Introduction: Pocket Transistor Amplifier

Picture of Pocket Transistor Amplifier

In this instructable I will show you how to make a simple amplifier that can be used for a variety of different things. Because the 8ohm speakers are very quiet, it wasn't worth connecting a 1/4 in. jack to it for my cello pickup or my dads guitar, so I decided to make ipod speakers with it. When I tried directly connecting the ipod to the speakers the signal was very weak, so I decided to connect it to a very simple transistor amplifier. 

Step 1: Materials

Picture of Materials

Materials:
-altoids tin
-small audio transformer (too big makes it sound fuzzy)
-0.1µf ceramic disk capacitor (104)
-2n3904 NPN transistor
-two 8ohm speakers
-battery case for one AAA batttery
-3.5 mm male audio jack
-wire
-SPST switch
-10k potentiometer 

Tools:
-pliers
-metal cutters
-drill
-soldering iron

Step 2: Cut Holes

Picture of Cut Holes

First you need to cut holes in the altoids tin (or other enclosure you are going to use) for the speakers, the jack, switch, and potentiometer.

Step 3: Make the Circuit

Picture of Make the Circuit

Build the circuit according to the schematic. Connect the capacitor to the base of the transistor. connect the wiper pin of the pot to the base of the transistor, and connect one of the outside pins on the pot to the power supply and the other outside pin to ground. connect the emitter to ground, and the collector to one of the outside wires on the side of the transformer that has three wires coming out. connect the other outside wire on the three wire side to  the the power supply. connect both wires of the speaker to the both of the wires on the side of the transformer that has two wires sticking out of it. don't worry about the audio input just yet. (if you are going to use two speakers connect them in parallel.)

Step 4: Connecting the Jack

Picture of Connecting the Jack

When I stripped the wire from the airline head phones, i found one black, one red, and one yellow wires. connect the yellow and red wires. You are going to connect the black wire to ground, and the yellow and red wires to audio in on the circuit from the previous step. Once this is done the circuit is complete.

Step 5: Put Everything in the Tin.

Picture of Put Everything in the Tin.

Cover the bottom of the altoids tin where the electronics are going to sit with electrical tape to prevent short circuits. glue your speakers in their holes, and put the pot in a hole drilled between the speakers (only put the knob through the hole.

Step 6: Listening to It

plug your ipod on, put your battery in the speakers, and play some music. you can adjust the volume with either the potentiometer or the ipod's volume control. If you turn the volume on the ipod up too much the sound will get fuzzy, so keep the volume on the ipod up about three quarters of the way, and adjust the volume with the pot. If you have any problems or questions feel free to leave a comment and I will try to help you out. Have fun!

Comments

systemus (author)2014-05-16

People have asked if some different part can replace the transformer. A 2N3906 or 2N2907 PNP transistor (silicon) could replace the transformer. Such a circuit would require at least a three-volt power supply. Nine volts would be even better. You would also need to provide bias for two transistors instead of one. If anyone is interested, I can go into more detail.

kartikkman (author)systemus2016-10-28

I am interested in making it without the transformer , i want to make it with the help of transistors only . So i just wanted to ask whether the speaker output depend on current or voltage , I mean for connecting two transistor should i use configuration like darlington or cascade connection

ItalianJack (author)systemus2016-05-31

That would be great if you could. I kind of would like to not have to buy materials, and I have 2 each npn and pnp transistors.

NW66 (author)systemus2015-12-16

could you post a schematic of that design?

rah187 (author)2016-08-06

Well, I made it... It sort of works, but I'm thinking I may have screwed up the audio connection and connected the tip to ground and the sleeve to the input instead of the other way around. This may explian the poor sound quality, and I am not hooking my ipod up to this thing again untill I fix it out of fear that my ipod won't like my reversed audio connection.

rah187 (author)2016-07-25

Is the sound quality any good? Also, what is the turns ratio on the transformer?

Mrinal 1 (author)2016-07-12

will this work for a 55 plasma speaker

mufaddalquaid.ghadiyali (author)2014-11-25

Kindly Can someone show me this circuit on breadboard.

This should work.

RahulM7 (author)2015-02-07

Can you give a circuit diagram also with battery to understand the circuit properly

ThomasT7 (author)RahulM72015-03-15

Both parts of the diagram which say 1.5v+ go to the positive battery terminal, and both grounds go to the negative side. The 1.5v+ are at the top of the drawing attached to the potentiometer and transformer, and the grounds are the triangular patterns of lines at the bottom, descending from the transistor collector and the bottom half of the audio jack.

RahulM7 (author)2015-02-07

Can you give a circuit diagram also with battery to understand the circuit properly

krstecd12 (author)2014-12-20

can you give me a circuit for amplifiler with 2-3 NPN transistors, and circuit for fm audio transmitter without trimmer capacitor. Thanks

GabrielP (author)2014-11-04

Connecting the + and ground with a 100 uf capacitor greatly improves the sound quality and volume.

AbdulM2 (author)2014-10-02

hey dear i cant understand the circuit please make it clear that what it ground and where i have to put the batteries ? and can you tell me is it works with an other kind of transformer use to transform current ? like those in mobile charger !! please help

Higgs Boson (author)AbdulM22014-10-05

The emitter of the transistor, one end of the pot, and the ground pin on the audio jack are connected to ground. Put the battery such that positive is V+ and negative is ground. Power transformers like those in phone chargers will probably not work. Your best bet is to buy a small audio transformer from radioshack, digikey, mouser, etc. The exact ratings aren't critical.

AbdulM2 (author)2014-10-02

hey dear i cant understand the circuit please make it clear that what it ground and where i have to put the batteries ? and can you tell me is it works with an other kind of transformer use to transform current ? like those in mobile charger !! please help

AbdulM2 (author)2014-10-02

hey dear i cant understand the circuit please make it clear that what it ground and where i have to put the batteries ? and can you tell me is it works with an other kind of transformer use to transform current ? like those in mobile charger !! please help

AbdulM2 (author)2014-10-02

hey dear i cant understand the circuit please make it clear that what it ground and where i have to put the batteries ? and can you tell me is it works with an other kind of transformer use to transform current ? like those in mobile charger !! please help

AbdulM2 (author)2014-10-02

hey dear i cant understand the circuit please make it clear that what it ground and where i have to put the batteries ? and can you tell me is it works with an other kind of transformer use to transform current ? like those in mobile charger !! please help

AbdulM2 (author)2014-10-02

hey dear i cant understand the circuit please make it clear that what it ground and where i have to put the batteries ? and can you tell me is it works with an other kind of transformer use to transform current ? like those in mobile charger !! please help

systemus (author)2014-05-16

The potentiometer is nice, but optional. You can set volume on the input device. Then substitute a voltage divider for the pot. The divider is a reliable and easy way to bias the 2N3904 transistor. Make the divider this way...

+ Buy a 10K and a 9.1K resistor. These should be quarter-watt parts.

+ Connect a 10K resistor to the battery +.

+ Connect the free end of the 10K to the 9.1K resistor.

+ Also connect this junction to the transistor base and 0.1 uF capacitor.
+ Connect the free end of the 9.1K resistor to ground (- battery terminal).

Higgs Boson (author)systemus2014-05-23

Thanks for the suggestions!

SUBHANKAR DAS (author)2014-04-27

Sir, i have searched all electronic store, but there have not any small audio transformer. I have a new LM386 N-1 IC. may i use it?? if yes. then where i attached it??

when I attached transistor's Base line to speaker's one side and power supply's positive side to speaker's other side, then its work. means sound coming. but very noisy/fuzzy sound also come.

when i give full volume then music sound not coming and fuzzy/noisy sound coming very loudly.

my power source is 10 volt Dc adapter with 500ma,,

please help me that i will success to make this circuit properly...

Thanx a lot for your help..................

Yeah, the transistor will output a signal, but it won't be of very good quality. The point of the transformer was to smooth the signal from the transistor and improve sound quaility. If no local stores carry any, you may want to try ordering it from any mail order electronics store. If you don't want to do that than the 386 will work, and you can look up countless numbers of circuits utilizing it that don't require the transformer. I hope you find something that works for you.

SUBHANKAR DAS (author)2014-04-24

Sir, I am facing a problem after completing this circuit. problem is :-

1) when i power on the battery and select pot as a full volume. then transistor become too hot and a burning smell come out from transistor.

2) there have no sound coming from the speaker.

I have used a nokia mobile battery. 3.7 volt. but other component same as u mention.

pls help me.

Make sure there are no shorts in your circuit. What are you using for your audio in? Too big of a signal may harm your transistor.

thanx for your valuable reply.. i am using " left, right, ground" 3.5 mm audio jack that i have purchased from local store..

i explain details.. if u find any mistake. then tell me......

1)I connect 104 capacitor's one side to audio in positive wire and other side Base of transistor. audio in negative side connect to power supply's ground side.

2) connect pot's wiper pin to base. one side pot's pin connect to power supply's positive side and other to negative/ ground side.

3) I use a small transformer that has one side 4 wire and other side 2 wire. i have take it from a mobile adapter.

4)connect the battery's positive side to transformer's one out side where 4 pin out. and other outer side connect the collector pin. speaker's 2 wires connected to transformer's 2 pin.

5) connect the emitter to battery's negative side.

when i power on my AA 1.5 volt battery. then no sound is coming...

i have tried to make this circuit about 5 times...

pls help me and tell me where is the mistake...

It sounds like you took a transformer from a power adapter. While this may work in some cases, it can often be difficult to get transformers to work in ways they were not originally designed for. I would look for a small audio transformer that you know is meant for this purpose. Also, just because a transformer has more wires on one side doesn't necessarily mean that that side is the secondary. You should always make sure you check the data sheet if you can.

thanx sir for ur reply... does my connection is right???? tomorrow i will buy a audio transformer.. but i want to know that may i correctly connect all component that a told you before????

Thanx in advance... :)

Everything seems right. Let me know if you have anymore problems and I will do my best to help. Good luck.

thanx again for your support sir. I try my best to make this circuit. i love very much to make different type of circuit. But my knowledge is not so good about electronics.. do u suggest me some book ????

There are a lot of good ones out there, but I think anything by Forrest M. Mims is really good. He walks you through everything from using resistors to discrete logic in his books.

SUBHANKAR DAS (author)2014-04-22

Very nice design.may i use a 9 volt battery on this circuit???? and may i use 40 watt 4 ohm speaker on this circuit????? if not. then pls give me the circuit diagram.. i want to make an amplifier on own. pls help me.

This amplifier is not really designed to handle the power coming for a 40 watt speaker your probably going to want a more robust Darlington transistor for that. A 9v would probably work, but may not be necessary.

SUBHANKAR DAS (author)2014-04-25

its not working. no any sound came from my 0.5 watt speaker. when i plug my speaker to 3.5 mm jack without any circuit. then very low sound is coming. but when i plug this circuit with speaker then no any sound come from speaker. also mobile volume automatically goes to zero and song automatically stop after some times later... pls help me....

lukeD (author)2012-02-17

is there a way to NOT use the transformer?

Higgs Boson (author)lukeD2012-02-17

Not with transistor oscillators. It is a major disadvantage of using transistors as amplifiers. If you wanted to not use the transformer, you'd probably want to look at some stuff with opamps. 386 would probably work well.

LazyH (author)Higgs Boson2013-12-09

well, you CAN make a class "a" transistor amp work if you add a few resistors and another capacitor, but the efficiency drops a lot, you can use a combination of npn and pnp transistors for class "b" and "ab" without a transformer but a transformer will generally increase the efficiency by a noticeable amount. For a diy project, understanding the concepts behind how these work, either with or without a transistor would be pretty cool to build, googling transistor amp can give a few more results on the concepts and some sample schematics even. For anything I'd use beyond just experimenting, I'd definitely go for a transformer in my amp.

LionBlood (author)2013-09-05

For the transformer, what do I ask for at the electronics store. I mean you can't just put anything there if you know what I mean???

Higgs Boson (author)LionBlood2013-09-14

Just ask for a small audio transformer. The specs aren't critical, and most audio transformers stores carry are pretty similar.

faziefazie (author)2013-04-18

If I don't used a transformer, what component should be replace it?

Higgs Boson (author)faziefazie2013-04-19

It has to have an inductor if you want it to sound good, so I don't think there really is a very good replacement for it. Sorry.

faziefazie (author)2013-04-18

If I don't used a transformer, what component should be replace it?

budhaztm (author)2012-09-17

Any ideas on turn ratio of the transformer?

agm88 (author)budhaztm2012-11-16

three wire side: 1000 ohm center wire is called center tap which is not used in this project.
two wire side: 8 ohm pretty simple
higgs if igot that wrong tell me








Higgs Boson (author)agm882012-12-21

Sounds about right.

Higgs Boson (author)Higgs Boson2012-12-21

Although that sounds like the impedance not the turn ratio. I am not sure what the turn ratio is.

agm88 (author)Higgs Boson2012-12-25

Me neither I onlyknow the impedence not exactly the turn ratio either

saba fazal (author)2012-12-21

i hve made this amplifier but its voice is low............and its potentiometer is not working.....what should i do??

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Bio: Science is my passion. I find myself constantly working on countless experiments, from low energy particle accelerators to good old simple electronics. I also like ... More »
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