This VU meter uses Nixie tubes as the display. There are a total of fourteen IN-13 bar graph tubes that map out the audio spectrum. The build uses purely hardware for the display; no microcontroller processing, or dedicated VU-meter chips were used.
Input begins with a dual Op-Amp along with a pair of potentiometers which allow the left and right channels to be balanced. Both channels are then each split into seven signals, which explains the layout of tubes seen above. Each signal is then fed through a voltage divider to envelope the output between 0V and 6V. There is also a low-pass filter to handle sudden volume spikes which don’t work well with the nixies. But as shown in the video after the break, all that work has paid off. Thr clip gives us a look at the green protoboards which host all of this filtering hardware. You’ll want to turn the volume down for the first couple of demonstrations which use a sweep to test the system.