As the name implies, the all-tube EVO 400 represents an evolutionary upgrade from the DiaLogue Premium preamp and one that the PrimaLuna team is particularly proud of. Probably the most significant (and obvious) new feature is balanced connectivity, implemented using mu-metal-shielded quadfilar line transformers to provide two stereo XLR inputs and one output. I should hasten to add that the audio gain circuitry is still single-ended from input to output. And if you don’t need balanced connections, there’s the less expensive single-ended EVO 300. Cost margins have been kept tight so that, as Upscale Audio’s Kevin Deal puts it, you don’t have to be some rich dotcom dude to be able to afford world-class performance.
Further enhancements include power supply upgrades, including an improved tube filament supply for reduced hum. Six expensive tin-foil capacitors sourced from a Swiss factory that typically only supplies the likes of NASA and Boeing are used in critical coupling locations. Swiss-made silver-plated OFC wiring is used in the signal path along with Japanese custom-built Takman resistors, which are known for their musicality. Instead of an input-selector switch, sealed relays mounted at the rear of the chassis (adjacent to the input jacks) are used for switching sources. This results in a secure low-noise connection. A soft-start circuit allows tube filaments to reach operating temperature before applying high voltage. Folks, this is truly a high-end preamp in terms of design, parts-quality, and execution.
I don’t think that Kevin Deal would forgive me if I failed to mention the AC Offset Killer, a feature common to the EVO line, which eliminates DC offset at the AC mains input and is effective in eliminating potential mechanical transformer hum. The chassis layout is dual mono, even when it comes to power supplies. This translates to two large toroidal power transformers potted in mu-metal shields and two separate tube rectifier and filter circuits with huge chokes. The only deviation from dual mono is the volume control, a stereo motorized Alps Blue Velvet conductive-plastic potentiometer. Sonically, this is a superior volume control that in previous listening tests had eclipsed IC-based digital controls. But since both channels are adjusted in tandem, there is no provision for channel balance.
One of the EVO400’s surprising aspects is its weight. Just try lifting it up and prepare to grunt. Despite a modest footprint, this thing is a beast at about 53 pounds! I guess the two massive toroidal power transformers are to blame. The preamp weighs more than many 60W tube power amps, and I’m hard pressed at the moment to think of another line preamp that exceeds the EVO400 in the weight category. Being overweight is not a bad thing when it comes to power supplies and is further proof of PrimaLuna’s commitment to excellence.
Despite the presence of three 12AU7 dual triodes per channel, the overall voltage gain is surprisingly only 10dB (about a factor of four). Of course, that’s by design, and Marcel Croese, former chief engineer at Goldmund, knows exactly what he’s doing. Less gain equates to less noise and tube hiss, a godsend for those of us with high-sensitivity loudspeakers. The signal-to-noise ratio is 93dB, an impressive figure for any linestage, especially for a no-global-feedback tube one.
Most digital sources generate output voltage levels of 2V to 4V, which are adequate to drive any power amp to full power. In such a scenario, a line preamp would be attenuating rather than amplifying incoming signals. In the old days, a line section was designed with much higher gain, typically over 20dB, to mate with an internal phono section. Most external phonostages today already have plenty of gain, so there’s really no good reason for a high-gain linestage in the digital age. To keep the overall gain low, the two triode sections of each 12AU7 are connected in parallel, reducing the triode count effectively from six to three per channel. Each paralleled 12AU7 has the same gain as before but with higher plate dissipation and increased transconductance. The first two triodes provide voltage gain while the third is configured as a cathode follower with low output impedance for driving long interconnect runs. Looking at the 12AU7 tube array, the input stages consists of the two middle tubes (V5 and V6), flanking them are V4 and V7 which make up the second gain stages. The outer tubes, V3 and V8, are the cathode followers. The volume control is sandwiched between the first and second gain stages. This unusual topology has been dubbed by PrimaLuna as “PerfectMatch.” In just about all preamp designs, the volume control precedes the input stage with the consequence that the input impedance varies with the setting of the volume pot. Here the input impedance remains a steady 220k ohms, irrespective of the volume setting.
In my experience, tube rectification has proven to be a prime requisite for approaching vintage tube sound. I’m very fussy about tonal weight and have found myself being disappointed in the past by solid-state-rectified tube preamps, so for me it’s discouraging to face the fact that tube-rectified preamps are rare birds these days. Thank you PrimaLuna and Lamm Industries for being true believers. Modern tube sound can best be characterized by a combination of transparency, imaging excellence, and refined detail, but the fly in the ointment is typically a lightweight lower midrange. Tonal gravitas is not easy to come by, but it became obvious pretty quickly that the EVO 400 has it in spades. To my mind, the 5AR4 tube rectifiers were at least partly responsible for the preamp’s tonal integrity, fleshing out the lower octaves and, in particular, infusing upright bass and cello with a full complement of tonal heft.