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Author How does a ladder filter work?
decklyn
 I went off the deep end and started building synth gear like crazy. I can understand most of the components of the synth fairly easily - egs, lfos, vcos, for example are fairly easy to understand. The filter to me is a complete mystery. I was chatting with a colleague today about how to frame this. How does a square turn into a sine, and we came to the conclusion that it's better to think of the harmonics that are cut rather than thinking of the waveform. And the resonant bump as an increase in harmonics at the point that the filtering starts and then cutting harmonics above/below. But I'm wondering if someone can help me understand exactly HOW eg a ladder filter achieves this through voltage control. It's really hard for me to wrap my head around and intuit. My friend has an educational background in electrical engineering but he's doing mostly digital and smd work - he could not explain how these things work to me :( so hoping I can get some help grokking this here!
Synthmatic
Dcramer
 I’m an idiot when it comes to building but I recently did some research into making little passive filters and they really relate to slewing. In other words, in the time domain, a square wave is rounded off into a sine (or tri) because the circuit slows down the rise and fall of the waveform edges. I really wanna build a simple little pair of passive hipass filters, but my DIY skills are like driving a truck through a sewing factory when trying to thread a single needle.
cornutt
 Dcramer wrote: I’m an idiot when it comes to building but I recently did some research into making little passive filters and they really relate to slewing. In other words, in the time domain, a square wave is rounded off into a sine (or tri) because the circuit slows down the rise and fall of the waveform edges.

Yep. Another way of looking at it, in the frequency domain, is that the low pass filter removes the upper harmonics that make a square wave square. Get the cutoff frequency down enough, and eventually all you have left is the fundamental, which by definition is a sine.

Your basic passive low pass or high pass filter consists of one capacitor and one resistor. The cutoff frequency (by definition, the point at which the output level has decreased by 3 dB from the passband) is given by: 1 / 2 * pi * R * C, where R is the resistance in ohms, and C is the capacitance in farads (not: not microfarads or picofarads, whole farads).
nigel
 cornutt wrote: Your basic passive low pass or high pass filter consists of one capacitor and one resistor. The cutoff frequency (by definition, the point at which the output level has decreased by 3 dB from the passband) is given by: 1 / 2 * pi * R * C, where R is the resistance in ohms, and C is the capacitance in farads (not: not microfarads or picofarads, whole farads).

And (in very simple terms) the transistors in a ladder filter are acting like variable resistors. Since they are connected by fixed capacitors, as the resistance changes, so does the cutoff frequency.
Navs
 Not sure if the question is about voltage control or filters themselves, but as far as the latter goes, I understand filters to be 'faulty' amplifiers, i.e. a LPF 'fails' to pass high frequencies. The user determines how much it 'fails' by.
decklyn
 Ahh thanks. That makes a lot of sense - the "slewing" eg slowing down via a capacitor via reactance. Thanks guys - I can intuit that.
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