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.

I remember reading an ad by a mfg. (Adcom?) who said to remove the top of the prospective amp and look at its main fuses. If the fuse is rated to blow at 20 amps (or whatever) how can the amp put out more? -Valid question...

Ghostrider, the rating and advertising claim is "capable of 40 amps pk-pk," and the claimed continuous wpc number into 8 ohms both channels driven is 30. This amp was actually one of the first I saw, this was 20 years ago, for which any current delivery claim was made. The point was it actually was a very robust little amp had not trouble driving very inefficient speakers that supposedly higher powered amps could not drive. So there must have been something to their claims. Nowadays, I use larger amps designed for high current delivery and stability into any load designed by people who I have spoken with and trust.

This is a great discussion. I used to think that if the amp doubles its power from 8 ohms to 4 ohms, it's capable of delivering a lot of current. True, this is a good sign, but there are high current amps that don't exactly follow this rule. If you are familiar with the Electron Kinetics Eagle 2 amp by the legendary John Iverson, it's rated at 125 wpc into 8 ohms and 200 wpc into 4 ohms. Yet, it's acknowledged as one of the real 'high current' amps. It can drive inefficient and difficult speaker loads with ease (such as the Apogee ribbon speakers) inspite of its relatively compact size. If you peek inside an Eagle 2 amp, you'll see some other signs of a typical 'high current' amp: a massive power transformer and two oversize electrolytic capacitors, each one the size of a beer can (80,000 mf x 2). I would also tend to agree that a heavy amp is a good sign because it usually means it has a hefty power supply. This is just my two cents worth.

Most any amp can provide a high level of "surge current" or momentary "peak to peak" output levels, but it is "long term muscle" that separates the "wanna-be's" from the REAL "brutes". Brief surge capacity can help amps sound cleaner on dynamic peaks, but they may still suffer loss of control / smearing on passages that are both loud and long in duration.

As for the Electron Kinetic's amps, the designer could have easily de-rated the amp to spec at 100 wpc @ 8 and 200 wpc @ 4 ohms. Instead, he chose to rate it more realistically and let the performance of the amp speak for itself. The fact that the Eagles have such large power supplies are what give them a big advantage when it comes to bass control and extended duration low frequency passages i.e. they are able to sustain high current levels longer than many other amps and don't "poop out" easily. Sean
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PS... My favorites are the specs that say something like "2000 watts PMPO" on computer speakers, boom boxes, etc... PMPO translates to "Peak Momentary Power Output" but really means "this is what the device is capable of at the point of thermo-nuclear meltdown" : )