Solid State Power Amp Reviews

A full set of measurements of the Lamm M1.1 was made in the unbalanced mode. I also checked the input impedance, gain, signal/noise, frequency response, and THD vs frequency in the balanced mode. All of the 8 ohm measurements were made with the impedance switch in the 6–8 ohm setting; the 2 and 4 ohm readings were made in the 1–6 ohm position, except as noted otherwise. Following the 1/3-power, one-hour preconditioning test, the M1.1's heatsinks were very warm, but I could touch them comfortably for an extended period without experiencing discomfort.

The M1.1 is noninverting in its unbalanced mode; in the balanced, pin 2 is configured as the positive leg, pin 3 the negative. The M1.1's input impedance measured 41.5k ohms (unbalanced) and 84.9k ohms (balanced). Its output impedance was under 0.17 ohms at either 1kHz or 20Hz, increasing to a maximum of 0.18 ohms at 20kHz. Voltage gain into 8 ohms measured 31.8dB unbalanced, and virtually the same balanced. DC offset was 16.2mV. Signal/noise (unweighted, ref. 1W into 8 ohms) measured 75.9dB, unbalanced and balanced.

Fig.1 shows the frequency response of the M1.1 at 1W into 8 ohms, unbalanced. The balanced response, as well as the response at 2W into 4 ohms, was virtually identical, and is not shown. The M1.1's 10kHz squarewave (fig.2) shows an excellent result, with a fast risetime and very little rounding of the leading edges. The 1kHz squarewave, not shown, was close to perfect, except for a slight rearward tilt of the tops of the waveform indicative of a very-low-frequency rolloff in the response.

Fig.1 Lamm M1.1, frequency response into 8 ohms in unbalanced mode (0.5dB/vertical div.).

Fig.2 Lamm M1.1, 10kHz squarewave.

The Lamm's THD+noise vs frequency curves are shown in fig.3. The distortion is virtually free of the typical rise at higher frequencies (with only a trivial rise into a 2 ohm load), and changes little with decreasing load impedance. The balanced distortion, not shown, is marginally higher than the unbalanced (by less than 0.002%). The 1kHz distortion waveform (fig.4) shows primarily second-harmonic content plus noise—a characteristic that was independent of load.

Fig.3 Lamm M1.1, THD+noise (%) vs frequency at (from bottom to top): 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms.

Fig.4 Lamm M1.1, 1kHz waveform at 2W into 4 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

The spectral response to a 50Hz input at an output level of 67W into 8 ohms (2/3 the rated power of 100W) is shown in fig.5. All of the distortion artifacts are reasonably low; the largest is the second at –61.7dB (about 0.08%), with the higher harmonics descending in order into the noise floor. [This characteristic almost always seems to correlate with a musical-sounding amplifier.—Ed.] Fig.6 shows the amplifier's output spectrum reproducing a combined 19+20kHz signal—the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies—also at 67W into 8 ohms. The largest artifacts here are at 18kHz and 21kHz (–56.5dB, or about 0.15%), and at 1kHz (–59dB, or about 0.11%). Into a 4 ohm load at the same power (not shown) the distortion was slightly higher at some frequencies (0.25% at 18kHz and 21kHz) and slightly lower at others (0.06% at 1kHz), but the trend of the artifacts was very similar.

Fig.5 Lamm M1.1, spectrum of 50Hz sinewave, DC–1kHz, at 67W into 8 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level at –61.7dB (0.08%).

Fig.6 Lamm M1.1, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 67W into 8 ohms (linear frequency scale).

The M1.1's 1kHz, THD+N vs output power curves are shown in fig.7. The distortion curve has features reminiscent of both tube and solid-state amplifiers. While the knee of the curve is relatively well-defined, there's a gentle but noticeable rise in the THD+N levels from 1W to 100W output. In this figure, it's clear that, with the impedance switch set to the most appropriate setting, the M1.1 puts out almost identical power into both 4 and 8 ohms. The M1.1's discrete clipping powers (at 1% THD+N) were 140W into 8 ohms (21.5dBW) (115V line); 138W into 4 ohms (18.4dBW) (114V line); and 230W into 2 ohms (17.6dBW) (115V line). Leaving the switch at the 1–6 ohm setting with an 8 ohm load, however, gives the result shown in fig.8—clearly a higher power output.

Fig.7 Lamm M1.1, distortion (%) vs output power into (from bottom to top at 1W): 8 ohms (impedance switch set to 6–8 ohms), 4 ohms, and 2 ohms (both with impedance switch set to 1–6 ohms).

Fig.8 Lamm M1.1, distortion (%) vs output power into (from bottom to top at 100W): 8 ohms with impedance switch set to 1–6 ohms; 8 ohms with impedance switch set to 6–8 ohms.

This curve cannot show all of the possible ramifications of using this switch setting, however; Lamm clearly must have had a reason for making the choice they did. If you choose to ignore the recommended settings in hopes of obtaining greater power output, you should at least watch for signs of amplifier distress—such as overheating. There are, however, very few loudspeakers with impedances which remain above 6 ohms—even those rated at 8 ohms. The 1–6 ohm setting is likely to be optimum with most real-world loads.

These test-bench results leave no room for criticism. The Lamm M1.1 is an unusually large amplifier considering its power rating, but its design goals clearly lie elsewhere. The measurements in no way contradict JS's positive assessment.—Thomas J. Norton