After painting my 2.1 subwoofer amplifier music system, I decided to paint the older boombox. I had created my 5-volt tablet-battery-powered dual-LM386 boombox stereo speaker system last year but it has remained unpainted. Recently, I fixed a long-standing problem with the speaker selector switch and added a LED sound level indicator. I decided the build could now be considered stable. It was time to paint.
As it was made of real wood (not plywood or particle board), I decided to apply a layer of shellac, rather than paint. This would highlight the natural contours on the wood and not be obscured by paint. Other options for shellac were polyurethane, wax+linseed oil combo, and lacquer. When I was a kid, I had seen a carpenter applying shellac on a door and so I knew how to do it. I decided to go with shellac this time.
Before applying the shellac, I had to choose a stain (color layer). The first store had red-brown, teak, charcoal and walnut. I assumed teak and charcoal were dark. I had no idea what walnut was. They had had no color swatch on paper for me to select the color from. The bottle had no color label either. I wanted it to look like the stock of an AK-47 but did not know how to describe or identify it with a name. At the second shop, they had yellow.
I sanded the wood with 120 silicon carbide paper. I have more nicer paper but I did not use those. I applied the yellow stain all over the wooden box and rubbed the excess off with a cloth. The red stain, which I tested on the bottom was too dark and I did not use it. I could have diluted it but did not know how. After five minutes, I applied a coat of shellac (Sheenlac) with a brush. I then waited ten minutes and applied another coat. There were some areas where the shellac was uneven. I ran them over the next day. Under some angles of light, my shoddy workmanship is visible but from a distance nobody is wiser.
The speaker used to look look like this.
LM386 is a mono amplifier IC. I couldn’t find a stereo version of it that was just as robust and simple. So, I designed a circuit with two LM386 chips. That was several months ago.
In the meantime, my BlackBerry Playbook refused to charge. Because of disuse, the battery deep-discharged. There are many videos online offering all kinds of solutions but none worked for me. BlackBerry decided, in their infinite wisdom, to control the charging process using a novel logic, which absolutely refused to charge the battery if the voltage went below some limit. After dismantling the tablet beyond repair, I realized that I could have saved the tablet by opening the plastic cover & directly charge the terminals.
Well… I then had two good tablet batteries and a broken tablet. I decided use it for something. These were high-capacity batteries but like phone batteries output only 3.7 volts.
I got a phone battery charging circuit board that took 5 volts in. It has two LEDs – charging & fully charged. The board also has two load terminals from which you could draw 3.7 volts just like a phone would.
My MP3/FM module requires 5 volts so I added a voltage boost (step up) circuit board.
My attempts to repurpose USB cables from discarded devices failed because they burned off internally during soldering. Hence, I fashioned my own USB cable.
I made a wooden housing for the player. The speakers have neodymium magnets and sound super loud. This is interesting because the stereo LM386 amplifier circuit runs off the same 5 volts. The MP3 module has FM, SD card, line in & USB. It is the same kind as those cheap Chinese FM/MP3 amplifier sets found in autorickshaws.
Add a 1000 uF polarised capacitor between the positive and negative rails to eliminate noise that occurs when the voltage source is from an AC-DC adapter.
The woofer is from an old 2.1 Philips PC desktop speaker. The speaker cone was replaced recently. The FM antenna is just a wire.
The mid & tweeter speakers on the rear are new. They are full-range speakers, not really mid-range tweeter speakers. Also, on the rear is a 2.1 mm DC plug socket.
The wood enclosure is cheap plywood on the front & back. Other four sides are MT boards.
The MP3 panel (Rs. 300) is the best part. It works splendidly – MP3 (USB disc & MicroSD card), radio and line in.
LM386-based circuit provides 20x amplification. A 3-way crossoverall board splits the audio to the speakers.
As operating voltage is 5 to 35v, anything from a PC USB or a mobile power bank or common mains power adapters can run the device.
Sound output is excellent with powerline noise eliminated with a 1000uf capacitor on the LM386 circuit.
I got lots of speakers, media players and amplifiers. I have two amplifiers in development. So, I moved my original LM386 DIY Boom Box to the kitchen where it is powered by a AC adapter. To cut the power when not in use, I have added a hanging bell switch. I had to slice the cable to add the switch. As I could not find a stand for the entire thing, I built a nylon fibre rope hammock for it.
A recurring theme in Tim Allen’s show-in-a-show Tool Time (Home Improvement) was MORE POWER. Tim Allen used to take regular devices and boost them up with more high-rated components. My last two DIY projects are like that.
A website was selling a portable USB speaker at a discounted price of Rs. 53. (The delivered piece does not look like the photo on the website.) I was not successful in applying the discount but a co-worker managed to do so. However, he was disappointed to find that that it used a puny speaker with almost no power in its output. I replaced the original speaker with a regular better-rated one. The circuit board uses a PAM 8002 speaker amplifier IC. It is originally meant for cell phone speakers but it is able to drive this speaker as well, but not as great as the PAM 8403.
I got this car radio replacement module mp3 media player but it did not come with a car. So, I used a pull box to provide housing for the mp3 player. The result is a Mini Boombox MP3 Player and FM Radio. The speakers are powered by a single mono LM386 IC 20x-gain amplifier minimal circuit.
I have a modded phone charger with a 2.1mm DC plug but I also created a USB-to-DC cable to power the boombox from a PC USB port. The module is rated at 12 volts but it can boot up at 5v from a USB port.
I can also power the boombox with a 9v battery. For that, I have created a modded battery clip with a you-guessed-it 2.1mm DC plug.
The boombox can play MP3s from SD cards, pen drives or any storage device with a USB port. It also plays FM radio. (I fashioned an aerial out of the antenna of an old RC toy remote.) The mp3 module had a line input connector and I added a 3.5 mm stereo plug for plugging in an external audio source such as a phone or a tablet.
The speaker parts that I had used are not the best or the loudest that I have. I had to settle for smaller ones that would fit inside the pull box. Still, they are quite loud. (If I had used a cap on pins 1 and 8 of the LM386 IC, I would have had 200x gain!) The MP3 player module provides very good pre-amplified sound and I did not want do meddle with with it. The car radio module cost me around 500; the pull box was 70 and rest of the parts (accessories not included) should have been less than 100.
UPDATE 1 (20-October-2013)
Front Panel Stereo Input/Output Connectors
I went to Chennai and got 3.5mm stereo panel jack connectors. I added two of them – one for audio input and another for output. I also added a switch so that the speakers are silent when the amplified signal is sent out.
UPDATE 2 (23 October 2013)
Now that I have a line input, I decide to create a powered mike with an electret microphone.
When combined with my 200x-gain LM386 speaker-in-a-box, the microphone becomes extra sensitive and can be held at a foot or two away for regular use.
I had made an LM386-based amplified speaker before. The sound quality was atrocious and I switched to the silky smooth PAM8403.
PAM8403-based amplfier boards had a limitation – 5 volts. No more or else the chip burns.
The LM386 has no such limitation. In fact, its sound output of an LM386 IC is directly proportional to the voltage supplied. It can be used to power very big speakers. So, I went back to that IC.
I ruined a few PCB boards before I realized that the solder by themselves will not form perfect connections on the back of the boards. The correct way is apparently to make connections using single-strand wire or the wires that form the body of resistors and capacitors.
My implementation is a text-book or datasheet version of 200x gain audio amplifier. Apart from that, I added a ceramic capacitor to eliminate noise from AC power sources and also a LED over the power input.
However, there is still noise from the audio signal input if a sound source is NOT connected. This happens only when power is flowing in. It is totally eliminated once I connect a sound source. Adding a ceramic capacitor eliminates the noise when power is flowing in but it introduces new noise when sound is flowing it. I think I need to introduce the capacitor on pins 3 and 4, instead of shorting it over the stereo input. I will do that later.
I also got a good box from the stationery section of a departmental store. I cut a square hole at the top and attached the speaker below it. The open hole is covered with umbrella cloth. I could not seal the hole from below because most glues do not form a perfect bond with the plastic material with which this box is made from.
Finally, my LM386 speaker sounds loud, suffers no noise, looks good, and is very compact and portable.