Lamm Industries ML1 monoblock power amplifier Measurements part 2

Fig.6 shows the spectrum of a 1kHz tone under roughly the same test conditions used to take fig.5. Again, the third harmonic is the highest in level, with the low odd orders predominant. However, if you look at fig.7, which was taken at a significantly higher power level, higher powers and lower load impedances both raise the level of the odd harmonics and introduce even-order distortion. The same is true at lower frequencies (fig.8), which I am sure correlates with J-10's noting that the amplifier had a "plummy"-sounding bass.

Fig.6 Lamm ML1, spectrum of 1kHz sinewave, DC-20kHz, at 10W into 8 ohms, 8 ohm tap (linear frequency scale).

Fig.7 Lamm ML1, spectrum of 1kHz sinewave, DC-20kHz, at 30W into 8 ohms, 8 ohm tap (linear frequency scale).

Fig.8 Lamm ML1, spectrum of 50Hz sinewave, DC-1kHz, at 30W into 8 ohms, 8 ohm tap (linear frequency scale).

Despite the highish harmonic distortion at high power levels, the intermodulation was low, at least regarding the level of the low-frequency difference product. Fig.9 was taken from the 8 ohm tap at 28W into 8 ohms, which was just below the visible clipping point with the very demanding 19+20kHz test signal. The 1kHz product lies at a low -60dB (0.1%), but the high-frequency second-order products are higher in level, at -46dB (0.5%).

Fig.9 Lamm ML1, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 28W into 8 ohms, 8 ohm tap (linear frequency scale).

As is usual with a design featuring low loop negative feedback, the ML1 was most linear at low levels: typically, less than 1W, which is where it will operate most of the time with most music program and speakers. But as you can see from fig.10, which shows how the distortion changes with increasing continuous output power, it gets less and less linear as the output power increases. The manufacturer's output power of 90W is specified at 2.5% distortion. Fig.10, taken from the 4 ohm tap, is typical and shows that the amplifier actually meets that spec at 3% THD. At our usual 1% THD+N clipping point, the maximum output power with a transformer-matched load was 60W (17.8dBW). This is still a useful output rating.

Fig.10 Lamm ML1, distortion (%) vs continuous output power into (from bottom to top at 2kHz): 4 ohms, 8 ohms, 2 ohms (4 ohm tap).

Looking at the Lamm's output power capability using not a continuous tone but a pulsed 1kHz tone with a low duty cycle (10 cycles on, 40 cycles off) proved interesting. With this signal, which more closely approximates a music signal, the amplifier appeared more powerful. Around 70W was available into a transformer-matched load for 1% THD+N, but at the more relaxed 3% figure, this increased dramatically, to around 107W. And as can be seen from fig.11, 135W was available into a load half the rated output tap. This graph was taken from the 4 ohm tap, and again the linear increase in distortion with increasing frequency can be seen, as can the increased distortion into lower impedances. As I noted above, this is typical of a design with low loop feedback. And as we have also found with such designs, the ML1 actually becomes more linear at very high powers, before the signal then clips for real.—John Atkinson

Fig.11 Lamm ML1, distortion (%) vs 1kHz burst output power into: 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), 1 ohm (green).