Preamplifiers

PrimaLuna ProLogue Premium Stereo Tube Preamplifier

ARTICLE INDEX

On the Bench

All distortion measurements were made within an 80 kHz bandwidth, both channels driven, unless otherwise noted.

At 1 kHz and 2 volts output, THD+N was 0.65%. The preamplifier will deliver 5 volts, but 2 volts is plenty for most power amplifiers. Notice that the distortion is primarily second order (you know, that "artificial" stuff that tubeophiles like). The noise floor is way, way down there at - 130 dBv for most of the spectrum, and - 100 dB below 1 kHz. This is one quiet preamplifier!

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The ProLogue does not reject that low level 20 kHz electrical noise that I have in my environment, and you can see it at the base of the test signal peaks. Some components reject it, and others don't. But, in any case, the purpose of this test is to look at the B-A peak at 1 kHz, which for the ProLogue is 40 dB below the fundamentals (19 kHz and 20 kHz sine waves). There are only two visible side peaks around the 19 kHz and 20 kHz peaks, and this represents good performance.

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IMD was 0.46% at 2 volts output (6 dBV corresponds to 2 volts RMS). The peak at 14 kHz is the second harmonic of 7 kHz, it is not part of the IMD calculation.

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THD+N vs. Frequency remained in the 0.5 to 0.7% range from 10 Hz to 20 kHz. This indicates a neutral sound rather than being bass-shy or overly intense in the midrange. Now there is a term called midrange lushness, and that refers to the addition of those second harmonics to the sound. The reason it is so noticeable in the midrange is that is where the amp is likely to produce low order harmonics that you can hear. At 10 kHz, on the other hand, which is not part of the midrange.

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The measured frequency response was 20 Hz - 20 kHz, - 1 dB.

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THD+N vs. Output Voltage indicates the sharp knee at 200 mv, and then a gradual increase to clipping (1% THD+N) at 4 volts.

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