I’ve always loved VU meters (favouring the LED types over the needle ones). When it comes to mixing (live performances or recording project) they prove to be a must have metering tool in my opinion.
Having dedicated hardware VU meter shows you your audio levels, even when you’re focusing on something else on your computer display. It also allows you to monitor any source of audio, even when your computer is off. For example, I find it quite really interesting to compare the level at which different albums are mastered to.
If you don’t know what Volume Unit represents, I would suggest reading this article on sound on sound as well as this article about audio levelling on wikipedia.
Of course you could buy an off the shelf VU meter, but why would you when you can build your own?
JLM audio sells a variety of audio products, including DIY kits that are definitely worth having a look at.
One of their kit is a 40 LED PPM/VU/GR Meter kit that can be stacked with another one to make it a stereo meter (I imagine you can stack more that two kits to build a multi-channel VU meter!).
Component list (sourced from JLM Audio)
The first LED kit has the +5V to +/- 12V SPMS option and powers the second kit.
The two boards are then stacked together using the provided spacers and female IDC 10 way connector.
You’ll also need the a few extra components:
The USB connector lets you power the device using USB (from a computer or a mobile phone charger).
The two XLR + Jack TRS combo sockets lets you connect any XLR or jack source. Using cinch to jack adapters you can even connect the VU meter to consumer audio products (CD player, hifi amp, etc).
The slim line rocker switch is the power on/off switch and finally, the ON-OFF-(ON) switch lets your select the peak mode for the meter (jumper 7 on the 10 way connector).
- Always ON position: peak hold with decay drop back. The maximum peak is hold for a few seconds before it starts decaying.
- OFF position: peak hold. The maximum peak of your signal is hold and never forgotten.
- Momentary ON position: peak reset. Lets you reset the peak when in peak hold mode.
When you receive your kit, you’ll receive all the components you need to assemble the boards, but you won’t find a user manual in the box (except solder and a soldering iron). It’s OK: all you need to know is available on the JLM Audio’s forum threads. There a forum thread for the current version of the kit which contains all you need to know to assemble one board.
To assemble two boards, also have a look at page two of the build thread for version 1&2 of the kit. There is a close up on how to solder the female IDC connector at the back of your master board so that you can stack two boards together! Very clever design!
This diagram summarises how I wired the two boards into the Hammond 1598BBK case.
Depending on how you install the stacked board in the hammond case you might have to install the jumper 4 (on the master board’s 10 way IDC connector) to reverse the direction of the metering on the LED bars.
Calibration is easy given you have a signal generator (also called function generator), hardware or software like the Test Oscilator plugin that comes with Logic.
- In logic, start an empty project with one audio track. On your track, insert a Test Oscilator plugin. All the tracks should already be set to 0dB, in the plugin make sure it outputs a 1kHz sine signal at 0dB.
If you have a function generator, use the 600 ohms output and generate a 1kHz sine at 1.228 VRMS (this is the 0VU or +4dBm reference level).
- Connect your JLM audio VU meter to your sound interface or function generator.
- Peak calibration: turn up RV1 (input trim) until the 40′s LED (red) just turns on.
- VU calibration: Then adjust RV2 (VU trim) until the 40′s LED (green) just turns on.
Repeat the calibration process on the second channel and you can start using the 40 LED stereo VU/PPM.