# School project/assignment

#### mitchelel

Joined Feb 23, 2009
6
Hello everyone i am currently building a DIY laptop stand for a school project. I just need some help with some electrical aspects of it. I am using a 12v power supply with two 0.2A fans attached a 50w amplifier that have 2, 20 watt speaker 8ohm impedance and a 120cm stretch of LED lights that need 0.75A. My question is would all of this run off the one 12v power supply? and if so how many Amps would i need to do so? Thanks

#### Wendy

Joined Mar 24, 2008
23,320
50W amplifier at 12V = 4.2A, give or take.

I guesstimate you need around 6A for the power supply, more is better. Reason, having components with extra usually allows them to last longer, since they aren't stressed as hard.

#### studiot

Joined Nov 9, 2007
4,998
Let us take a look at the figures provided.

8Ω speaker 50 watts power:

Irms = 2.5A
Ipeak = 3.5A
Vrms = 20volts
Vpeak = 28 volts
Vpeak to peak = 56 volts.

Max power available at 12 volts into 8 ohms is 2.25 watts

4Ω speaker 50 watts power:

Irms = 3.5A
Ipeak = 14
Vrms = 20volts
Vpeak = 20 volts
Vpeak to peak = 40 volts.

Max power available at 12 volts supply into 4 ohms is 5 watts.

These are the bald requirements for an amp delivering 50 watts into 8 ohms or 4 ohms (2 speakers in parallel)

It is a good job that the peak to peak requirements preclude your amp ever reaching this with 12 volts available. It is never a good idea for the speaker capability to be less than the amp's.

It was not clear you have two speaker and one amp? Do you intend to run mono or stereo?

#### mitchelel

Joined Feb 23, 2009
6
I am going to use stereo i have the cables to split the headphone jack into left and right so thats all sorted i just needed to know the power specs and if it is able to do what i need to do. Thanks for you help so far

#### studiot

Joined Nov 9, 2007
4,998
So how will you do this with only one amp?

Did you understand the significance of my posted figures?

Please ask if you need further amplification (ha ha).

#### Wendy

Joined Mar 24, 2008
23,320
I always assume the amp has something like transformers, being a finished product. Is the amp a commercial product?

#### Audioguru

Joined Dec 20, 2007
11,248
I always assume the amp has something like transformers, being a finished product. Is the amp a commercial product?
Audio amplifiers have not used output transformers for about 50 years, output transformers were used with vacuum tubes.
Car amplifier ICs are usually two amplifiers bridged together, so that the peak-to-peak voltage across the speaker is almost doubled. When the voltage is almost doubled then the current is also almost doubled for almost 3.5 times more output power into a speaker. Usually the speakers in a car are 4 ohms for plenty of current. When the battery is 13.2V then the power at clipping into a 4 ohm speaker is 14 Watts, or 25 Whats.