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 & H1, AURALiC Vega, Aqua Hifi Formula, 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, Questyle QP1R; Bakoon DAC-21 [on review]
Preamplifier: Nagra Jazz, Wyred4Sound STP-SE MkII, Vinnie Rossi LIO (AVC module), COS Engineering D1
Power & integrated amplifiers: Pass Labs XA30.8; FirstWatt SIT1, F5, F6, F7; Bakoon AMP-12R; Crayon Audio CFA-1.2; Goldmund Job 225; Aura Note Premier; Wyred4Sound mINT; Nord Acoustics One SE Up NC500MB; Linnenberg Audio Allegro [on loan], Goldmund Telos 590NextGen [on review]
Loudspeakers: Albedo Audio Aptica; EnigmAcoustics Mythology 1; soundkaos Wave 40; Boenicke Audio W5se; Zu Audio Druid V & Submission; German Physiks HRS-120; Eversound Essence; Audio Physic Avanti [on review]
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 redlevel 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 15m open floor plan with two-storey gabled ceiling, wood-sleeved steel trusses and stone-over-concrete flooring
Review component retail: $19'590

Genesis: size, power and heat. "At this moment we are working mostly on the internal construction and structure of our new 50-watt amp. It will share the concept of the HPA-21 and HPA-01M. It is actually quite big though relatively low in height. It relies on considerable internal heat sink surface. I remember Nagai-san and I had a discussion with you when we visited you in Switzerland in mid 2015. Unlike manufacturers who claim high power from small enclosures/heat sinks, we must rely on large heat sinks to guarantee our full continuous power at rated output. Attached find the specification of our new amp's heat sinks. We are using two of these at 72mm height. That's quite big. But even at 50wpc (perhaps 30-35wpc continuous), it hits 40C° already. That would operate them at 65C°/ea. for 25C° ambient room temperature. Because we're using thick 8-15mm aluminium panels for our casing, those work as additional heat dissipaters to manage thermal issues. But without added heat sinks, Nagai-san says our case would only be good for up to 20wpc at full power. Hence our skepticism over small non class-D amplifiers with claimed high power specs. I notice that FirstWatt too always use rather big heat sinks despite relatively low power ratings." That was Soo In Chae of Bakoon chatting about their AMP-51 in late 2015. He couldn't know that by 2017, it would have morphed into a very different beast to win their rare 'R' suffix for 'reference' model.

Functional prototype still at 35wpc and without display or remote control.

By then Keng Koe Ho, ex Sony, had joined their staff. Bakoon had built a new R&D centre in Kagoshima, Japan. The AMP-51R now did 100/200w stereo/mono. Its internal heat sinks had become pure copper bars. It had added a sophisticated Oled display. It had a 50-step MPU-control attenuator with 0.1% tolerance resistors and 25ppm/C° temperature coefficient. That is functionally different. It sets the gain factor of Akira Nagai's circuit. His Satri IC's gain is fixed with an output resistor. Thus Bakoon always used a 10K Alps or Tokyo Ko-On pot. It replaces what otherwise would be a fixed resistor with a variable one.

"The conventional method uses attenuators at the circuit's very input. This reduces incoming signal strength but compromises S/NR and sound quality. Altering the circuit's amplification factor instead is lossless." After reworking the main board five times, Keng Koe had perfected a new precision bias control circuit. With Akira Nagai he'd worked out a revised driver stage for what over the prototype's bipolar transistors had become Exicon lateral Mosfets.
Merely half of the circuit boards in the AMP-51R's development cycle.

Prototype heat sinks.

SMD selection was "mostly from aviation and aerospace" parts catalogues. Plitron supplied a customized shallow 625VA power transformer with their proprietary low-noise technology. Soo In sweated new casework. Team Bakoon were chasing their masterpiece. Inside the final product are the signatures of Akira Nagai and Keng Koe Ho, electrical engineers; Soo In Chae, mechanical engineer; and June Huh, industrial designer. And there is this famous quote by Michelangelo Buonarroti: "The greater danger for most of us lies not in setting our aim too high and falling short but in setting our aim too low, and achieving our mark."

Akira Nagai facing us, with Soo In Chae working on AMP-51R. At right, finned radiator footers and 'heat pipes' for the output transistors.