Amplification Reviews

When I measured the tiny CIA D-100 for Wes Phillips' review in August (p.113), I found that while this class-D power amplifier met its 100W power specification into 8 ohms, it failed to do so into 4 or 2 ohms. I thus recommended that the amplifier not be used with speakers whose impedances dipped below 4 ohms. CIA's Dusty Vawter addressed this point in his "Manufacturer's Comment" in that issue (p.141): "the low figures shown at 2 and 4 ohms are not due to lack of headroom, but are a result of premature current limiting. This has been addressed in our current production model, and the D-100 is very comfortable driving difficult loads."

I asked Vawter to send me samples of the revised D-100; when they arrived, I hoisted them onto my test bench. Again, with no signal present, the amplifier's output still featured around 250mV of ultrasonic noise, this resulting from the switch-mode output stage. But I noticed a difference when I started the 60-minute conditioning period, running the D-100 at 1/3 power into 8 ohms. Whereas the original sample had started out cold with about 0.05% THD+noise (including the ultrasonic spuriae), this increasing to 0.1% after an hour, the new sample began at 0.027% and increased only very slightly, to 0.0285%: a promising beginning. It also was a little warmer at the end of the hour than the first sample had been.

I then measured the power available at clipping (1% THD+N) into loads ranging from 8 ohms down to 2 ohms (fig.1). The amplifier delivered 110W into 8 ohms (20.4dBW), but now 200W into 4 ohms (20dBW) compared with the original's 105.5W (17.2dBW), and 300W into 2 ohms (18.75dBW) compared with 55W (11.4dBW).

Fig.1 Channel Islands Audio D-100, revised sample, distortion (%)vs 1kHz continuous output power into (from bottom to top at 1W): 8, 4, and 2 ohms.

I then connected a sixth-order low-pass filter set to 30kHz between the output of the amplifier and the analyzer in order to eliminate the ultrasonic noise, and examined how the D-100's percentage of small-signal audioband THD+N varied with frequency. The results are shown in fig.2 as the lower pair of traces. The upper traces show the results of this measurement for the original sample. The second sample is clearly very much better—10 times better into all loads!

Fig.2 Channel Islands Audio D-100, original sample (top) and revised sample (bottom), THD+N (%)vs frequency at 2.83V into (from bottom to top): 8, 4, and 2 ohms, with sixth-order low-pass filter at 30kHz in front of analyzer.

With its adjusted current limiting, the Channel Islands Audio D-100 now meets its specified output power into lower impedances. I withdraw my caveat about using it into such loads. The D-100 is a nice little amplifier that appears to offer the benefits of a switching output stage—small size, cool running—with none of the downside.— John Atkinson