Introduction: Portable Retro Hi-Fi Speaker

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NOTICE:
The electronics for this project are now available as a kit from Jameco electronics!  Includes a blank PCB and all components/connectors required to build this project. 

DISCLAIMER:  Batteries can be dangerous.  They can deliver high current if shorted out and start fires.  If they are improperly charged it is possible they can explode.  Please do not attempt anything that you are not familiar with and do not feel safe doing, and double-check your work.  I am not responsible for any damage or injury due to misuse of these instructions.

Portable audio is a pain.  Its either a huge boombox that eats D batteries like M&Ms, a bad-sounding iPod dock, or a virtually silent mini speaker set.  None of these will do for a true music lover.  Those who are so inclined and adequately skilled can build one that meets their exacting demands with relative ease.  Now you'll be all set for a trip to the beach or camping. 

A couple years ago while building an amplifier project I accidentally burned out the main driver from a set of Minimus 7 bookshelf speakers.  Radioshack has sold this pair of speakers in various forms for over 30 years, and they are well known for their great sound and low price.  I was quite bummed out that I was left with only one, collecting dust in my storage room, unlikely to ever be of use to me as a Hi-Fi speaker again. 

Then I was inspired to turn it into a portable Hi-fi speaker by installing a rechargeable battery, charger circuit and amplifier.  I had most of the parts needed lying around and managed to put the device together for only $15 additional costs.  A thrifty shopper could probably do the project for around $60.  Great value indeed!

In this guide, I will show you how I converted this speaker into a battery powered portable mini Hi-Fi system.  It works great!

Step 1: Parts and Materials

Heres the list of goodies you'll need, and some details about them.
  • A bookshelf speaker.  You're going to want something that will work well on only 10-20 watts of power, otherwise the battery life will be very, very short.
  • Sealed Lead Acid battery.  I know these are heavy, but they're also cheap and very easy to charge.
  • Circuitry.  The full bill of materials is on Step 3.  The circuit is based on the reference design for a TDA2003 10 watt audio amplifier IC and a generic lead acid charging circuit using a LM317 variable voltage regulator, used to control charge current for the battery.
  • 18V laptop power supply.  It is important that it is 18V.  The lead acid battery charger circuit needs 18V and the amplifier doesn't like continuous operation above 19V.  Hopefully you can find one of these for cheap or free.  2A or more is plenty of current for this.
  • Sheet metal.  Something to fill the hole in the back of the cabinet to make a new control panel out of.
  • 2.1mm DC input jack.  For the input power from the laptop supply.
  • 3.5mm stereo jack.  For your audio source input.
  • Toggle switches.  For switching between wall power (charge) and battery power modes, and for on/off control.
  • Potentiometer (optional).  To control volume, if you want the ability to control it separately from your device.  Controlling it from your device will save on your device's battery power, though.  The TDA2003 only uses the power you hear so there is no waste of power having it at maximum volume all the time.
  • Chip heatsinks.  Definitely a good idea to add some small heatsinks for the ICs.
  • Handle.  Something to make it easy to carry.
  • Rubber feet.  To protect the wood and make sure the device doesn't slide away.

Step 2: Fabricating Some Parts

Remove everything from the speaker cabinet.  If you are using a multi-speaker cabinet like mine, separate the crossover since you will need it later. 

Mark and drill holes in the side of the cabinet for the handle.  This will be easier to do now than later.

Use a ruler to measure the spacing and diameter of the hole on the back of the speaker.  If the one you are using has permanent wires or something, you'll probably want to drill a large hole to facilitate switch and jack mounting, since the 3/8" or 1/2" thickness of the wood is going to make switch and jack mounting impossible.  If your enclosure is metal then you'll probably be fine without a plate at all.  Just mount everything right through the cabinet itself.

After making various measurements of components, I drew a drill template in AutoCAD to print out and transfer to some aluminum sheet metal.  Lots of materials can work for this purpose.

I also fabricated a small bracket out of the same aluminum to retain the battery inside the case, as seen in picture #3 on this step.

Step 3: Circuitry Explanation

I made a PCB for this project to save space.  I've supplied everything needed to build your own PCB in PDFs.  One is the schematic.  One is an image with a silk screen to help you build the board and show you where to place a few jumper wires.  The last one is a ready to print transfer that is pre-flipped for iron-on PCB making methods. Use these to fabricate by your favourite PCB method. 

Alternatively, you can build it on perfboard or however you like to do it.  Personal preference weighs heavily in these things. 

Wire the switches and inputs with enough wire they can reach where you are externally mounting them.  Use 22 gauge wire or higher for the switches and power input to ensure the wires don't get hot.  If you want to get fancy, use shielded cable for the headphone input. 

The amplifier is lifted directly from the datasheet for the TDA2003A 10W car radio audio amplifier IC.  This IC is really simple and works well with a line-level input from an iPod, phone or laptop.  Since this is a mono speaker and most sources are stereo, I added R9 and R10 to serve as a passive mixer circuit, combining the stereo signals into a mono signal.

The lead acid charging circuit uses a LM317T variable voltage regulator IC.  It functions by controlling the current that the battery can receive from the laptop power supply to an optimum level for charging.  When the battery approaches full, the battery voltage increases and the circuit delivers a reduced amount of current to "trickle charge" the battery further, and maintain the full charge without damage.

If you don't want to include a volume control and would rather control the volume from your device, simply solder a wire from the center potentiometer hole to the other potentiometer hole not connected to the ground plane. This will simply pass the circuit through.

PLEASE NOTE it is important to tune the trimpot RV1 while measuring the current through the battery with an ammeter so that the current through the battery is 1/10th the Amp-hour (Ah) value of the battery.  In my case, my 1.3Ah battery should be charged with 130mA of current for optimal battery charge and lifespan.  Over-current charging can cause the battery to vent hydrogen gas, which in a confined area can trigger an explosion.  Do be sure to adjust the trimpot correctly.

For speakers with a crossover, you'll need to solder some wires to where the old wire hookups were so that the crossover can be connected to the PCB.  For LC crossovers, the inductor goes in series with the woofer and the capacitor goes in series with the tweeter.  Try to leave the crossover wired as it was originally, those engineers (probably) knew best.

Here is the full parts list for the PCB:

Category,Reference,Value
Resistors,"R1",39,
Resistors,"R2",220,
Resistors,"R3",2.2,
Resistors,"R4",1,
Resistors,"R5",100,
Resistors,"R6",120,
Resistors,"R7",470,
Resistors,"R8",0.5,
Resistors,"R9",1k,
Resistors,"R10",1k,
Resistors,"R11",470
Capacitors,"C1",10uF,
Capacitors,"C2",470uF,
Capacitors,"C3",100uF,
Capacitors,"C9",100uF,
Capacitors,"C4",100nf,
Capacitors,"C6",100nf,
Capacitors,"C5",39nF,
Capacitors,"C7",1000uF,
Capacitors,"C8",0.22uF,
Integrated Circuits,"U1",TDA2003,
Integrated Circuits,"U2",LM317T,
Transistors,"Q1",2N3904,
Miscellaneous,"D1",LED
Miscellaneous,"J1",T Block
Miscellaneous,"J3",T Block
Miscellaneous,"J4",T Block
Miscellaneous,"J2" Wi
Miscellaneous,"RV1",1k Trimpot
Miscellaneous,"RV2",100k, Potentiometer
Miscellaneous,"SW1",SW-DPDT,
Miscellaneous,"SW2",SW-SPST,

Attachments

Step 4: Testing the Circuit

Hook it all up and check that it works!

When testing the battery charger, turn the potentiometer maximum counter-clockwise to minimize current to the battery.  Connect 18V DC to the circuit, and connect the battery with an ammeter in series with it.  Turn the potentiometer clockwise until the charge current reaches 1/10th the Ah rating of the battery.  As the battery charges the current should slowly drop.  Also, the current will be flowing in reverse than how you would expect when you connect the (+) of the ammeter to the (-) of the battery.  This is how it works and not a sign something is wrong, as it may intuitively seem.

To test the amplifier, connect the speakers and a music playing source.  If you're unsure about your circuit building skills (or my design skills) use a volt meter to check the voltage present at the 3.5mm jack, or test with an old device.  If you hear music, everything works great!

Step 5: Assembly

Time to assemble!

First, put the battery in place.  I used #4 wood screws for mounting everything, and drilled 5/64ths pilot holes where possible.  If no pilot hole is possible it is okay since the particle board can take a small screw without cracking.

Mount the PCB on some small plastic standoffs so that it isn't sitting directly against the wood.  I had some small 3/16" tall nylon washers which worked perfectly.  Try and keep it away from the speakers so that there is minimal interference, both physical and electromagnetic.

Route the LED up through a hole in the speaker face of the cabinet and secure it with the LED holder.  I just pulled one of the dust screen mounts out with pliers and put it in there.

Route the switches and jacks out the hole in the back and attach them to the plate.  Screw the plate in place on the back of the cabinet.

Put hot glue on the crossover and glue it inside so all the wires can get to where they need to go.

Connect the speakers and screw them in.

Stick some little rubber feet on the sides you want it to stand on.

Plug it in and test it out!

What to check if it doesn't work:
  • If it works on wall power but not on battery, check the battery polarity and charge.  Reverse polarity on the battery is really bad for the circuit so try and not do that one.
  • If it sounds bad, make sure you didn't get the wires for the tweeter and woofer mixed up.  This would cause some really odd sound.
  • Sometimes T-blocks don't get a good grip on wires and they fall out after handling.

Step 6: Finished!

Take pride in knowing you put an old unloved speaker to good use, and enjoy the high quality audio now at your disposal anywhere, anytime. 

Improvements:
  • A low battery indicator.  I thought of this after I was nearly finished but a transistor and voltage divider could activate an LED when the battery drops below a particular voltage.  Could definitely come in handy and would also help prevent over-draining the battery.
  • A different battery technology.  A lighter or more powerful battery technology would increase the battery life and weight quite a bit.  They are, of course, more costly and require special charging circuit otherwise they pose a major fire hazard.
  • Bigger more powerful amplifier.  The TDA2003 design is scalable, so a more powerful amp circuit could be devised, but the battery life will take a hit unless upgraded as well. If someone wanted to build a large cabinet with a full car battery inside then a 100W amp may be required.  High power amplifiers for car audio can be purchased as kits for relatively cheap on the internet.
Please feel free to ask any questions or post suggestions.  Thanks for reading!
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