December 2001

Krell FPB-300c Stereo Amplifier: Measurements

All amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test amplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

• Measurements were made with 120V AC line voltage.
• Power output plotted with both channels driven.
• Gain: 20.9x, 26.4dB.
• Output noise, 8-ohm load: wideband 0.227mV, -80.0 dBW; A weighted 0.054mV, -92.5 dBW.
• AC line current draw at idle: 4.7A, 3.3A when driven hard and hot.
• Output impedance: 0.069 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 470W
• 8-ohm load at 10% THD: 520W
  
  
• 4-ohm load at 1% THD: 800W
• 4-ohm load at 10% THD: 950W

General

The Krell FBP-300c has a wide bandwidth of over 200kHz. Its output impedance is typical of many solid-state power amplifiers and, consequently, the difference between open circuit and 4-ohm loading is quite small. This can be seen in Chart 1. This means that the frequency response into a speaker load will be quite invariant with frequency. This design has what is termed by Krell as "Sustained Plateau Bias II," with the consequence that the amount of output stage current is increased in several steps as the signal level increases. In Chart 2 this has the effect of changing the amount of distortion as these transitions occur. Chart 2 also shows that the rated power of 300W into 8 ohms and 600W into 4 ohms is easily met before clipping occurs (where the distortion rises abruptly). Chart 3 illustrates the desirable characteristic of a low amount of rise in distortion as frequency increases. The low output impedance translates in to a high damping factor as seen in Chart 4. In Chart 5, the harmonic structure for the 10W signal is desirably simple in nature with no harmonics rising above the noise level above the 5th harmonic.

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load

(line up at 1W to determine lines)
Top line: 4-ohm THD+N
Second line: 8-ohm THD+N
Bottom line (red): 8-ohm SMPTE IM

4-ohm output loading
Cyan line: 600W
Blue line: 100W
Magenta line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 4-ohm load

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