Tube Preamp Reviews

All measurements were made through the Lamm L2 preamplifier's Direct inputs (which bypass the input switching) and at both the balanced and unbalanced outputs. The input impedance was a reasonably high 42.4k ohms. The maximum unbalanced voltage gain was a moderate 8.83dB, this increasing by the expected 6dB, to 14.84dB, at the balanced outputs.

The absolute polarity was preserved from both sets of outputs with the front-panel switch set to "0 degrees," confirming that the XLR jacks are wired with pin 2 hot. Setting the polarity switch to "180 degrees" inverted the unbalanced output but not the balanced output. The unbalanced output impedance was a low 124 ohms in the midband, increasing inconsequentially to 134 ohms at 20kHz, but significantly to 3.35k ohms at 20Hz, due to the limited size of the output coupling capacitor. (These figures doubled from the balanced outputs.) The Lamm L2 should not be used with power amplifiers having an input impedance of less than around 22k ohms if the low frequencies are not to sound lightweight.

The dual volume controls are well-matched and operate in reasonably accurate 1dB steps over most of their range. The unity-gain setting was 1 o'clock balanced and 3 o'clock unbalanced. The 12 o'clock positions gave an attenuation of 20dB from both outputs. Interchannel crosstalk (not shown) was unmeasurable, while frequency response (fig.1) was identical from both balanced and unbalanced outputs, and flat within the audioband. The ultrasonic rolloff reached -2dB at 200kHz.

Fig.1 Lamm L2, balanced frequency response at unity-gain volume-control setting (0.5dB/vertical div.).

Levels of distortion into our standard 100k ohms load were very low, and identical at all audioband frequencies. The unbalanced figure at 2V output was 0.006%, the balanced at 4V around 0.002%. And as well as being so low in level, the harmonic spectra were low-order. Fig.2 shows the spectrum of a balanced 50Hz tone driven at 4V into 100k ohms: the only harmonic to poke its head above the -100dB level is the third (150Hz), at -93dB. As balanced operation cancels even-order harmonics, it was not surprising to see the second harmonic (100Hz) join the third from the unbalanced jack (fig.3), at a level of -83dB. This spectrum was again taken into 100k ohms; the second harmonic rose by 10dB into 10k ohms (not shown), but even into a demanding 1k ohm load (fig.4), the harmonics remained below -60dB (0.1%) and retained their subjectively consonant relationship.

Fig.2 Lamm L2, balanced spectrum of 50Hz sinewave, DC-1kHz, at 4V into 100k ohms (linear frequency scale).

Fig.3 Lamm L2, unbalanced spectrum of 50Hz sinewave, DC-1kHz, at 2V into 100k ohms (linear frequency scale).

Fig.4 Lamm L2, unbalanced spectrum of 50Hz sinewave, DC-1kHz, at 2V into 1k ohm (linear frequency scale).