Last night I was looking at some possible analog circuitry again, on the Natural Radio side, specifically filters to track Schumann Resonances. The frequencies involved are around 7-30Hz. To check the response of these and other filters, I could do with a good sweep generator and a true RMS voltmeter. After sleeping on it I remembered that I worked on exactly these (and various other) mostly audio-oriented circuits in articles I wrote for this magazine, way back in 1993. Unlike digital circuits, for the everyday hacker the analog circuit state of the art hasn’t really changed from then.
These were my first published works, helped to pay for my first IBM compatible PC. I was so chuffed that I got the cover feature with The Twisted World of Non-Linear Electronics (PDF). And what a cover!
Circuits in there include exp/log converters, an RMS converter, an (audio) dynamic range processor (compressor/expander) and a couple of chaotic circuits – that make a horrible noise!
The other article I have a scan of is The Versatile World of OTAs (PDF) – I think I wrote others, but don’t appear to have scanned copies. That’s operational transconductance amplifiers. They are closely related to regular op amps, but instead of producing an output voltage, they produce an output current. What makes them really useful is that they usually feature an additional input that controls the level of the output current. These things are found pretty much everywhere you might want something voltage-controlled, such as voltage controlled oscillators (VCOs) etc. in analog music synthesizers.
Circuits in there include a tunable active loudspeaker crossover, a couple of voltage controlled filters and a VCO. And…a bat detector. That worked a treat – made one, I with an LM380 or similar amplifier, out on a summer night, chirp, chirp!