MonoBox Powered Speaker

By Ross Hershberger
Category: Electronics, Music
Time Required: 6 hours
Difficulty: Difficult

MonoBox is a small, inexpensive powered speaker that amplifies the output of your headphone music player. It’s little but it’s loud! All the circuit parts are available from RadioShack. The speaker and cabinet are left to your preference.

You’ll learn how to assemble and solder an audio power amplifier using an integrated circuit (IC) chip, and how to choose a speaker and install it in a cabinet with the amplifier.

The core of MonoBox is a compact and efficient audio amplifier based on the LM386 power amp chip. It will run on 200mA of current using power supplies from 6V–15V DC. This gives you the flexibility to power it from a wall adapter, a 9V battery, or a car accessory outlet.

You’re probably thinking, “Sure, but it’s so small. Does it rock?” Fair question. The prototype has been exhaustively tested and it does indeed rock. Maximum volume output is 90dB, and with the added bass boost your socks will be rocked clean off!

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Steps

Step #1: Select your cabinet.

There are many options for housing your MonoBox. I've used nice old wooden cigar boxes; you could also use a lunch box or small toolbox. Choose something between 1/8 and 1/4 cubic foot (equal to 6"×6"×6" and 6"×6"×12", respectively). Make sure it's deep enough for your desired speaker, and has one surface suitable to mount your speaker on.
NOTE: A box that can be tightly sealed against air leaks will provide the best bass sound.
Wood and plastic are good cabinet materials, as they're easy to work. Metal is more challenging. Almost anything relatively rigid can be used. Construct a box from cardboard in any shape you like, and cover it with colored duct tape!
RadioShack sells two project boxes that are about the right size. They're made of black ABS plastic that's very easy to work. They measure 7"×5"×3" for model 270-1807 (shown here), or 8"×6"×3" for model 270-1809.

Conclusion

Going Further

What if you really want stereo? Make 2 MonoBoxes! In each, connect only one channel (left or right) of the input jack directly to the amplifier circuit board, and use a SPST switch to connect or disconnect the other channel, so each box has its own mono (or 1-channel) selector switch.

Set both boxes to 1-channel, then use a stereo headphone splitter to connect both MonoBoxes to your music player.

65 Responses to MonoBox Powered Speaker

Peter123 on December 1st, 2012 at 2:45 am said:

To power the speaker you could also have a solar panel on top so during the day you can switch from battery power to solar power by just adding a switch.

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fontmark on December 2nd, 2012 at 1:43 am said:

Found the diagram in the PDF.

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Nick Normal on December 3rd, 2012 at 8:31 pm said:

hi saulx350z, the diagram is one Page 2 of the project: http://makeprojects.com/Project/MonoBox+Powered+Speaker/2396/2 – it’s Step 15, or, if you can’t see that for some reason, here is the image: http://guide-images.makeprojects.org/igi/X3WB2Tny1O2xSlQb.large – hope this is what you were looking for.

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Ross Hershberger on December 4th, 2012 at 2:20 am said:

Volume: I measured peaks 90dbs at 1 meter distance before distortion set in. This was with an 8 Ohm speaker of mid/high efficiency. Not enough to throw a disco party but way above a typical computer speaker. I recommend a wall-wart power supply rather than a 9V battery for extended high volumes. Uses a lot of current.

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Ross Hershberger on December 4th, 2012 at 2:24 am said:

It depends on the speaker. A larger speaker may have reduced high frequency response unless it’s designed as a full-range driver. If you’re unsure I’d say build the amp circuit alone and try it with that speaker mounted on cardboard as described in the article. if you don’t like it you can change to a different speaker for the final assembly.

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Ross Hershberger on December 4th, 2012 at 2:25 am said:

I wish I could give a full answer. Do you mean it plays at low levels but stops completely when the volume is turned up?

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Ross Hershberger on December 4th, 2012 at 2:29 am said:

Note that a number of the Radio Shack parts like the circuit board and some of the resistors come 2 or more to a pack so you won’t have to duplicate the whole list when making 2 amps.

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ambrosehone on December 8th, 2012 at 12:46 pm said:

Also, I guess I would also need to adapt the power for Australian sockets.

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ambrosehone on December 8th, 2012 at 1:02 pm said:

*Complete Battery Overkill*
Also, I guess I would also need to adapt the power for Australian sockets. Hmm…..

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Ross Hershberger on December 8th, 2012 at 2:09 pm said:

Stereo is covered at the end of the article. The LM386 amp chip has only one channel of amplification. The easiest way to go stereo is to build two of them. You have to make a small modification to the circuit at the input jack because the stock mono circuit connects right and left together. You would want one built with only the right channel connected and the other built with the left connected. Option: install a switch at the jack so you could switch to mono operation by connecting in the other unused channel.

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tripsofjacks on December 10th, 2012 at 6:39 am said:

I went into radio shack today and they had no problem helping me find all the items on the list. I’m going to build this over the next couple days. I found this on flipboard and thought it sounded too awesome not to try… I have never done a project like this, but your instructions appear well done. I will post on the outcome when I am done. Thanks for sharing this project.

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tripsofjacks on December 12th, 2012 at 5:38 am said:

My project is a sucsess, my amp is working well, and is really cool!!! Much thanks to the author for sharing these awesome and easy to follow plans!!!

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Ross Hershberger on December 16th, 2012 at 2:27 am said:

Note from the author here. I’m abruptly out of the country for work and have unreliable internet. Apologies for the lack of replies. I will read all responses and answer as soon as I’ve sorted out certain industrial laser issues and made my back north of the border. Thanks. Ross Hershberger

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- rkan665 on March 13th, 2013 at 10:25 pm said:
  
  so, when i plug in my amp via 9V battery or the enercell adapter 9V(dc) it makes a popping noise.. can anyone explaing that? regards, ryan
  
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Michael Lewis on January 5th, 2013 at 2:50 pm said:

Looks like I’m going to answer my own posting: The author clearly intended to indicate by use of a triangular symbol that the LM 386 is the IC in the circuit. However, by adding the + and – signs to the input leads, he created the symbol of an op amp. This is a mistake that only a person with limited electronic knowledge would notice, since the square wave output of an op amp makes it clearly unusable as an audio amplifier.

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Scott W Vincent on January 6th, 2013 at 2:51 am said:

Hauke, you need a log (audio) pot for audio signals. I don’t what exactly would be best for this circuit, but it seems that values between 10k and 50k are most common these days, though I’ve seen some amps that use 100k pots.

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Scott W Vincent on January 6th, 2013 at 2:57 am said:

That is very nice! I like how the lion artwork surrounds the speaker, looks great!

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Fergal on January 9th, 2013 at 6:36 pm said:

Tim – Vocals are normally in the centre of a stereo recording. This means that they are present in exact same amounts on the left and right channels. Instruments tend to be placed more on one side than the other.

When making a mono speaker like this, the left and right signals get summed together. However, if at some stage during the construction, the polarity of either the left or the right channels is inverted, then anything in the centre, like the vocals, will get lost. This is because the vocals are now present as exact opposite signals on the left and right, and perfectly cancel out when summed.

This phenomenon doesn’t affect the instrumentation as they tend to be in different amounts on the left and right, and so do not cancel out when one side is subtracted from the other.

I can’t help you with regards the specifics of the circuit (I haven’t looked in detail at it yet), but a good place to start would be to look at the wiring of sockets and jacks – have you swapped a shield and a tip, for example?

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Arnoldas Gribas on January 13th, 2013 at 12:14 am said:

setup would be the same
yes it would be capable powering a bit more powerful driver, but don’t expect much because its only 1W

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Arnoldas Gribas on January 13th, 2013 at 12:22 am said:

very nicely done
distortion is boosted by increasing gain?

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Hauke on January 13th, 2013 at 11:28 am said:

i made a stereo version because i had enough parts left: https://www.dropbox.com/s/fqv7p5zpogspep2/lm386-stereo.jpg

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- Pete on April 5th, 2013 at 7:37 pm said:
  
  Is there a schematic for the stero circui? I built the mono curcuit and am building a second but I dont know how to connect the two for stereo.
  
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  Zorro on April 29th, 2013 at 8:01 am said:
  
  build two circuits. stereo line in has ground, right and left channel. use ground for both and left for one circuit and right for the other. the original version has left and right together in one circuit.
  
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  Pete on April 30th, 2013 at 10:51 am said:
  
  Awesome. Worked great. My only issue is that the volume is half as loud in one speaker. I swapped speakers ad determined it is the circuit. Any immediate ideas. I’m assuming it is my craftsmanship to blame. I’ll probably have to make a new circuit.
  
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  Havok on May 4th, 2013 at 10:17 pm said:
  
  Really want to build a stereo Do this :
  Make 2 monoboxes In each, connect only one channel (left or right) of the input jack directly to the amplifier circuit board, and use a SPST switch to connect or disconnect the other channel, so each box has its own mono (or 1-channel) selector switch.
  
  Set both boxes to 1-channel, then use a stereo headphone splitter to connect both MonoBoxes to your music player.
  (I got this from the down of the article!)
  
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Scott W Vincent on January 13th, 2013 at 2:25 pm said:

Very nice! The pot is a great addition. I’m thinking of doing another some day with an option for line in level sources with a volume control.

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Scott W Vincent on January 13th, 2013 at 2:26 pm said:

That’s interesting – thanks for sharing. I might try the 1 watt chip on my next build.

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Ross Hershberger on January 26th, 2013 at 2:24 am said:

Check your email inbox.

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Jakub Czarnecki on January 26th, 2013 at 5:39 am said:

I would say check that there is no short and make sure the chip is not in upside down

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Hauke on January 28th, 2013 at 1:29 pm said:

yes

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Ross Hershberger on February 8th, 2013 at 5:43 pm said:

I’ve checked with Parts Express and they have the $0.98 oval speaker tha twe used in some of the prototypes still in stock.

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Ross Hershberger on February 8th, 2013 at 5:44 pm said:

Capacitor voltages are not critical, as long as they are at least 50% higher than the power supply that you’re using.

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Ross Hershberger on February 8th, 2013 at 5:45 pm said:

The bass boost in the circuit can exceed the voltage swing capability of the amplifier. If you get distortion on bassy music consider using the EQ on your music player to turn the bass down. Or refer to the article and disable/remove the bass boost parts.

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Ross Hershberger on February 8th, 2013 at 5:54 pm said:

We found in testing that some portable headphone players delivered a lot of noise when loaded with a high impedance. They need a lot of current drawn to shunt out and damp the high frequency hash in particular. That’s why the 100 Ohm input resistor. Terminating the headphone output with a higher resistance will result in higher noise on some source devices.

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Ross Hershberger on February 8th, 2013 at 5:55 pm said:

Builder’s issues solved via email. He built it right, but had the worng IC chip. See his further comments below.

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- rkan665 on March 13th, 2013 at 10:26 pm said:
  
  can you help me?
  
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Alec Spicer on March 13th, 2013 at 9:31 am said:

I soldered, drilled the box, and the whole nine yards. I finally hooked up the power to see if it works but there is no audio. All I’m getting is a little bit of noise when I hook the power chord into the power jack. Ask me questions about the problem so I could give more detail and so I can get this awesome project to work.

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- Jake T on March 29th, 2013 at 2:21 pm said:
  
  I’m having the same problem. I’ve double checked all my work but everything looks like it’s in place. The IC chip I bought the LM386N from radio shack. I don’t know if that might be a problem.
  
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  Havok on May 18th, 2013 at 6:22 am said:
  
  Your IC might be upside down. (I think.)
  
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praveen on March 13th, 2013 at 11:05 am said:

I think its very helpful for me .

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jerry on March 24th, 2013 at 12:51 pm said:

I want to have the option of my box being powered by a battery(in the box) and a wall plug. I know i’ve seen the instructuions here somewhere. can someone point me in the right direction?

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Phil O on March 26th, 2013 at 9:59 am said:

Awesome guide! I’m using a Nightfire Electronics LM386 kit I bought for $10 from Amazon. Saved me a little trouble assembling all the components. One comment on the circuit: looks like you bridged the input signals without resistors. I’ve read that this causes distortion and can even damage the source, because the two signals are essentially fighting to set “their” voltage on the same wire. It’s a good idea to run each signal through a 10k resistor before connecting them. But it seems that lots of people are following these instructions with no complaints, so maybe it’s not that important?

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Pete on April 4th, 2013 at 8:14 pm said:

i did it. Awesome. powerful

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gary green on April 7th, 2013 at 8:48 pm said:

Where Can I add a pot so I can have volume control?

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- jeromymurphy on April 24th, 2013 at 7:09 am said:
  
  Can you just use the volume control on your mp3 player?
  
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- Zorro on April 29th, 2013 at 7:57 am said:
  
  Ad pot at audio line in.
  
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jeromymurphy on April 24th, 2013 at 7:08 am said:

I had a great time building this. Great project for an amateur. My radio shack was sold out of the 8 pin connector so I used a 14 pin instead. Same cost, I just had to be careful what I plugged into and soldered. I used a speaker driver scavenged from an old Samsung surround sound. The speaker was flush with the face of the box so I used the metal lid from a canning jar with fabric glued in to finish it out.

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Havok on April 24th, 2013 at 7:30 am said:

Is it necessary to use an ic pc board? Reply!

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bonsai171 on April 26th, 2013 at 4:37 pm said:

Can this be modified to make a preamp and drive an amp instead? How hard would that be?

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Havok on April 26th, 2013 at 11:59 pm said:

Hi in the schematic there are two triangles pointing down so, i am a newbie to this and schematics. so can you help me?

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- bonsai171 on April 27th, 2013 at 3:09 am said:
  
  Those two arrows are ground connections.
  
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  Havok on April 30th, 2013 at 10:00 am said:
  
  Thanks bonsai171, But there is only one positive connection.
  
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  bonsai171 on May 1st, 2013 at 3:29 am said:
  
  Ground is negative
  
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  Havok on May 2nd, 2013 at 8:48 am said:
  
  But there are 2 ground connections and 1 positive connections.Right?
bigredlevy on April 27th, 2013 at 11:30 pm said:

I made a veroboard layout:
http://2.bp.blogspot.com/-NnoOf5NSBOU/UXzBR6jy68I/AAAAAAAAAVQ/-4bTuU2QCM4/s1600/MAKE+Monobox+(2).jpg

my build:
http://levysounddesign.blogspot.com.au/2013/04/make-monobox-trash-speaker.html

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Havok on April 30th, 2013 at 8:28 am said:

Thanks bonsai171, But there is only one positive connection.

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Havok on May 4th, 2013 at 11:45 am said:

Hey, Ross there are 5 caps in the photos but in schematic there are 4 caps. But how?

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- Pete on May 4th, 2013 at 2:17 pm said:
  
  Two of the caps “stack” together to get a higher value.
  
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Havok on May 4th, 2013 at 10:14 pm said:

Thanks, Pete that helped a lot.

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SkipF on May 5th, 2013 at 4:26 am said:

I found an electronic crossover PCB via ebay in china…And I’m using a pair
of aluminum 2 way speakers; each speaker has it’s own amp. Power is a bit
funky. 2 18650 lithium batteries for the power, plus 2 14500 LBatteries for the
negative supply of the Xover.

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Havok on May 5th, 2013 at 6:53 am said:

Is it necessary to use an ic pc board?

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SkipF on May 5th, 2013 at 11:29 am said:

No… but it’s a whole lot easier. I used to build prototype cameras @Polaroid. Once we
got BOARDS, they worked a lot better than the WWrap or P2Point vector boards we originally
modeled the circuits with. 2 boards for $13.00…

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Tommy on May 6th, 2013 at 10:43 am said:

This is a cool project. I actually used a circuit similar to this to build a guitar amplifier into a cigar box (same workhorse LM386 and all). Only difference is that you would use a pot to control the gain. For any guitar players out there, you can very easily modify this exact same build (just sub out a couple of parts) to get a sweet-looking portable guitar amp. The following link shows the schematic (though it’s not my site or my work).

https://lh3.googleusercontent.com/-X8sUApMtOhM/UNEB4o8zwOI/AAAAAAAAArY/oQHvUsvDA2U/w500/amp-diagram.png

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Doug on May 8th, 2013 at 7:43 am said:

I used a pvc pipe to house everything! its great

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Havok on May 15th, 2013 at 10:37 am said:

Can i use a 4 0hm 100watt speaker driver? Will it run on a 9 volt battery ?

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