Reviewer: Srajan Ebaen
Financial interests: click here
Sources: 27" iMac with 5K Retina display, 4GHz quad-core engine with 4.4GHz turbo boost, 3TB Fusion Drive, 16GB SDRAM, OSX Yosemite, PureMusic 3.01, Tidal & Qobuz lossless streaming, COS Engineering D1, Metrum Hex, AURALiC Vega, Aqua Hifi La Scala MkII, Fore Audio DAISy 1, Apple iPod Classic 160GB (AIFF), Astell& Kern AK100 modified by Red Wine Audio, Cambridge Audio iD100, Pro-Ject Dock Box S Digital, Pure i20
Headphone amps: April Music Eximus DP1, Aura Vita, Bakoon AMP-12R, Questyle CMA-800R (x2), Eversound Essence, Vinnie Rossi LIO
Headphones: Forza Audio Works recabled Audeze LCD-2/XC & Sennheiser HD800 & MrSpeakers Alpha Prime; stock-cabled HifiMan HE1000 (3.5mm, 6.3mm, 4-pin XLR); ALO-rewired Beyerdynamic T1 & T5p; Aëdle VK-1; Meze Headphones 99 Classic [on loan]; Final Sonorous III [on loan]
Preamplifier: Nagra Jazz, Esoteric C-03, Vinnie Rossi LIO (AVC module)
Power & integrated amplifiers: Pass Labs XA30.8; FirstWatt SIT1, F5, F6, F7; S.A.Lab Blackbird SE; Crayon Audio CFA-1.2; Goldmund Job 225; Gato Audio DIA-250; Aura Note Premier; Wyred4Sound mINT; AURALiC Merak [on loan]
Loudspeakers: Albedo Audio Aptica; EnigmAcoustics Mythology 1; soundkaos Wave 40; Boenicke Audio W5se; Zu Audio Submission; German Physiks HRS-120; Eversound Essence
Cables: Complete loom of Zu Event; KingRex uArt, Zu and LightHarmonic LightSpeed double-header USB cables; Tombo Trøn S/PDIF; van den Hul AES/EBU; AudioQuest Diamond glass-fibre Toslink; Arkana Research XLR/RCA and speaker cables [on loan]; Sablon Audio Petit Corona power cords [on loan], Black Cat Cable Lupo
Power delivery: Vibex Granada/Alhambra on all components, 5m cords to amp/s + sub
Equipment rack: Artesania Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands [on loan]
Sundry accessories: Acoustic System resonators
Room: Rectangular 5.5 x 15 metres, double-high gabled ceiling, stone tile over concrete flooring
Review component retail: $8'785 configured as just DHT preamp (no DAC, no headphone output, no phono stage, no Mosfet amp)
I love the sound we achieved with our all-tube designs. With them, soundstage depth, presence and musicality are astounding. And, pure solid-state designs still have greater speed, dynamics and detail. There is a trade-off. Both do specific things very well. I am hoping to achieve the ultimate amp by combining our best tube gain stages with our best transistor current amplifier."

That was Dan Wright of ModWright who works both sectors. His sentiments echo mine to perfection. That makes this quote a suitable segue into today's assignment. To wit, the tantalizing promise of combining a DHT aka direct-heated triode preamp with a first-rate transistor amplifier. By letting the tubes see an amp's fixed input impedance rather than a speaker's wildly variable one—which additionally is peppered with back electromotive force and fussy phase angles as is always the case when DHT amplifiers drive loudspeakers—the glow bottles are free to sing, not stress out. They're exploited for voltage not current gain. It's what nearly all designers agree on is the proper work-load split between valves and transistors. DHT preamps are quite rare though. They need to be free of noise which isn't easy. And, they're expected to have low output impedance with high bandwidth, an often mutually exclusive ambition.

One version of a fully tricked-out LIO prior to the DHT module option.

Enter Vinnie Rossi's wildly accoladed modular LIO platform which customers build to suit and can retrofit forever after. As did Anthony Gallo's original $3'000/pr Reference 3 loudspeaker, LIO has won top marks from each and every reviewer who wrote it up. In nearly each case it also netted an award. From us it received our most prestigious and rare Lunar Eclipse distinction. Being highly interactive with his online community, Vinnie collected wishful feedback on features that would go above and beyond his original à-la-carte menu of:

  • resistor-ladder or autoformer volume control with or without small-signal triodes
• headphone amp with 6.3mm or 4-pin XLR output
• analog input module
• balanced i/o module
• 25wpc Mosfet power amp
• phono stage.

One of the requests was for a DHT module which just bowed as a ~6dB-gain minimalist grounded-cathode design. "The beauty of this approach is that you hear what these DHT tubes really sound like without the usual driver tubes, output transformers, feedback, cathode resistors or bypass caps. Going spud means the fewest parts in the signal path, just one cap and the operating point set with a single nude Vishay Z-foil grid resistor, no cathode follower or OPT. The real challenge is that tubes configured like this pick up tremendous noise and hum. That's where feeding the B+ from our ultracap supply comes in. The cathodes are heated by a custom Belleson super regulator called SPVR (Belleson Super Power Vinnie Rossi), one per tube fed from an external linear power supply. You don't want to throw in any old 4-pin DHT but something closely matched and carefully screened for tube rush and hum. But one needn't spend a fortune on NOS exotica. I'm getting great sound and very low noise with the $70/ea. ElectroHarmonix gold-grid 2A3 and am thinking of offering those standard (or possibly the equivalent 300B). I'm still experimenting with a variety of current production tubes including Emission Labs mesh plates and of course will encourage NOS types as well. Our forum will be helpful for owners to share their favourites."

Focusing on the classic 4-pin bottles 2A3, 45, 300B, 71A, PX4, PX25, SV572, SV811, 101D or 205D*, a filament voltage switch accessible without cover removal allows for easy rolling. As LIO's biggest module yet, the DHT assembly with Yamamoto Teflon tube sockets plugs into the space otherwise occupied by the tubestage and HPA locations. Just so, the latter can be relocated atop the new board. A special version of the RVC module with Z-foil resistors is available with the DHT module. That nets the following usage scenarios:

  • DHT preamp with/without DAC, with/without phono
• DHT/transistor hybrid integrated with/without DAC, with/without phono
• DHT/transistor hybrid headphone amp with/without DAC, with/without phono
• DHT DAC with fixed output à la LampizatOr or Tobian Sound Systems
• DHT phono stage with fixed output.

Being off the grid where two banks of ultracaps power cycle indefinitely (one plays whilst the other charges), LIO's power supply is ultra quiet and puts out stout current. That's an ideal backbone for microphony-prone direct-heated triodes. And because LIO is hifi lego, the concept involves return credits when customers outgrow existing modules and wish to swap them. That makes upgrading far less lossy than usual. One never wipes out the core investment into LIO's posh casing, full-metal remote wand and sophisticated ultracap power supply with its smart charge controller. One just returns a module to Vinnie and gets another one for the price difference after the trade-in value. Or, one keeps the original module to plug 'n' play with multiple options. In DHT mode, one also gets a new top cover with the necessary holes to build a small skyscraper; and a new linear PSU. To my way of thinking, LIO was all set to continue its astonishing victory lap with a real exotic run. How many DHT preamps and/or headphone amps and/or DACs do you know of?

* "The output impedance with most 2A3, 300B and PX4/25 will be <1kΩ; and <2kΩ with most 45, 71A and SV811/572. The most hum-prone 101D/205D raise that to 4-7kΩ. With the LIO Mosfet amp's >100KΩ input impedance, the first two groups of tubes are ideally matched. Interfacing LIO DHT as preamp into an external amp wants to see an input impedance of >20KΩ like our own VR120 with its 50KΩ figure." Whilst output transformers could obviously lower the DHT module's Z-out, they'd also limit its bandwidth. Without them, Vinnie reports nearly ruler-flat 4Hz to 650kHz bandwidth -3dB for a 300B. That's utterly unheard of for transformer-coupled 300B SETs. As we already learnt, with these tubes it's either exploded bandwidth or low output impedance, not really both. Vinnie wanted the former. Users should check that the input impedance of their amp is roughly 10 x higher than whatever tube type they opt for. With 47KΩ a quasi amp standard, that should include a large variety. This ratio between lower source and higher load impedance mirrors the ideal of zero output impedance (perfect voltage source) and infinite input impedance. Since neither are possible, the 1:10 approximation has become a more realistic rule of thumb to most home hifi users.