watts or joules?

Watts. The output power of an amplifier is never specified in joules. Joules refers to the amount of energy storage that is in the filter capacitors of the power supply. Generally a higher number is better, but will not tell you much except in comparison to other amplifiers of similar power.

Being that there are a variety of other things that can also affect the way an amplifier sounds, I would not put too much weight on joules alone. Like any other human endeavor, it pays to educate one's self about what is possible rather than hang your hat on one specification over all others!

Above all have fun!

my question still remains,why does the arc ca-50 sound not no where as good as the arc vt-50,arc tell me that they are identical except for the joules,and that joules or current will give you transients,attack,and hidden nuances,and that makes all the diff,. Rogue audio make a tempest at 90 watts per but they do not address the joules question,I am let ato believe that while many amps are great arc is the best!,Nick

"My question still remains ..."

My simplistic response is that an amp with a 50 watt rating might not be able to handle certain music peaks. 50 watts alone might not be enough. The 50 watt rating is how much power the amp will deliver on a steady state basis into a purely resistive 8 ohm load (versus a more complex load like a real speaker) at a mid-band frequency. What happens with a complex load or a load where the impedance drops below 4 ohms? The amp might run out of steam. The joule rating applies to the power supply, actually to its capacitors, and gives a measure of how resilient the amp is when dealing with complex music material with significant peaks when driving a complex load like most speakers. I won't repeat all the fine explanations of the preceding posters regarding what a joule is. My bottom line is that an amp rated at 50 watts might not be powerful enough for some music peaks. Whether it can perform well depends on design decisions that can be partially described by certain statistics, like the power supply's joule rating.

The VT 50 has a better power supply.....

http://www.arcdb.ws/VT50/VT50.html

http://www.arcdb.ws/CA50/CA50.html