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 Forum index » DIY Hardware and Software » Developers' Corner
Most simple VCF on earth
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2kohm



Joined: Feb 19, 2013
Posts: 49
Location: germany
Audio files: 4

PostPosted: Mon Mar 25, 2013 11:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

hope to hear and see it ´Smile



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umschmitt



Joined: Jun 29, 2011
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Location: brrlin
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PostPosted: Thu Mar 28, 2013 12:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sounds nice !
Do you have a schemo with the values you used for this filter ?
(apart from the blue abstract picture you posted)

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2kohm



Joined: Feb 19, 2013
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PostPosted: Fri Mar 29, 2013 12:48 am    Post subject: Reply with quote  Mark this post and the followings unread

THX,

I used the schematic from Page 1 Off this Topic.
And the Second Breadboard schematic.


P.s ich komme aus München Wink

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2kohm



Joined: Feb 19, 2013
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PostPosted: Wed Apr 10, 2013 6:04 am    Post subject: sound demo Reply with quote  Mark this post and the followings unread

here is my version of the filter


9V-VCF_TEST.wav
 Description:
9v powered super simple vcf

Download
 Filename:  9V-VCF_TEST.wav
 Filesize:  8.41 MB
 Downloaded:  221 Time(s)


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Infrablue



Joined: Dec 29, 2011
Posts: 123
Location: Utah

PostPosted: Sun Apr 14, 2013 1:00 am    Post subject: Reply with quote  Mark this post and the followings unread

For simple filters, I really think this one may take the prize.

I think I want to make an outboard version of this to route synths through for live performance.

www.youtube.com/watch?v=qecPQ0FB-64
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beep



Joined: May 05, 2013
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Location: Germany

PostPosted: Sun May 05, 2013 1:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

if I would put two of this in series, calibrating the cutoffs and using a stereo pot for resonances, will I have 24db/oct or will the cutoff cvs respond
too differently? should I build 200 and then select a matched pair? :DDD
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reflex



Joined: Mar 04, 2011
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PostPosted: Mon May 06, 2013 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've got a version of this circuit running with a couple of 0.01uF caps and a 220k resistor across the op-amp. The cutoff works beautifully, but adjusting the resonance knob only alters the sound for the last 1/8th of the knob travel (it's a 10k pot). I've attached a sample of my filter in action.

Does anyone have hints or tips for scaling the resonance range?


twin-t bass.mp3
 Description:
Twin-t LPF driven be dual digital oscillators (PWM).

Download
 Filename:  twin-t bass.mp3
 Filesize:  1.46 MB
 Downloaded:  164 Time(s)

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the19thbear



Joined: Apr 19, 2007
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PostPosted: Tue May 07, 2013 2:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Way cool!
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reflex



Joined: Mar 04, 2011
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Location: Canada
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PostPosted: Fri May 10, 2013 10:55 am    Post subject: Reply with quote  Mark this post and the followings unread

OK, here's what I've discovered after some experimentation:

My version of the circuit has 0.01uF poly caps and a 272K resistor around the op-amp, running on a 9V power supply. There is a slight but audible difference between the poly caps and cheap ceramic ones.

The choice of op-amp makes a significant difference in audio quality.

LM324 (quad): Nice filter cutoff throughout the filter range, the resonance knob will self-oscillate if turned past about 90%, below that the resonance is a bit thin. There's a bit of unfiltered "sparkle" in the high end. Will operate at 5V, but I hear significant distortion/clipping.

LM358 (dual): Sounds the same as the LM324 (not really a surprise). Self-oscillates when resonance is turned up full.

TL074CN (quad): Resonance knob doesn't self oscillate when cranked, sounds similar to the audio sample I posted. HOWEVER, the cutoff has an audible bump as it's turned all the way down. This was a showstopper for me.

TL071CP: The winner. Similar resonance response to the 074, but the cutoff is much smoother throughout the full range. Not sure why there's such an audible difference between the 071 and the 074, given the similar chip specs.

I've breadboarded a little voltage doubler circuit that scales 0-5V cv from my microcontroller's DAC to 0-9V to allow full range control of the filter cutoff.

Last edited by reflex on Fri May 10, 2013 12:48 pm; edited 1 time in total
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elmegil



Joined: Mar 20, 2012
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PostPosted: Fri May 10, 2013 11:52 am    Post subject: Reply with quote  Mark this post and the followings unread

reflex wrote:

TL074CN (quad): Resonance knob doesn't self oscillate when cranked, sounds similar to the audio sample I posted. HOWEVER, the cutoff has an audible bump as it's turned all the way down. This was a showstopper for me.


That is VERY interesting.

I have a Thomas Henry VCF from AS21C which is supposed to self-oscillate, but does not. Tried a bunch of ideas from Dave Dixon on improvements to the circuit, still does not self-oscillate.

Guess which Op Amps I used? Smile

I wonder whether there is even variation across batches, since the original specs TL074's....
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reflex



Joined: Mar 04, 2011
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PostPosted: Fri May 10, 2013 12:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:

I have a Thomas Henry VCF from AS21C which is supposed to self-oscillate, but does not. Tried a bunch of ideas from Dave Dixon on improvements to the circuit, still does not self-oscillate.

Guess which Op Amps I used? Smile

I wonder whether there is even variation across batches, since the original specs TL074's....

Yes, I'm deeply suspicious of differences across batches and between the various production runs.

Have you tried LM324s in your TH VCF? It would be really interesting to see how it compares (might be awful, might be gold).
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elmegil



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PostPosted: Fri May 10, 2013 1:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well that's pretty much exactly what I'm getting at. I hadn't tried, but I plan to. Either 324's or any of a number of other quad op amps with compatible pinouts (LF444 comes to mind, though I only have 1 left).
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elmegil



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PostPosted: Fri May 10, 2013 10:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, both the LM324 and the LF444 made it quite a bit more aggressive, but the high end of the pot (past about 3 o'clock) breaks up and sounds like crap, and it still doesn't really self-oscillate in any manner that tracks the frequency knob.

Of course along the way, one of the sockets on my cheap leaf-spring socket gave up the ghost, and I had to troubleshoot that too. Nothing like stuff you built before you realized particular pitfalls Smile.

But in the end the TL074, while not as aggressive, has the best range with the pot, and since none of the options self oscillate, I've gone back to it.
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jackdamery



Joined: Apr 26, 2010
Posts: 72
Location: UK

PostPosted: Wed May 29, 2013 5:23 am    Post subject: Reply with quote  Mark this post and the followings unread

What is the function of the diode in the OP's circuit?
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reflex



Joined: Mar 04, 2011
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PostPosted: Wed May 29, 2013 1:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

The diode works as a voltage controlled resistor.

See richardc64's comment here: http://electro-music.com/forum/post-348389.html#348389
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beep



Joined: May 05, 2013
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PostPosted: Thu May 30, 2013 12:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Breadboarded it. It's a very nice filter. Had a few problems in the beginning,
cause I used LM741 instead of TL071. Changed a resistor.

If I connect a LM386 Little Gem amplifier, it oscillates. the more gain, the more oscillation. connected to soundcard input, there was no oscillation. Is this because of the load of the 386 amp?

Tried to buffer the output with a simple voltage follower,
but that didn't help.

Had a lot of fun with it. two videos. first one with little gem speaker output,
second one with soundcard input, but little gem still connected.
controlled by lfos, sample&hold and simple envelopes.

Simple Saw VCO as input.









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