Intro: Pocket Stereo Amplifier
a while a go i came across this instructable: Rechargable pocket sized amplifier
and loved the idea of a pocket sized audio amp but was disappointed but its lack of capability, namely only using mono amplification and only having 1 output option
hence the idea for the stereo amplifier was born
doing a little digging on amplifier IC's I found the LM 386 a mono amplifier ic with some extra capability's like bass boost and 200 times gain settings (but more on those later)
So how do you take a Mono amplifier and make a stereo? easy, stack 2 side by side
this amp also contains 3 output modes: internal speakers, external 3.5mm jack and RCA
Step 1: Tools and Materials
Materials and components:
2 - LM386 (mono-audio amp)
2 - 8 pin ic holder
2 - 3.5 mm audio socket
2 - speakers
2 - 2 pole 2 throw switch
2 - 33nF capacitors
2 - 47nF capacitors
2 - 10µF capacitors
2 - 220µF capacitors
2 - 10Ω resistors (colour bands brown, black, black)
2 - 10KΩ resistors (colour bands brown, black, orange)
1 - double RCA socket
1 - 2 gang linear 10k potentiometer
1 - potentiometer knob
1 - 2 pole 3 throw switch
1 - 1 pole 2 throw switch
1 - 9V battery
1 - 9V battery clip
1 - ultra mini experimenters board or other small perf board
1 - project box
- mutli- coloured single strand copper wire, (not shown)
- masking tape (not shown)
2 - screws (not shown)
- soldering iron
- hot glue gun
- solder sucker
- wire cutters
- screw driver set
- dremel or a drill
- ruler (not shown)
- sandpaper (not shown)
- glue (not shown)
Step 2: Design
In this step i am going to discuss how i designed the amp, if you don't plan on designing similar circuits please read the important notes, print the circuit diagram and move onto the build
due to software limitations i was unable to merge the 2 pole switches, so note the 3 throw switch above both amps is the 2 separate poles of the same switch, this switch controls base boost and gain so it is important both amps get the same setting at the same time
to avoid cluttering the diagram i have avoided hooking all the ground points together, although in the finished circuit they will all be joined
designing circuits based on an IC such as the LM386 starts with the datasheet
the LM386 datasheet can be found here
datasheets contain lots of useful information, such as internal schematics, operation conditions and performance characteristics
however by far the most useful is the application hints and the typical application diagrams
it is from the typical application diagrams that this project really came to life.
while gazing at the standard gain =20 amplifier circuit (image 2) i realized that the only difference between it and the gain =200 circuit (image 3) and the bass boost circuit (image 4) was a change in the way pin 1 was connected to the rest of the board.
by this stage i was already contemplating multiple outputs (and hence multiple switches) and decided one more switch could not hurt.
so i merged all 3 circuits and the amplifiers final form was born
A FEW NOTES:
it was only after building the amp that i discovered the bass boost feature works by reducing the amplification of the higher frequency (the singing, instruments and such) and as such a signal without a lot of bass played with the bass boost just sounds softer then the standard gain setting, or the max gain setting.
the max gain setting can play havoc with cheap, small travel speakers (such as the ones i put into my amp)
Step 3: Designing a Face Plate
measure all your components for length, width and hole size
make sure to take into account the room required for a components leads (if they are not back facing)
my measurements where:
box - 3cmx6.5cm (this gives the outer edges of the plate)
on/off switch - 0.8cmx0.5cm with a 0.5 cm hole (this is the 1 pole 2 throw switch)
in/out switch - 0.9cmx0.8cm with a 0.5 cm hole (one of the 2 pole 2 throw switches)
3.5mm/RCA switch - 0.9cmx0.8cm with a 0.5 cm hole (the other 2 pole 2 throw switche)
potentiometer - 1.2cmx1.0cm with a 0.6 cm hole
base/g=20/g=200 - 1.3cmx1.2cm with a 0.6 cm hole (the 2 pole 3 throw switch)
then using an illustration program witch allows you to give parameters to your shapes
construct each of the switches and the potentiometer in the program and set them up any why you like. (image 3)
I set my pate up in a kind of left to right fashion, on/off->input->pot->bass/20/200->internal speakers/ external->3.5mm/RCA->output 3.5mm jack
once everything is in position and set remove the switch plates and leave the positions for the holes (image 4)
mark what each switch is used for and print out
Step 4: Cutting Holes for the Face Plate
sand down the front panel of the box (image 1)
cut out and glue on the paper face plate (image 2)
use a drill or dremel to cut all the holes needed (image 3)
remembering that some switches and potentiometers have locating nodules which need cuts on the inside of the box to keep them straight and positioned during use
on the back of the box, add a strip of masking tape (image 4)
line up and mark the location of the RCA pugs
cut holes for the back of the plugs (image 5)
place the RCA plate in the holes and drill holes for the screws to hold it in place (image 6)
Step 5: Test Fitting
to make sure all of you components are going to fit
place in all switches and face plate components (bottom to top left to right when the box is open end up, face plate facing away)
next hold the RCA plate in and fit the speakers and battery (image 2)
finally try to fit the circuit board if it does not fit (like mine) (image 3)
use wire cutters and sand paper to bring it down to size (image 4)
Step 6: Speaker Placement
Now that everything has a place its time to cut the holes which will allow the speakers to be heard outside the box.
Run a strip of masking tape to the top of the box and place the speakers in there place.
Next draw around them with a permanent marker to create guides (image 1).
Remove the speakers and divide each circle in to quadrants draw 7-8 dots these are the drill locations (image 2).
Being as neat and orderly as possible drill out all marked holes (image 3).
Finally remove the masking tape and deburr (image 4).
Step 7: How Components Work
this is just a quick run through of how components work to aid in the soldering which is about to come.
If you are use to electronic components, how they work and how the pins are numbered feel free to move on.
Switches take a power or signal in through the middle pin and connect it to the pin the switch is pointing at
ie in image 1 the bottom of the switch toggle is pointing at the right hand pin hence the right hand pin is connected.
in image 2 he bottom of the switch toggle is pointing at the left hand pin so despite the toggle being "right" the right hand pin is not connected.
potentiometers or variable resisters if you move the knob on the potentiometer counter clock wise as far as it will go the movable arm attached to the middle pin is closest to the left pin and farthest away from the right pin, hence (read from the middle pin) the left pin has low resistance and the right pin has the pots rated resistance(image 3). conversely if you move the knob all the way clock wise the left pin will read rated resistance and the right pin will show very little (image 4). In this circuit set up maximum volume occurs when there is minimal resistance between the middle pin (output) and the signal.
hence due to the orientation of my potentiometer my signals are connected to the right hand side terminals of my potentiometer
IC chip pins are numbered as shown in (image 5)
image 5 sourced from http://en.wikipedia.org/wiki/Dual_in-line_package
Step 8: Component Placement
This step is a bit more personal preference then rule.
I usually place all components in before starting to solder.
This is for a number of reasons but mostly efficiency and sanity checking (for example the capacitor i put in backwards (image 4)).
place the IC holders into about the middle of the board you are going to need space either side of them.
As a side note soldering is why I use IC holders, its very easy to over heat and damage an IC when soldering it directly to the board where as it is almost impossible to damage it by soldering in a holder first.
next wrap the power cable through holes in the circuit board. if you board doesn't have holes it is a good idea to drill your own as this prevents any battery movements from destroying your solder joints. (image 1 and 2)
once those are in place fold the legs down on the IC holders to hold them in place and similarly put a kink in the power leads
all the on board components for this build are connected into pins 5 and 8 (the first and last pins of the right hand side) (image 3)
to connect all components into pin 5 bend the leg of the 220µF back towards pin 5, when soldering solder this to each pad it crosses (image 4)
Step 9: SOLDER!!!
the most important part of this step is colour coding your wiring.
I used yellow and white wires for the left hand side amp and blue for the right hand side.
soldering is best done in stages:
Start with the RCA plate and solder both signal and ground wires in.(image 1)
Then solder up the single switches (image 2-3).
Next solder up the output switches (the 2 pole 2 throw) leaving the RCA plugs un-soldered
to solder these up also solder the out put 3.5mm jack (image 4-5) and the speakers(image 6)
Wire up the input, which involves the potentiometer and the input 3.5mm audio jack (image 7)
once all these are soldered as per the circuit diagram CHECK COLOUR CODING!!!!! this is the most important part of this step make 100% sure that all colour codes are correct and that you have not changed the colour on any switch or component.
Place each component in its spot and trim the wires to length.
Once again being careful with your colour code, solder the wires and components into the circuit board (image 8)
once all wires and components are in, find a few legs cut off components and use them to create wire bridges for connecting pin 6 to power, and pins 2 and 4 to ground
Step 10: Test!
This step is quite important.
Despite not having the RCA soldered in the Amp should now work.
So plug the LM386's into place, add a battery and a music source
hopefully it now plays music! if not go back and check the circuit diagram, trace each signals path and try to find the fault
once it is working
double check that the switch positions line up with with your face plate, if not, you have two options re-do your faceplate or re-wire the switches, either way will work its purely an aesthetic choice.
Step 11: Final Solder Joints
Once the circuit is working and the switches have been checked it is time to solder in the rear RCA plugs.
To do this place the wires through the holes in the box and solder into their correct positions (image 1)
Step 12: The Final Pack
the Amp is now fully electrically operational all that is left is to secure it into the box.
pack the front panel components as before (botttom to top, right to left this time) applying hot glue as you go to hold them all in place
glue the speakers into place the add the battery and finally the circuit board.
Step 13: Operation and Notes
the amp if now finished!!!
Time to enjoy the music!
just to reiterate a few notes from the design step
the bass boost feature works by reducing the amplification of the higher frequency (the singing, instruments and such) and as such a signal without a lot of bass played with the bass boost just sounds softer then the standard gain setting, this could foreseeably be fixed by using both 200 gain setting and bass boost at the same time,
the max gain setting can play havoc with cheap, small travel speakers (such as the ones i put into my amp) but works really well on larger better quality speakers
Third Prize in the
Pocket Sized Electronics
Finalist in the
Second Prize in the
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Second Prize in the
DIY Soundhack Contest