Created
05/22/06

Related Items
Hard Drive Speaker

Materials
- Plastic cutting board
- two 4xAA battery holders
- two matched hard drives
- standoffs and screws
- amplifier parts
- music source
- 3.5mm stereo cable

Some time ago I turned a hard disk drive into a speaker, inspired by articles on the internet, all of which seem to be traceable to The Afrotech Hard-disk Sound System. But why stop at one speaker? Just because I'm willing to put up with the scratchy, jittery sound of a new hard drive low-fi system, that doesn't mean I'm willing to give up the novelty of stereophonic sound!

I actually began my hard drive stereo months ago, when I was working on my mono hard drive speaker. I built a second amplifier board that had two channels. It was wired up in ultra-hideous fashion, and exhibited an inexplicable short-circuit on one channel. I only figured out that it was due to what I thought was a dual-gang potentiometer, not in fact being a dual-gang potentiometer, when I ripped the board apart to recycle the stereo input jack. So I guess this circuit was needlessly put to death. Oh well, its successor looks much nicer. Also pictured is the pile of parts from which the new amplifier would be built.

The schematic is pretty simple. No reason to get complicated for something that's going to be driving a hard drive. This is just two standard 386 audio amp circuits, one for each channel. By 'standard' I mean so standard that it's printed on the LM386 datasheet.

I built the amplifier on a piece of high-quality pre-tinned protoboard, trimmed it, and drilled two new corner holes for mounting later. Each audio channel has its own volume control potentiometer so that I can provide proof of stereo-ness to skeptical unbelievers.

For testing purposes I hooked up a pre-butchered hard drive, probably one of the ones I used to test the mono speaker idea.

Here is the amplifier being tested both in mono and stereo. I am driving it with the headphone signal from a Sandisk Sansa MP3 player, which at first seemed to be a decidedly weaker signal source than the CD player I used for the last project. Then I realized that the volume control is nonlinear to the human ear, and gets louder faster toward the high end of the volume scale. It can actually put out a bigger sound than the CD player could. The waveform is pretty damn cool looking.

I mounted the amplifier to a plastic cutting board with some old standoffs I found in my miscellaeneous hardware drawer. Apparently I didn't have enough metal ones; I see a single nylon post on there. The battery holders were bolted down and wired in series for 12 volts, and a power connector was fashioned from female header. I have a diode across the power input to protect against reversed polarity.

I picked up two almost identical dead hard drives from a local computer store for free. After careful consideration I decided that there were no pre-existing mount holes which I could use, at least not with the screws I had available. There were four cyclindrical recesses near the corners of the drive cases, however. I expanded and lengthened these with a drill press. I then tapped the holes the hard way - with a screw. Being that the case was aluminum and the screws were steel (?), this wasn't too difficult. It did take a while, though: for every full rotation of the screw, I backspun it at least half a rotation. One screw got stuck and snapped in half.

Positioning the drives was a task much simplified by a carpenter's square and a Sharpie permanent marker. Holes were quite easy to drill in the cutting board. Screws were threaded up through the board and into the drive cases to hold them firmly in place.

Once I had the drives mounted, I realized that I needed to drill holes for the audio cables to the head actuators, so I had to dismount them again. A small hole was drilled in each drive on the face of the short end at a point near the head actuator connections. Twisted pair wire that had just moments previously been the power cable to a computer power supply cooling fan was cut, stripped, and inserted through this hole. I strung two short pieces of heatshrink tubing onto two short lengths of wire-wrap wire, soldered the wires to both the head actuator connections and the ends of the twisted pair cable, and shrunk the tubing to insulte the junctions.

Strain relief was provided by making a grommet from hot glue. I also applied some hot glue to the twists near the other end of each audio cable to prevent un-twisting. These ends were then attached to the amplifier output terminal block.

It looked pretty good, but it needed something extra...

It needed an attached music source! I marked hole locations around the perimeter of my Sansa MP3 player when layed down on lower left corner of the cutting board. At these locations I drilled holes through the cutting board that were slightly smaller than the width of my remaining stock of long screws. When these were screwed in, the formed a cage of just the right dimensions to allow the sansa to be slid in and out from the bottom of the board. Because two of the screws were at either side of the headphone connector, connecting an audio-out cable would lock the MP3 player in position.

All that remained was to put some nice labels on it. The number on the smaller label is a project serial number based on the date it was completed, 4/10/2006.

The sound is much, much improved on that of my first hard drive speaker. I have added hinges to the hard drive lids, so that I can close the drives to soften scratchy noise and jitter, but open them at a moment's notice to show curious friends the inside of the drives. The convenient carry handle on the cutting board is nice. I took this to school one day and carried it vertically by the handle all day long. It is pretty fun to put it up on my shoulder and act like it is an ultranerdy version of an 80's boombox. I get a lot of confused stares when I do that.