Help w/ Introduction to Active Filters & Audio (VCF)

[michael.everett]

Hi all,

I'm looking for beginners information regarding building very simple (and hopefully into slightly more complicated) active filters for use in audio signal paths. I'm specifically wanting to learn how to make a very simple lowpass VCF (preferably in a breadboard first). Either links to good resources (I've been looking and haven't find anything that is truly a "from the ground up" guide.), or people with personal experience who wouldn't mind me bouncing some questions or ideas off of them.

I suppose a little introduction and background is in order, so here goes. I've been working in one form or another with synthesis and electronic music for over a decade and am fairly familiar with electronic music, synthesis, and production techniques. Although I do most of my work in software now, I still have a few semi-classic synths around (Alpha Juno 2, MKS-70, etc.).

I'm a software guy and program for a living. Recently about a year and a half ago I decided to make the switch to the world of electronics and hardware, and although I really only know the basics, I can get around pretty well wiring things in breadboards and deciphering IC datasheets. I've done a handful of fun electronics projects, of which some of the audio one's have included creating a 9V battery powered line amplifier based around an LM386 op amp, and a really lo-fi digital synthesizer by programming a LPC2138 ARM microcontroller (using basic DDS techniques and small wave tables, full MIDI IN implementation for control, and using its 10 bit DAC connected to an RCA jack).

Coming from a software background I've been a little more comfortable writing firmware on microcontrollers thus far as opposed to wiring up any circuitry that's beyond basic (at least understanding what it all does). But I'm really wanting to push a bit further into the linear/analog realm with audio.

In searching around the net I've found a few projects and circuit designs on sites like www.musicfromouterspace.com and of course www.paia.com, which have been useful in getting me in somewhat the right direction in terms of providing proven circuit schematics, etc. I've also seen a bit of the theoretical information on Wikipedia and in some books I have. But what I'd like to do is get some information that floats somewhere in between.

The filters on www.musicfromouterspace.com for example, are a little more complex that I was hoping to start out with. I can follow the circuit diagrams along a good bit, but not completely, and not knowing what everything does.

What I'd like to do is either find a how-to, guide, tutorial, or someone I can talk to who can guide me through starting with about as simple a decent lowpass filter as possible (single pole, perhaps fixed cutoff and Q), and explain what's going on (how to select component values, etc.), and then slowly lead into more complex design (for example adding in voltage tunable cutoff and Q, and cascading for multiple poles, etc.). And then of course when I've got an idea for a design and some components, it'd be nice to get some experience feedback of any pitfalls I may not be hip to in the design.

Ultimately this is one learning stop on the way to having some sort of DIY sound module that combines a microcontroller and DAC for a DCO and perhaps some digital sound processing routines (and MIDI control), fed into some sort of VCF(s), and with an op-amp at the end for gain and overdrive (at least that's the vision now). Also an input for sound processing would be nice (I'm in love with the idea of using the microcontroller's ADC and DAC for some bit-degradation effects on a line-in as well).

So in that regard I'd also love to get people's opinions on whether or not I should even be bothering with creating VCFs by hand, or if I should just be using prebuilt/semi-build/or off the shelf solutions. For example I noticed www.synthtech.com carries CEM ICs, which seem to be a one-stop shop for user adjustable cutoff-res, and would save me a lot of hassle in the ultimate design (are there any present day equivalents to these ICs?). Part of me really likes the idea of learning how to build VCFs, and how different components will change/shape the characteristics of the filter, but another part of me says to stay focused on the bigger picture of the project goals and not to get side tracked where I don't have to just for the sake of being able to say "I did it".

If you took the time to read this post, thanks. And thank you even more if you took the time to reply.

Happy Synthing,
-Mike

[Antimon]

Great question! I would be an interested reader of any replies here, although it might be more at home in a DIY section maybe?

/Stefan