Krell Full Power Balanced 350mc monoblock amplifier:
Measurements part 2
Fig.4 reveals that at low levels, this distortion is primarily third-harmonic. (The amplitude of the lower trace in this graph is very much exaggerated for clarity; its absolute level was below 0.005%.) At very high levels (fig.5), the third harmonic is joined by a regularly decreasing series of higher odd harmonics, though these are all at or below -80dB! Intermodulation distortion (fig.6) was also low, even at very high power levels—this graph was taken just below the visible clipping point into 4 ohms.

Fig.4 Krell FPB 350mc, 1kHz waveform at 53W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 Krell FPB 350mc, spectrum of 50Hz sinewave, DC-1kHz, at 414W into 4 ohms (linear frequency scale).

Fig.6 Krell FPB 350mc, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 342W into 4 ohms (linear frequency scale).

Finally, fig.7 shows how the FPB 350mc's THD+noise percentage changes with output power into 8, 4, and 2 ohms. Below 1W, the upward slope of the traces with decreasing power indicates that the measured percentage is dominated by noise; above that level, harmonic distortion dominates and reveals a maximum in the 10-20W region before the amplifier starts to clip, revealed by the knee in these curves. Formally defining clipping as the 1% THD+noise figure gave maximum power delivery of a monstrous 475W into 8 ohms (26.8dBW), way above the specified power. 850W was available into 4 ohms (26.3dBW) and while I measured 1060W into 2 ohms (24.2 dBW) rather than the specified 1400W, the AC line in our Santa Fe office was sagging significantly for this measurement.

Fig.7 Krell FPB 350mc, distortion (%) vs continuous output power into (from bottom to top at 2kHz) 8 ohms. 4 ohms, and 2 ohms.

While digital sources these days can achieve better than 16-bit dynamic range, amplification components have been lagging behind, in my opinion. But Krell's massive FPB 350mc joins that select few band of analog components that have a measured performance equal to the demands of high-resolution digital.—John Atkinson