Portable Stereo Amp

My wife loves watching television programs and movies by streaming them online. She used to use a large and heavy laptop. For Christmas I gave her an Asus Eee Transformer tablet so she has more portability. It streams the shows fine, but the speaker in it are awful – they are very quiet and sound like crap. In addition, a lot of stuff online has very low audio levels (maybe digital stuff that was mastered at -20, and never boosted when converted and posted online). We have a pair of regular computer speakers, but then she will be tied to a power outlet and have two speakers with several wires to carry around. More portable speakers are powered from USB, which will drain the tablet battery, and many of them also have many wires to manage. I decided to make a custom speaker box for her that met several goals.

Speakers

Goals

Portable
Lightweight and easy to carry.
Stereo
This is more for me when I watch shows with her.
Large Amplification
The signal from the tablet is low, and many videos online have low audio. It has to have more amplification than most PC speakers.
Power Indicating LED
So it is obvious when the unit is turned on.
No LED on Front
The power LED shouldn’t be on the front of the case because it will be annoying to have the light shining in her face.
No Power Cables
Powered by a single 9V battery, which is easy to replace with batteries she can get from the dollar store.
Only One Wire
A single wire to connect to the computer. No USB connector, and no wire to run between the right and left speakers.
Audio Jack on the Left
The output jack is on the right side of the tablet, so the input jack should be on the left side of the speakers. This makes it easy to set the tablet and speakers side by side.
Stand Up or Lay Down
It should stand up for ideal usage, but also be able to work laying down when she uses it in bed.
***UPDATE*** – DC Power Adapter Option
We were going through a lot of batteries and wanted an option to use a DC power adapter when we were near a plug. I added a jack for the adapter and a switch that toggles between the two power sources.

Inside Speakers

Parts

Case
$2.95 case with handle from surplus shop.
Speakers
From a Yamaha PSR-150 that I used for parts for another project.
Amplifier
Two LM386 amplifier IC’s that were $1.25 each.
Speaker Screen
Mesh placemat from the Dollar Store.
Ports
$1.00 speaker cover fasteners from surplus shop.
Knob
$1.00 large volume knob from surplus shop.
LED Power Switch
$0.75 from surplus shop.
Miscellaneous Electronic Components
Stuff I had in my workshop – stripboard, dual pot, resistors, caps, IC sockets, 7805 power regulator, 1/4″ stereo jack and wire.

I find a nice plastic case with a carry handle that was the perfect size at a surplus shop. For the ports, I used some weird hollow fasteners that were labelled as ‘speaker grill fasteners.’ They are shaped like little cups with a wide brim at the top. I cut off the bottom of the cup, and they make perfect little ports.

I split the stereo signal and send each channel to its own LM386. I wired them up using the 200 times amplification circuit in the datasheet (but added 4.7uF caps from pin 6 to ground). Both channels are controlled using a single dual pot.

***UPDATE*** – The 200 times amplification was too much for most usage. Luckily, it was as simple as cutting the leads on a single capacitor to change the circuit to 20 times amplification. I had to remove the capacitor from both the left and right channel circuits.

Thanks

A huge thanks to EdisonRex from Electro-Music.com who helped me debug a problem. The LM386’s were self oscillating. He helped me trouble shoot it in the electro-music chat room and found out that the problem was that I needed 4.7uF caps from the power pin (pin 6) to ground.

Comments (2)

  1. Ted Mieske

    I have many circuits for the LM386, so I’m asking for your schematic
    so I can add it to my list.

    Reply
  2. Jacob Watters (Post author)

    Hello Ted. The schematic that I used is the 20x Gain schematic in the datasheet. I made two of them (one of each stereo side). I used a dual-gang pot on the input to control the volume of both amps at the same time.

    Reply

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