Instructables
Picture of Pocket transistor Amplifier
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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. 
 
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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 3: Make the circuit

Picture of Make the circuit
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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.)
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systemus3 months ago

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)  systemus3 months ago

Thanks for the suggestions!

systemus3 months ago

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.

SUBHANKAR DAS4 months ago

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..................

Higgs Boson (author)  SUBHANKAR DAS4 months ago

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 DAS4 months ago

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.

Higgs Boson (author)  SUBHANKAR DAS4 months ago

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...

Higgs Boson (author)  SUBHANKAR DAS4 months ago

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... :)

Higgs Boson (author)  SUBHANKAR DAS4 months ago

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 ????

Higgs Boson (author)  SUBHANKAR DAS4 months ago

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 DAS4 months ago

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.

Higgs Boson (author)  SUBHANKAR DAS4 months ago

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 DAS4 months ago

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....

lukeD2 years ago
is there a way to NOT use the transformer?
Higgs Boson (author)  lukeD2 years ago
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 Higgs Boson8 months ago
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.
LionBlood12 months ago
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)  LionBlood11 months ago
Just ask for a small audio transformer. The specs aren't critical, and most audio transformers stores carry are pretty similar.
faziefazie1 year ago
If I don't used a transformer, what component should be replace it?
Higgs Boson (author)  faziefazie1 year ago
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.
faziefazie1 year ago
If I don't used a transformer, what component should be replace it?
budhaztm1 year ago
Any ideas on turn ratio of the transformer?
agm88 budhaztm1 year ago
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)  agm881 year ago
Sounds about right.
Higgs Boson (author)  Higgs Boson1 year ago
Although that sounds like the impedance not the turn ratio. I am not sure what the turn ratio is.
Me neither I onlyknow the impedence not exactly the turn ratio either
saba fazal1 year ago
i hve made this amplifier but its voice is low............and its potentiometer is not working.....what should i do??
Higgs Boson (author)  saba fazal1 year ago
A few things.
1. Check your circuit for any mistakes, shorts or missing connections
2. Check the wattage of your speakers. This amp is very low power so almost anything will work but below a quarter watt you may possibly run into clipping, and something that is higher (in the area of a few watts) may require more power.
3. Check and make sure the transformer is not too big for this application
4. try playing with the volume of the ipod/input device as well as the pot to see if you any results
5. You may also try a higher voltage on the input. To be honest when I made this I was expecting at least 3 volts to be required, and depending on your components (speakers, transformer, etc.) and input device the amount of power needed to operate may vary.
saba fazal1 year ago
i have made this amplifier but voice is low and potentiometer is not working.......plz give me suggestion
saba fazal1 year ago
i m going to make this circuit if i need any help i will contact u
Higgs Boson (author)  saba fazal1 year ago
Sounds good.
agm881 year ago
hey higgs im trying to put an LED synce curcuit in this amp but cant figure out where to put it when i test synce curcut it works on high volume and when i put a speaker to it the LED blinks very dimmly any ideas will surely hepl
Higgs Boson (author)  agm881 year ago
So if I understand what doing, your just trying to make the LED's brightness match the volume of the music right?
im trying to get the amp to play music
(this amp)(wich i havent built yet but will next time i go to radio shack)
and have the LEDs synce with the music but i test the LEDs it works ON MAX VOLUME wich realy sucks and when i put a speaker to the curcuit the LED blicks very dimmly/ shorter answer:im trying to get the LEDs to blink with out them going dimm from the speaker or the amp
Higgs Boson (author)  agm881 year ago
I would try to putting it across both the secondary and primary coil of the audio transformer, and seeing if one or the other works better. I also might try playing with the input voltage, but if your problem is the LED is drawing too much of the speaker's power, or vice versa, I would put the base of another transistor on the output, and wire up the LED to the transistor drawing power directly from the battery.
okay will try when i get every thing built and test to see if every thing works corr\ectly
Higgs Boson (author)  agm881 year ago
Good luck.
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