Integrated Amp Reviews

"Hybrid" technology—specifically, mixing tubed and solid state in the same amplification device—stirs a deep desire for many enthusiasts. It has the potential to embody a perfect blend: the tonal liquidity, presence, and spaciousness of tubes coupled with the power and dependability of solid state. It's potentially an end-game technology.

Still, I have long remained skeptical. I am, I confess, a certain kind of audiophile, a blend of purist and traditionalist. I favor older technologies and simpler circuits. Amplifiers—including integrated amplifiers—should be tubed, input to output. Rectification? Tubes of course. I've even entertained OTL designs—the idea of them at least, though my experiences have been mixed.

Still, my mind and ears are open. Countless hours reviewing for Stereophile have shown me that our technology, though quite mature in most ways, nevertheless continues to evolve. A shuttered mind risks ending up ignored like a derelict amplifier on a shelf in the back of a home entertainment superstore, or languishing forgotten in an audiophile rest home. Because what matters isn't what we think but how the music makes us feel, and this can work both ways, old school or new-tech.

A recent example of the hybrid approach is the EVO 300 Hybrid, which was realized by a unique braintrust. Marcel Croese, who designed the highly regarded PrimaLuna EVO 400 tubed preamplifier, contributed his touch to the integrated amp's tubed input stage. Jan de Groot of Floyd Design (footnote 1) applied his skill to the solid state output stage. Herman van den Dungen, CEO of Durob Audio, assisted.

Tube/solid state integrated amplifiers have a rich history. Pioneering designs like Bascom H. King's HCA from 1979, which delivered 150Wpc into 8 ohms, paved the way. Harvey "Gizmo" Rosenberg's New York Audio Labs expanded the concept with the Moscode amplifier from 1984, which combined a tubed input stage and a FET output stage and was available in several power configurations (footnote 2). Today, the legacy continues with the Vinnie Rossi L2i-SE, Supravox Vouvray, Copland CSA150, BAT VK-3500, Riviera Levante (reviewed by Herb Reichert), and Vincent Audio SV-737 (reviewed by me). And others.

Design
The EVO 300 Hybrid is, at 63lb, substantial, but it is also quite compact: 15.9" wide × 8.1" high × 15.2" deep. Its steel construction, which includes both a 1.5mm-thick case and a 3mm-thick chassis, is the same as that found on other PrimaLuna amplifiers, including the EVO 400 integrated that I used for comparison in this review. Also like other PrimaLuna amps, the EVO 300 is finished in five coats of automotive lacquer, hand-sanded between coats.

The heart of the EVO 300 Hybrid lies in its tube/solid state design. The preamp section features six PrimaLuna-branded dual-triode 12AU7 tubes, two handling the gain stage and four acting as drivers. This curated selection lays the groundwork for the robust output stage. Here, Linear Systems JFETs join forces with custom-built pairs of MOSFETs from Exicon, operating in a dual-mono, class-A/B configuration. This synergistic pairing delivers 100Wpc into 8 ohms and 160Wpc into 4 ohms.

The EVO 300 Hybrid utilizes independent power supplies throughout. The preamp section draws power via two dedicated toroidal transformers; the amplifier leverages a separate 500VA toroidal transformer. All transformers are wound in house, housed in metal, and potted in nonmicrophonic resin to minimize noise and protect the windings.

Beyond the tubes, transistors, and transformers, the EVO 300 Hybrid incorporates high-quality components including pink Japanese-made Takman resistors; DuRoch, Nichicon, Kemet, and Rubycon capacitors; and an ALPS Blue Velvet potentiometer-based volume control. The amplifier section is flanked internally by two large heatsinks, for the main PSU and for the MOSFETs.

The EVO 300 Hybrid is primarily hardwired, like the majority of PrimaLuna amplifiers. In "the critical signal path," the wire is Swiss-made, silver-plated, oxygen-free, continuous crystal (OCC) copper with a Teflon dielectric, "for superior signal speed and increased clarity," the company website states. Executing the EVO 300 Hybrid's auto-bias function, though, required circuit boards, which are said to be of the highest quality: 2.4mm thick with 105µm gold-plated copper traces. Another feature common to PrimaLuna amplifiers: the "AC Offset Killer" circuit, which "ensures that DC on the mains never causes the transformers to hum."

"The AC Offset Killer is used as a filter to correct nasty AC," said Kevin Deal, owner of PrimaLuna's US distributor Harmonia Distribution. "Herman van den Dungen had that circuit designed back in the '80s to help quiet the mechanical hum of the toroidal transformers in a certain very high-end amplifier."

It's no longer only about mechanical hum, Harmonia Distribution Service Manager Josh Phelps added in a separate interview. "Bad AC is like putting bad fuel into your vehicle. AC is like water so even when a power conditioner is used, the AC Offset Killer makes the AC that much cleaner."

PrimaLuna also sought to minimize noise in the EVO 300's input-switching mechanism, which operates via sealed relays mounted on the back panel. When an input is selected, "that relay closes, giving you the best connection possible," the website states. "All the other relays are left open, so noise and signals from other sources can't leak in." Because the signal path is now essentially closed, there are no wires to pick up noise, in theory.

Curved air
PrimaLuna's signature visual style is easy to see in the EVO 300. A curved aluminum tube cage showcases the 12AU7 tubes, which are also set in a curve and nestled in porcelain sockets. The brushed-aluminum faceplate is adorned with two control knobs, for volume and input selection, and a 6.35mm headphone jack. The power switch is on the left side of the amp; the switch for choosing between loudspeaker and headphone outputs is on the right. The feet are outfitted with a protective plate to safeguard surfaces. The EVO 300 hybrid comes with a substantial remote control made of black steel and wrapped with two rubber rings for a better grip.

The EVO 300 offers five line-level inputs and tape and subwoofer outputs, the latter supported by a mono/stereo switch. A separate small case suspended from the bottom of the main cases accommodates the optional phono preamp (and a sixth input, for phono) if it's installed. All jacks are outfitted with plastic caps.

All this functionality converges on a single pair of gold-plated, solid-shaft WBT loudspeaker terminals. The EVO 300 can accommodate an optional PrimaLuna MM phono board, which was not included in my review sample. If you wish, you can bypass the preamp section and use the EVO 300 as a pure power amplifier.

Setup
The EVO 300 slid easily into my Salamander rack, in the spot previously occupied by the EVO 400 integrated amplifier. A smaller PrimaLuna, the EVO 100 phono stage, sat on an upper shelf. A 1m pair of AudioQuest Pegasus RCA interconnects connected the phono stage to the EVO 300. A 10' pair of AudioQuest William Tell speaker cables mated the EVO 300 to the Volti Audio Razz loudspeakers (spec'd at 97dB/2.83V/1m). I used the amp's stock power cord straight into a Synergistic Research outlet.

Like its PrimaLuna brethren, the EVO 300 exhibits a careful approach to power management. When you press the power button, a red LED illuminates, indicating a soft start cycle that takes just over a minute. To ensure a pristine startup, all controls remain muted until the LED transitions to green, signifying the amplifier's readiness to make music.

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Footnote 1: Floyd Design is part of Durob Audio.

Footnote 2: Wes Philips reviewed the Moscode 401HR in 2006.