Tube Power Amp Reviews

With its twin triple-nippled power tubes, Lamm's ML1 looks both intriguing and industrial. It was also reassuring to find that, following its transcontinental trip to my lab, the meters on the sample I was measuring indicated that its operating parameters of bias, balance, and plate idle current were all healthy: no further adjustment needed. I let it cook for an hour at one-third full power into 8 ohms—the top plate and transformer covers were merely warm to the touch—before I took any measurements.

Getting the essentials out of the way: The amplifier was non-inverting, and its A-weighted noise floor was very low, at -99.8dB (ref. 1W into 8 ohms). This rose to -77.2dB with an unweighted wideband (10Hz-500kHz) measurement window, which is still low. The ML1's input impedance at 1kHz was a highish 42k ohms, while its output impedance varied according to the output transformer tap used. As expected, the 8 ohm tap featured the highest source impedance: 1.15 ohms at 1kHz, rising a little at the frequency extremes. The impedance was 0.62 ohms from the 4 ohm tap, this dropping to 0.4 ohms from the 2 ohm tap. While higher than you get with a good solid-state design, the ML1's source impedances are on the low side for a tube design, a tribute to that unusual 6C33C-B tube's low internal impedance. Any modification of the amplifier's frequency response due to the interaction between its source impedance and the impedance of the speaker will thus be relatively mild.

This can be seen in figs.1 and 2, which show the small-signal frequency response from the 8 and 4 ohm taps, respectively. Even using the 8 ohm tap (fig.1), the response variation with our dummy speaker load is less than ±1dB, this dropping to ±0.4dB from the 4 ohm tap (fig.2). The extreme high frequencies roll off gently, being just 0.2dB down at 20kHz. But note the slight blip at 150kHz in the 8 ohm trace, which indicates a well-suppressed parasitic mode of some sort from this tap. This blip could not be seen in the 4 or 2 ohm response graphs, and a 10kHz squarewave taken from the 2 ohm tap (fig.3) was textbook-perfect, the slight rounding of the leading edges corresponding to the slight response rolloff at 20kHz. (Ignore the LSB toggling in this graph, which is an artifact of the digital 'scope I used.)

Fig.1 Lamm ML1, 8 ohm tap, frequency response at (from top to bottom at 1kHz): 2.828V into simulated loudspeaker load, 1W into 8 ohms, 2W into 4 ohms (1dB/vertical div.).

Fig.2 Lamm ML1, 4 ohm tap, frequency response at (from top to bottom at 1kHz): 2.828V into simulated loudspeaker load, 1W into 8 ohms, 2W into 4 ohms (1dB/vertical div.).

Fig.3 Lamm ML1, 2 ohm tap, small-signal 10kHz squarewave into 8 ohms.

Fig.4 shows how the Lamm's small-signal THD+noise percentage varied with frequency. The 4 ohm tap was used for this graph; the results from the 8 and 2 ohm graphs are similar. The amplifier is less linear at low frequencies and into lower impedances, but throughout the important midband the harmonic distortion content is impressively low for a design with minimal loop negative feedback. And as can be seen from fig.5, the distortion at low levels into kind loads is predominantly the benign third harmonic.

Fig.4 Lamm ML1, 4 ohm tap, THD+noise (%) vs frequency at (from top to bottom at 6kHz): 4W into 2 ohms, 2W into 4 ohms, 2.83V into simulated loudspeaker load, 1W into 8 ohms.

Fig.5 Lamm ML1, 1kHz waveform at 5W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).