- Written by John E. Johnson, Jr.
- Published on 10 May 2010
On the Bench
Distortion measurements were within an 80 kHz bandwidth. Except where noted, the yellow graph line represents the left channel, and red is the right channel.
At 1 kHz and 2 volts output, THD+N was a mere 0.008%. I did note some high frequency noise in the 20 kHz range at about - 90 dBV, which would be inaudible, and it is lower than the - 84 dB noise level below a 2 volt signal specified by Lamm.
At 10 kHz, distortion rose a bit, which is not unusual.
Using 19 kHz and 20 kHz as the test signal, the B-A peak at 1 kHz was 66 dB lower than the fundamentals.
IMD was 0.02%, using 60 Hz and 7 kHz as the test signal.
The measured frequency response was 20 Hz to 20 kHz, flat. It rolled off 2 dB by 200 kHz. Using a 600 ohm load resulted in a severe bass rolloff below 400 Hz. However, one would never encounter this type of load in the real world.
THD+N vs. Frequency showed a flat response out to about 3 kHz, then rising to 0.07% at 50 kHz. This is excellent performance!
The measured output indicated a knee at 2 volts, then rising to a second knee at 70 volts before going into clipping (1% THD+N) at 80 volts. This is a very powerful preamplifier. It means that during transients, which may demand quite a bit of voltage (they last a few milliseconds, at the beginning of a steel guitar string pluck, or a gunshot for example), the Lamm will not falter. Again, the 600 ohm load shows definite stress, even though it does not really mean much. Other publications show component performance with this load, so we might as well do it too. But, I wish Audio Precision would add a 10 kOhm load and a 47 kOhm load to the menu options. Those are more likely to be encountered than 100 kOhms or 600 ohms.