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Reviewed on: SoundStage! Solo, October 2019

I measured the Lehmannaudio Linear USB II using a Clio 10 FW audio analyzer and a Neutrik NL-1 Minilyzer. Note that my focus with these tests is on measurements that confirm these devices’ basic functionality. I used the analog inputs; unfortunately, I’m currently unable to interface Clio’s coax digital output to USB-only DACs.

Frequency response

This chart shows the Linear USB II’s frequency response with 1mW output into a 32-ohm load in the left and right channels. Channel matching is excellent, with the right channel measuring just 0.01dB lower in level than the left, and the two having neatly overlapping curves out to 40kHz.

Frequency response

Here you can see how the Linear USB II’s frequency response differs into 32-, 250-, and 600-ohm loads. Into 32 ohms, the response measures -0.01dB at 20Hz, -0.14dB at 20kHz, and -1.18dB at 75kHz. Into 250 ohms, the numbers are -0.02dB, -0.11dB, and -1.15dB, respectively. Into 600 ohms, the numbers are -0.02dB, -0.11dB, and -1.16dB, respectively. From a frequency-response standpoint, the Linear USB II’s response can be described as load-invariant, meaning the amp’s tonal balance won’t change depending on the headphones you use.

THD vs. power output

This chart shows the output of the Linear USB II vs. total harmonic distortion (THD) into 32-, 250-, and 600-ohm loads with a 1kHz signal. Rated power is 400mW into a 60-ohm load and 200mW into 300 ohms, both at unspecified distortion at an unspecified frequency. Into 32 ohms, the power/distortion curve looks peculiar because it rises rather quickly to a plateau of typically 1.6% THD, and doesn’t hit its “clipping knee” until 1.35W at 2.1% THD. It hits 0.5% THD at 127mW, and 1% THD at 176mW. This is the only deviation I found from excellent measured performance. With higher-impedance loads, the Linear USB II performs more as I’d expect. Into 250 ohms, output at 0.5% THD is 327mW, and output at 1% THD is 347mW. Into 600 ohms, output at 0.5% THD is 141mW, and it’s 149mW at 1% THD.

Distortion

Here you can see the harmonic distortion spectrum and noise floor of the Linear USB II, referenced to 2.59Vrms (210mW) output at 600Hz into 32 ohms. This is pretty typical of a conventional solid-state amp, with much stronger odd-order (3rd, 5th, 7th, and so on) harmonics than even-order (2nd, 4th, 6th, etc.) harmonics. Odd-order harmonics are more objectionable because they occur at non-harmonic intervals to the fundamental tone, but this measurement was done at a far higher level than you’d encounter in normal listening.

Output impedance at 1kHz measures 5.6 ohms, close to the rated 5 ohms. This relatively low output impedance ensures that the amp’s output impedance will have very little interaction with the reactance of the headphones or earphones, so the headphones or earphones will deliver the frequency response they were designed for (assuming they were designed using an amp with low output impedance).

. . . Brent Butterworth
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