Sonic Frontiers Power 3 monoblock power amplifier Measurements

Sidebar 3: Measurements

A full set of measurements was made on the Sonic Frontiers Power 3 in its balanced mode, with selected measurements repeated in unbalanced (single-ended) mode. Unless noted otherwise, all measurements are for balanced configuration.

Following its one-hour warmup, the Power 3 was typically hot for a tube amplifier. Its input impedance measured a high 198k ohms (99.3k ohms unbalanced), its output impedance a maximum of 0.17 ohm at 20Hz and 1kHz, increasing to a maximum of 0.22 ohm at 20kHz. These are very low—and very good—values for a tube amplifier, and indicate that the Power 3's performance should be less affected by real-world loudspeaker loads than most of the breed.

The Power 3's voltage gain measured 22.9dB (22.8dB unbalanced) with the output transformer set for 4 ohm operation. This is lower than that of most amplifiers; you should ensure that the preamplifier you intend to use will provide adequate overall gain in your system.

DC offset was unmeasurable. Pin 2 of the balanced input is positive, and the Power 3 is noninverting from its unbalanced SE+ input. The signal-to-noise ratio measured 84.5dB from 22Hz to 22kHz, 83.1dB from 10Hz to 500kHz, and 96.3dB A-weighted. The unbalanced results were within 1dB of these figures.

Fig.1 shows the Power 3's frequency response at 1W into 8 ohms. Note how small the deviations are into our simulated load. The 10kHz squarewave is shown in fig.2. There is a small overshoot, and some ringing at about 110kHz. The latter is also visible on the 1kHz squarewave (not shown), but should have no audible consequences.

Fig.1 Sonic Frontiers Power 3, balanced, frequency response at (from top to bottom at 6kHz): 1W into 8 ohms, 2W into 4 ohms, and 2.828V into simulated loudspeaker load, (0.5dB/vertical div.).

Fig.2 Sonic Frontiers Power 3, small-signal 10kHz squarewave into 8 ohms.

The Power 3's THD+noise vs frequency performance is plotted in fig.3—a good result for a tube amplifier, though the inevitable reduction in linearity above the audioband makes its presence known in this graph. The waveform of the distortion at 2W into 4 ohms (at 1kHz) is shown in fig.4. The result is heavily second-harmonic, plus noise.

Fig.3 Sonic Frontiers Power 3, balanced, THD+noise (%) vs frequency at (from top to bottom at 6kHz): 4W into 2 ohms; 2.83V into simulated loudspeaker load; 2W into 4 ohms; and 1W into 8 ohms.

Fig.4 Sonic Frontiers Power 3, 1kHz waveform at 2W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 shows the spectrum of the Power 3's output in response to a 50Hz input, taken at a 147W output into a 4 ohm load. The second harmonic (100Hz) is -54.2dB (about 0.2%), the third (150Hz) is -63.6dB (0.07%). Fig.6 shows a similar spectral analysis, using a combined 19+20kHz input signal at an output of 96W into 4 ohms. (The amplifier will clip above this power with this input signal.) The largest artifact, at 1kHz, is -56.3dB (0.15%). The results of figs.5 and 6 are respectable for a tube amplifier, but not really exceptional.

Fig.5 Sonic Frontiers Power 3, spectrum of 50Hz sinewave, DC-1kHz, at 147W into 4 ohms, 4 ohm tap (linear frequency scale).

Fig.6 Sonic Frontiers Power 3, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 96W into 4 ohms, 4 ohm tap (linear frequency scale).

The THD+noise vs continuous output power measurements are plotted in fig.7; the discrete clipping levels are shown in Table 1. (It didn't prove possible to carry out pulse testing using the Miller Audio Research Amplifier Profiler.)

Fig.7 Sonic Frontiers Power 3, distortion (%) vs continuous output power into (from bottom to top at 2kHz) 4 ohms, 8 ohms, and 2 ohms (4 ohm tap).

Table 1 Sonic Frontiers Power 3: Clipping(1% THD+noise at 1kHz)

Load ohmsW (dBW) (L)
8143.7 (21.6)
4203.3 (20.1)
2229.6 (17.6)

The Power 3's test-bench results are good for a high-powered tube amplifier. The single exception is that it fell short of its specified power into 8 ohms, even at the 3% THD+noise level. This is presumably due to the fact that, as supplied for review, the amplifier was configured for 4 ohm operation.—Thomas J. Norton

Sonic Frontiers
205 Annagem Blvd.
Mississauga, Ontario
Canada L5T 2V1
(905) 362-0958