Hovland Sapphire power amplifier Measurements part 2
Fig.7 Hovland Sapphire, 4 ohm tap, 1kHz waveform at 1.8W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Hovland Sapphire, 4 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 27W into 8 ohms (linear frequency scale).
Things look a little better at higher frequencies and slightly lower powers (fig.9), but, as confirmed by the demanding high-frequency intermodulation test (fig.10), the Sapphire is not a low-distortion amplifier. Even though this test was taken a couple of dB below the point where visual clipping of the waveform peaks sets in on the oscilloscope screen, the 1kHz difference component lies at a high -55dB (0.18%), and the higher-order products at 18kHz and 21kHz are at -45dB (0.55%).
Fig.9 Hovland Sapphire, 4 ohm tap, spectrum of 1kHz sinewave, DC-1kHz, at 22W into 8 ohms (linear frequency scale).
Fig.10 Hovland Sapphire, 4 ohm tap, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 17W into 4 ohms (linear frequency scale).
Figs.11-13 show the change in percentage of the Sapphire's THD+N from its 4, 8, and 16 ohm taps, respectively, as the output power is increased into 8, 4, and 2 ohm loads. Again, the amplifier delivers respectably low distortion at useful output powers only when the load is not much less than the rated output-transformer tap. And if clipping is defined as the usual 1% THD+N—which equates to easily visible squaring of the waveform on an oscilloscope screen—the Sapphire fails to meet its specified power delivery of 40W. Only if the definition of "clipping" is relaxed to 3% THD+N does this happen, with 40W available into 4 ohms from the 4 ohm tap (12.9dBW), 49W available into 8 ohms from the 8 ohm tap (16.9dBW), and 53W into 8 ohms from the 16 ohm tap (17.25dBW).
Fig.11 Hovland Sapphire, 4 ohm tap, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, 2 ohms.
Fig.12 Hovland Sapphire, 8 ohm tap, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, 2 ohms.
Fig.13 Hovland Sapphire, 16 ohm tap, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, 2 ohms.
Overall, while I agree with Michael that the Sapphire is drop-dead gorgeous, its measured performance is disappointing. Just because an amplifier uses tubes doesn't mean it also has to have hum, an ultrasonic resonance, or fairly high levels of distortion.—John Atkinson