I stumbled onto this video of a music synchronized light show and thought that it seemed very well done. Noticing that it was all analog, I was pleasantly surprised to see the different colors isolated pretty well, responding to different frequencies. Usually people tend to take the easy way out on these. I contacted Mike6798k and got more details including a schematic!
So basically all of the input resistors are the same value, they are there to mix the incoming stereo signal to mono, and should be chosen to match the source impedance (ipod or whatever, can be like 5 to a couple hundred ohms).
i used LM741 op amps with a single ended power supply (9v). perhaps using a dual rail supply would give a better effect, but this was much easier to do! the negative feedback resistor was chosen to be 10-50x higher than the input resistors to give a strong output.
that output was the filtered by three, simple first order RC filters. the low-pass filter was chosen to have a cutoff frequency of 120Hz (1/(2piR*C) to set the values of compoenents), the high pass 10kHz, and the bandpass (1000hz-5000hz).
the filtered outputs then drive a high gain transistor to power some bright LEDs!
Let me know if you need any clarification, I think the most important part is choosing good frequencies for the filters so that when you listen to music you can visualize different parts of the music
You’ll find the schematic after the break, or you can get it on Imgur. Thanks Mike6798k!