Sorry for not posting in a while. I’ve been acquiring more modules and making a little noise. Check it out…I was working on some incidental (background) music, and came up with this little ditty:

Not much to it, but I like it.

Anyway, on to today’s topic…

Filters (also known as VCFs)

If you read through my earlier post, The Basics of an Analog Synthesizer, you will be familiar with the fact that most analog synths have something known as a filter. You’ll also know that there are a few types of filters. For more details about that, please refer to the earlier post. For this post, I’m going to talk about a few different styles of filters and how to control them.

First, and perhaps most famous is the…

Moog Ladder Filter

This type of filter is detailed pretty well here by Timothy E. Stinchcombe in much better detail than I can provide, so for those interested, head over there for a read of all the technical details. Technically, this is just a low pass filter. Well, okay, perhaps not just a low pass filter, but it doesn’t do all the fancy stuff of a multi-band or band pass filter. However, what it does do is sound friggin’ fantastic. Get one. You’re rack will thank you, and you will be happy. In fact, instead of letting all the specs on the myriad filters out there get to you, I’d say just ignore them for right now and get one of these. They are, seriously, just that useful. Here’s a great one that I picked up from Doepfer. I was longing for that fantastic, fat filter that my Prodigy gives me in a Eurorack format. This is pretty much it!

As you can see, there’s a control for frequency cutoff, CV 2 & 3 attenuation, input level, and resonance. This is a very simple filter that takes the sum of the CVs and controls the frequency cutoff with the result.

What’s interesting about filters (this one being no exception) is that there is a CV for a 1 volt/octave. “Hey, wait a minute,” you might say, “I thought that’s something you use to set the frequency of an oscillator.” Yes, it is. Why on earth would you control a filter like that? Well, think about it this way…a filter lowers the volume of a given set of frequencies, yes? Well if the frequency cutoff is set at, say 220 Hz, then it will have a distinctly different sound if you played at 110 Hz vs 440 Hz. At 110Hz, assuming there are significant harmonics, you’d still hear the fundamental and it’s first set of harmonics. However, when you play at 440 Hz, you will only hear a muted tone, as the fundamental is an octave above the cutoff frequency of 220 Hz. So, by adjusting the cutoff frequency based on the same frequency control that the VCO pitch is set, the filter will always pivot around the note being played. With this one, you can also have a couple of other VC inputs that will modulate that, too, as you often use an envelope generator to create that great WOMP sound from a Moog. The second VC input allows you to use an LFO or other CV to further modulate the cutoff frequency. Very simple, and very useful.

Vactrol-Based Filters

Vactrol-based filters aren’t exactly a different style from others, but the way they’re controlled is a bit different. Because of this control, they do, indeed, sound different. Take, for instance, the stellar Make Noise MMG (left). In addition to this being a stellar filter, it is controlled via vactrol.

What’s a Vactrol?

Basically, it is a completely light-sealed device with a light-emitting portion and a photo-resistor (light-dependent resistor, LDR, for short) whose resistance goes down when more light hits it. Nowadays these are built with LEDs for their quick response. In earlier times, small incandescent lamps were used and before that lightning bugs in a jar. That last bit might only be a rumor, though.

But, a resistor is a resistor, right? Well, yes, and no. While the LDR does, indeed, look just like a resistor to the rest of the device, it has some interesting characteristics. First, since it has no electrical connection to the CV, it is completely decoupled from noise that may enter via CV. Second, while LEDs may be very fast to respond, LDRs are less so. The result is what some folks refer to as creamy, soft, or mellow sounding. It’s definitely audible, and I, personally, really like it.

Other Filters

There are, obviously, other filter types out there, and, to be totally honest, you just have to play around with them to see what you like. Several, like the Intellijel Korgasmatron/Corgasmatron are two filters in one unit that can be cross-faded or used separately. These are very handy and sound great.

Here’s the Deal With Filters

There are so many wonderful filters out there, you’re not going to just have one in your rack. So, play through as many as you can get your hands on. As I mentioned earlier, I highly recommend that you get at least one ladder filter, as they always sound wonderful, then broaden your horizons with the various other amazing units out there. You almost can’t go wrong.