Tube Power Amp Reviews

Sidebar 3: Measurements

The Woodside MA50's chassis was hot over the output tubes following the 60-minute, 1/3-power preconditioning, but no more so than the typical tube amplifier. Its input impedance measured just over 92k ohms, its voltage gain into 8 ohms, 29.7dB. The Woodside is non-inverting; a positive-going input results in a positive-going output. S/N ratio measured an excellent 110dB (unweighted, 22Hz–22kHz referenced to 1W output into 8 ohms). The amplifier's DC offset was unmeasurable, as is almost always the case with transformer-coupled designs.

The Woodside's output impedance was respectably low for a tube amplifier: 0.28 ohms at 20Hz, 0.23 ohms at 1kHz, and 0.18 ohms at 20kHz. With a loudspeaker cable of a reasonably low resistance, I would expect the Woodside's frequency response driving a typical loudspeaker load to be relatively unaffected (altered by less than 1dB, footnote 1).

Fig.1 shows the MA50's frequency response. It holds up well at lower frequencies. At the top end, it begins rising at 10kHz, rising to a peak just above 60kHz (a slightly higher frequency into 4 ohms). Though not severe in magnitude, this peak is reflected in the leading-edge overshoot and oscillation in the 1kHz squarewave response (fig.2). It's also visible in the 10kHz squarewave (fig.3) as a damped oscillation at the top and bottom of the waveform occurring at approximately 60kHz. This type of behavior is uncommon in today's better amplifiers, both tube and solid-state, and is most likely an output transformer problem.

Fig.1 Woodside MA50, frequency response at 2.83V into 8 ohms (top at 20kHz) and 4 ohms (0.5dB/vertical div.).

Fig.2 Woodside MA50, 1kHz squarewave at 1W into 8 ohms.

Fig.3 Woodside MA50, 10kHz squarewave at 1W into 8 ohms.

Rising distortion at the frequency extremes is visible in the THD+noise curves in fig.4, though it remains reasonable—at least into loads down to 4 ohms. The THD+noise at low power (not shown) is primarily second-harmonic. However, at higher power (41W) into 8 ohms (fig.5), a heavy fifth-harmonic component appears. Into 4 ohms, the low-power THD is also primarily second-harmonic; but as power increases, the third harmonic begins to dominate (fig.6, 25W). Into 2 ohms (not shown), the third harmonic is clear at powers as low as 10W.

Fig.4 Woodside MA50, THD+noise vs frequency at (from top to bottom): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms.

Fig.5 Woodside MA50, 1kHz waveform at 41W into 8 ohms (top); distortion and noise waveform with fundamental notched out (bottom).

Fig.6 Woodside MA50, 1kHz waveform at 25W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom).

Fig.7 shows the distortion spectrum with the MA50 reproducing 50Hz at 35W into 4 ohms. While it will become apparent by the end of these measurements that the Woodside is at its best into 8 ohm loads, it is specified to put out its rated power into 4 ohms. There's only one set of output terminals, with no apparent provision for internal reconfiguration of the MA50 for other load impedances (as with the Sonic Frontiers SFM-160, reviewed elsewhere in this issue). The results in fig.7 show a peak distortion of —8dB (or 1.2%) at 150Hz. The artifacts decrease with increasing frequency, and at this power level, the dominance of the odd-order harmonics can be seen. This is a fairly high level of distortion, considering the relatively modest power being asked from the amp here; its audible significance might be an added warmth with music heavy in low frequencies.

Fig.7 Woodside MA50, spectrum of 50Hz sinewave, DC–1kHz, at 35W into 4 ohms (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level, at —8dB (1.2%), with the odd-order harmonics dominant.

A similar spectral plot in fig.8 shows the intermodulation (IM) at the output resulting from a combined 19+20kHz input, for an output level of 23W into 8 ohms (just prior to visible clipping of this waveform). The 18kHz and 21kHz artifacts are at about –48dB (or about 0.4%). The 1kHz distortion component is –58dB (or about 0.12%). There are also considerable low-level artifacts visible below 2kHz, these probably due to 60Hz AC-supply modulation with this demanding signal. Into a 4 ohm load at similar power (not shown), the results are similar: slightly higher at low frequencies (0.25% at 1kHz), slightly lower at high frequencies (0.3% at 18kHz).

Fig.8 Woodside MA50, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 23W into 8 ohms (linear frequency scale).

The THD+noise vs output power results (at 1kHz) for the MA50 are shown in fig.9. As with most tube amplifiers, there's a gradual rise in distortion without a well-defined break-point, or "knee," in the curves. The design here is clearly optimum with an 8 ohm load, though above about 58W, the results with a 4 ohm load show a slight superiority. The discrete-clipping measurements for the MA50 are shown in Table 1. Here I measured the output at both our normal standard for clipping—1% THD+noise—and at 3%. Finally, our measurements showed the MA50 to be about 5W (0.5dBW) short of its rated power output into 8 ohms.

Fig.9 Woodside MA50, distortion vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

Table 1: Clipping Power

Load Impedance	1% THD+noise	3% THD+noise
	W (dBW)	W (dBW)
8 ohms	44.8 (16.5)	45.8 (16.6)
(line voltage)	115V	115V
4 ohms	29.7 (11.7)	49.6 (14)
(line voltage)	115V	115V
2 ohms	13.6 (5.3)	20.5 (7.1)
(line voltage)	116V	116V

The test-bench results of the Woodside MA50 Renaissance were reasonable, though nowhere near exceptional, for a moderately powered tube amplifier.—Thomas J. Norton

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Footnote 1: See "Questions of Impedance Interaction" in the January '94 Stereophile (Vol.17 No.1, p.109), and John Crabbe's letter in response to this article (April '94, pp.25–26), for more details on the possible frequency-response aberrations in amplifiers of various output impedances driving real-world loudspeaker loads.