Most simple VCF on earth

metal_head_82 · Posted: Tue Nov 23, 2010 8:04 am Post subject: Most simple VCF on earth

Hey there!

I thought I share a little VCF-circuit I've been working on.
It's a pretty simple circuit using just a single opamp, some resistors two caps and a diode. I'm pretty sure someone else already had the same idea but I still think it's worth sharing. Wink

It's not a very accurate filter but it can be powered from a single 9V battery. And for those who are looking for cheap and simple VCFs this might be the solution (or at least a pointer to the right direction).

I ran some tests with some drum- and synthloops.
It should be ok especially for noise boxes, drones and other crazy and noisy stuff Cool

Enough talk!

Here is the schematic and a little recording with a drumloop and a version with an additional synth.

Have fun!
Greetz

Hendrik

UltraSimpleVCF.mp3

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A quick demo with a drum loop running through the filter

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UltraSimpleVCF_2.mp3

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Demo recording with drumloop and synth

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UltraSimpleVCF.png

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This is the revised version of the schematic

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synthmonger · Posted: Sun Nov 28, 2010 10:06 pm Post subject:

Those 1k CV summing resistors should at least be 100k.

And your VCF looks a lot like Tim's

http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/simplevcf.gif

metal_head_82 · Posted: Mon Nov 29, 2010 4:25 am Post subject:

Hey Snthmonger,

I lowered to summing resistors on purpose.
They were 100k but that didn't gave me the filter sweeps I wanted - so I lowered them down. The circuit itself has also changed. But it's not finished yet. I'm still trying to improve the frequency range.

The filters you posted look pretty cool. I think I'll have to breadboard them.
My Idea for this filter came from some simple, passive VCF circuits which didn't offer Resonance control. So I tried to include the technique used there in an active design.
I was pretty sure that I came up with something new Sad

But as I already mentioned: I'm sure that someone else had the same Idea.

Had the same issue with a 2 op amp LFO I thought I designed.
But I realized that it was such a simple standard design - it has been posted several times in many forums. So no post here.

I'm currently building a little filter toy with my design. Perhaps the filters you posted work out better. I will give it a try.

Thx for the hint! Smile

umschmitt · Posted: Thu Dec 22, 2011 6:21 am Post subject:

Hej metal_head_82. I know this is an old topic, but by chance did you succeed in improving the frequency range of you filter ? I'm using it - roughly adapted to 5 volts ! - in this modded circuit
http://electro-music.com/forum/phpbb-files/mutant_ds_7_prelim_schem_525.png
and it sounds good, but I'd like to be able to make it growl… So which parameters should be changed ? Do you have an idea of the equations involved here ? (is it even possible ?)
Thanks in advance !

metal_head_82 · Posted: Mon Jan 02, 2012 9:23 am Post subject:

This is an active bridged-T filter.
Just google for bridged-T filter and you should find the equations. I selected the values by trial and error Wink

To improve the frequency range you could exchange the diode with a garden variety transistor. e.g. BC547.

Connect the emitter to the junction of the two caps and the collector to ground (0V). Apply the CV to base. This should improve the behaviour to cv and give you some more control - especially with low CVs. You should then rise the summing resistor values to at least 100k. To limit the filter's frequency response when no CV is applied you could try to place a resistor in parralel to collector and emitter.

I think this is the same way Ray Wilson achieved a VCF in one of his designs. I'm pretty sure I saw this method in one of his old devices. I came up with this when I modified my WSG. Now it has some nice "quasi-vcf" Wink

Hope this helps a little.

Greetz

umschmitt · Posted: Mon Jan 02, 2012 5:56 pm Post subject:

Thanks. I ended up doing the same, using a transistor, on the advice from people here, and it works great ! I tried using 100k resistors for the CVs, without much success I'm afraid. I had very little range on the controls. So back to 10k. Putting a resistor across the transistor is great too, actually it answers an issue I had concerning the pots' travel…
So. Besides, I had put this transistor the other way round, like I thought it should have to be (as I mentioned on your VCO topic). The fact is that both work. I won't even ask questions here. I just don't understand.
By the way, your filter is among others discussed here.

mrkva · Posted: Tue Feb 07, 2012 2:19 pm Post subject:

Any news with this one? Any upgrades of the schematic / values? I really like the simplicity!

tokyomatik · Posted: Thu Mar 01, 2012 9:53 pm Post subject:

i tried to improve the sweep and cv following the tips here on this thread, but didn't worked...i would be also curious to see new schematics
i'm trying to use this filter to build a baby 8 for a friend, with internal vco and vcf controlled by the sequencer
kind of self contained groovebox
pretty funny till now, but sure could be even better
when i feed the signal to the filter the volume goes drastically down
and even with the cutoff completely open, i don't have the original sfrequency spectrum of the original, i loose part of the bottom and start to sound almost like an hipass
plus the pots don't have a linear response, too less and then too much if u know what i mean, should i try with a couple of logs ?

anyway the sound is already more interesting than the wsg filter for example, at least the resonance is not only perceived

keep up the good work

peace

MrSparkle · Joined: Mar 11, 2012 Posts: 1 Location: Portland, OR

ELEKTRICGASCHIEF · Joined: Jul 17, 2012 Posts: 2 Location: SCOTLAND

Hi, I'm new so forgive me for butting in!

I made a similar filter, using an LED and a 47K resistor, I got quite a good range from it but it was very unstable bursting into HF oscillation and causing the output to clip. Is this an inherently unstable circuit? Perhaps I need better supply decoupling...

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MY FILTER

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JingleJoe · Posted: Tue Jul 17, 2012 1:41 pm Post subject:

The polarized capacitor you have on your LED might be to blame, if you get oscillation you may need to adjust the resonance control or input amplitude.

Rolbista · Joined: Nov 17, 2012 Posts: 23 Location: Poland

Hey all, first post here, slapped this VCF together and works quite nice, but i have a problem with hiss and noise with cutoff and resonance set low (as in, cutoff completely open, and minimal resonance). At frst I thought it was ceramic caps for those two 100nF, but swapped them for film ones and the noise is only a littke better. I used 10k resistors for CV, and 100k for input, there is also a BC546 instead of the diode. Should I trim the input signal more? Or maybe it has to be like this? I used the schematic at the top of this thread.

JingleJoe · Posted: Thu Nov 29, 2012 3:41 am Post subject:

What op amp are you using? people usually blame op amps but maybe you need some high pass filtering at the input/output?
Also your connections between the circuit and amplifier, or whatever you are listening to it with, could be sheilded poorly, you could even be in an electromagnetic field hot spot Laughing

Rolbista · Joined: Nov 17, 2012 Posts: 23 Location: Poland

op amp - TL071cn , i used normal guitar cables and a small guitar amp. In the meantime I tried it during a band practice, and we are so loud we barely hear that noise. It would be nice however, to have it clean for studio purposes. Also, what voltage swing from an LFO should I use to modulate it from full to no cutoff, 0-9V?

JingleJoe · Posted: Fri Nov 30, 2012 1:07 pm Post subject:

There is another thread about simple VCFs somewhere else on this forum but I don't know what it was called anymore (I think umschmitt posted there too), anyway a very similar circuit was used but with a transistor in place of the diode, this should help with noise problems and allow you to tailor the control voltage range.

max92 · Joined: Aug 04, 2010 Posts: 17 Location: Sweden

I've just built this circuit using the original circuit at the top, only exception being that I use a tl072 instead of the tl071, but I guess that shouldn't matter.

My plan was to use this as a pedal for a bass guitar.

I've got two problems though:
1. There's a lot of noise when the filter is "open".
2. When the filter is "closed" it still let pretty much through. I would guess it is between 2-5 khz

So the noise. My first guess is that is because of the weak signal coming from the bass guitar. Could this be fixed with any coupling capacitors or change of current?

The cutoff frequency I guess should be a matter of changing a resistor somewhere or?

elmegil · Posted: Sun Dec 16, 2012 11:06 pm Post subject:

pre-amp stage?

wahoah · Joined: Jan 27, 2013 Posts: 6 Location: USA

I have the same problem with the "white noise" that my VCF produce

HELP!

bubzy · Posted: Mon Jan 28, 2013 6:22 am Post subject:

analog_backlash · Posted: Mon Jan 28, 2013 4:24 pm Post subject:

Hi everyone involved.

This may be a really idiotic comment, but I can't seem to find any mention of screened connections to the input and output jacks, which I would do as a matter of course. As I say, this may be stating the "bleedin' obvious" but it depends on how new you are to electronics. I certainly would not have thought of it when I first started out.

Just an observation Very Happy

Gary

wahoah · Joined: Jan 27, 2013 Posts: 6 Location: USA

2kohm · Posted: Sun Mar 24, 2013 11:22 pm Post subject:

mh... i use a 470K input resistor ( tested with iphone audio input ) and with my lm324 osc. smaler value bring me dist and noise

sounds nice an clean, but i think the filter add´s a lot of ultra high frequencies.
i used resistors in series with the pots to get a nice scale.

anybody else tried this filter or have some modifications ?
i like the sound off this filter!!!

bubzy · Posted: Mon Mar 25, 2013 5:42 am Post subject:

2kohm · Posted: Mon Mar 25, 2013 6:19 am Post subject:

intersting filter

this filter sounds nice and i think we can mix the circuits

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baudrate · Joined: Mar 19, 2012 Posts: 27 Location: Utah