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!).


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 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.

iMac mat bezel mod – glass is history

At work I have an core i3 iMac 21.5″ from Mid-2010.

Unibody iMac computers are great machines but I cannot stand the glass panel: It looks great but it’s a nightmare to have a piece of highly reflective glass between your screen and yourself.
I don’t really understand why apple did not offer the option to have the computer’s glass panel replaced with a black mat plastic bezel. I’m not talking about the LCD screen, but just the glass panel that can easily be removed with suction pads to get inside the computer.

I asked my boss if he would mind if I removed the glass panel altogether and left the computer’s LCD screen and edges exposed. He reply he would mind adding he wouldn’t really like the look of it and also so that the computer would gather dust and that maybe water would eventually find it’s way inside the computer…

A couple of weeks later had a brainwave: I should be able to use diamond cutting discs and my dremel to cut out a window into the glass panel thus eliminating the annoying reflections.

I did so and… it was a bit a disaster.

It’s virtually impossible to cut a piece of glass with a dremel and cutting discs without breaking the glass. By the way, cutting glass with a rotating tool is dirty and dangerous business:

  • Wear a breathing mask unless you want glass dust in your lungs.
  • Wear protection glasses (I felt on several occasions small shards of glass hit my hands and face!).

The good news is behind the black bezel of the iMac’s glass panel is a thin metal frame that sticks to the glass and holds it together even when the glass is broken.

Once the window cut out of the panel I used a large sheet of black vinyl to cover the sharp edges of the panel and render it mat.

Check out the end result bellow. I am really thrilled with the results: No more headaches and annoying reflections between the LCD screen and my eyes!

There are still reflections from the glossy LCD screen but these are not a problem at all as the surface of the screen is treated just like macbook air screens’ are.

USB Powered Magic Trackpad

Batteries on the Magic Trackpad last an amazingly long time. I would say about 6 months for my usage (on alkaline cells).
Nevertheless, eventually running out of batteries is always annoying so the other day I decided to power the Magic Trackpad using an USB cable.

This mod has already been documented on’s forum. My twist on it was to drill a hole in the stainless steel battery compartment cover instead of leaving wires exposed or drilling through the trackpad’s body.

Be warned that drilling stainless steel requires a lot of patience. You also need a drill that can rotate a slow speed and Cobalt drill bits. It is really important to take your time and not to drill for too long otherwise the heat will turn the stainless steel into super hard steel you will not be able to drill through.
In the case of that battery cover, being small it will get hot very quickly so I drilled for 30 seconds at a time, waiting a minute or two in between to allow for the piece to cool down.
It took me a whole afternoon to drill through the cover!

I used three 1N4001 diodes in series to step the voltage down a bit (from 5V to about 4.2V). You could use an extra two to bring the voltage closer to 3V, but the trackpad doesn’t seem to mind being powered by tensions over 3V (don’t try with more that 5V though) (I’m pretty sure Apple’s engineers used regulators to protect the trackpad’s internals).

In the process, I discovered that the positive end of the battery ersatz have to reproduce the shape of the + side of an AA battery. If the + end is flat it will not work. There is clever mechanism that doesn’t close the electrical circuit to prevent from damaging the trackpad if you insert the AA batteries in the wrong orientation.


  • A small piece of soft wood, cut down to the dimension of two AA batteries in series
  • Electrical tape
  • A beer cap (for the negative pole)
  • The positive tip of an AA battery
  • A couple of 1N4001 diodes to step down the USB voltage

Bellow are a few pictures of my mod. Enjoy!

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.

DIY pro (+4dBu) to consumer (-10dBv) audio level transformer

Pro audio equipement such as mixers and sound interfaces work with signal levels referenced up to +4 dBu whereas consumer audio equipement (such as hi-fi amplifiers or cd players) works with signals refereced up to -10 dBu.

The following diagram represent a very easy to build transformer that will let you connect the output of a ‘pro’ mix-table to an ‘consumer’ amplifier for instance. Please note that I deliberately left the output to be balanced. You need to make sure the the input of the ‘consumer’ equipment you are connecting the output of this circuit to has a balanced input. If it hasn’t simply connect pins 3 and 1 of the output together to make the line out unbalanced.

+4dBu to -10dBv level adapter

Notes: 1 dBu is the voltage level which delivers 1 mW of power in a 600 ohm resistor. dBu and dBv mean exactly the same; dBv came up first but it was confusing people with dBV so dBu is now in use.

I used the following schematics from Jensen Transformers to make the diagram above:

I found it very difficult to buy Jensen transformers from UK but luckily I found an alternative with OEP audio transformers (I used an OEP A262A2E wired in 4:1).

Intrinsically, a transformer will not introduce ground loops and other undesirables to your sound path! And it won’t make your sound sound weak like resistor attenuators tend to do.

It would also be very easy to rewire the transformer into a 1:1 configuration if all you need is to de-couple two systems together without introducing ground loops (like in a sound stage to front PA scenario for instance)…

The two circuits transformers are housed in an aluminium box
End result with the circuit above (x 2) housed in an aluminium box

Mac Mini inside a Powermac G4: Wiring

Following my previous Powermac-mini post, I would like to should you how I wired and coupled the mini to the powermac.

Inside the powermac-mini: wiringThe challenge here was to have the ATX power-supply put inside the powermac to start at the same time the mini is started (and to stop when the mini stops) and also to get the front LED and buttons of the powermac to work with the mini. Grincheux’s dedicated blog gives all the info you need to do all that so I won’t go too much into details about it.

Bellow are pictures showing how I modified the original powermac quicksilver’s front buttons board in order to be able to wire it to the mini. The idea is to wire the power-button in parallel with the reset switch so that it’s easy to wire the reset wires to the on/off momentary connector on the mini’s main-board.
Then on the other side I wired the two unused wires on the 10 pin IDC connector straight to the LED.


I am thrilled with this mod. The powermac quicksilver case looks stunning under my desk, I’ve got plenty of room for hard drives, the mini runs cooler and I can’t hear it any more (if you’ve got a mini sitting on your desk you know what I’m talking about)!

Best of all is arguably the increased expandability the PCI-express 1X expansion slot gives.

Mac Mini inside a Powermac G4: Body work

After having swapped my mac mini’s internal SATA hard-drive for an external 3.5″ one I got really pleased with the performance boost but also progressively more and more annoyed with the noise produced by the bigger hard-drive, sitting right next to me on the desk.

A few month later I happen to browse Grincheux’s powermacmini blog and got instantly seduced by the concept: Replacing the guts of Powermac G4 with a mac mini! Why not any other PC case for this mod? Well, it’s mostly a question of aesthetics: I really like look of powermac computers.

powermac_g4_quicksilver powermac_g4_quicksilver_open

I chose to experiment on a Powermac G4 quicksilver and it didn’t took me long to get one from eBay. You will find bellow a few pictures of the body work and installation of the mac mini inside the case. For more detailed information please jump to

Hole cutting

The first thing that had to be done was digging out a hole at the back of the G4’s case in order to be able to connect things to the mini when it is positioned inside.

Internal support

Then, I started converting an old aluminium plaque into a support that would maintain the mini in place inside the G4 case. The left picture bellow shows one of my first satisfying attempts. The one the right shows the final version of the plate (with the paint totally removed so that the plate can act as a passive heatsink for the mini by dissipating the heat that builds up on the mini’s rubber pad). I also cut a section out the aluminium plate to make room for a PCI express 1x female socket…

Powermac optical drive issue

I didn’t foresee something quite frustrating: with the mini mounted like illustrated on the pictures above, it’s impossible to close back the Powermac’s case since the end of the optical drive conflicts with the mini. In the end I was forced to cut a small portion of the mini out. This is the only irreversible modification I performed on the mini, but although irreversible, it doesn’t interfere with the mini’s operation at all, and in fact you can even put the mini’s case back on it and you can’t see a thing (unless you look at the mini upside down).