04.05.2024
It is no secret that I love to listen to music or an audio book on the go for which . I use my smartphone in combination with some good old wired earphones. . The only problem sometimes is though, that the loudness of certain audio books can be a bit . quiet even if I crank the volume up to the maximum. . So what do we do if we want to increase the loudness of an audio signal? . Of course; We build an audio amplifier! And in this video I will show you how to create . this super simple and awesome sounding audio amp that plugs right into your phones headphone jack . and USB type C port in order to boost the volume of the audio signal coming out of your phone. . Along the way I will not only tell how to easily build such a Class A audio amplifier . but I will also tell you how to get 5V power from your phones USB Type C port..
Let's get started! This video is sponsored by JLCPCB . which is a PCB manufacturer with very high standards for their product quality. . It is so high that their complaint rate is lower than 0.2%. So why not try out their service by . uploading your Gerber files today and thus ordering affordable and high quality PCBs quickly. . First off let's gather all the components we need for a Class A audio amp. . As you can see we only need one BC337 NPN BJT, 4 Resistors and . 2 Capacitors which we have to connect to one another according to this schematic. . And since we want to amplify a stereo signal we will need to build up this circuit twice. . But before actually testing the circuit on a breadboard, let's firstly talk about how the . amplifier works and how I chose the values for its complementary passive components..
As already mentioned the heart of the amplifier is a BJT or Bipolar Junction Transistor. . I already created a video about this component . which I highly recommend you to watch because I will not go over all the basics once again. . But in a nutshell we can say that when a base current is flowing, . then a collector current can flow with a value of the gain factor multiplied by the base current. . So if the base current is a music signal then the collector current will also be a music signal but . with a higher current value which is basically the loudness amplification we are looking for. . Sounds pretty straightforward, but of course such a simple setup does not work just yet. . As you can see while using a sine wave created by a function generator on the base input of the . transistor, pretty much no current flows through the collector and there is also only the DC supply.
Voltage visible at its collector emitter path. The problem is the voltage drop across the . base emitter path which is around 0.7V. So we have to increase the sine voltage . maximum above this value before anything happens on the output side of the BJT. . But if we have a look at a common audio signal of a phone, we can see that it comes with maximum and . minimum voltage values of around +/1V. That means that pretty much all of the . input audio signal would get ignored by the BJT which is definitely not what we want. . Instead we have to add such a resistor network to the base of the transistor in order to . bias the base with a DC voltage and current. This way we can let the input voltage/current . oscillate around a DC offset value and thus all of the input signal gets used for the amplification..
The only problem of such a biased class a audio amp design is that the transistor is now always on . since a quiescent current determined by the resistor network is always flowing. . Class B or AB amplifiers however which I partly presented you in a previous video . do not have this problem since they use two transistors which both handle . more or less one half wave of the input signal. That means they are more efficient but can have . problems with crossover distortions or turnon times and are always a bit . more complex to design and build. Class A amps however are simpler . to build and feature a super linear amplification with very low distortions. . But then again their maximum efficiency is usually around 25% which is horrible . but since we are only using them for headphones so a small load and with a relatively low.
Quiescent current, it will be just fine. And with that being said the input side of . the amp should be clear but what about the two resistors on the output side. . Well, they are basically used to set the operation point of the amplifier. . They define the collector current, the base current and the DC offset voltage . at which the output voltage will oscillate around so that it has enough . space between the upper and lower voltage limits. FYI though; if the output voltage wants to exceed . those limits then the audio begins to clip which looks like this and sounds horrible. . Last but not least we also got two capacitors to basically get rid of the DC voltage . for the audio input and output signal. And with the basic functional principle . out of the way, let's start to select values for the components..
To do that I firstly had to decide on a collector current . which later directly determines how loud the amplified music will be. . I did calculations and practical tests for 1mA, 10mA and 20mA and I have to say . that 1mA was too quite, 10mA featured clipping at the maximum amplification . and 20mA was pretty much the sweet spot with no distortions and loud enough amplification. . To prove it, here is how the max audio level of my phone usually sounds like..and . here is the amplified version with 20mA..... . And now that we got the Collector current of 20mA, . we can calculate Rc which is the supply voltage divided by the collector current and by 2, . so that the output music signal swings around half of the supply . voltage, and divided by the collector current. This equals a value of around 130Ω.
The supply voltage by the way will be provided through my phone and is around 5.2V. . Next the emitter resistor usually drops around 10% of the supply voltage and since we know . that the collector current is flowing through it, we get a resistor value of 26Ω. . Now we can calculate the base current by simply dividing the collector current through the gain . factor which is around 170 at 20mA. The resistor values for R1 and R2 . can then be calculated through the base current, supply voltage and base GND voltage which gave me . values of 3.4kΩ for R1 and 1kΩ for R2. Last but not least we can throw in . some common 10µF capacitor for decoupling and we are basically done. . Now this class A amp is rather basic since you can add way more components to improve . it and my calculations might not have been the most precise and accurate ones but.
I was still very happy with the audio quality of my built amp and that is all that counts, right? . But if you want to dig deeper into the subject then feel free to have . a look in the video description where I linked some pretty useful articles. . Anyway after I was done testing my amp on a breadboard . it was time to move it underneath my phone. To firstly get power from it though I ordered . myself such a USB Type C breakout board. But after plugging it in, I realized that . the supply voltage and GND pin of the board didn't feature a usable voltage. . The solution was to basically desolder its 56kΩ resistor and replace it with a 5.1kΩ one . which is connected to GND. This way we apparently added a 5.1kΩ resistor . between the CC pins and GND and thus we now got 5V between the voltage bus and GND pin..
So I soldered this connector along with an audio jack to a piece of perfboard around which I then added all of the components for the stereo class a amp which I then soldered to one another according to this finalized schematic. After the soldering process was complete, I added a bit of hot glue for stability, designed a fitting enclosure for the project in Fusion 360, 3D printed it with my Prusa 3D printer and added the housing to the circuit. And just like that you can make your own Class A Audio Amp for your phone. I hope you enjoyed this video, if so don't forget to like, share, subscribe and hit the notification bell. Stay creative and I will see you next time.
