Tube Preamp Reviews

You know I'm a lucky guy. I maintain two separate audio reviewing systems.

The core component of my beloved, daily-driver desktop system is a Mytek Brooklyn DAC-preamp-headphone amp. Through this system I play high-resolution files and Internet sources (Tidal, Qobuz, Netflix, and YouTube). One of the Brooklyn's two line-level inputs delivers NPR news and baseball from my Kenwood KT-990D FM/AM tuner. I mostly use this system with headphones, but currently, the Brooklyn's line-out feeds a pair of Bel Canto Design's compact e.One REF600M monoblocks driving the shelf-mounted Dynaudio Excite X14 speakers I use to play movies and videos.

My bulkier, more elaborate floor system employs a modified Home Depot rack as a totem and a variety of moderately priced cables as fetish objects. This system is not anchored by a newfangled, multipurpose device like the Mytek Brooklyn; instead, its core component is an old-school, line-level preamplifier I rely on for selecting sources, adjusting volume, and, most important, setting the entire system's gain, tone, and temper. The sound quality of every source component is affected and established by the sound quality of this single active device. Likewise, the sound character of every amp and speaker I connect to it. That is a huge audio system responsibility—so huge that most audiophiles, especially those whose only source component is a computer, are delighted to abandon separate preamps.

In my floor system I prefer to experience the texture and vivid intimacy of tubed gear—such as Rogue's RH-5 headphone amp/preamp driving their own Stereo 100 amplifier (100Wpc, KT120 tubes); or PrimaLuna's ProLogue Premium preamp driving their own ProLogue Premium amp (35Wpc, EL34 tubes). Occasionally I go all solid-state, with Pass Laboratories' HPA-1 preamp and headphone amp, and either Pass's XA25 amp or the Bel Canto e.One REF600Ms.

But what I like best of all is a tubed preamp driving a solid-state power amp. Lately, I've spent a lot of time with a PrimaLuna preamp feeding the solid-state Pass XA25. The PrimaLuna's tubes add a touch of feminine flush and glow, generating deeper reproductions of the sounds of recording venues, more distinct voices, and richer timbres—while the supertransparent Pass XA25 grips the speakers more firmly than the ProLogue Premium amp. This unusual hybrid system of tubed and solid-state separates gives Harbeth's Monitor 30.2 speakers satisfying balances of yin/yang, masculine/feminine, disegno/colorito.

Unfortunately, when I use the PrimaLuna ProLogue Premium preamp with my newly beloved First Watt SIT-3 amplifier, the sound becomes a little too flushed with second-harmonic distortion, resulting in a tiny amount of dark murkiness that's most noticeable in orchestral climaxes. This slight dullness is caused not only by the SIT-3's unusually high (for a solid-state amp) component of second-harmonic distortion (it has no feedback), but also by its unusually low gain of 11.5dB (most power amps have at least 25dB). The PrimaLuna has only 12dB of gain, which means that it and the SIT-3 provide a total gain of only 23.5dB—which leaves me about 10dB shy of a well-managed, sparkly-dynamic condition. If I use low-sensitivity speakers—eg, Harbeth P3ESRs or Magnepan .7s—this problem is exacerbated.

That was all before the Rogue RP-7 preamplifier ($4995) entered my system. In single-ended mode, the RP-7 sports 14dB of gain—only slightly more than the PrimaLuna ProLogue Premium. But unlike the PrimaLuna, the RP-7 is also a fully balanced design, in which mode it delivers 20dB of gain. That's great—but the SIT-3 has only unbalanced input jacks (RCA). However, because the RP-7 is fully balanced, I could run the HoloAudio Spring "Kitsuné Tuned Edition" Level 3 DAC (which has no volume control) into one of the RP-7's balanced inputs and pick up another 3dB of gain. Which worked quite nicely.

Best of all, the RP-7 sounded drier than the PrimaLuna. It didn't flood the First Watt with second harmonic. Like Rogue's Stereo 100 power amp, the RP-7 barely sounded as if it was running on tubes.

Description
One of my favorite things about the Rogue RP-7 is that I can easily read its azure-tinted OLED display from 9' away. Rogue's RH-5 preamp and headphone amplifier, which I reviewed in November 2017, used a less-geezer-friendly VFD display.

The RH-5 measures 15" wide by 4" high by 13.5" deep and weighs 19 lb. The RP-7 is slightly wider, higher, and deeper—18.5" by 4.5" by 17"—and heavier, at 30 lb. It looks and feels sturdy and substantial, and, as with Rogue's Stereo 100 amplifier, the style of its steel case and thick aluminum faceplate is unaffected and timeless.

The RP-7 uses four 12AU7 dual-triode tubes and has 17 (!) separate power supplies—including an individual regulated filament supply for each tube. According to Rogue's founder and chief engineer, Mark O'Brien, the RP-7 uses military-spec circuit boards with heavy copper traces, populated with oil-filled Mundorf capacitors, Vishay resistors, and Vishay HEXFRED diodes. All of which are genuinely expensive, audiophile-certified bits.

Although bigger and wider than the RH-5's, the RP-7's front panel has the same number of buttons and knobs. The big knob at far left is the Balance control, which, late at night in the dark, I always mistake for the Volume control—which, counterintuitively, is the big knob at far right. The volume and balance knobs are stepped in increments of 0.5dB, but on the remote, balance operates in 1dB increments down to –15dB, then jumps to –90dB. This permits fine adjustment where it's most typically needed, yet facilitates the extinguishing of the channel without going through all the intermediate steps. When the Balance control is used, the OLED display shows the level of each channel separately.

Each of the nine small, round buttons strung in a row across the bottom of the front panel is topped by a little blue status LED, to make it easy to recognize and assess from afar. Close up, their tiny labels are almost legible—from left to right, Display, Proc Loop, Line 1, Line 2, Line 3, Bal 1, Bal 2, Mono (I love Mono buttons!), Power—but it doesn't matter: The display tells you, in large characters, which input is selected.

I almost never touched the RP-7's front-panel buttons—they're too high-quality and positive in their actions. I prefer the remote's cheesy plastic "lazy buttons." Unfortunately, if you want to extinguish the RP-7's bright display, you have to get up and directly push the Display button. I'd have been happier had this important feature been included on the remote. Likewise, though there's no Mute button on the front panel, the remote has one: when I hit it, a little red LED just left of the Volume knob lit up.

The chief difference between a headphone amp that includes a preamp and a preamp that includes a headphone amp is easy to spot. A proper headphone amp, such as Rogue's excellent RH-5, has separate, three-pin left and right XLR jacks, plus a four-pin XLR jack, for balanced headphone listening, and two ¼" (6.35mm) jacks for single-ended headphones. The RP-7 has only a single ¼" phone jack, and no balanced options. The owner's manual explains: "For performance reasons the headphone jack does not include a switch to break the audio signal. To mute the output from the preamp either mute the preamp or turn off your power amp."