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help demistify vcf's please
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tegleg



Joined: Feb 07, 2014
Posts: 3
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PostPosted: Fri Feb 07, 2014 5:30 pm    Post subject: help demistify vcf's please
Subject description: having trouble getting my head round a few things
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hello
sorry for the vague title.

i can make a basic hi or lo pass filter with a resistor and a capacitor, the resistor of course controls the cutoff.
i can make for example a 'decay' circuit that smoothly drops from +9v to 0 that is triggered from a sequencer.

how would i (in as few analog parts as possible) make that voltage controll the filter cutoff?

i can't get my head round how to replace a resistor with a voltage in a circuit like that. i can add some pictures is somethings not clear.
thanks for any help
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JovianPyx



Joined: Nov 20, 2007
Posts: 1257
Location: West Red Spot, Jupiter
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PostPosted: Fri Feb 07, 2014 5:48 pm    Post subject: Re: help demistify vcf's please
Subject description: having trouble getting my head round a few things
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tegleg wrote:
hello
sorry for the vague title.

i can make a basic hi or lo pass filter with a resistor and a capacitor, the resistor of course controls the cutoff.


Both the resistor and the capacitor control cutoff.

tegleg wrote:
i can make for example a 'decay' circuit that smoothly drops from +9v to 0 that is triggered from a sequencer.

how would i (in as few analog parts as possible) make that voltage controll the filter cutoff?

i can't get my head round how to replace a resistor with a voltage in a circuit like that. i can add some pictures is somethings not clear.
thanks for any help


First, the filter you described above is the simplest and least interesting of all filters. It is a single pole filter. If you want the kind of sounds you hear from modular synthesizer filters, then you want a filter with more poles - at least 2. There are many types. It depends on what you're after as to which filter is best. An easy to build filter that has the control aspects you described (voltage control over cutoff) would be the state variable filter. This can be done with an LM13700 IC - there are many many schematics for this filter type on the web. Just google "analog state variable filter" as a starting point.

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tegleg



Joined: Feb 07, 2014
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PostPosted: Sat Feb 08, 2014 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

thats exactly what i needed thank you
its sometimes difficult to find something if you dont know what its called.
Smile
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