Class A single-ended transistor amplifier from phone charger components

An Intriguing Project

Here is an intriguing project. A transistor amplifier from components inside a phone charger. The device is a single transistor amplifier called a single ended amplifier. Many YouTube videos show the construction and testing of such a device with sound quality and amplification that sounds fairly clear. This actually works, I have tested this out on a breadboard.

The first step is to find a broken phone charger, of which there are usually many lying around. I found some in my box of electronic parts box.

Update: July 18, 2021: Project on hold due to low output and complicated set up.

Parts List

After locating the transistors, the next step is to find a circuit diagram of a working circuit that uses these transistors.

You tube video

Parts List:

1. C945 transistor and 13001 transistor from an old phone charger

2. 1 uF capacitor

3. 68 K Ohm resistor

4. 9V battery

5. 9V battery connector

6. Connecting wires

7. RCA headphone output jack

8. Audio source

Project Updates (scroll down for earlier entries )

18 July 2021

Returning to this project after the near completion of the LM386 dual board amplifier, I was able to get the amp working, however sound volumes were very low. I recall that sound was loud enough and on the YouTube demos the sound was loud enough to rattle objects placed on the small speakers.

If the sound output is not high enough the project will have to be abandoned.

UPDATE: The amplifier works, however the sound output is not very loud at all, a little louder than PC speakers with a 3 inch 4 inch and 6 inch speaker, the sound quality is good, however too low an output with a tested 9V battery power.

With the high power consumption, heat generation and general power supply problems, this has project will be postponed until I am able to work on a class A amplifier again.

4 December 2020

When testing a different set of speakers I noticed that the speaker cone moved a set amount on the application of power. There is a need to keep DC currents out of the speaker. There are two options here - one is to use a transformer and the other to use a capacitor. After checking several capacitors I ended up with a 3.3F (3.3 Farad) Capacitor that let the sound pass through without much distortion audible. These are large in value, but not very expensive, however these may be difficult to obtain.

There is a nice article on output capacitors here:

1 December 2020

The amplifier is in testing right now. Sound quality, which is the main concern here, is good, but at low volumes. I am running off a 9 Volt battery that is down to about 4 volts. One big step was replacing the 13001 transistor with the C945 transistor and obtaining the same type of output sound. I now have the second channel.

Overheating

The amplifier is run at about 5 volts per channel, and it runs cool at this level. Running the amplifier at 9 Volts results in a transistor that is too hot to touch, in fact one that is burning hot. A heat-sink needs to be added, but it is not how effective it will be. The final set up will be a 9V supply running two channels at 4.5V per channel off a power adapter. Amperage is not known.

The next step is the design of the circuit board ( no soldering will be used at all ) and the outer case. Some simplification of the wiring has been achieved.

24th November 2020

The amplifier is to be set up on a new type of board - solderless, and using plastic screw connectors. I have used these connectors for speaker wires and they work very well. The intention is to build the complete amplifier and connect all the components without using any solder at all. The test of the system will be the sound quality of the audio output.

16 November 2020

The amplifier works! The transistor gets very hot, but it works, draining the battery from 9V down to 4V in a short period of time.

I used the circuit here:

https://theorycircuit.com/simple-single-transistor-audio-amplifier-circuit/

Here is an image of the transistor on a breadboard: the rest of the circuit uses crocodile clips. Changes to the circuit include a variable resistor between the emitter and base, which has to be carefully adjusted for a gain / distortion balance.

The input capacitor is a 470uF which can be omitted without much change to the sound.

12 November 2020

Testing of the circuit showed no improvement over a the un-amplified output. To make matters worse, the circuit produced some sound even when the transistor was disconnected. Testing or the transistor continued, and although the initial tests using the transistor test function on the multi- meter showed no result, testing the transistor in the diode function showed the typical readings between base and emitted and high resistance between emitter and collector.

I decided to set up a circuit where the transistor is used as a switch in order to test the operation of both transistors I had in my possession, both salvaged from a phone charger and a electronic project bought from a student many years ago.

The amplifier will find its use with my PC speakers, since the LM 386 units will be moved to the main stereo when the case is complete. It will be an unique pleasure listening to salvaged transistors at my PC.

Links: class A amplifiers

https://www.electronics-tutorials.ws/amplifier/amp_5.html

https://www.diyaudioprojects.com/Solid/IRF610-Class-A-Headphone-Amp/

https://320volt.com/en/lamba-mosfet-karisimi-class-a-kulaklik-anfisi/

Darlington Transistor : 5-12V :

https://usamodelkina.ru/10249-prosteyshiy-usilitel-zvuka-na-odnom-tranzistore-za-15-minut.html

Nice explanation:

https://www.learnabout-electronics.org/Amplifiers/amplifiers12.php