Legend Audio Design Starlet integrated amplifier Measurements part 3
Fig.7 Legend Starlet, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 6W into 4 ohms (linear frequency scale).
Out of morbid curiosity, I captured the waveform of this signal (fig.8). The 19 and 20kHz tones sum to produce a combined waveform with a peak amplitude twice that of either wave on its own, and the waveform's envelope is modulated at a frequency of 1kHz. (Because the 1kHz modulation affects only the envelope of the waveform, there should be no actual 1kHz content in the amplifier's output.) As I hope you can still see at the scale at which this graph must be reproduced in the magazine, the waveform as output by the Starlet is severely asymmetrical, its positive-going half being significantly larger than its negative half. Whenever you see such asymmetry, there will always be even-harmonic content, which is actually more typical of a single-ended design than the push-pull Legend. (Perfect push-pull operation cancels even harmonics by ensuring signal symmetry.)
Fig.8 Legend Starlet, HF twin-tone waveform from fig.7.
Finally, I examined how the Starlet's distortion varied with output power, both with continuous 1kHz tones with both channels driven (fig.9) and with a low-duty-cycle 1kHz toneburst and only one channel driven (fig.10). In either case, the Legend amplifier's output at the usual 1% THD+N point is laughably low: between 1W and 2W! Relaxing the definition of clipping to 3% THD+N gives figures closer to the manufacturer's specification, especially under toneburst conditions—33.5W into 8 ohms (15.25dBW), 52W into 4 ohms (14.2dBW), with a maximum current of 3.2A RMS—but this is clearly very nonlinear amplifier design.
Fig.9 Legend Starlet, distortion (%) vs continuous output power into (from bottom to top at 2kHz): 8 ohms, 4 ohms, and 2 ohms (both channels driven).
Fig.10 Legend Starlet, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).
Or is it? Early on in the measurements, I became suspicious of the active preamp stage and made sure I used small input signals with the volume control full up, even though this maximized the contribution of the hum. And yes, when I specifically checked the effect of the volume control after I had finished the usual set of measurements, it turned out that the Starlet's active preamp stage is actually a distortion generator: The smaller the input signal and the higher the volume-control setting, the less distortion is produced. (Both channels behaved identically, so I assumed this is probably not due to something being broken. However, Legend later claimed that a preamplifier tube was not working properly—see their "Manufacturer's Comment" letter.) With the volume control set to 2:00, an input level of 100mV had 0.4% of added distortion—this is acceptable, given the strong second-harmonic nature of the distortion. But when the input level was increased to 1V and the volume control backed off to give the same output level, the distortion increased to 1%. And when I fed the amplifier with 2V (the typical maximum output of a CD player), and backed off the volume control even further, again to keep the output level constant, the distortion was a distressingly high 6.6%!
The test conditions for figs.9 and 10 involve keeping the volume-control setting constant and driving the amplifier with increasing input levels. But with what appears to be a saturating preamp section, the Starlet's graphs are actually dominated by the preamp's nonlinearity, at least until the hard-clipping point. I therefore suspect that the Starlet's power-amplifier section is actually more linear than this set of measurements suggests. But add a variable distortion generator in front of that power amp—Chip was correct to call it a "tone machine"—and the result is anybody's guess. With a high-level source like CD, each musical peak will be accompanied by bursts of extreme second-harmonic and intermodulation distortion that might sound "right" and even compensate under some circumstances for the prematurely rolled-off top octave, but will ultimately be fatiguing. In my opinion.—John Atkinson