Building the simplest, best sounding, yet most inefficient Audio Amp! || Class A Audio Amp Tutorial

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 turn- on 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.

Channel: GreatScott!

Views: 386,015

Keywords: audio, amp, amplifier, class, class a, class b, class ab, tutorial, guide, make, your, own, diy, do, it, yourself, project, beginner, beginners, calculate, design, bjt, bipolar, junction, transistor, switch, resistor, capacitor, simple, super, best, sounding, quality, efficient, efficiency, schematic, compare, comparison, bias, dc, direct, current, voltage, music, smart, phone, usb, type c, typ c, perfboard, pcb, 5V, headphone, earphone, portable, power, powered, solder, 3D, print, printer, greatscott, greatscott!, electronic, homemade

Id: 7Pv-j_R3YP4

Length: 10min 45sec (645 seconds)

Published: Sun Feb 14 2021