Sounds nice !
Do you have a schemo with the values you used for this filter ?
(apart from the blue abstract picture you posted) _________________ ::U::N::S::C::H::N::E::L::L::
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
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).
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
Joined: Mar 20, 2012 Posts: 2179 Location: Chicago
Audio files: 16
Posted: Fri May 10, 2013 11:52 am Post subject:
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?
I wonder whether there is even variation across batches, since the original specs TL074's....
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?
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).
Joined: Mar 20, 2012 Posts: 2179 Location: Chicago
Audio files: 16
Posted: Fri May 10, 2013 1:10 pm Post subject:
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).
Joined: Mar 20, 2012 Posts: 2179 Location: Chicago
Audio files: 16
Posted: Fri May 10, 2013 10:27 pm Post subject:
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 .
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.
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.
hi!
i know this post seems very dead! but i made a stripboard version of the vcf for a microcontroller based application and i want to hear your feedback. Any mistakes?
i am pretty new and not very into electronics. only getting started
vcf mit verkabelung.png
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i think it would be better to post schematics instead of a layout as most of us (especially myself) are a bit lazy...
schematics are better to look at and see to see if something looks suspect..
first of all i would add in Input resistors for all external Inputs... and then i'm not sure if a 741 will run on 9V... i think it's supposed to be run on a + and - supply... the LM3900 or LM324 can be run on single supplies i know...
but maybe i'm talking nonsense and the 741 will do the job... well as i said i'm not sure... _________________ https://soundcloud.com/bastian-j
thanks for the reply. its the conversion of the picture of breadboard on bottom first page of this thread.
resistor to input. check.
i will check the ic issue. _________________ Sound: https://soundcloud.com/stream Art: http://ketamuworld.blogspot.de/
I'am trying to breadboard this vcf and i'm having some difficulties as my oscillator (made from cd40106 chip) seems to be sending some signal to the vcf even when the output of my oscillator is not wired to the input of the vcf.
Have you experienced this problem or do you have any workaround or explanation ?
absolutely not, the lm741 can handel up to 18V supply rails
Maybe you Need to switch some other resistor values _________________ Shop Videos Stromakustik.de
Joined: Aug 10, 2020 Posts: 1 Location: Uk
Audio files: 1
Posted: Sun Jan 31, 2021 2:20 pm Post subject:
I’ve made this today and everything seems to work fine except the cv in, I’ve tried modulating it with my Crave’s LFO but there is a pulsing sound that I can’t get rid of. Can any one help, please ? I’ve included a video with the annoying sound.
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