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, 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
Preamplifier: Nagra Jazz, Esoteric C-03, Vinnie Rossi LIO with DHT module, COS Engineering D1, Wyred4Sound STP-SE Stage 2
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; ; Aura Note Premier; Wyred4Sound mINT; Nord Acoustics One SE UP NC500MB; AURALiC Merak [on loan]
Loudspeakers: Albedo Audio Aptica; EnigmAcoustics Mythology 1; Sounddeco Sigma 2; soundkaos Wave 40; Boenicke Audio W5se; Zu Audio Druid V & Submission; German Physiks HRS-120; Eversound Essence, Rethm Bhaava [on loan]
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 15m open floor plan with two-storey gabled ceiling, wood-sleeved steel trusses and stone-over-concrete flooring
Review component retail: ~€6'600 ex VAT

May the forte be with you! For Cees Ruijtenberg, painting outside DAC boundaries is second nature. In fact, his hifi career began with designing electrostats. Under his Metrum Acoustics brand, he's simply been known for NOS converters. Until now. The Forte stereo amp finally puts his personal reference into production. Sorta. You see, Cees' own amp—and thus also the amp which he used for all his design work on the Octave, Hex, Musette, Pavane and Adagio digital—is a 12AX7 hybrid. Yet the Forte is pure transistor. Still, it sounds the same, with superior specs. Huh?

"It's an idea from two years ago. At the time I had hybrid amps for pleasure listening and critical evaluations. What I did not like was the fact that triodes suffer aging problems. In simulation software, I started to imitate triodes with Fets. The good thing with triodes is their low gain and high bandwidth without needing feedback. That was my goal. It took me a few weeks of simulations but I hit upon the desired result. It falls somewhere between a 12AU7 and 12AT7 on gain but with a bit more bandwidth. The disadvantage is that it runs on the same high 350V rails as real tubes would. From there we drive a class A/B output section (10 watts in class A, 50w/8Ω, 100w/4Ω, 170w/2Ω). Damping factor is 200. Compared to my old hybrids, there's no sonic difference, just less noise. Two years later, we've now turned this into a commercial product with a lot of added protection because during the startup process, we must charge up 176'000µF. And that, in a nutshell, is the Forte platform."

Cracking the nut, we learn that "the signal passes two Fets run on 350VDC in full class A before the power stage. The rest is protection. The first amplifier I made was without output relay or any kind of protection. That's been running trouble free for 3 years in my living room. But going commercial, I had to avoid all risk in case of malfunction. I added a protection circuit which disconnects the mains completely. This is built on the front panel with its own power supply. As long as the mains is connected, it's always active. In standby, the amplifier is still live but quiescent current is zero. Ditto for the high-voltage section. Hence the capacitors are charged at all times but power consumption is reduced to the 5 watts of the control unit. In our second prototype, we added this protection circuit. Though mains power now disconnected safely, a lot of charge remained in the 176'000µF capacitive storage. In the event of a huge offset on the amp's output, these caps could still damage a loudspeaker. Therefore the last correction was a 16A output relay for fast protection. In case of malfunction, the error LED will flash and the amp cannot switch on again until a hard reset by reseating its power cord."