How to spot a high current amps?

Paulwp, welcome to the world of marketing by specs.

Remember, power = current squared times resistance. Using 40 amps and 25 watts, this says that:

25 = (40)^2 * r where r = resistance. solving for r we get a value of about .015 ohms.

In other words, if those numbers are correct, the amp only delivers 40 amps into essentially a dead short. What is the minimum impedance the amp is rated to drive?

At 8 ohms the amp could deliver about 1.8 amps, and at 4 ohms about 2.5 amps. This assumes the amp delivers 25 watts at both impedances, which is probably not true.

In short, these numbers don't really make sense.

I also like the weight test mentioned in previous posts.

As always, trust your ears.

Paulwp-

I'm not saying it's a bad amp. It sounds like it was well engineered for its price point. But that doesn't stop the marketeers.

Your last post sounds like the old "Instantaneous Peak Power" ratings that amps got in the 70's, before the FTC stepped in. You had 1 watt amps delivering 2000 watts of instantaneous peak power.

Refering to my previous post, the power delivered to a load of resistance r with a current of 40 amps is 40^2 times r, or 1600 times r.

So if they claim delivery of 40 amps at 8 ohms, that implies 12,800 watts delivered.

At 4 ohms, that's 6,400 watts.

At 2 ohms, 3200 watts.

At 1 ohm, 1600 watts.

In other words, just saying the amp can deliver 40 amps doesn't really tell you anything by itself. You need to know for how long, into what impedance, and over what bandwidth.

Otherwise it might mean that the amp can deliver 40 amps for just the quarter second it takes for the line fuses to blow, or the protection circuitry to activate.

PS - to Dweller

The fuse argument is a good one, but not as you state it. Remember, the amp's power supply converts power line voltage up or down as required by the design (tube or transistor). You can't compare line current directly to speaker current.

So its more correct to say that the fuses limit the continuous power the amp can provide. Power is also equal to voltage times current, so a 10 amp fuse at a line voltage of 115 would limit power to a maximum of 1150 watts.

This number is an upper bound but not a least upper bound, and doesn't take into account efficiency, which would limit power (and thus current) further.

Of course fuses take time to blow, so the instantaneous power drawn from the AC line could exceed this number.