Tube Preamp Reviews

When I unpacked the PrimaLuna EVO 400 preamplifier, it looked very familiar. It appears identical to PrimaLuna's ProLogue Premium line preamplifier that Herb Reichert reviewed in our June 2017 issue (footnote 1). But on closer inspection, while the earlier preamp had single-ended inputs and outputs, the EVO 400 adds two pairs of balanced input jacks and a pair of balanced output jacks. And while the EVO 400 still uses two 5AR4 rectifier tubes, there are now six 12AU7 twin triodes compared with the ProLogue Premium's four 12AU7s.

I measured the PrimaLuna EVO 400 preamplifier with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The gain from the unbalanced inputs to the unbalanced output with the volume control set to its maximum was slightly lower than the specified 10dB, at 9.1dB. From the balanced inputs to the balanced output, it was 7dB. The preamplifier preserved absolute polarity (ie, was noninverting) from all the inputs and outputs. Specified as offering a very high input impedance of 220k ohms, the unbalanced inputs measured 175k ohms at 20Hz and 1kHz, dropping to a still very high 105k ohms at 20kHz. The balanced inputs had a much lower input impedance, at 4k ohms at 1kHz and 3.5k ohms at 20Hz and 20kHz. While this will not be a problem with sources having a direct-coupled, solid-state output stage, it will result in lightweight low frequencies with tubed sources that have a capacitor-coupled output.

The PrimaLuna EVO 400's output impedance is specified as 256 ohms, which is significantly lower than the ProLogue Premium's 2800 ohms. The EVO 400's balanced output impedance measured 1050 ohms at 20kHz, 590 ohms at 1kHz, and 2600 ohms at 20Hz. The unbalanced output impedances were 240 ohms at high and middle frequencies, rising to 2785 ohms at the bottom of the audioband. The EVO 400's unbalanced frequency response into the demanding 600-ohm load (fig.1, cyan and magenta traces) was therefore down by 3dB at 100Hz, compared to –3dB at 15Hz into 100k ohms (blue and red). The high frequencies were flat almost to 20kHz and reached –3dB at 90kHz This graph was taken with the volume control set to its maximum; the excellent matching between channels was preserved at lower settings, and, commendably, the frequency response didn't change. However, for balanced operation, the EVO 400's ultrasonic output peaked by 2.3dB at 40kHz (fig.2). This behavior is typical of a circuit using transformers to convert single-ended signals to balanced—looking at the preamplifier's interior revealed a small coupling transformer for each balanced input and output. Fig.2 was taken into 100k ohms, the specified input impedance of PrimaLuna's new EVO-series power amplifiers, and with the volume control set to its maximum. The peak was preserved at lower setting of the volume control but was reduced in amplitude as the load impedance was lowered.

Channel separation was the same as that of the earlier preamplifier for both balanced and unbalanced operation. It measured >77dB in both directions below 1kHz, but worsened to 50dB at 20kHz, due to capacitive coupling between channels (fig.3). The wideband, unweighted signal/noise ratio, taken with the inputs shorted to ground but the volume control set to its maximum, was 72.4dB ref. 1V (average of both channels). This improved to a superb 91.6dB with a 22Hz–22kHz measurement bandwidth, and to 100.9dB with an A-weighting filter in-circuit. The noise floor was dominated by spuriae at mainly the 60Hz AC line frequency and its odd-order harmonics (fig.4). These are probably due to magnetic interference being picked up by the tubes' steel pins, but these are all very low in level in absolute terms and were identical for balanced and unbalanced operation. Even-order harmonics of the power-line frequency, which stem from internal grounding issues, were even lower in level, which implies excellent circuit layout.

As with the ProLogue Premium, the EVO 400 had relatively high levels of distortion once the output level rose to 1V, even into high impedances. Fig.5 shows how the percentage of THD+noise increases as the output voltage rises into 100k ohms. Although the THD+N is very low below 350mV, the percentage rises almost linearly as the output increases, reaching 0.6% at 2V and 1%—our definition of waveform clipping—at 3.05V. This graph was taken from the unbalanced output; the balanced output behaved similarly, though it clipped at 2.7V. Both these clipping points will probably be above the maximum voltage the EVO 400 will be required to deliver in practical use.

As with the ProLogue Premium, the distortion remains constant with frequency (fig.6) as well as with load impedance. But also like the earlier PrimaLuna preamplifier, the EVO 400 is a second-harmonic generator. Fig.7 shows the spectrum of the EVO 400's balanced output—the unbalanced output behaved identically—while the preamplifier drove a 50Hz tone at 1V into 100k ohms. The second harmonic is the highest in level, at –50dB (0.3%), with the third harmonic at –60dB (0.1%). Higher harmonics, however, are all much lower in level. Significantly, the harmonic signature didn't change with frequency, and as the output level is reduced, while the second harmonic remains the highest in level, the third-harmonic content was reduced.

When it came to intermodulation distortion, the "bent" transfer function responsible for the second-harmonic distortion results in a relatively high level of the second-order difference product at 1kHz with an equal mix of 19 and 20kHz tones—0.3% or –50dB (fig.8). However, only a few higher-order products are visible in this graph, and these lie close to –70dB (0.03%).

As with PrimaLuna's ProLogue Premium preamplifier, the EVO 400 measures well for a tubed design, other than having a second-harmonic distortion signature that will fatten up the sound.—John Atkinson