Ok, this has been gone over again and again, so I spent a lot of time thinking about this at work, and here is what I believe.

While it isn't a 100% effective way of determing an amps power, it IS a good way of getting a general idea of the RMS power an amp can do before the fuse blows. Volts * amps = watts.

Example:

Power Acoustik Gothic OV2 1200

270.2 @ 4Î© RMS

300.2 @ 2Î© RMS

600.1 @ 4Î© RMS bridged

Assumption:

My Power Acoustik has two 30 amp fuses. Assuming I have only 12 volts, and this amp is at 100% efficiency, and doing 1Î©, I will have 12*60 =720 watts power bridged. Divide that in half and you get360 watts per channelsconstantly going out of the amp before those fuses gives out.

Prediction (2 channels):

This isn't the case, instead it's a 2Î© load per channel, I have 14.4 volts, and being at full gain, I'm going to call it 85% efficiencyfor example.

14.4*30 = 432 / 2 = 216 * .85 =183.6 wattsconstant per channel.

Prediction (Bridged in series):

Now, let's bridge it. I have 60 amps, 14.4 volts, 85% efficiency, and a 4Î© load.

14.4 * 60 = 864 / 4 = 216 * .85 =183.6 watts.

Okay, now my amp is REALLY over-rated. What the hell? That is not right.

So, where does this amp stand? If I can, I will get it benched. It's a long drive to somewhere that can accurately do that, so it could take a while. The only place close attaches them to a 25 amp power supply, and wonders why they never get far past 300 watts.

Example 2: Mobile Authority (lesser MA Audio) Rockbox.

25x2 @ 4Î©

Bridgable? Some say yes, some don't, I don't know if I want to try.

It has a 5 amp fuse.

at 100% efficiency, 12volts, and 4 ohms, I should get 15 watts / channel out of it.

at 85% (full gain), 14.4volts, and 4 ohms, I should get 15.3 watts, RMS.

My only conclusion is they are assuming roughly 100% efficiency, 14.4 volts, and the lowest impedance possible.

My other conclusion, although I can't really back this up, is that multiplying the fuse rating by 6 will give a decent RMS rating at impedance.

30*6 = 180 / channel, or the wattage of my Power Acoustik at 2 ohms.

5*6 = 30 (15 / channel), or the wattage of my Rockbox.

This is my thoughts regarding fuse ratings of amplifiers with UNREGULATED POWER SUPPLIES. Amps with DC/DC converters can work at far higher voltages, and this would drastically change how this system works.

My usual 2 cents.

UPDATE: I was not able to bench my amp. Building a sub box is first priority. That's a different story. Edited for coherency.

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