Solid State Power Amp Reviews

One mbl Reference 9007 amplifier (10125) was preconditioned in balanced monoblock mode by running it at one-third the specified clipping power into 8 ohms for one hour. At the end of that period, the heatsinks were just too hot to keep my hand on, implying a temperature of around 60°C; the chassis was much cooler. The initial THD+noise figure at this power level was 0.016%; after 60 minutes, this had dropped very slightly, to 0.013%.

The 9007's voltage gain into 8 ohms was 27dB in both balanced (mono) and single-ended (stereo) modes, and the amplifier preserved absolute polarity; ie, was non-inverting in both modes. Both balanced and unbalanced input impedances were to specification at all audio frequencies, at 20k ohms and 10k ohms, respectively. The output impedance for mono operation was a little on the high side for a solid-state design, at 0.33 ohm at low and middle frequencies, 0.38 ohm at 20kHz. This is to be expected for a balanced design in which the output impedances of the two amplifier halves are effectively in series. The 9007's output impedance in single-ended, stereo mode was exactly half this figure, as anticipated.

With this highish output impedance, the mbl 9007's frequency response into our standard simulated speaker load (fig.1, top trace at 2kHz) featured some ±0.3dB of variation. This was halved when the amplifier was used in stereo mode (not shown). The 9007's small-signal bandwidth can be seen from this graph to be very wide, with a –3dB point at 200kHz into 8 ohms. The bandwidth decreased a little into lower impedances, with a very slight degree of top-octave rolloff into 2 ohms. The single-ended bandwidth (not shown) was even wider, with the output into 8 ohms down by just 1.65dB at 200kHz. As a result of this wide bandwidth, the 9007's reproduction of 1kHz and 10kHz squarewaves (figs.2 and 3) was superb, with no hint of overshoot or ringing.

Fig.1 mbl 9007, balanced 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 mbl 9007, small-signal 1kHz squarewave into 8 ohms.

Fig.3 mbl 9007, small-signal 10kHz squarewave into 8 ohms.

Used as a stereo amplifier, the 9007 met its specified output power, with the clipping points (1% THD+N) of: 137W into 8 ohms (21.4dB), 231W into 4 ohms (20.6dBW), and 396W into 2 ohms (20dBW). As can be seen in fig.4, the distortion was very low in level before clipping, especially into 8 ohms. The upward slope of the traces with decreasing power below 10–20W in this graph is due to the fact that the actual distortion is below the noise floor in this region. Fig.5 plots the THD+N percentage against output power for balanced mono operation. The clipping power has increased significantly, to 444W into 8 ohms (26.5dB), 600W into 4 ohms (24.8dBW), but 515W into 2 ohms (21.1dBW). The distortion also starts to rise at lower powers than with single-ended operation; however, even in the worst case, into 2 ohms, it remains below 0.1% below 150W.

Fig.4 mbl 9007, single-ended distortion (%)vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms.

Fig.5 mbl 9007, balanced distortion (%)vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms.

As MF noted, the 9007 is a very quiet amplifier: its wideband, unweighted signal/noise ratio, ref. 1W into 8 ohms in balanced mode, was 90.9dB, increasing to 100.6dB when A-weighted. In conjunction with the very high power, this low noise floor endows the mbl with a greater dynamic range than almost any other amplifier that has passed through my test lab. In fig.6, I plotted how the balanced THD+N percentage varies with frequency at 5V output, which is where the actual distortion starts to rise out of the noise floor. Though the distortion rises with decreasing load impedance, it is very low in level and doesn't change with frequency, which suggests that the 9007's circuit has a very wide open-loop bandwidth.

Fig.6 mbl 9007, balanced THD+N (%)vs frequency at 5V into (from bottom to top): 8, 4, 2 ohms.

In addition to the low level of the distortion, its content is predominantly the subjectively benign third harmonic (figs.7 and 8). Note the low levels of even-order harmonics in fig.8, which suggest that the characteristics of the two amplifiers comprising the balanced 9007 are closely matched. This graph was taken into 8 ohms; into 4 ohms (not shown), the third harmonic rises only slightly, by 4dB, to –70dB (0.03%)—but the second harmonic rises from –90dB (0.003%) to –80dB (0.01%). No power-supply–related spuriae appear in any of these high-power spectra, however, suggesting that the 9007's power supply is up to the demands made of it. Finally, even just below visible clipping on the oscilloscope screen, the 9007 produced low levels of intermodulation distortion (fig.9).

Fig.7 mbl 9007, balanced 1kHz waveform at 28W into 4 ohms (top), 0.0175% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.8 mbl 9007, balanced spectrum of 50Hz sinewave, DC–1kHz, at 250W into 8 ohms (linear frequency scale).

Fig.9 mbl 9007, balanced HF intermodulation spectrum, DC–24kHz, 19+20kHz at 420W peak into 4 ohms (linear frequency scale).

I love measuring an amplifier that is as well-engineered as the mbl 9007. But there is nothing in its measured performance, other than the much higher output power, that would indicate why Michael Fremer preferred its sound in balanced mono mode.—John Atkinson