Silicon Arts Si2 Mini Line pre & power
To simplyfy a bit, here's what I've thought about so called 'digital' or Class D amplifiers until now: The related techniques is so complicated (eg. timing accuracy of the switching transistors) and demanding that only a handful of manufacturers have resources to do any real R&D work. That's why most Class D amplifiers carry chips or modules by a few reliable companies be it ICEPower, Tripath etc., and therefore the true value added of different Class D amps is limited to factors that have a minor importance to the sound quality. This would explain why all Class D amps that I've auditioned so far have sounded, if not identical, still somehow similar, sharing many and not always complimenting sonic characteristics.
Against this background, I was surprised of the undisputable sonic difference between the Class D Silicon Arts Micro Line Si2 power amp and the much cheaper Class D Firestone Big Joe IV power mono blocks (Silicon Arts Si2 Preamp as a pre). I expected a minor difference. Possible explanations?
Masataka Tsuda, the head designer of Silicon Arts Design and Concert Fidelity, stresses the importance of keeping the number of parts and the length of the signal path at minimum, without which there won't be the best sound quality. With the Si2 Micro Line components Tsuda has taken this design philosophy to its extreme: the Silicon Art Si2 Micro Line components are among the smallest on the market. But so are Firestone DACs and amps, so the explanation hardly lies in the shortness of the signal path. And I assume there can't be a significant difference in the number of parts either, but here I may err, of course.
Si2 Preamp has an electronic volume control and an electronic switch to choose from the two inputs. The rear panel has two pairs of RCA input jacks, and a pair for the pre out. The line stage is non-inverting and powered by a separate DC power adapter.
The Si2 Power amplifier outputs 14 W into 4 ohms, and 10 W into 8 ohms in a bridged configuration. The front panel has only a mute switch - a very useful switch in practice. The back panel shows a pair of RCA inputs, and inlaid banana jacks for the speakers. The DC power adapter is rated 12V/6A.
What exactly goes inside the boxes, I have know idea. Better to proceed with listening. Silicon Arts recommends not longer than 1 meter interconnects between the preamp and the power amp. My choice was the Gregitek interconnect, a very good one. I also wanted to use Gregitek's superb speaker cables, and did in the beginning, but soon had to resort to the lighter and more flexible Kimber 8TC: the thick Gregitek cable dragged the light Si2 power amp where it wanted, and even small dictionaries (French / English) on top of the cases didn't help.
The manufacturer recommends pluging the units in and wait 24-48 hours before listening. So I did. In fact I think it's advisable to let the amps be powered all the time. I tried several loudspeakers including my super sensitive horns but ended up using middle sensitive 2-way monitors - a likely option in practice, although not necessarily in theory, for this Silicon Arts Mini Line combo.
Si2 combo vs. Firestone Big Joe IV as the power
When I replaced the Big Joe IV with the 12W Silicon Art Si2 power amp, the Silicon Art Si2 per as a preamp, the difference was obvious from the first note. Among the most obvious differences were the greater dimensionality and refinement of the sound of the Si2 combo. It sounded more mature, slightly softer, had better separation between the instruments, and better defined timbre. The two Si2s together made great vocal and instrumental music, and with this music especially was preferable to the significantly cheaper Firestone Big Joe IV. Hurray for the small Class D differences! As compared to the Si2 combo, the Big Joe IV mono blocks talked loud and high, were pure and clean, but something was missing, some finess, and smoothness, and the whole presentation was more 2D. In these regards, the Si2 thus bettered the performance.
Si2 combo vs. Almarro 2050A
As compared to Almarro 2050A (EL84 based SET), as an integrated amplifier, the latter made the sound, midrange especially, more organic, as they say, but the Si2 combo had a tighter bass and was also fine in areas where tube amps typically excel. This comparison wasn't, however, really valid because the Almarro 2050A only delivers 4-5 watts to 6-8 ohms, and require much more sensitive speakers than the ones I had chosen for the two Si2.
These Si2 units are designed to be used together, and that is how I mostly evaluated them. But I was too curious to know what's the meaning of the Si2 power amp in the final sound, so I replaced the Si2 power amp with the Almarro 2050A, and by using low playback volume, listened to the effect. Relative to the Si2 power amp, the Almarro 2050A (now used as a power amp) sounded quite soft and warm, a bit slow and noisier than the Si2 amp, ie. not what this small power Almarro amp is capable of at its best (it's a very interesting amp indeed; wait for the review). More interestingly, although the Si2 power amp was brighter, it didn't sound like a typical solid state amp or Class D amp. There are elements in the sound of the Si2 that take it closer to the world of tube amps, closer than the Class D amps I've heard before. At the same time, the difference is still there. And I think the same applies to the Si2 preamp: it's a good preamp but doesn't do to the sound what a high quality tube preamp would do, eg. in terms of transparency.
Si2 combo vs. Wadia 151 DAC & digital amp
This comparison gave highly interesting results. The sound of the Si2 combination was more fragile, more 3D - music coming from behind the center line of the speakers, and more or less equal in the bass. But the Wadia 151 had sonic virtues, eg. purity of the sound, that the Si2 combo didn't provide (a review coming). This difference may be due to the Wadia 151's integrated DAC section, and therefore comparing the two products is not straightforward or fair. Also, although both the Si2 combo and the Wadia 151 deliver the same amount of power, the latter sounded more powerful whereas the Si2 sounded dynamically more 'complex'.
Conclusion
Silicon Art Si2 Micro Line combo did not hide its sonic strengths, most notably some kind of sophistication of the sound. In particular I was impressed by the fact that, despite my initial scepticism, it seemed to depart from the Class D standard, ie. had more variation in its sound than the Class D amps I've heard have had on average.
Having said this, there are some limitations that have to be taken seriously. The speaker sensitivity is one. The Si2 package generates 10 watts into 8 ohms and 18 watts into 4 ohms per channel, and if used in a normal stereo system the speakers better be of high sensitive. But not the only this. If one wants to play music loud the speakers need be really high sensitive, 90 dBs is not enough. The reason is that, probably due to the limited power supply of the power amp (12V/6A), the sound gets easily distorted during dynamic peaks of, say, piano music. I mean, audibly distorted. Therefore, in order to avoid this happen, not only the speakers should be sensitive, the playback level should be reasonable and preferably the listening distance shorter than in a normal stereo setup.
This all points to the fact that Silicon Arts has presumably designed the Mini Line combo first and foremost for a computer system or similar near field setup. And for that purpose they fit perfectly, and I'm sure, sound much better than most other desktop amps on the market. What will limit their use in this function is the price. Only very few have more than 3000 euro to invest in a desktop amplifier, even though it clearly is a highend amplifier.