- Written by John E. Johnson, Jr.
- Published on 01 December 2011
The Pass Labs XP-30 Preamplifier On the Bench
All distortion measurements were made within an 80 kHz bandwidth. I used the XLR inputs and XLR outputs for all tests. The load was 100 kOhms, unless otherwise specified. Both channels were driven on all the tests.
At 1 kHz and 2 volts output, distortion was a miniscule 0.002% (it was actually less than that, but I rounded it up). Notice that the height of all the harmonics is very similar. This is the spectral pattern that resulted from listening to the amplifier with various changes to the circuitry, and choosing the variation that sounded best to everyone at Pass Labs. If you compare this with the XP-20, you will see that it has only two harmonics, a second and third, with the third being slightly higher than the second.
I also tried a 600 Ohm load, which one would never encounter in the real world, and still, the distortion was vanishingly low.
At 5 volts output (into 100 kOhms), the results were pretty much the same as with 1 volt.
Using 19 kHz and 20 kHz test signals, the B-A peak at 1 kHz was 120 dB below the fundamentals. This is truly fine performance.
At 5 volts, the B-A peak was 95 dB below the fundamentals.
IMD was a low 0.001% at 1 kHz and 2 volts output.
At 5 volts, the measurement was still 0.001% IMD, although you can see that the peaks are a bit broader beyond 7 kHz (IMD measures the amount of peaks within a band 250 Hz on either side of the 7 kHz fundamental).
THD+N vs. Frequency showed low distortion throughout the audible spectrum, rising to 0.007% at 20 kHz and 2 volts output, and 0.004% at 20 kHz and 5 volts output.
THD+N vs. Voltage Output with 100 kOhm loads as well as 600 Ohm loads is shown below. With a 100 kOhm load, the sharp knee was at 5 volts, with clipping (1% THD+N) at 30 volts, and with a 600 Ohm load, the sharp knee was at 1.7 volts, and clipping at 12 volts. This preamp has plenty of power to drive any power amplifier. If you compare this graph with the one I got for the Pass XP-20, you will see that the XP-30 has a much greater ability to drive a low impedance load.
The frequency response was flat out to 20 kHz, and was down 1.5 dB at 100 kHz.