Good evening to all! These days I would like to make a sound amplifier with a power of 20W! I've built an amplifier, I think 2W, AB class push-pull, made by HackTheWeek or otherwise he called, and worked perfectly for me! I was very pleased when i finally made an amplifier that really work! Of course I would like to make one of 5W, 7W, 10W, 15W, 20W ... But all of these I think are related to what kind of transistors I use, the voltage / current ratio, and the current / resistance ratio. But there are many things that are pending, which I do not know all about. These are the following:
1) The "gain"
2) The "hFe"
3) The "Power Dissipation" (ie is the power that the transistor needs to operate, or is it the power it produces?)
So I have done different calculations on how to produce 20W, or at least that's my reasoning. My calculations were simply multiplier:
20V * 1A = 20W
10V * 2A = 20W
5V * 4A = 20W
4V * 5A = 20W
2V * 10A = 20W
1V * 20A = 20W
Now if any power supplies are circulating at any of these values ... (If the power of an amplifier does NOT depend solely on the current/voltage ratio, I would be very happy to learn if there are other factors that may affect the performance of an amplifier).
I have a computer power supply in my home, basically i have 3 computer power supplies, but I'm so confused, I do not even know what these labels say. I will try to transfer everything that says the label of a model available from one of my power supplies I have:
Name: eXcelent Power {Switching Power Supply} Incl. PFC (Power Factor Correction)
Model: ITT 350AC
Input 230V / 6A ~ 50Hz max: 350W
+ 3.3V (orange): 28A
+ 5V (red): 35A
+ 12V (yellow): 16A
-5V (white): 0.3A
-12V (blue): 0.8A
+ 5VSB (purple): 2A
And somewhere say: "Combined Power 3.3V + 5V max 200W".
(A question irrelevant to this theme, before we talk about the topic. How it could be 5V to produce 35A? Or 12V to produce 16A? Or 3.3V to produce 28A! This is very strange! How little resistance you have to have so as to generate so much current According to the Ohm's Law, R = V / I => 5V / 35A = 0.142 Ω, but such resistance to trade does not exist! Then what is the logical explanation in this? My opinion is, maybe PPC produces a high current eg 40A, and the power supply only distributes this current to the whole circuit? Since Kirkhoff law says that the total current entering circuit be equal to the total flow exiting from it. If i answered my own question, i covered myself, but if not, what is the explanation )
Lets return to our theme! If I use let's say the value of 12V/16A (if it really produces that much ampere), how will I make it 10V / 2A? What resistance (s) do I need to have this voltage / current ratio? (If there is a better solution you can share it). Finally, if you have any 20W amplifier circuit plan, could you send it? I've tried, despite many amplifier designs, but I've got tired so much on being always a "fish" for every fake circuit that I can't handle that anymore! I have spent many moneys, buying parts such as transistors, capacitors, resistors, coils, etc for every fake circuit! I would like to build a normal operative amplifier of over than 10W for one time in my life at least! Thank you very much

P.S If i tell something wrong here, or something doesnt match together on this theme, feel free to say that! Thank you very much!

 

In audio circuit, the load is the loudspeakers or headphones. And for exampel you have a loadspeakers with impedance equal to 8Ω then for 20W of power deliver in this loudspeakers you need a voltage across the loudspeakers be equal to V= √(2*Pout*RL) = √(2*20W * 8Ω) ≈ 18Vpeak (36V peak to peak).
And the peak curret of about 18V/8Ω = 2.25A peak (loudspeakers power need to be larger the the amplifier power).

The gain is the amplification factor if G = 10V/V the you will get a 10V at the output if the input voltage is 1V.
hfe - is a BJT transistors current gain, also known as a beta ( β ).
Power Dissipation - is amount of power wasted by BJT (loss of power) and all this power is burned off as heat.

The load resistance determines the current that is drawn from a PSU.
https://forum.allaboutcircuits.com/threads/ohms-law-for-noobies-or-the-amp-hour-fallacy.69757/