Notebook Speaker System

Like most people, you probably own a portable MP3 player by now. These devices are great when you’re driving or jogging. But, what if you want to play some tunes while you’re working in the yard? Or maybe, like me, you travel quite a bit and would like to listen to music in your motel room without using headphones? What if you have one of those portable DVD players and would like a pair of speakers with a little more kick? It was for these reasons that I decided to build a Notebook Speaker System. Why a notebook? Well, I wanted something that would be flat and reasonably compact. If necessary, I could put it in my briefcase along with my laptop. So, with requirements in hand, the quest began.

The first thing I did was to tear apart a couple of sets of cheap computer speakers to see what parts they used. For this exercise, I sacrificed a set of Dell speakers and a pair of house-brand speakers from Micro Center. Let me note that according to the spec sheet, the Micro Center speakers are rated for 100 Watts total maximum output (PMPO). I’m pretty sure that if I pumped 100 Watts into these speakers they would burst into flames. Both sets of speakers contained small circuit boards with amplifier chips. The Pro Sound speakers were built around an 8-pin TDA2822M from STMicroelectronics. This device can pump about 1.3 Watts into an 8 ohm speaker. The Dell amplifier board was built around TDA1517P which is manufactured by the company formerly known as Philips (now called NXP). This class B device can push about 3 Watts into a pair of eight ohm speakers.

The TDA1517P would make a good amplifier for this application since 3 Watts would be more than adequate. And, I was tempted to just drop the Dell board into my enclosure. But, I wanted something unique and maybe a little more cutting edge. So, I went to the T.I. web site and checked out their selection of Class D amplifiers. At first I was a little frustrated since most of their devices require a regulated 5 volt supply. What’s the point of using a Class D amplifier in a battery powered device if you have to add a 5-volt regulator to use it?

I couldn’t find what I wanted so I placed a phone call to Keith, my local Arrow representative. He directed me to the TPA3005D2. Bingo - this device requires a minimum of 8.5 volts and can drive a pair of 8 ohm speakers with up to 6 Watts. And, the best part is, it doesn’t require filters on the output side like most Class D amplifiers. But there was just one catch: the part is a 48-pin quad flat pack with a large pad on the bottom that requires connection to the PCB for optimal thermal performance. I wasn’t looking forward to laying out a board and so was very happy to discover that T.I. makes an evaluation board for the TPA3005D2. After considering the time and expense of designing my own board, I figured that the $50 price was a bargain.

So, this was my plan: I would use six AA batteries to generate 9 volts, the TPA3005D2 evaluation board as an amplifier, and build some small enclosures that would incorporate the Dell speakers. It was time to collect the raw materials.

My first stop was OfficeMax where I bought a Mead Five Star® 1.5" School Ensemble Binder. Then it was to Hobby Town USA where I purchased a 1’ X 2’ piece of 1/8” laminated plywood and some super glue. After drawing a schematic, I went to Austin Electronics, my local emporium where I picked up the discrete components necessary to finish the project. While there, I told the owner, Lloyd, about my project. He directed me to some small preassembled speakers that measured roughly 4” X 4” X 1.5” – perfect! Finally, I went to Home Depot for some small brass hinges, some silicone-based glue, four ¼” spacers, and some ¾” #4 machine screws and nuts. Assembly then began.

First I built a small circuit board to hold the amplifier assembly, the battery contacts, and the dual potentiometer that I scavenged from the Dell amplifier board. The nice thing about this pot is that it has an audio taper and an intrinsic power switch.

Next, I spread all the parts out and figured out where things were going to go. I decided to glue the speakers directly to the base on the left and the right and leave opening in the middle for the MP3 player. Then I decided to place the amplifier and the battery pack in a small box towards the rear of the assembly. So, the wood working began.

To make the walls of the box I shaved 1/8” strips off the side of a 2X4 with a table saw. If you don’t have a table saw or don’t want to generate copious amounts of saw dust, you can buy 1.5” X 1/8” strips of balsa wood at a good hobby shop. As it turned out, I only needed about sixteen inches. However, if you choose to make your own speaker enclosures, you will probably need more than that.

After making the walls, I cut a 9¾ X 11¾ piece out of the 1/8” laminated plywood to form the base. To accommodate the tabs used for opening the binder rings, I cut notches into the top corners. Holes were then drilled for the rings and the speaker wires.

Next, the box walls were glued to the base and allowed to cure overnight. The next day, holes were drilled into the box walls for two RCA jacks and a coat of flat black paint was applied. Contact cement was used to apply a layer of felt to the top of each speaker (they were a little ugly). And the speakers were then glued to the front corners of the base with silicone adhesive. These were held in place with C-clamps and left to cure overnight.

The next day, assembly was completed. The lid was attached via two small brass hinges. Then the circuit board was placed inside the box and attached with the #4 screws and ¼” spacers. The RCA jacks were added and routed to the circuit board along with the speaker wires. The battery was connected and dropped into place behind the circuit board and the lid was closed.

The assembly was then placed inside the three-ring binder and a Microsoft Zune MP3 player was attached to the audio inputs. Power was applied and the Play button was pushed. Music was heard and life was good.

Like any project there is room for improvement. I’m sure somebody could come up with a more robust speaker design. And, if you’re really ambitious and are willing to sacrifice battery life, a five volt switch-mode regulator could be added to power a Zune. For an iPod a 12 volt boost regulator will be required.

Now you know what I did during Christmas break. As you can see, it’s been a busy week, so I think I’ll relax and listen to some music.

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