Portable speaker for MP3 player

I made this portable speaker in the summer. It mixes the stereo input into a mono signal, which is then amplified. It's powered by a six pack of AA batteries. The batteries are held in an external battery pack, so they are easy to change on the fly, without the need of a screwdriver.

Completed speaker pumpin' out some bass

I was going to a cabin with the guys to celebrate juhannus (midsummer) the next day, and there was going to be no electricity. Juhannus without music wasn't an option, and I'm not too impressed by the portable speakers available in stores (overpriced and shit sound). So I built my own blaster that evening.

The circuit is a modified version of the portable guitar amplifier I've built many times. I found the original circuit years ago from the great site Red circuit designs. The circuit is modified to have a simple stereo-to-mono mixer in the input, consisting of resistors R1 and R2. I chose 1k as their value, that should be enough to isolate the left and right channels from each other, since the headphone output on an MP3 player has a low impedance.

There is no volume control, instead I just use the volume control of the MP3 player. If someone wants to add one, just replace R3 with a potentiometer and connect "In+" of the TDA7052 to the middle pin of the potentiometer...

Schematic of the amplifier.

Going through my sizeable e-trash collection I found an old pairless computer speaker. It would get a new life soon.

The victim, a pairless computer speaker.

It was a left speaker, with just a speaker inside and an RCA jack on the back. I was going to use that as the power jack.

An RCA jack on the back was going to be the power input.

I took the metal leg off to access the screws and opened the case. Next I cut the wires off the speaker. There was some foam padding inside, which I removed for a while.

Case opened and wires cut.

I soldered all the amplifier circuit on a small piece of stripboard and then the speaker and the jacks. A single screw hole in the back of the case was used to fasten the thing there.

The amplifier circuit board is fastened to the wall with a screw.

I glued some mouse pad pieces to the most critical places to ensure there's no unwanted vibrations. To top it off I put back the original foam padding.

Some pieces of old mouse pads ensure the wires don't vibrate around.

I put also the original foam padding back to further eliminate vibrations.

Here is the backside of the finished speaker. I sticked an inspirational sticker on the back. The RCA jack on the left is for power and the 3.5mm jack is a stereo line input.

Connectors on the back.

I used some part of an old projector as a vise when gluing the battery pack together. Very handy. Another gem saved from the trash.

Gluing the battery pack together...

The power packs are also 100% recycled: the battery holders and the piece of RCA wire were also found in e-trash.

Finished test battery pack, with random AA's I had at hand.

The speaker was a hit in Juhannus and got very good feedback from it's bassy and clean sound. Just have enough batteries with you - it ate 3 six-packs of AAs during the 3-day trip and would have eaten even more. Of course, the speaker was on all the time. When the batteries are running low the sound starts to get distorted but turning down the volume a bit helps. This way you can also extend the battery life a bit.

That's all folks!

Links
Red free circuit designs -  Mini Portable Guitar Amplifier
My mini guitar amplifier build