Product Review
 

Lamm M2.2 Monoblock Hybrid (Tube/Solid State) Power Amplifier

Part IV

May, 2006

John E. Johnson, Jr.

 

On the Bench

I tested the M2.2 using its XLR balanced input. I also set the M2.2 to its upper toggle position, meaning 1 to 6 ohms, even though I used 8 ohm loads as well as 4 ohm loads. This was because I did not want to add a second variable, namely, the toggle position. The toggle basically changes the bias and limits the voltage output. However, the 5 volt output is within the Class A output for both toggle settings, and 20 volts output is out of the range of the Class A output for both toggle settings.

At 1 kHz and 5 volts output into 8 ohms, THD+N was 0.024%. Signal-to-Noise (S/N) ratio was 84 dB (unweighted).

At 20 volts into 8 ohms (50 watts RMS), THD+N went up to 0.06%. S/N was 94 dB (unweighted). I could not hear any hum or hiss at all with the CD player on pause.

Moving to a 4 ohm load, THD+N was 0.02% at 5 volts output.

At 20 volts output, and a 4 ohm load, THD+N was slightly more than 0.1%.

In the following graphs, I used a standardized SMPTE/DIN Intermodulation Distortion test, which has input sine waves of 2 kHz and 60 Hz. IMD readings represent the peaks immediately surrounding the 2 kHz peak (i.e., ± 200 Hz). In this case, IMD was 0.060% at 5 volts output into 8 ohms.

At 20 volts output into 8 ohms, IMD was 0.247%.

For the sake of comparison, here is a graph of the IMD for 20 volts output into 8 ohms, using the lower toggle setting (8 - 16 ohms). IMD was 0.228%, not significantly different than with the upper toggle setting (1 - 6 ohms).

Click Here to Go to Part V.

© Copyright 2006 Secrets of Home Theater & High Fidelity

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