Posted by hjp on December 03, 2004 at 16:16:58:
In Reply to: A-3 W/ KRELL FPB 300 posted by mike brilla on December 02, 2004 at 18:20:47:
I breifly owned a FPB-600. It had sufficient power to play well over 100 dB at my listening seat (U-1's c:2001). Sound was quite good, only a little "hard" for my tastes (I'm a tube guy). I maybe could have lived with it but the one thing I wouldn't tolerate though was the temperature it ran at.
Since it employs ramped biasing, musical demands in the 90dB + range caused it to bias at or nearly at the full class A 600 watts/channel (actual power is 900+ watts/channel). Krells with this biasing ramp up instantly but ramp down over a 30 second or so delay to prevent "pumping", hence they hold high bias for extended periods if you use their power, even for momentary peaks. By the way, if you actually attempt to use the amp's power, you will have to install a large dedicated 120 V circuit for it (even if operated at 240 volts) to keep from poping 20 amp breakers (just calculate the wall outlet current draw for 1800 watts of output power [input will be much higher].......approx. 900 watts output power for the FPB-300 ).
This caused the amp to run at over 160 degrees F with a 24" window fan blowing directly on the amp from 2 feet away at the highest speed!! Rediculous!! I was worried adjacent furniture would be scorched from being near it. If you touched it, even momentarily, you might get burned!! Krell confirmed everything was working fine. Too much for me. I sold the amp.
The moral? If you actually use this much power on a highly reactive capacitive load like SoundLab, since there is little to no actual resistance in the speaker to dissipate the power into, much of it will be reflected back to the amp and disipated in its own output stage. If its a big class A amp, its going to get very very hot. Word to the wise