Solid State Power Amp Reviews

Before performing tests on an amplifier, I thermally stress it by running it for 60 minutes at one-third the specified power into 8 ohms, which is the worst case for an amplifier with a class-B or-AB output stage. Eyeballing the Musical Fidelity Supercharger 550K, it doesn't appear to have enough heatsink area for a 550W design. This, in fact, proved to be the case—it shut down after 18 minutes of operation at 191W into 8 ohms. The chassis was way too hot to touch and the amplifier's orange Temperature LED was illuminated. Letting it cool down for 30 minutes brought the 550K back to life. In the Supercharger's defense, it is intended to increase the dynamic range of its owner's regular amplifier by allowing large-amplitude transient peaks to be reproduced without clipping, so the reduced amount of heatsinking is justifiable.

The voltage gain into 8 ohms was high from the unbalanced line input, at 29.9dB, but, as specified, was much lower from the speaker-level input terminals: 5.5dB. This is a little low to allow smaller tube amplifiers to drive the 550K fully to its limit. However, looking at the Supercharger's clipping power into 8 ohms (see later), it sets an upper limit of around 175W for the driving amplifier, which is fair. The input impedance at these terminals was 54 ohms, which will not load down the driving amplifier to any significant extent. However, some tube amplifiers will develop an ultrasonic peak in their response when faced with such a load; such amplifiers should be used from their highest-output transformer tap with the Supercharger. The input impedance at the RCA line-level jack was a usefully high 49k ohms. Unusually, this input inverted signal polarity. (I neglected to measure the polarity at the speaker-level inputs, but as these are floating with respect to ground, it should be possible to connect them either way around.)

The amplifier's output impedance was a low 0.1 ohm at low and midrange frequencies, rising slightly to 0.2 ohm at 20kHz. As a result, the Ohm's Law modification of the amplifier's frequency response, due to the interaction between the output impedance and that of the speaker, was very small, at ±0.1dB (fig.1, top trace at 2kHz). This graph reveals that the Supercharger has a wide small-signal bandwidth into 8 ohms, with a –3dB point of 148kHz, this correlating with the excellent shape of a 10kHz squarewave into 8 ohms (fig.2). However, the –3dB frequency drops to 55kHz into 2 ohms, with a 0.75dB drop at 20kHz (fig.1, bottom trace).

Fig.1 Musical Fidelity Supercharger 550K, frequency response at 2.83V into (from top to bottom at 2kHz): simulated loudspeaker load, 8, 4, 2 ohms (0.5dB/vertical div.).

Fig.2 Musical Fidelity Supercharger 550K, small-signal 10kHz squarewave into 8 ohms.

Measured via its line input with that input short-circuited, the Musical Fidelity's unweighted wideband signal/noise ratio (ref. 1W into 8 ohms) was a little on the low side at 69.6dB, but this improved to a superb 90.3dB when restricted to the audioband, and improved even further, to 94.1dB, when A-weighted.

And this is without taking the 550K's very high output power into account. Fig.3 plots the percentage of THD+noise in the amplifier's output against continuous output power into loads ranging from 2 ohms (top trace) to 8 ohms (bottom). Defining clipping as our usual 1% THD+N, the Supercharger exceeded its specification by clipping at 650W into 8 ohms (28.1dBW), 850W into 4 ohms (26.3dBW), and 1kW into 2 ohms (24dBW). (The AC supply voltage, which I don't regulate, dropped from 125V at idle to 123V at clipping with the final measurement.) The flattening-out of the traces above 10W in this graph suggests that that is where the distortion begins to emerge from the amplifier's noise floor. I therefore plotted how the Supercharger's THD changes with frequency at a higher output level than usual (20V, equivalent to 50W into 8 ohms). The results are shown in fig.4: the THD into 8 ohms (bottom trace) is very low in the midrange and bass regions, but rises both into lower impedances and at higher frequencies, where the amplifier's open-loop gain margin starts to diminish. Even so, in the worst case, 20kHz into 2 ohms, the THD percentage is still low, at 0.07%.

Fig.3 Musical Fidelity Supercharger 550K, distortion (%)vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms.

Fig.4 Musical Fidelity Supercharger 550K, THD+N (%)vs frequency at 8V into (from bottom to top): 8, 4, 2 ohms.

The spectral content of that distortion is heavily third-harmonic in nature, which will further work against audibility (fig.5). Increasing the output power to a couple of dB below clipping into 4 ohms, the third harmonic is joined by some other low-order harmonics (fig.6), but the levels of these harmonics remain low. Note also in this graph that the 120Hz power-supply component just kisses the –100dB line (0.001%), which suggests that the Supercharger's power supply is hardly breaking a sweat with this low-frequency signal.

Fig.5 Musical Fidelity Supercharger 550K, 1kHz waveform at 20W into 4 ohms (top), 0.00214% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Musical Fidelity Supercharger 550K, spectrum of 50Hz sinewave, DC–1kHz, at 600W into 4 ohms (linear frequency scale).

Repeating the spectral analysis with the very demanding equal mix of 19 and 20kHz tones, again at 600W into 4 ohms (fig.7), the 1kHz difference component lies more than 90dB down from the signal level (<0.003%), though now 120Hz sidebands appear around the two high-frequency fundamental tones.

Fig.7 Musical Fidelity Supercharger 550K, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 600W peak into 4 ohms (linear frequency scale).

Like Musical Fidelity's kW750 monoblock—on which, I understand, its circuit is based—the Supercharger 550K offers a superb set of test results. In fact, its measured performance is actually a little better than that behemoth's. In its intended role of "supercharging" its owner's low-power amplifier, the Supercharger should have no problem performing as promised.—John Atkinson