[Logistics]

So I was looking at the power supply crash course on TnT, and he calculates the required voltage this way:

Sq. Root of: (power * 2) * load impedance = peak voltage
Power being in Watts multiplied by two channels, and the impedance is 8 Ohms.

How did he get the following result?
Sq. Root of: (50 x 2)x8 = 28,284271V

Did he leave part of his math out? I cannot get this result!

Here is the link for reference:

http://www.tnt-audio.com/clinica/ssps2_e.html

 

[rb67]

He got the number by

sqrt(50 * 2 * 8) = 28.284271V

 

[Logistics]

How do I replicate it on a calculator? I still can't figure it out.

 

[rb67]

just find the square root of 800.
What kind of calculator are you using? On most calculators there should be a square root function.

Btw, he calculated it using the following relation

P = V^2/R
P_T * R = V^2
V = sqrt(P_T * R)
since power is over two channels, its 2*P
V = sqrt(2*P*R) = (2*P*r)^0.5

(P_T = total power, P = power per channel)

 

[Logistics]

I can use the Windows calculator or my little TI-30X Solar*, which I think is much more advanced than the Windows calculator, but I don't understand how to do this problem with it. It appears to me that I just need to know the Wattage and the impedance to find the voltage. Sorry, I am no good at math so no matter how many times I try to figure out what you've posted, I don't come up with the result he did.

*This guy hxxp://www.elexp.com/calc/02TI30X.jpg

 

[rb67]

The button just to the left of the blue division symbol is square root.

for this example
wattage : 50 watts per channel, 2 channels >> 100 watts
impedance : 8 ohms

8 * 100 = 800

then you use the square root function on your calculator. (Press the symbol with the "x" in it to the left of the division button and put the value you want to find the square root of)
sqrt(800) = 28.284271V

Also , you may use google as your calculator. Just in the search box type

sqrt(50 * 2 * 8) and it will give you the answer you are looking for.

Also, for reference, TnT writes it as 28,284271V using a "," instead of a ".". It is common in other countries (outside America) to use "," as a decimal point.

another example:
You want to power 1 speaker to 80 watts.
The impedance is 4 ohms

the max voltage is
sqrt(80 * 4) = 17.8885438 V

 

[Logistics]

Ahh, I didn't understand that when you see SqRt: it means the SqRt of all the numbers. I got some guys in Chat to teach me how to do this on paper, I'm still a little baffled by it, but it makes sense now. Thanks for your help, even they didn't know what the button would look like lol

 

[rb67]

Actually, you can blame TnT

He made a mistake in his parenthesis.

Sq. Root of: (50 x 2)x8 would mean sqrt(50 * 2) * 8 = 80.

The proper way of square rooting "all numbers" is
Sq. Root of: (50 x 2 x 8) >> sqrt(50 * 2 * 8) = 28.284271V
Just goes to show what a little typo can do

 

[Nisbeth]

Overall, I think that's a pretty poor article. The word "effciency" never appears (quite a useful concept if you're calculating output power requirements...), there's not description of how current requirements vary between class A and AB amplifiers and there's no mention of the fact that music signals contain less energy than sinewaves which also affects power supply requirements (look in any midpriced commercial amplifier to confirm this).


/U.