Slew Rate?

Another spec closely related to slew rate, and usely more widely published, is bandwidth. Amplifiers with wider bandwidths are faster and equate to larger valued (faster) slew rate specs. An amplifier that can reach 100kHz is faster, and would have a better slew rate spec, than one that goes only to 50kHz - the higher the number the better.

Current is the electrical property that is needed to drive speakers well, not voltage. Producing lots of current for a given power rating is part of what makes better amplifiers heavier and costlier. Current specs are rarely published, so one way to get an idea of an amp's current capacity (besides opening the amp and looking at the fuses) is to compare power ratings at different speaker impedances. An amp rated at 100W into 8ohms and 165W into 4ohms has more current capacity than an amp with 100W into 8ohms and 125W into 4 ohms. Please keep in mind though, that this only works if manufacturers aren't misleading when publishing their amp's specs. Product reviews where specs are measured are a good source of data for comparisons.

I have Magnepan planer speakers and use amps with high current capacity to drive them, with excellent results. Keep in mind that when choosing a high current amp, planer speakers need a certain amount of minimum overall power as well. I would recommend 100W as a minimum, as long as current capacity is there, but more would be better.

Hope this helps. :o)

The slew rate states the voltage increase the amp is capable of in one microsecond. This is important for very dynamic recordings (classical music has the greatest dynamics)and becomes more important the more inefficient the connected speakers are.
It is in no way related to an amplifiers bandwidth.
I've seen amps with a bandwidth of 10Hz-35kHz and a slew rate of 45V/microsec and amps that do 1Hz-175kHz and a slew rate of 23V/microsec.

Wattage can be increased by either increasing the voltage capability OR the current capability OR ideally both! The defining part for this in an amp is the powersupply, unfortunately this is also the most expensive part of any amp. The best indicator for a good powersupply is how close the 4ohm output is to the mathematical ideal of twice the 8ohm output.
I know of only one amp that achieves this: the asr emitter. It actually goes one step further and is capable of quadrupling the output into 2ohm.
For something cheaper try QUAD amps, they are designed to run electrostats.

If you like cars you see engines have two ratings torque and horsepower. Sheer horsepower effects top end spped. torque effects low end power.

Similarly,amps have a power output and a current output. This is why when playing a tube amp and solid state amp of similar power the tube amp appears to have more power. Because it generaly puts out more current.
For some reason planar speakers seem to require more current to move thier large diaphrams. The ET is reknown for it's inefficiency. I have found the conrad johnson to be very good in this area. Early Krell and Levinson(company not the man) where also excellent.

The only way that an amplifier can deliver current is by applying voltage to the load. You cannot deliver amperes without volts, so it is not an either/or proposition. However, an amp with a "weak" power supply might be designed to deliver a high voltage that permits signal peaks without clipping, but which, when applied to the load, will result in current that the amp cannot sustain very long. This is why "music power" ratings can be so much higher than continuous rms power ratings. A "weak" power supply produces high rail voltages when the signal is small, but these voltages drop greatly when a large signal persists, because the transformer and rectifier are too small to keep the capacitors charged up. Larger capacitors can help here. "Weak" power supplies actually do make sense because they are economical, and to tell the truth, music signals are characterized by peak voltage far in excess of that which corresponds to the rms voltage. It can't hurt to have a power supply that is capable of pumping out the amps without strain, but unless the audio signal is loud, compressed and peak-limited (pop music perhaps) this capability will not come into play.

The slew rate of an amp will tell you how fast the amp is capable of reproducing an output signal with respect to the input signal. The output signal of an amplifier is similar to an enlarged xerox image of the input signal's magnitude. But the input signal also has a time factor - that is, it has a voltage rise is over a period of time. If the amplifier can produce the enlarged image just as fast as the signal enters the amp, then the slew rate is infinite. If there's a time lag, then that's the slew rate. The higher it is, the quicker it generates the output signal.

The relationship to performance is that the slew rate is related to the amp's POWER bandwidth - not signal bandwidth, as most amps are capable of exceeding 20kHz. The power bandwidth is defined as the highest frequency the amp can reproduce at one-half it's rated power. So the higher the power, the higher the power bandwith and the higher the slew rate needed to accommodate it.

Don't look to slew rates as a measure of how an amp will sound because no one spec will be the defining factor. Slew rate is not a stand-alone indicator of quality; however if an input signal is so quick that the amp can't keep up with it, then there will be transient distortion which may be audible. Two amps with the same power rating but different slew rates may sound different due to slew rate - but more importantly, the one with the lower slew rate will probably have other shortcomings that account for the lower slew rate.