top Top



Joined: Feb 02, 2010
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PostPosted: Wed Sep 08, 2010 9:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Here is one I designed. It is a "vocal" filter, which is two filters working inverse to eachother... inspired by Tim Escobedo's Ghost Dance filter (which is a single 4069 inverter wah schem). Uses two inverters on a 4069.

You could omit the transistor gain stage in the front if using it for loud signals like a lunetta, though it can do some cool wave-shaping as well.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Some samples (they're all on guitar, but it sounds pretty cool with synths as well).



If you wanted it under voltage control or something similar, you would need to add a vactrol type thing in place of the pot. What you hear in the video is under pedal control. There are still 4 open inverters, which could be used to create an LFO with two outputs 180° from eachother...

I personally have no problem with filters in combination with CMOS. Whatever the sound you are looking for... go for it.

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corex



Joined: Mar 02, 2010
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PostPosted: Sat Sep 11, 2010 12:01 am    Post subject: Reply with quote  Mark this post and the followings unread

That's an interesting extension of the Ghost Dance idea. The transistor gain stage is interesting too. Thanks!
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Psyingo



Joined: Jun 11, 2009
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PostPosted: Sat Sep 11, 2010 7:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

i posted these on the other lunetta forum, the sounds of logic; ill repost here as its relevant to the topic.
its probably the easiest multimode vcf's i have seen. it is called a sallenkey. i n the schematics i have used vactrols for the voltage control.

the first one is a sallenkey filter transcribed from this article: http://experimentalistsanonymous.com/diy/Schematics/Filters%20Wahs%20and%20VCFs/Sallen%20Key%202P%20Filters.pdf
the second is one with some modifcations, including an output stage with a gain control.


Original
Posted Image, might have been reduced in size. Click Image to view fullscreen.
Modified
Posted Image, might have been reduced in size. Click Image to view fullscreen.[/url]
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corex



Joined: Mar 02, 2010
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PostPosted: Sat Sep 11, 2010 10:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

I looked at quite a few Sallen-Key designs but most of them were more theoretical (no component values) and I wasn't sure if they had resonance.

These schematics look easy and practical. I'll definitely give these a shot.

Thanks!
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Silesius



Joined: Feb 12, 2010
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PostPosted: Mon Sep 13, 2010 9:44 am    Post subject: Reply with quote  Mark this post and the followings unread

I've breadboarded the Mouthmeistor (without the gain stage for the moment) and it's really nice and easy. I'm thinking on building a CMOS filterbank with 3 or 4 filters from the 4069, with the possibility of connect it in series or in parallel.
I've noticed that the resonance pots are not very useful; I've used it always at maximum resonance. The thing is: is there any way to make the filters more resonant, with selfoscillation? That would be great. I've been trying changing some components, but I'm not very good at electronics, so I don't know which ones are responsible for the resonance. Any ideas?
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Silesius



Joined: Feb 12, 2010
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PostPosted: Mon Sep 13, 2010 11:19 am    Post subject: Reply with quote  Mark this post and the followings unread

I've never seen this one here:

http://www.synthforum.nl/forums/attachment.php?attachmentid=59705&stc=1&d=1276610908

http://www.synthforum.nl/forums/attachment.php?attachmentid=60956&d=1280577622

It's easy, low parts, single power supply, and very interesting. The possibility of change the capacitor is nice. That changes the response and the sound, and with some caps it will oscillate. I use it with a 9v battery in a pocket sized Lunetta and it works great.

Also, I forgot to mention: filters are great, in Lunettas and everywhere! Wink
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MirlitronOne



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PostPosted: Mon Sep 13, 2010 12:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

"...pocket sized Lunetta..."

I'm intrigued! Photos anywhere? Very Happy

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Silesius



Joined: Feb 12, 2010
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PostPosted: Mon Sep 13, 2010 12:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok, maybe a little offtopic, but...

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Four oscillators from one 4093 chip + that Hilltree filter + passive mixer.
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inlifeindeath



Joined: Apr 02, 2010
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PostPosted: Mon Sep 13, 2010 12:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

cool knobs, where'd you get them, if you don't mind me asking?
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Top Top



Joined: Feb 02, 2010
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PostPosted: Mon Sep 13, 2010 3:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Silesius wrote:
I've breadboarded the Mouthmeistor (without the gain stage for the moment) and it's really nice and easy. I'm thinking on building a CMOS filterbank with 3 or 4 filters from the 4069, with the possibility of connect it in series or in parallel.
I've noticed that the resonance pots are not very useful; I've used it always at maximum resonance. The thing is: is there any way to make the filters more resonant, with selfoscillation? That would be great. I've been trying changing some components, but I'm not very good at electronics, so I don't know which ones are responsible for the resonance. Any ideas?


Yea, in this configuration, the term "resonance" is actually a little misleading.

I didn't know what else to call it, it is more like a volume for presence of each filter. In the case of a guitar pedal it makes more sense, for how wah/vocal you want it to be, but I also tend to leave it maxxed out almost all the time. When you turn it down, you are actually just bleeding off that filter to ground.

By default with "resonance" up all the way, you have 1M to ground, and at least for me, it is right near self oscillating at that point. If you make the resistance higher, or, for example, possibly 2M, or take out the pot completely (so that the resistance to ground is infinite), it will self oscillate each filter I believe. Messing with the cap that is right by the resonance control, as well as the .1 and .0022uf caps (the ones at the front of each filter behind the 680pF/470Ω parallel part), is another way to alter how resonant it is. Actually, all the resistors and caps in each filter, in their own way, act on the frequency and resonance of the filter.

You could also try a resistor in series with the pot, to have the resonance not entirely drop the filter out when it is down, but allow more oscillation when it is up. You'd have to try some different values, I wouldn't know them off hand.

I have a layout with four filters on one 4069 (no mixing or input gain stage, but you probably don't need that anyway), that I can post later when I am home if you want.[/right]

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Psyingo



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PostPosted: Mon Sep 13, 2010 3:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

Top Top wrote:
I have a layout with four filters on one 4069 (no mixing or input gain stage, but you probably don't need that anyway), that I can post later when I am home if you want.


yes please.
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DGTom



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PostPosted: Mon Sep 13, 2010 3:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

I imagine there will be differances between 4069s, but they will scream like a screamy thing if deployed right;

http://dl.dropbox.com/u/252415/Havok_Pulser_Notch_2.mp3

this is a clip of an all 4069 audio path. 2 4069 VCOs & a 4069 State Variable Filter. The SVF is more or less a textbook design;

Posted Image, might have been reduced in size. Click Image to view fullscreen.

http://sound.westhost.com/articles/dwopa3.htm

...but with the inverters replacing the op-amps.

I'm using a pair of vactrols to control frequency, but, it sounds just as awesome with a dual gang pot. They produce quite a nice (for CMOS) sine wave with no input & with a short blip on the input will "ring" quite well.

@ Top Top, I like the sound of that "vocal" filter, will have to put one of those together Very Happy

I bet that Hilltree filter would sound badass with a 4052 / 4066 switching the capacitors around - that's my favourite way to get "voltage control" with CMOS Twisted Evil
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Psyingo



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PostPosted: Mon Sep 13, 2010 7:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

@DGTom, how did you set up the inverters in that filter? just an inverter with feedback or annythign more specific?

i tried makign a sallen key filter using 4069's a while ago and i got quite a bit of bleed of the original signal. i wasnt sure if that was due to the inverters, or the way i constructed the filter. there is are examples here:
http://www.ruinsite.jomdom.net/storage/ladyfafa.mp3
http://www.ruinsite.jomdom.net/storage/ladycmosfafa.mp3

you can hear the original signal, unfiltered when the cutoff goes lower. do you experience that in your design? it's hard to tell with your sound clips.

also, what resistors were cutoffand resonance in the schematic you posted?
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DGTom



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PostPosted: Mon Sep 13, 2010 8:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

basically just like the above schem, just no connections to ground.

That sample was of the notch ouput... which doesn't really filter all the way off, this one is the lowpass out;

http://dl.dropbox.com/u/252415/Havok_Pulser_LPF_1.mp3

all the silent gaps are when the filter cuttoff goes way down.

take a look at the Ken Stone version of the classic EDP filter which used 4069s

http://www.cgs.synth.net/modules/pic/schem_cgs49_twf.gif

just remove the OTAs (CA3080s) & the 1K & 33K input resistors, stick a dual gang pot in their places & use a simpler feedback path for resonance. I think I'm using 56K where that schem has 33Ks.... as long as they are all the same it shouldn't be too critical. To get it going really low with a 100K pot you might need to bump the 51Ks up or increase the 1nF caps.

& run it off normal power Razz
that one is running of -5V...
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Psyingo



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PostPosted: Mon Sep 13, 2010 8:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

what about the first opamp ? how is that connected as an inverter?
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DGTom



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PostPosted: Mon Sep 13, 2010 9:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

just connect them all to the one input. tbh thats (I think) the only time I've seen the resonance hooked up like that.

Usually R3 is just connected to the inverting input & non-inverting to ground. That cct. will work that way as well, use a higher value resistor & stick your resonance pot there.
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Top Top



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PostPosted: Mon Sep 13, 2010 10:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Psyingo wrote:
Top Top wrote:
I have a layout with four filters on one 4069 (no mixing or input gain stage, but you probably don't need that anyway), that I can post later when I am home if you want.


yes please.


Here it is. This is basically four of the the "Ghost Dance" filter but with altered values. The 1M resistors are in place of the resonance pots from my design (it is high in resonance but not self oscillating, at least in my bread-boarding). I have built this on perf but haven't actually powered it up to test it yet, but I did look over the layout pretty carefully.

You could just sub in some different cap values and get filters with different ranges. If you put in sockets in place of caps you could even pop them in and out... or else breadboard it and note values that you'd like to use for the caps.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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Top Top



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PostPosted: Mon Sep 13, 2010 10:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

DGTom wrote:
basically just like the above schem, just no connections to ground.

That sample was of the notch ouput... which doesn't really filter all the way off, this one is the lowpass out;

http://dl.dropbox.com/u/252415/Havok_Pulser_LPF_1.mp3

all the silent gaps are when the filter cuttoff goes way down.

take a look at the Ken Stone version of the classic EDP filter which used 4069s

http://www.cgs.synth.net/modules/pic/schem_cgs49_twf.gif

just remove the OTAs (CA3080s) & the 1K & 33K input resistors, stick a dual gang pot in their places & use a simpler feedback path for resonance. I think I'm using 56K where that schem has 33Ks.... as long as they are all the same it shouldn't be too critical. To get it going really low with a 100K pot you might need to bump the 51Ks up or increase the 1nF caps.

& run it off normal power Razz
that one is running of -5V...


That filter sounds great.

I am trying really hard to understand this...

The schem of the EDP filter looks a lot more complicated, with a lot more inverters, than the one posted above.

This one:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Which resistors in that schem would be replaced with the vactrols? And which one becomes resonance?

Also, you said that you eliminate ground connections, which makes sense in general for an inverter, but what about the 5K to ground on the first opamp input. Do you keep that one?

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Psyingo



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PostPosted: Mon Sep 13, 2010 10:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

it appears that R3 is resonance and R6 and 7 are cutoff.
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DGTom



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PostPosted: Tue Sep 14, 2010 12:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Indeed.

Don't worry about the 5K to ground, just tap off the 2nd inverter, run that thru a pot as an attenuator & use a larger resistor for R3 - this will depend on how much screaming you need Very Happy

R6 & R7 govern frequency... interesting things start to happen if you use 2 seperate pots rather than a dual gang...

the WASP filter uses 3 gates as the filter core. One gate is used to create the Notch output by mixing the HP & LP outputs. I'm using that set-up in the samples, 4 gates - the other 2 gates in that linked schem are a seperate distortion / overdrive.
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