Lamm Industries LL2 Deluxe preamplifier Measurements
The Lamm LL2 Deluxe preamplifier offered a maximum voltage gain of 17.97dB, this the same from both the Line and the Direct inputs. The unity-gain setting of the volume controls was 11:30. The LL2's input impedance measured 50k ohms in the bass and midrange, dropping slightly to 45k ohms at 20kHz, which is still within the specification. The LL2 preserved absolute polarity; ie, was noninverting.
The LL2's output impedance was a low 245 ohms in the midrange and treble, this rising to 3.3k ohms at 20Hz, presumably due to the finite size of the output coupling capacitor. As a result, the preamp's frequency response suffered from a premature low-frequency rolloff into the low 600-ohm load (fig.1, lower pair of traces). Into the higher 100k-ohm load (fig.1, top traces), the response is flat from 10Hz to 50kHz and just 1dB down at 200kHz. This graph, taken with the twin volume controls set to their maximum positions, shows excellent channel balance. At the controls' unity-gain setting, the right channel was 0.25dB higher than the left, however. Channel separation (not shown) was better than 90dB below 4kHz in the L–R direction, but significantly less good from R to L, at 63dB at 1kHz.
Fig.1 Lamm LL2 Deluxe, frequency response at 1V into (from top to bottom at 200Hz): 100k ohms, 600 ohms (0.5dB/vertical div.).
Provided it is used into high-impedance loads, the Lamm LL2 offers enormous dynamic range. The unweighted, wideband signal/noise ratio, taken with the input shorted and the volume control at its maximum, is good rather than great at 72.6dB, this figure increasing to 95.2dB when A-weighted. But, as shown by fig.2, which plots the THD+noise percentage against output voltage, the LL2 can swing more than 60V into 100k ohms before clipping! The clipping point (1% THD) drops to 40V (!) into 10k ohms and 2.4V into 1k ohms, though it is fair to note that the last load is way below what the Lamm will be required to drive in real life. The important point to draw from fig.2 is that the Lamm's distortion remains below its noise floor up to 3V into 100k ohms and 1.5V into 10k ohms, which are around the maximum voltages it will be asked to deliver with typical power amplifiers. At 3V into 100k ohms, the THD remains below 0.01% across almost the entire audioband, with just a small rise apparent above 10kHz (fig.3).
Fig.2 Lamm LL2 Deluxe, distortion (%)vs 1kHz output voltage into (from bottom to top at 10V): 100k, 10k, and 1k ohms.
Fig.3 Lamm LL2 Deluxe, THD+N (%)vs frequency at 3V into 100k ohms.
Of greater subjective importance than the level of distortion is its spectrum. Fig.4 shows that, even into a low 8k ohm load, the LL2 offers almost pure second-harmonic distortion, which, even if it were higher than the –75dB seen here (0.02%), would still be subjectively benign. Increasing the load impedance to 100k ohms drops the second harmonic to –90dB (0.003%, not shown). Intermodulation distortion was also acceptably low (fig.5), even given the small decrease in linearity at the top of the audioband revealed by fig.3.
Fig.4 Lamm LL2 Deluxe, spectrum of 1kHz sinewave, DC–10kHz, at 1V into 8k ohms (linear frequency scale).
Fig.5 Lamm LL2 Deluxe, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 1V peak into 8k ohms (linear frequency scale).
As I am coming to expect from Vladimir Lamm's designs, the LL2 offers excellent audio engineering. Its measured performance indicates that the preamplifier is best used with power amplifiers offering a load impedance of at least 20k ohms; Lamm's own power amps offer a load of 41k ohms, which will be fine.—John Atkinson