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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
CMOS VCF!
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Psyingo



Joined: Jun 11, 2009
Posts: 247
Location: Canada

PostPosted: Sun Nov 18, 2012 11:37 pm    Post subject: CMOS VCF!
Subject description: Using some inverters
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Hi there,

I've been working on this VCF in transistor form for about a month now. It is using the ever popular twin t configuration for bandpass filtering. I found that disconnecting part of the twin t circuit, thus creating a sort of bridged t, would give a pretty decent lowpass response. the resonance on the lowpass isnt SCREAMING, but its pretty nice. Resonance, on bandpass, however, will oscillate. The diode for voltage control is taken from paia's early circuits. they LOVE using diodes for voltage control.

the inverter IC1B and associated circuitry is just an output amp that can be pushed into distortion. if this does not interest you, you can take the signal output from R3 and forget about the rest.

All in all, this filter sounds pretty decent for the parts count. It is by no means a great filter, but i like it. it adds a nice character to things. It is very raw and crude, right up my alley.

hope you enjoy this...

EDIT:

it has come to my attention that 5 pots may be a lot for a filter. you could get rid of the volume pot (VR5) on the input and the CV depth pot (VR3) and just have CV go straight through the 220k resistor. Also, you could remove the gain stage on the output, and you'd be down to a good ol' 2 knob filter. use your noggin' and decide what is best for you.

EDIT2:

This has come up recently with me, swapping in and out various logic chips:

The gains of different 4069's between manufacturers differs greatly, or any logic ic for that matter. since they arent mant as amps, there really is no gain matching, as long as it inverts the way it should to spec.

so when you make this or other analog cmos stuff, try swapping out different manufactur chips, from saya TI to a National, or something... or maybe even try some of the same manufacturer. the filter may self oscillate with some but not others.

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

Last edited by Psyingo on Wed Nov 28, 2012 7:29 pm; edited 1 time in total
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tjookum



Joined: May 25, 2010
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PostPosted: Mon Nov 19, 2012 12:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, that looks great!

Have you tested it at different supply voltages? Im guessing it's the regular 9v supply?

With some extra parts you could make a LFO out of the other inverters from the same chip.

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Psyingo



Joined: Jun 11, 2009
Posts: 247
Location: Canada

PostPosted: Mon Nov 19, 2012 6:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

yes the supply voltage that i made it for was 9V, but i dont imagine too many difficulties at other voltages.

you could do a multitude of things with the extra gates... oscillators, mixer, distortion, envelope follower... you could probably make a whole synth out of just one 4069.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Mon Nov 19, 2012 7:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Do you know if this will work using nand gates wired as inverters? If not, I will get around to trying it at some point (lots of real world stuff in the way right now though).
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Psyingo



Joined: Jun 11, 2009
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Location: Canada

PostPosted: Mon Nov 19, 2012 7:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Do you know if this will work using nand gates wired as inverters? If not, I will get around to trying it at some point (lots of real world stuff in the way right now though).


re: the schematic:
"4069 can be replaced by any CMOS inverter (Not Schmitt Trigger). 4049, 4011, 4001, etc. Experiment"

yes. i've used the 4011 and 4001 with satisfactory results. the only thing i would recommend is to use UB types, so CD4011UB. the UB stands for UnBuffered.
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elmegil



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PostPosted: Mon Nov 19, 2012 7:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry, due to previously mentioned real world stuff, my attention span is not what it ought to be. Cool beans, I will definitely give this a shot.
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Psyingo



Joined: Jun 11, 2009
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PostPosted: Wed Nov 21, 2012 9:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

This has come up recently with me, swapping in and out various logic chips:

The gains of different 4069's between manufacturers differs greatly, or any logic ic for that matter. since they arent mant as amps, there really is no gain matching, as long as it inverts the way it should to spec.

so when you make this or other analog cmos stuff, try swapping out different manufactur chips, from saya TI to a National, or something... or maybe even try some of the same manufacturer. the filter may self oscillate with some but not others.
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JL



Joined: Jun 25, 2012
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PostPosted: Wed Nov 28, 2012 6:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

I built this today, but put the output gain stage before the filter, since I might be using this with more quiet sources like guitar. I also used the other half of the 4069 for an LFO, using this schematic: http://home1.gte.net/res0658s/fatman/4069_lfo.html

I got rid of the volume control, as it was a bit redundant since I already had an adjustable gain stage in front of the filter. I also removed the CV input depth knob, is was also redundant because the LFO has an intensity knob. It sounds good when it works. Sometimes it gets really quiet and the resonance and cutoff knobs don't work right. It could be a loose solder joint, a mistake with the veroboard layout, or maybe the 4069CN I'm using isn't ideal. If I remember, I'll try and post a picture tomorrow.

My advice to those considering this project: build it for the bandpass. The lowpass mode isn't spectacular, although with the low parts count and simplicity it is definitely worth it!
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Psyingo



Joined: Jun 11, 2009
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PostPosted: Wed Nov 28, 2012 7:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

JL wrote:
My advice to those considering this project: build it for the bandpass. The lowpass mode isn't spectacular, although with the low parts count and simplicity it is definitely worth it!


I agree that the lowpass isnt spectacular, and your results with the lowpass will definitely vary depending on your device being used.

have you tried swapping out different ic's? different versions of the 4069 will yield more or less gain, and this will effect the goodness of the lowpass.

i've built it to get a pretty decent lowpass sound but you need to pick your ic, since these arent made as linear amps their gains are all over the place.
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Psyingo



Joined: Jun 11, 2009
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PostPosted: Wed Nov 28, 2012 7:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

oh... heres a demo.
you can hear me switching between bandpass and lowpass.

http://www.voxinfinitus.net/~psyingo/electronics/filter_demo.mp3
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JL



Joined: Jun 25, 2012
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PostPosted: Wed Nov 28, 2012 8:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Psyingo wrote:
have you tried swapping out different ic's? different versions of the 4069 will yield more or less gain, and this will effect the goodness of the lowpass.


I just ordered some 4069UBE's, maybe they will give better results, and hopefully stabilize the LFO a bit. Tomorrow I'll try disconnecting the preamp section to see if that is causing any weirdness.
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JingleJoe



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PostPosted: Thu Nov 29, 2012 2:40 am    Post subject: Reply with quote  Mark this post and the followings unread

A problem with your filter is that the resonance only occurs on the bottom half of the wave, the falling edges.
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Psyingo



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PostPosted: Thu Nov 29, 2012 7:48 am    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
A problem with your filter is that the resonance only occurs on the bottom half of the wave, the falling edges.


Why do you say that?
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JingleJoe



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PostPosted: Thu Nov 29, 2012 12:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Because it is the truth.
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Psyingo



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PostPosted: Thu Nov 29, 2012 1:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
Because it is the truth.


i meant could you please explain?
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JingleJoe



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PostPosted: Thu Nov 29, 2012 2:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

Why didn't you just ask plainly? Laughing
I could hear it first, because I've made a filter similar to this before and so I recognise that sound, secondly I could see it as I have an oscilloscope in my media player (It is comparable to and better than some digital scopes I've used).

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Psyingo



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PostPosted: Thu Nov 29, 2012 3:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
Why didn't you just ask plainly? Laughing
I could hear it first, because I've made a filter similar to this before and so I recognise that sound, secondly I could see it as I have an oscilloscope in my media player (It is comparable to and better than some digital scopes I've used).


yeah, the resonance does become asymmetrical depending on the amplitude of the signal being fed in, but i have been able to get pretty symmetrical resonance.

i like the sound it has when you drive it a bit too hard and it starts to saturate.

i wouldn't consider it a problem, as you said.

maybe i should have used a better clip to illustrate the filter

asymmetrical resonance: it's a feature.
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JingleJoe



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PostPosted: Thu Nov 29, 2012 4:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

I knew you would say that, still seems more like a glitch to me though. I fixed it in my filters Razz
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Psyingo



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PostPosted: Thu Nov 29, 2012 4:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
I knew you would say that, still seems more like a glitch to me though. I fixed it in my filters Razz


this isn't a competition, joe. why don't you share how you fixed it?
if you have nothing CONSTRUCTIVE to say, please refrain from posting.
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JingleJoe



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PostPosted: Fri Nov 30, 2012 1:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Maybe you should take a leaf out of your own book. Stating that I fixed it is constructive if you ask me, if I were you I would have replied in earnest with eagerness to find out how I fixed it, not by sending me insulting messages. I think that says a lot about the kind of man you are and that's why I don't really want to help you anymore.
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-minus-



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PostPosted: Fri Nov 30, 2012 3:48 am    Post subject: Reply with quote  Mark this post and the followings unread

The fact you did NOT reply with your solution says a lot about the kind of attention seeking imbecile you are. What, we all have to lick your dirty steampunk gaitered boots do we? I think not.
Last edited by -minus- on Fri Nov 30, 2012 8:48 am; edited 1 time in total
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richardc64



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PostPosted: Fri Nov 30, 2012 5:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Right on cue, the toady jumps in.

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

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elmegil



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PostPosted: Fri Nov 30, 2012 6:06 am    Post subject: Reply with quote  Mark this post and the followings unread

Come on guys. Everyone needs to stop assuming the worst possible interpretation of the other's statements and escalating to this ridiculous point. This ain't high school any more (sorry Joe, I don't remember offhand what the equivalent is in the UK, but you get my point I'm sure).
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JingleJoe



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PostPosted: Fri Nov 30, 2012 6:42 am    Post subject: Reply with quote  Mark this post and the followings unread

richardc64 wrote:
Right on cue, the toady jumps in.

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

the image link didn't work but I gather from the URL the jist of it; and that is exactly what I thought when psyingo messaged me with such an insulting message.
I'm really surprised minus isn't banned, he is clearly a troll.

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JingleJoe



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PostPosted: Fri Nov 30, 2012 9:09 am    Post subject: Reply with quote  Mark this post and the followings unread

I'll post my solution to the resonance problem because psyingo apologized via pm;
You can fix the resonance issue by lowering the signal amplitude, it has probably hit one of the power supply rails you see? Also by changing the offset voltage of the wave, however I am not certain how you'll do that with a 4069, can you simply adjust the input wave biasing?
Sometimes un-evenness in the filtered signal is due to the diode effect causing only one half of the wave to be filtered, there could be internal diodes in the 4069 causing this or it's own transistors can cause it (trannys being a lot like diodes in themselves).

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