How to spot a high current amps?

Hmm. I have amps with current rating, eg., 45 amps, 40 amps, 60 amps. One of them is not difficult at all to lift. Very light in fact, an old Mission Cyrus One integrated that feels like it weighs a pound or two. Yet it was rated at 40 amps, but only 25 wpc. Now, I'm sure you think that's just marketing talk, but the fact is that I used to drive a pair of KLH 5's with it(about 77db efficiency) and it's never had trouble with any pair of speakers. It is a little light on the very bottom, a little warm in the upper bass and seriously rolled off in the treble, but still a very musicallly dynamic little amp.

In general, however, the heavier amps do seem to be rated for higher current.

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.