My plan is to build another analogue synthesizer, but this time a modular one. I’ve been collecting the needed parts for a few years now, and am nearing the point where I can fire up the ‘scope, chauffe the old soldering iron (or “soddering iron”, as it’s known here) and start building circuits.
Something like this is what I’m thinking of (but probably not quite as professional):
In an idle moment this afternoon I dusted down a spectrum analyzer I’d made years ago, opened it up, and took a look inside. It was built for 220 Volts mains, and I’d included a 9V rectifier. So although I couldn’t plug it in to try it, I instead hooked it up to a 9V DC supply. It worked! I was chuffed.
The wiring is what I would call “Boutique”, as you can see from this photo:
The audio input is fanned out into five separate bandpass filters of my own design. Each filter is based around a 741 opamp, and tuned to a different frequency from the rest. The output from each filter is fed to a set of five LM341 LED driver ICs which each turn a bank of 10 LEDs on depending on the output level. The 341 can run in two modes: the first mode lights just the LED corresponding to the output voltage, the second lights that LED and all below it. I had a switch to select between the two.
In this image the LEDs are in the second mode, and the Analyzer is responding to a 5kHz sine wave input.
My design had several flaws, most serious was that there was intereference between the lowest two banks of LEDs. I think the filters are very wide, and I was too ambitious in tuning their centre frequencies. The result is that, for low frequency inputs, the lowest LED banks oscillate in brightness.
I powered up my thirty year old home brew sine/square wave generator tonight, and it worked first time. It hasn’t felt a hot electron since the 70′s. (Which is more than can be said for my penis, but that’s the subject of a different post.)
This was from a design based on a Wien Bridge, by the great J. Linsley Hood.
Perhaps if I mention it on my Web page he will send me an email. I would die.
Which reminds me that I should tell you I’ve had quite lengthy email correspondence with Mr. Shaw, he of Synthesizer fame (come along, come along, you mean you aren’t following these geeky posts of mine?), and he has shipped me a circuit board and design book. It will be coming all the way from Scarborough in Yorkshire, England.
Ee By Gum!
Q: “Are you going to Scarborough Fair?”
A: “No, it’s just the way my trousers hang.”
