High Current / Low Current / No Current ??????

I found the explanation below on the B&K Website in their FAQ's and think it's good. (I copied it directly from their FAQ - so credit to B&K).

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Question: “What is a ‘high-current’ amplifier and why is it better?”
A “high-current” amplifier is capable of delivering power into low impedance loads (speakers)
without going into protection and/or shutting down. Ideally, an amplifier’s output power would double
every time the load presented at its output is halved. For example, an “ideal” amplifier rated 250
Watts @ 8 Ohms would deliver 500 Watts @ 4 Ohms, noting that the load has halved and the
power has doubled. In actuality, a “real-world” high-current amplifier capable of 250 Watts @ 8
Ohms might be delivering about 425 Watts @ 4 Ohms.
An easy way to identify a “high-current” amplifier for use with home consumer electronics is
to look at what happens to the power rating as the impedance of its load (speakers) drops. The
closer the amplifier comes to approaching the “ideal” amplifier scenario, the more current the
amplifier is capable of delivering and the better the sound reproduced by the speaker.
In a typical amplifier-speaker circuit, voltage and current are delivered to a very complex load
consisting of speaker drivers, resistors, inductors, and capacitors. Voltage may be thought of as the
potential to do work, and current as what actually flows to do the work. Although basic power may
be calculated by the simple multiplication of voltage and current, it is the delivery of the power from
the amplifier to the complex speaker load that accounts for why two amplifiers may have the same
power rating into identical impedances (speaker loads), but still be significantly different from one
another in sound quality. Some of these sound quality differences may include perceived loudness,
depth, and clarity. Amplifiers that are designed to operate with high voltage as opposed to high
current are typically much better suited to high impedance loads, typically 8 Ohms and higher.
Lower current rated amplifiers have been said to sound dynamically limited and “harsh” at high
listening levels.
High quality “audiophile” speakers can have nominal impedances from 8 – 2 Ohms, and
during very dynamic passages in source material can easily dip below 2 Ohms. These super low
impedance drops can easily choke a high voltage amplifier with limited power storage capacity. For
a high-voltage amplifier, this will likely cause problems and trigger its protection, not to mention the
less than nice sound it may produce. A high-current amplifier operates with much less effort and
typically does not have any problem with these types of speakers.
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Ohm's law (V=I*R and W=V*I) is in operation here. Say a typical transistor amp is capable of delivering 25 volts into an 8 ohm load. That'd give 3.125 amps of current, which is about 80 watts.

Hook the same amp to a 4 ohm speaker and the current demand is now double at 6.25 amps needed. However, if the amp can't supply the extra oomph, they you have a current limited amp. If the amp can deliver the extra current, you now have a 160 watt amp into 4 ohms instead of an 80 watt amp.

Magnepan 1.2 and 1.6 speakers are 4 ohm units so will demand more current from a transistor amp than an 8 ohm speaker. Some transistor amps will be challenged by this and not respond well.

Most tube amps run their output tubes through a transformer that changes the output voltage to match the impedence of the speaker. The max current stays constant whether 4 or 8 ohms (assuming the output transformer has both taps) but the max voltage changes. An 80 watt tube amp will deliver the same watts & current to both a 4 ohm or 8 ohm speaker, but the 4 ohm speaker will get double the voltage.

The max current stays constant whether 4 or 8 ohms (assuming the output transformer has both taps) but the max voltage changes. An 80 watt tube amp will deliver the same watts & current to both a 4 ohm or 8 ohm speaker, but the 4 ohm speaker will get double the voltage.

Not exactly, max power stays the same but both voltage and current change.

The 4 ohm doesn't get double the voltage of the 8 ohm, that would produce 8 times the power.

The 4 ohm gets .707 of the voltage that an 8 ohm speaker gets and draws 1.414 times more current. This keeps the power the same for both.

Herman, thanks for the correction. Shows the perils of replying off the top of one's head.