Final Laboratory Music-4 phono preamplifier, Music-5 line preamplifier, & Music-6 power amplifier Measurements part 3

Because the Music-6 is battery-powered, I suspected that it would not have much in the way of output-stage bias current. That this appears to be case is shown by fig.15, which shows the waveform of the distortion riding along with a 1kHz sinewave in the amplifier's output with the feedback at minimum. Although the THD percentage appears moderately low, at 0.13%, note that every time the sinewave crosses the zero line, a large spike overlays the residual waveform. Such classic crossover distortion has been found to be very audible because of the high-order harmonics it introduces. Fortunately, crossover distortion, which is due to the output transistors being completely turned on and off each cycle, decreases as a percentage of the signal as the signal level increases.

Fig.15 Final Music-6, minimum feedback, 1kHz waveform at 1W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.16 shows what happens when the negative feedback is maximized. The measured distortion level has decreased slightly, to 0.098%. However, the distortion waveform has become more spikey-looking, which means that the feedback is shifting energy into the higher-order, more subjectively annoying harmonics. This nicely correlates with Art finding the Music-6 to develop "some unwanted texture or 'grain,' audible up top" with maximum feedback.

Fig.16 Final Music-6, maximum feedback, 1kHz waveform at 1W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Figs. 13 and 14 show that the Music-6 produces low distortion at low frequencies. Fig.17, the spectrum of the amplifier's output while it drove 50Hz at 1W into 8 ohms, shows that the highest-level harmonic is the second, at -71dB (0.03%), with the higher-order harmonics linearly decreasing with increasing order. A quarter-century ago, Franco-Japanese writer Jean Hiraga showed that such behavior correlates with pleasing sound quality. But note what happens at higher frequencies (fig.18): the second harmonic is still the dominant, but has been joined by a regular picket fence of odd-order harmonics. Ugh! A similar picket fence can be seen in the high-frequency intermodulation spectrum (fig.19). This was taken with minimum feedback; the picture worsened with increasing feedback.

Fig.17 Final Music-6, maximum feedback, spectrum of 50Hz sinewave, DC-1kHz, at 1W into 8 ohms (linear frequency scale).

Fig.18 Final Music-6, maximum feedback, spectrum of 1kHz sinewave, DC-20kHz, at 1W into 8 ohms (linear frequency scale).

Fig.19 Final Music-6, minimum feedback, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 4W into 8 ohms (linear frequency scale).

I do have some good news to report: With one channel driven, the Music-6 exceeded its power specification. Fig.20 plots the continuous output power against THD into 16 ohms (red trace), 8 ohms (black), 4 ohms (blue), 2 ohms (green), and 1 ohm (magenta). Defining clipping as 1% THD (noise is ignored by this FFT-based measurement), the Music-6 puts out 17.8W into 8 ohms (12.5dBW) at clipping, and a little more, 20W, into 4 ohms (10dBW). Although the 1 ohm power is just 11.75W, this is equivalent to an RMS current of 3.43A—quite respectable, considering the Music-6's power supply consists of 36 D cells connected in series!

Fig.20 Final Music-6, maximum feedback, distortion (%) vs continuous output power at 1kHz into 16 ohms (red), 8 ohms (black), 4 ohms (blue), 2 ohms (green), 1 ohm (magenta).

Not only was I disappointed with the Music-6's measured performance, it shows that the feedback control can't be used to optimize such things as output impedance without degrading the amplifier's high-frequency linearity. But given the enormous changes that can be wrought in the Music-6's behavior by this control, perhaps the audibility of the things the amplifier's errors can be reduced for a specific loudspeaker. Which is perhaps why Art Dudley's auditioning comments were more concerned with what the amplifier did right than what it did wrong. It is also relevant to note that with Art's very sensitive Lowther-based speakers, he will only be asking the Music-6 to deliver at most a few hundred milliwatts, which will sidestep the amplifier's poor high-frequency linearity problems at higher levels (though the crossover distortion will still be an issue).

Overall, their measured performances strongly suggest that the Final Laboratory components should not be with other manufacturers' products. Their behavior appears to be optimized when they're used with each other.—John Atkinson

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