Several years ago I had a rack of stereo components that I had collected over the years. Tape deck, FM unit, dedicated amps, CD player, the works. I always though it would be kind of cool to have a nice analyzer unit for the audio. Instead of buying one, I decided that I was going to build one using some already designed circuits and some interesting displays.
I had originally intended to use what I've heard called "Pinball Displays" recovered from old HP equipment. Instead of (expensive!) Nixie Tubes, these were just neon bulbs inserted into an array marked 0-9, which lit up to indicate the digit. I had planned on replacing the neon bulbs with multicolor LEDs. 10 channels of bouncing lights with the option to have rectified outputs to drive analog meters.
The analyzer channels themselves were simply copied from a consumer unit that was available cheaply. The rest of the circuitry, including the precision instrument rectifier board and dot-driver was adapted from reference designs. Everything was going to be mounted modular and connected with DB connectors for easy installation and removal.
One side of the 10-channel filter board and it's accompanying precision instrument rectifier board:
However, like a lot of things, it changed. My audio rack gave way to a laptop with a good DAC, which gave way to a streaming audio device connected to a remote network share. All of that old equipment simply became dead weight, and was either sold, or donated because it no longer held value.
I was never able to get enough of the displays to complete the project, and other things like 1N34 germanium diodes became harder to find. I decided to shelve the project shortly after selling off the audio equipment, and I've been trying to figure out what to do with these things ever since - beside just using them as a talking point on job interviews...