Krell Full Power Balanced 350mc monoblock amplifier Measurements

Sidebar 3: Measurements

Of necessity, all the measurements on the FPB 350mc were performed using its conventional, voltage-drive balanced input. The voltage gain into 8 ohms was 26.3dB and the amplifier didn't invert signal polarity (pin 2 of the XLR wired as "hot"). The input impedance was usefully higher than specified, at 187k ohms, while the output impedance was 0.18 ohms at 1kHz and 20kHz, 0.15 ohms at 20Hz. This is a little higher than we usually find with solid-state designs, indicating low overall negative feedback, and resulted in ±0.1dB of response modification with our simulated loudspeaker load (fig.1).

Fig.1 Krell FPB 350mc, frequency response at 1W into 8 ohms, 2W into 4 ohms, and 2.83V into dummy loudspeaker load (5dB/vertical div.).

The small-signal response was otherwise flat across the audioband into resistive loads, with an ultrasonic -3dB point above 150kHz. The 1kHz squarewave response (not shown) was essentially perfect, as was the 10kHz waveform, though there was just a hint of overshoot on the very fast leading edges (fig.2), which later turned out to be due to the digital oscilloscope drifting out of adjustment.

Fig.2 Krell FPB 350mc, small-signal 10kHz squarewave into 8 ohms.

This is a very quiet amplifier, with an A-weighted S/N ratio of more than 95dB. This is referenced to 2.83V output; this is equivalent of more than 130dB referenced to the amplifier's clipping point into 8 ohms! Even without the A-weighting network and extending the measurement bandwidth to 500kHz only worsened the S/N ratio to 84dB (again re. 2.83V).

Levels of harmonic distortion were also very low, particularly into higher impedance loads. The bottom trace in fig,3, for example, was taken at 2.83V into 8 ohms, and doesn't vary from 0.005% across the band. Reducing the load to 4 ohms increased the THD figure slightly to 0.007%, though into 2 ohms, the distortion went up to 0.03%.

Fig.3 Krell FPB 350mc, THD+noise (%) vs frequency at (from top to bottom at 4kHz): 4W into 2 ohms; 2.83V into simulated loudspeaker load; 2W into 4 ohms; and 1W into 8 ohms (right channel dashed).

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