How To Measure "Current" In An Amplifier?

What is probably the best indication of an amp's ability to drive low impedances and/or impedances that have highly capacitive phase angles at some frequencies (which would also increase the amount of current that has to be supplied) is if the amp has a continuous power rating into 4 ohms that is twice its rating into 8 ohms, and if it has a rating into 2 ohms (if that rating is provided or is indicated in the measurements section of published reviews) that equals or approaches twice its rating into 4 ohms.

Current specs that may be provided in manufacturer literature should be ignored in most cases. They usually represent how much current can be supplied into a dead short (zero ohms) for some unspecified number of milliseconds (thousandths of a second). And according to a post I recall that was made a while back by Atmasphere in another thread on this topic, that spec is commonly not even based on the short being placed at the output of the amp, but at the output of the power supply instead.

Regards,
-- Al

Mitch, @almarg is pretty much dead on.

There are a couple of other indicators. A square wave into a simulated speaker load, but that's usually only provided by Stereophile, as well as the damping factor. A high damping factor is an indicator of low output impedance, and, by inference, high current capabilities.

But these really are just indicators.

My personal view is that very few speakers justify being designed to be so difficult.  Electrostatics one of the few exceptions. Otherwise, I'd say steer clear of "discerning" or difficult to drive speakers as they usually have some other underlying issue which a simpler design would avoid.

Best,


Erik

Clarification!

Sorry, I forgot.  Yamaha used to, and may still, provide square wave signals in their manuals, indicating very good frequency and current capabilities into difficult loads.

Others may as well, but in the consumer market I haven't seen any manufacturer provide that level of performance detail.

Best,

Erik

Erik, re: the last paragraph of your last post, one could argue that other considerations mandated speaker designs with such an impedance, and that one might rather just avoid amplifiers that weren't robust enough to handle the task at hand.