# Powering 100W amplifier with 12V rails from ATX power supply

I've got some old speakers that I'm planning to power using this 2x50W amplifier(link)(I'm open to suggestions of a better quality to price amp). To power the amp I was thinking of using an old ATX 400W Power supply(link) and connecting the four 12V wires.

My calculations:
100W(power requirement)/12V(supply) = 8.3A(being drawn from the psu)

Therefore, if the psu uses 20AWG wire(with about 5A ampacity):
4(wires) * 5A = 20A(maximum usage)

Would this be enough of an amperage gap to prevent wires from overheating, or does this design pose a large fire hazard?

Thanks for the help.

• For clarity, it's a 2x50W amplifier. A 100W amplifier driving an 8 ohm speaker load would need PSU rail voltages of a lot more than +/-24V. – user207421 Feb 16 '15 at 22:43
• @EJP Thanks for clarifying, I edited it to avoid further confusion. Also, would that mean that since only 12V are supplied, that instead of drawing more current, each channel would only be powered by 18W? (12V)^2 / 8ohms == 18W ??? – Pierino Feb 17 '15 at 0:19
• See ti.com/product/tpa3116d2 that board is a straight reference board for the tpa3116d2 – Passerby Feb 17 '15 at 0:48

You need to rethink your calculations.

Let's assume that the amplifier you're using has negligible losses. Then the very greatest power you can provide to the speaker is (12 volts x 12 volts / 8 ohms), or 18 watts. Music, though, is essentially sinusoidal, so the maximum sustained AC power, technically called RMS power, is 0.707 times the peak power. This means that your amplifier can only produce 12 watts. Note that the data sheet you linked to suggests a 22 volt power supply. Checking this against the previous calculation gives (22 x 22 / 8) x .707, or 43 watts RMS. And using the absolute maximum of 24 volts gives 51 watts. Close enough.

So the peak current will be 12 / 8, or 1.5 amps per channel, for a total current of 3 amps, and a total power supply requirement of 36 watts.

So, to answer your question, the unit does not pose a fire hazard. It doesn't provide nearly as much power as you thought it would, but that's a different question.

• I thought my calculation wasn't right. Thanks for clarifying. So that means instead I could just use one or two 12V wires and run the amp & speakers at 36W until I find a power supply with 24V? – Pierino Feb 17 '15 at 1:32
• On a side note, would the following amp also only require 36 watts or would it draw more current, since the input says DC 12V, thus using the full 100 watts? Thanks again. Link to another amp – Pierino Feb 17 '15 at 1:35
• As to running just one or two wires - yes. Although all you'll get (in RMS terms) is 12 watts apiece. – WhatRoughBeast Feb 17 '15 at 2:07
• For the new amp. You need to be aware that although volts and amps can be traded off, power can't be cheated. 100 watts at 12 volts is 8.5 amps. Power equals current times voltage. Plus, I'm not sure how efficient the amplifier is, so you should figure on 10 amps. – WhatRoughBeast Feb 17 '15 at 2:10
• But would I be able to use the second amp as I originally intended to use the first (4 wires) and it would supply the full 50W (43W RMS) – Pierino Feb 17 '15 at 2:17