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The Sub THE SUB

I have two Tannoy Reveal 5A studio monitors which sound perfect and flat, but when you're creating music it's always good to get it sounding big. To help my two monitors in the low frequencies, I built a subwoofer with an internal amplifier and crossover. The amplifier was originially supposed to be powered by two LM3886 chips in a bridge configuration, for a total power of around 100W. A second version of the PCB was done using a AB-Class amplifier, with the schematic inspired from Elliot Sound Products. Left and right XLR inputs are mixed to mono and then a 4th order Linkwitz-Riley filter limits the frequency range from 20Hz to 150Hz.

The Sub in it's environment

The Sub in it's environment

Aluminium backplate with panel mounted connectors

Aluminium backplate with panel mounted connectors

I got a hard time finding valuable information on speaker building on the internet. It's a very complex art and there's a factor of luck and randomness as you can't control all of the parameters. I've chosen to make a sealed enclosure, so I would save the trouble of trying to tune the box and vents. Sealed enclosures tend to give less sound output than properly vented enclosures, but are alot simpler to design and build.

The internal volume of the box, measured in liters or cubic feet, is a critical factor in the design. Speaker manufacturers provide the VAS, which is the equivalent volume of air the box should contain. You can find a couple of calculators or formulas to get the exact volume your box should have, depending on the driver type, box type (sealed or vented), the resonant frequency of the driver and other parameters. To be honest, I didn't even follow these rules, my box being a little under the calculated volume. I did noticed that if your make a sealed box, you really need to get it sealed properly to get the right sound. In fact, my subwoofer sounded awful until I installed the backplate to seal the box. The other tip I followed was to add an internal structural piece to prevent the box side panels from warping under the pressure. If you simply create a square box without any internal structure, your box might wobble and sound ugly.

Below are the CAD drawings made to see if everything would fit together and to provide the cutting patterns for the CNC.

Front view of the CAD drawing

Front view of the CAD drawing

Interior and back view

Interior and back view

The speaker box was cut using an AXYZ4008 3-axis CNC from Robocut in 1/2" MDF sheets. The whole thing was glued together, then I routed the edges with 1/2" round cutting head to give smooth curved edges. 3 coats of black paint completed the box. I added 4 rubber feets to support the sub and prevent unwanted vibrations in the floor. I then cut the backplate in 1/8" annodized aluminium and laser-etched the lettering.

The speaker itself is a Dayton 8" dual 8Ω voice-coil, bought on Parts Express. The amplifier PCB was hand-made using toner transfer and a HCl + H2O2 etching solution.

Amplifier PCB

The rubber feet and the 8" subwoofer

Amplifier PCB

Amplifier PCB, for LM3886

© Charles Gervais-Dumont 2013