Reminder: how to tell current from an amp's specs?

At a given impedance, say 8 ohms, the current of the amplifier is directly related to the specified wattage. If your speakers are rated at a flat 8 ohms across the frequency spectrum, more watts equals more amps, i.e. current.

The issue becomes more complicated when a speaker has a widely varying impedance rating across the frequency spectrum. For example, my Thiel CS6 speakers range from about 2 ohms up to around 8 ohms depending on the frequency. In order for the amplifier to supply the necessary power to allow the speaker to have a flat frequency response it needs to double its output with each halving of the impedance. My Krell KSA 300S outputs 300 watts at 8 ohms, 600 watts at 4 ohms, 1200 watts at 2 ohms, and 2400 watts at 1 ohm. When people talk about a high current amplifier this is what they mean.

Current relates to amps. The more amps your amplifier produces the more "current" it has. You can calculate the amperage of your amplifier from the wattage specs at the rated impedance at those wattages. Amps are calculated by dividing Watts by Volts. 300 watts divided by 120 volts = 2.5 amps. 2400 watts divided by 120 volts = 20 amps. Big difference.

If an amplifier outputs 500 watts at 8 ohms and 500 watts at 4 ohms it would not be considered a high current amplifier - at least in my world. It doesn't matter how big the power supply is or how expensive the parts are. It simply cannot output the amps that a speaker with a demanding impedance curve requires to produce a flat frequency response. There's nothing wrong with that, however, if you are running a speaker with a flat impedance curve and the amp has the watts to drive the speaker to an acceptably loud level. Devore speakers, for example, are designed to have a flat impedance curve so they can be driven by a wide range of amplifiers that don't produce high current.

People get wrapped around the axle about "current" but most speakers present a fairly benign load and do not require the heroic output of a Krell, Pass, D'Agostino, or Levinson amplifier. But if you are running certain Wilson Audio, Thiel, Apogee, or other speakers that present a difficult load then a high current amp is necessary for them to sound their best.

Sorry for the long post. I hope this helps.

The REGA Osiris integrated amp is one of these exceptions. While it is a beefy hi-current and high WPC amp that doesn’t fully double down, it still has LOTS of current because of its high-end build and design. This is provided by those four Sanken output transistors per channel, that insure that no speaker is too hard to drive.

This statement is incorrect. The extra transistors make sure the output section can support the current without damage but if the power transformer lacks the current capacity it really won't make any difference.

 To me, a big honkin’ transformer (sorry Ralph) and the ability to double down to 4 Ohms and maybe double again down to 2 Ohms is a good indication an amp has the balls to power most speakers

This is generally correct but if the speaker does not have any low impedance dips it could be a waste of money, especially on Maggies which do not have such low impedance dips.

I should point out also that not all speakers that dip to 2 Ohms need a lot of current capacity in the amplifier; Wilsons are a good example. They can easily be driven by tube amplifiers even though the low impedance dip is in the bass. Generally speaking Wilson loudspeakers have traditionally been easy to drive.

If I can point something else out: Just because your amplifier can drive to 2 Ohms and is able to double power as it does so does not mean its sounding its best when doing so. All amplifiers make higher distortion when driving lower impedances! If you think that distortion is inaudible think again- the increased distortion is audible as increased brightness, harshness and a reduction of detail (distortion obscures detail) because most of that added distortion is unmasked higher ordered harmonics.

I have to disagree with atmasphere on the issue of low impedance Wilsons doing fine with tube amps. Any speaker that has a demanding impedance curve will not give a flat (to the best of its ability) frequency response if it is not driven by an amp that provides high current (doubles output with halving of impedance).

I'm not saying that you can't use a tube amp with a big Wilson but the speaker will not produce the frequency response that it was designed to produce. You are basically introducing an unpredictable tone control for the speaker. You may like the sound or you may not but you are not getting the sound that the designer heard when he voiced the speaker.

There are speakers like Devore Fidelity that are intentionally designed with a flat impedance curve so they can be run by virtually any amp. My Thiels would sound anemic with a tube amp because they drop to around 2 ohms in the bass region.

I have to disagree with atmasphere on the issue of low impedance Wilsons doing fine with tube amps.

Yeah, good luck with that. 
 

If I can point something else out: Just because your amplifier can drive to 2 Ohms and is able to double power as it does so does not mean its sounding its best when doing so. All amplifiers make higher distortion when driving lower impedances! If you think that distortion is inaudible think again- the increased distortion is audible as increased brightness, harshness and a reduction of detail (distortion obscures detail) because most of that added distortion is unmasked higher ordered harmonics.

Like I said before, I defer to Ralph always.

@soix ,

Ditto.

Bob