Solid State Power Amp Reviews

I measured the Moscode 402Au using Stereophile's loaner sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). Before performing any tests on an amplifier, I run it for 60 minutes at one-third its specified power into 8 ohms, which is thermally the worst case for an amplifier with a class-B or -AB output stage. The Moscode 402Au got very hot during this period, with its shrouded heatsinks way too hot to touch and the chassis overall reaching around 122°F (50°C). It kept working, however, and after an hour the measured distortion in its output had risen only slightly, which suggests that the amplifier had reached thermal equilibrium and thus has adequate heatsinking.

The voltage gain will depend on which front-end tubes are used. With the supplied 6H30Pis and 6GU7s, the gain was 29.4dB into 8 ohms, a little higher than usual for US-designed amplifiers. The output preserved absolute polarity; ie, was non-inverting. The Moscode met its specified input impedance of 100k ohms at 1kHz, but was somewhat lower at the frequency extremes: 71k ohms at 20Hz and 76kHz at 20kHz. This will not be significant in practical use, as all the values are high enough not to load down the partnering preamplifier in any meaningful way.

The 402Au's output impedance was a little on the high side for a design with a solid-state output stage, at 0.22 ohm across the audioband. Even so, the modification of the amplifier's frequency response that results from the interaction of this source impedance with how a loudspeaker's impedance varies with frequency is modest, at ±0.15dB (fig.1, gray trace). Notable in this graph is the fact that, especially into higher impedances, a slight peak develops between 60 and 100kHz. The exact frequency of the peak and its height depend on the load impedance, and this peak gives rise to a degree of overshoot in the amplifier's squarewave response (fig.2), though this is well damped.

Fig.1 Moscode 402Au, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (0.25dB/vertical div.)

Fig.2 Moscode 402Au, small-signal 10kHz squarewave into 8 ohms.

Channel separation was excellent, at 85dB in both directions below 2kHz. Though the separation decreased at high frequencies, it was still 76dB L–R and 69dB R–L at 20kHz, which is good. The 402Au was quite quiet, with its unweighted, wideband background noise lying at –77.8dB (ref.1W into 8 ohms with the input shorted). Reducing the measurement bandwidth to 22Hz–22kHz improved the signal/noise ratio to 95.5dB and adding an A-weighting filter increased it to 100dB, both factors suggesting that there is some ultrasonic noise present in the amplifier's output.

The Moscode 402Au more than met its specified power outputs of 200Wpc into 8 ohms (23dBW) and 300Wpc into 4 ohms (21.8dBW), clipping (defined as 1% THD+noise) at 210Wpc into 8 ohms (23.2dBW) and at 301Wpc into 4 ohms (21.8dBW). Fig.3 shows how the THD+N percentage varies with output power into 8 and 4 ohms (but not 2 ohms, due to the amplifier having problems driving loads below 4 ohms). The distortion is very low below 1W but rises in a fairly linear manner with output power. It is better behaved into 8 ohms than into 4, the latter rising above 0.1% THD+N above 15W output.

Fig.3 Moscode 402Au, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4 ohms.

Fig.4 shows how the THD+N percentage changes with frequency, taken at a power level that fig.3 had indicated was where the actual distortion was higher than the background noise. While the THD+N was low with both channels driving 8 ohms, the left channel (blue trace) was very much better behaved than the right (red). Peculiarly, the situation reversed into 4 ohms, with the left channel (cyan) now a little less linear than the right (magenta). Commendably, the 402Au's THD+N percentage doesn't increase by much in the top audio octave into higher impedances; but into 2 ohms, the amplifier is definitely in trouble even at this relatively low output level, its THD lying at at least 3%.

Fig.4 Moscode 402Au, THD+N (%) vs frequency at 10V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).

What matters more about distortion than its absolute level is its spectral content. At fairly low powers, that content is predominantly the relatively benign third harmonic (fig.5), though some higher harmonics are present. At higher powers, however, a veritable picket fence of harmonic products is present (fig.6), with the right channel (red trace) showing more higher-order, even-numbered harmonics than the left (blue), which suggests that the matching between the complementary output devices is less good in that channel. The right channel was also less linear than the left on the demanding high-power high-frequency intermodulation test (fig.7), with more high-order products evident. Even so, the highest-level intermodulation products, at 18 and 21kHz, lie at almost –70dB (0.03%), and were still below –60dB (0.1%) at higher powers into 4 ohms.

Fig.5 Moscode 402Au, 1kHz waveform at 9.6W into 4 ohms (top), 0.066% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Moscode 402Au, spectrum of 50Hz sinewave, DC–1kHz, at 109W into 8 ohms (left channel blue, right red; linear frequency scale).

Fig.7 Moscode 402Au, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 100W peak into 8 ohms (linear frequency scale).

The Moscode 402Au has some measured idiosyncrasies, but its problems are generally mild enough in degree or in nature not to lead to audible problems. However, this amplifier should definitely not be used with speakers having impedances that drop below 4 ohms.—John Atkinson