Tuesday, December 4, 2012

Simple Multi-Type Analogue Filter

Ok, so I cheated on this filter, I really did. But, it does some cool stuff!

So here's the filter:
Let's look at this in sections. The bottom section (those two components) is a passive RC high-pass filter. Vary the cutoff with the resistor. The rest of the schematic is an opamp-based Butterworth slope low pass filter. Vary the cutoff with the resistor labelled 'var'. This could easily be a highpass by swapping the var resistor and the .01u capacitor.

So, how do I get multi-type filter out of just this? Well, let's start with the details.

The highpass is 3dB/oct, meaning that every octave the cutoff frequency moves, the volume of the attenuated frequencies is 3dB lower than normal. This is a very subtle filter, but has a rather drastic effect.

The lowpass filter is 12dB/oct, which is quite drastic in all respects, minus filters with more attenuation (obviously).

Now, let's talk about actual filtering. More so, let's talk about the four main types of filter: lowpass, highpass, bandpass, and band reject. In this configuration, all of these are possible. Firstly, the low and high pass filters are naturally there. The band reject is easy, too: set the lowpass to about 25% and highpass about 75%, and BAM, instant band reject. Variable in all respects, too.

Now, for a band pass, you'd need a slight modification: a double pole, single throw switch, and a diode. You'd need to install it like this:
Imagine those three unconnected but close wires is the switch.
 
To the left, you get the above variation. To the right, however, you have a band-pass filter. The low pass output goes into the highpass, and that can create a bandpass. However, you can't do much else with this configuration. I think a different switching type should be in order, but for now it works well.
 
So, there you have it. A simple multi-type analogue filter. And, you can also create interesting combinations, like some of those found on Doepfer's A-106-6. It really is pretty useful. And, you can actually use switches to rearrange the filters and, thus, change combinations. You can even include feedback loops, should you want.
 
Only problem? No Q. No Peak. No resonance. But, hey, who needs that? Just set up an RLC resonator somewhere. I'm still learning how to use inductors, though.

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