I had this idea to try to make an instrument where the tonalities of the instrument were created using feedback. This was somewhat inspired by the fact that I bought a shit ton of cheap 8 ohm speakers from Electronic Goldmine. This is the first test toward making the instrument. Essentially I wanted to see what frequencies I could get with just an electret microphone and small speaker. Predictibly the feedback is fairly ear piercing, but it may not be too bad with some polyphony (that, or an order of magnitude worse).
I built a preamp circuit based on some disqruntled guy's schematic on minidisc.org. Apparently the preamps int he Sony R91 clip at ridiculously low levels and he needs something with a little more headroom. Of the few preamp schematics I tried, this one seemed to be the one with the least amount of noise. I also built a simple speaker driver circuit using a LM386 to drive the output and help create the feedback.
Below is the schematic for the microphone preamplifier curtosy of minidisc.org
Below is the schematic for the speaker driver. I just connected the output of the preamp to the input of the driver.
And below is a video of the first test of the feedback organ
Another interesting quirk I discovered when I was building this was the ability to control the receiving of radio frequencies. When I first breadboarded the circuit I accidently switched the polarity on the biased electret mic. Only when I touched ground (the side of the microphone) did the radio transmit through the speakers. My body acts as an antenna amplifier. Below is a video of me demonstrating that. Apparently Radio Disney is strong in my apartment.