DIY Hardware Stereo LED VU and Peak Program Meter

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!).

Build

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!

The diagram below summarises how I wired the two boards into the Hammond 1598BBK case.

 

Stereo VUPPMGR40v3 wiring

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

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.

  1. 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).
  2. 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.

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Miller 597 – Vintage Rear Dynamo Light / LED modification

Recently, I found a gorgeous looking bicycle rear light on eBay.

It was a bit rusty but after cleaning it the following markings on it became readable:

  • Miller
  • 597

Unfortunately, I couldn’t find much about on Miller as a bicycle light manufacturer and the different models they used to make… If anyone knows about it feel free to leave a reply at the bottom of this post.

597 rear lights are bottle dynamo lights so there is only one piece of wire coming out of the back them, but I converted this one to work of my hub dynamo!

With a  bottle dynamo fitted on your bike, the frame is the return path of the electrical circuit and you only need one piece of wire that runs form the dynamo to the lights to bring the current to the light bulbs.
With dynamo hubs, the frame is not part of the electrical circuit and you need to lay a 2-core wire from the front hub to the lights (usually the front light has a switch that the front and the rear light on & off).

It was quite easy to remove the original one core piece of wire to replace it with a  2-core one: I drilled into the rear of the light through the 1-core wire and also removed all the tar that was used to seal the unit and make the lightbulb holder stay in place.

Next step was looking into how to replace the lightbulb with an LED. LEDs on a bike are advantageous over incandescent lightbulbs because they are absolutely immune to vibrations (there is no filament to break). Incidentally they consume a lot less current but that’s not something you worry much unless you power them out of batteries.

Generators (dynamo) are AC generators. Their output tension is not regulated (the faster you cycle the more energy they produce) so I had to think about voltage and current regulation before using LEDs.

Here is a diagram of the circuit I built and managed to fit inside the 597 (these lights are small!).

The circuit can be broken down in to the following elements

  1. The hub AC generator
  2. The diode bridge that turn AC into DC.
  3. The zener diode that limits the tension to 5.5V
  4. The 0.47F (yes half a Farad) memory backup capacitor that smoothes the DC tension.
  5. The 150mA / 3.3V positive voltage regulator that drop the tension and limits the current in order not to fry the LED
  6. A miniature 1uF capacitor to prevent the regulator from oscillating
  7. A 30 Ohms resistor to lower the tension down a bit before feeding the LED

The design above has the advantage to give a “stand light” facility. That is to say, when you stop cycling (at a red light for instance), the 0.47F capacitor is charged an keeps powering the circuit. Depending on how much current is draw by your LED the light should stay on few minutes after you have stopped.

Design considerations if you if you want more that one LED:

  • Make sure you don’t exceed 150mA of current draw (or use regulators in parallel or a bigger one)
  • Make sure there is always a resistor before each LED if you use LEDs in parallel

List of components:

  • 4 x silicon rectifier diodes (more or less any type will do)
  • 1 x 5W 5.5 Volts Zener Diode
  • 1 x 5.5v 0.47F memory backup capacitor
  • 1 x 3.3V / 150mA positive voltage regulator (TS2950CT or equivalent)
  • 1 x 1 uF electrolytic capacitor
  • 1 x 30 ohms 1/4 watt resistor
  • 1 x red LED

It should cost you less than £10 to source the parts above. If you are based in London, the people at Cricklewood Electronics should have the components needed in stock!

Bellow are two pictures of the modified Miller 597 installed on my road bike.