Digital Do Main

From Hi-Fi World - December 2009 issue

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Field of Dreams

Following in the footsteps of japan's legendary 1970s MOSFET amplifiers, Noel Keywood assesses the new Digital Do Main B-1a stereo power amp sporting special Field Effect Transistors...

Drawn by the eerie purple glow of a corona discharge, I went into a room at the Munich High End show earlier this year to see the Lansche Audio No3 loudspeakers. Seated in the gloom of this low lit room, however, I saw to my right a very unusual looking amplifier, the Digital Do Main B-1a you see pictured here. Beside was an earnest looking Japanese gentleman, Kazuhiko Nishi, eager to explain to me the inner workings of a radical amplifier that used custom made, large area audio power FETs (field effect transistors) able to give better sound quality than we get from today's general purpose industrial power transistors. With words like "silver vapour deposition", a "mirror image FET power supply" using the same devices, "gold plated heatsink contact areas" and other esoterica, the Digital Do Main B-1a sounded like one highly specialised solid-state design to me, potentially able to overcome all the ills that afflict the breed, I thought, and hoped...

2SK278B, 2SK78 and 2SJ78 low power

static induction transistors used in the B-1a.

Aware that the amplifier has been little seen in Europe or the UK, I beseeched our Japanese speaking editor to phone 'em in Hamamatsu and get one if he could. I don't know what he said in Japanese but it worked, and a B-1a was delivered through UK importer ABC Audio. Well, collected in fact, from the ABC room at Audio 09 Show a few weeks ago by myself. I was again nervous, as with the wonderful Ortofon Cadenza moving coil cartridges I nabbed at Munich,  that the B-1a might not make it back to Hi-Fi World towers if anyone found out about it at the Show, but I was lucky enough to make it home Sunday night with a single B-1a securely housed in a sturdy flight case, adorned with Japan Airlines stickers. Yes, this unit was straight from the factory and the rear panel carries a large 'SAMPLE' sticker.  It is also a 100V version so was accompanied by a step-down transformer, one of surprisingly small dimensions. It struck me immediately that this would affect bass quality, likely for the worse, something to bear in mind in this review.

Our review sample was a single stereo power amplifier. But this amplifier can be set to bridge mode to double power output, in which case it becomes a monoblock and two are needed, doubling available power for those that need more than 150 Watts per channel. With rear mounted input level controls for each channel, plus a big volume control in the centre of the front panel, a preamplifier isn't needed, so we did not use one, running a Stello DA-100 DAC straight in for CD purposes and an Eastern Electric Minimax phono stage for vinyl. Loudspeakers were our Spendor S8e benchmark references, chosen for a nicely damped, colouration-free midband.

Weighing 35kgs – and much more in its flight case – our B-1a wasn't going to be spirited away from me too easily I must admit and I had to get two security guards to carry it into a London guarded store. Although it has a compact fascia measuring 218mm wide and 176mm high, it is an enormous 550mm deep.  The front carries a power switch and a centrally placed volume control illuminated by a small escutcheon. A master power switch sits on the rear panel. This also carries loudspeaker sockets able to accept bare wires, 4mm plugs and spade terminals. Balanced XLR inputs and unbalanced phono inputs are provided, each channel having its own input level trim control able to reduce volume to zero. These can be used to lower sensitivity (although at 1.25V it isn't high) or adjust channel balance. The amplifier switches on and off silently.

At the heart of this amplifier lies something known as a Static Induction Transistor. Originally described by J. Nishizawa in Japan in 1975 it is in fact a Field Effect Transistor, with Drain, Source and Gate, as usual. Digital Do Main manufacture the 2SK77b high voltage / current output device used in their B-1a, as well as smaller signal devices. It seems that the Static Induction Transistor (SIT) is difficult to fabricate and its excellent properties are only really needed in high voltage transmitters, where it can replace valves, and audio amplifiers – not a sufficiently large market to sustain profitable manufacture. So the SIT has been in and out of use for a long time, never having quite managed to secure its future. Its voltage transfer characteristics are triode-like, it does not suffer thermal runaway and it is able to work up to 225 degrees C.

All of this looks very good, but mismatching between devices will affect crossover distortion, which still exists, so feedback remains necessary. And also, the sound of 'triodes' isn't really something easily determined, as the few main types in use (300B, 211 and 845) all sound quite different and, in my experience (I use a 300B at home and 845 at work, so I suppose I am Mr Triode!) is nothing like any solid-state amplifier I have ever heard, including this one. Since the current density and dielectric within thermionic triodes both differ radically from any solid-state device, and since the latter don't suffer secondary emission, saying they sound the same is like saying a Mini and Ferrari are the same because they both have round wheels.

Best to forget simple but misleading comparisons like this; the B-1a can justify its existence without it and there is a need for solid-state amplifiers built around custom designed, linear audio transistors I believe. This is why I was so excited to hear about the B-1a in the first place, and why in audio it is a radical amplifier: this is the only amplifier I know to have output transistors optimised for audio use. By way of contrast, any 300B or KT88 based valve amplifier enjoys dedicated audio valves inside – almost an unfair advantage!