"High Current"

Two equations tell the entire story. Watts=VoltagexCurrent, and Watts=Current(squared)x Resistance.

So as has been stated earlier an amp that is not current limited will double its power as the impedence is halved. So if an amp is rated at 200 watts into 8 ohms it must produce 400 watts into 4 ohms and 800 watts into 2 ohms.

But that really is only half of the story. To answer the question about how this impacts the sound you have to know what the impedance curve is for the speakers you have. I used to own Thiel 3.6s. They are nominally 4 ohm speakers but there is a point in the lower midrange where they dip to about 2 ohms as I recall. That means if you are driving a current limited amp (say 100 watt into 4 ohms rated amp) close to its limit, the speakers will be demanding 200 watts in this midbass region. If the 100 watt amp is only rated at 150watts into 2 ohms you could be running into clipping and a clear audio distortion. So high current designs are not necessarily required for many well behaved speakers. There are lots of high end speakers some cause trouble other wont.

Gregm,

The Krell Class A "Full Power Balanced" cx-series "double-down"
all the way to 2 ohms.

Take the "baby" of that line-the FPB-300cx: 300 wpc at 8 ohms,
600 wpc at 4 ohms, and 1200 wpc at 2 ohms. See:

http://www.krellonline.com/html/m_ClassA_p_FPBs_300cx.html

Dr. Gregory Greenman
Physicist

Morbius: Those are typical factory specs, nothing like what we are discussing here. On top of that, the voltage swing on that specific Krell is pretty low at their rated 138 v peak to peak. My Perreaux PMF-3150, which is also rated at 300 wpc @ 8 ohms, is rated at 180 v peak to peak. I would like to see what the Krell REALLY clips at into various loads. I have NO doubt that it is a sturdy amp, but exactly how sturdy, i don't know.

As a general rule, so long as current levels are reasonably up to snuff, the amp with the highest rail voltages will typically sound the cleanest and least compressed when driven hard. This is why tubes, with their very high plate voltages, can work quite well. This is true even though they are typically quite current limited in comparison to a "big brute" SS amp. The reason? So long as they have "enough" current to deal with the speaker load at hand, they've always got gobs of headroom due to the high rail voltages. Sean
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The reason current is the issue is that high rail voltages collapse under demand and low impedences. Hi voltage measured with high impedence has nothing to do with an amps performance.

Keis: Your comment makes it look like high rail voltages have the potential to suffer more than a lower rail voltage. That simply is NOT the truth. ALL voltages collapse under demand IF there is a lack of current. The use of a higher impedance load simply reduces the need for as much current, but one could still run into sag IF the circuit was still not "beefy" enough OR the circuits response time was not quick enough. As such, rail voltages have little to do with sagging whereas they have a LOT to do with the dynamic capabilities / headroom of the circuit.

If one needed 1 amp, 10 amps or even 50 amps of current, so long as the circuit could deliver it on a timely basis, there would be minimal voltage sag. The idea behind a "TRUE" high current amp is that it can handle any load at any time due to always having an abundance on reserve. That "any time" part takes into account that the circuit is responsive enough to deliver the needed power as needed, not lagging and therefore sagging under demand.

This is why i said that an amp with high rails, high current, high speed, high stability and wide bandwidth is the ultimate amp. It can deal with any load or signal related condition thrown at it and do so with finesse. Sean
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