# A little question on audio amplifiers..

Discussion in 'Audio' started by MizziTDK, Jan 21, 2015.

1. ### MizziTDK

7
0
Jan 21, 2015
Hi guys! I have been planning to create a similar project as to this one.

I definitely know how the circuit works. The audio signal from the P2 connector turns on the led by saturating the transistor, but I was wondering how do you know the peak of the input signal?

I mean how do you know the line level of an audio amplifier? All I can see is power ratings. I can't see any peak to peak output voltage of the audio output. Thanks!

Last edited: Jan 21, 2015
2. ### davennModerator

13,654
1,888
Sep 5, 2009
Hi there
welcome to EP

for this little project, you wouldn't want to feed any signal level greater than around the headphone audio level

Dave

MizziTDK likes this.
3. ### MizziTDK

7
0
Jan 21, 2015

Oh, I mean I'd like to modify this circuit a bit. I do know a little about how transistors work. I was planning on creating such circuit for bigger audio amplifiers. Let's just say for 100W amplifier. I was wondering how will I be able to know the output of the line level so that I would know as to what are the components that I should have in order to saturate the transistor to turn on the LED. THanks!

4. ### BobK

7,682
1,686
Jan 5, 2010
P = V^2 / R

V = SQRT(R * P)

So a 100W amp driving an 8 Ohm speaker, the RMS voltage is SQRT(800) = 28V.

But that voltage will only be attained when the amp is putting out max power. Typical voltage is likely < 10V.

Bob

MizziTDK likes this.
5. ### MizziTDK

7
0
Jan 21, 2015
Thanks! So does that mean the amp can attain power only when its volume is full, right? Does that mean it will output a peak to peak of about 40V, right?

6. ### Gryd3

4,098
875
Jun 25, 2014
The numbers you are spouting would be what the amp is 'capable' of, not what it typically runs at.
Same is true for your car's HorsePower rating... It won't actually generate those numbers unless you push it to it's limit.

7. ### MizziTDK

7
0
Jan 21, 2015
Hmmm. so what would you suggest? Im still new to this. What would be the safest way to approximate it? Thanks!

8. ### Gryd3

4,098
875
Jun 25, 2014
You don't really... it depends on how they user will be using it.
I like to go fast on my bike, my brother does not. If we had the same model. I would be putting out more power on my bike then he is.
The best you can do is determine it's capabilities.

9. ### BobK

7,682
1,686
Jan 5, 2010
You design the circuit such that it can handle the max output, but saturates at a lower output, maybe half the voltage. Or, you add a sensitivity control that allows one to adjust to the current listening level.

Bob

10. ### MizziTDK

7
0
Jan 21, 2015
Hmmmm.. Thanks guys these are definitely good tips that I should consider!

By the way, I have one last question. Uhm, how do you find out the minimum Beta for saturation. When I look at the datasheet there are a lot of values for Hfe. For example, the 2n2222a bjt has a lot of hfe values on its data sheet. Which one should I consider?

Thanks again!

Last edited: Jan 25, 2015