Lamm Industries M1.2 Reference monoblock amplifier Measurements
Before I perform any tests on an amplifier, I run it at one-third power into 8 ohms for an hour. For an amplifier with a class-B output stage, which the vast majority are, this power level thermally stresses the amplifier to the maximum extent. At the end of that time, even with its heavy class-A bias both the vertical heatsinks and the chassis of the Lamm M1.2 were almost too hot to touch, implying a temperature of around 55ºC. However, the measured distortion level had dropped slightly, from 0.065% when cold to 0.058% when fully warm.
The Lamm was noninverting when driven via its balanced XLR input, which appears to be wired with pin 2 hot, and via the red unbalanced RCA jack (with the other, white jack shorted to ground with the supplied plug). The input impedance was to specification at 41k ohms (unbalanced or each phase of the balanced input) across the audioband. The balanced voltage gain at 1kHz into 8 ohms was high, at 31.7dB in both the High Impedance (Hi-Z) and Low Impedance (Lo-Z) bias conditions. The unbalanced gain, with the other input shorted, was the same, instead of 6dB lower, as expected.
The output impedance was consistent across the audioband, at 0.26 ohm, regardless of impedance setting. As a result, the modification of the amplifier's frequency response by the loudspeaker's impedance will be relatively small. With our simulated speaker load (fig.1, top trace at 2kHz), the response variation remained between ±0.25dB limits. This plot also reveals the M1.2 as having a wide, small-signal bandwidth, the top-end response being down 3dB at a very high 164kHz. At the other end of the spectrum, the amplifier was flat down to below 20Hz.
Fig.1 Lamm M1.2, Low Impedance setting, balanced frequency response at 2.83V into (from top to bottom at 2kHz): simulated loudspeaker load, 16 ohms, 8 ohms, 4 ohms, 2 ohms (0.5dB/vertical div.).
This wide bandwidth is reflected by the sharp corners and short risetimes in the shape of a 10kHz squarewave (fig.2). Both of these graphs were taken with balanced drive and were identical for both impedance settings. When I repeated the tests with unbalanced drive signal, I got a very similar result. Fig.3 shows the M1.2's small-signal frequency response into 8 ohms with an unbalanced input (shorting the unused signal phase to ground with the supplied shorting plug). It is the same as fig.1.
Fig.2 Lamm M1.2, balanced small-signal 10kHz squarewave into 8 ohms.
Fig.3 Lamm M1.2, High Impedance setting, unbalanced frequency response at 2.83V into 8 ohms (0.5dB/vertical div.).
Perhaps due to the higher-than-normal gain, the Lamm's signal/noise ratios were good rather than great, at 72.2dB ref. 1W into 8 ohms (unweighted, wideband). Switching in an A-weighting filter increased this figure to 79dB. Fig.4 shows how the THD+noise percentage present in the M1.2's output varies with output power with the Hi-Z bias setting into loads varying from 2 to 16 ohms. The amplifier comfortably exceeds its rated output power, giving out 180W into 8 ohms (22.6dBW), 305W into 4 ohms (21.8dBW), and 490W into 2 ohms (20.9dBW), all at 1% THD. For comparison, fig.5 shows what happens with Lo-Z output-stage biasing: the maximum output power is almost halved, but the signal benefits from significantly lower distortion into low impedances.
I looked at the measured THD+N percentage at an output power where figs.4 and 5 suggest the distortion spuriae are starting to rise out of the noise floor. Fig.6 shows that with the Hi-Z setting, the amplifier's good linearity is not significantly affected by either frequency or output current with loads of 4 ohms or above. In the Lo-Z conditions (fig.7), the THD+N trace into 2 ohms is as low as those into higher impedances.
Fig.4 Lamm M1.2, High Impedance setting, distortion (%) vs 1kHz continuous output power into (from bottom to top at 1W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.
Fig.5 Lamm M1.2, Low Impedance setting, distortion (%) vs 1kHz continuous output power into (from bottom to top at 1W): 16 ohms, 8 ohms, 4 ohms, 2 ohms.
Fig.6 Lamm M1.2, High Impedance setting, THD+N (%) vs frequency at 5V into (from bottom to top): 16 ohms, 8 ohms, 4 ohms, 2 ohms.
Fig.7 Lamm M1.2, Low Impedance setting, THD+N (%) vs frequency at 5V into (from bottom to top): 16 ohms, 8 ohms, 4 ohms, 2 ohms.
Of more important subjective importance, the distortion at low levels or into higher impedances is low-order, mainly second-harmonic (fig.8). Dropping the load impedance or increasing the power introduces some third-harmonic content (fig.9), but the THD is commendably free from higher-order harmonics. This is graphically shown in fig.10, the spectrum of a low-frequency tone taken at a very high level into 8 ohms (Hi-Z biasing). The third harmonic is a little higher than the second, but both are below –60dB (0.1%), while the fourth and fifth harmonics are at or below –80dB (0.01%). You can also see the 120Hz power-supply component in this graph, though it lies at –100dB, which won't worry listeners. Fig.11 shows a similar spectrum taken with the amplifier driving a 50Hz tone at 195W into 4 ohms. Intermodulation distortion was also relatively low, even close to clipping into 4 ohms with the Hi-Z bias setting (fig.12).
Fig.8 Lamm M1.2, High Impedance setting, 1kHz waveform at 16.7W into 8 ohms (top), 0.033% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.9 Lamm M1.2, High Impedance setting, 1kHz waveform at 31.6W into 4 ohms (top), 0.032% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.10 Lamm M1.2, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (linear frequency scale).
Fig.11 Lamm M1.2, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (linear frequency scale).
Fig.12 Lamm M1.2, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 250W peak into 4 ohms (linear frequency scale).
As I have found with other amplifiers designed by Vladimir Lamm, the M1.2 offers fundamentally good measured performance.—John Atkinson