Wadia, founded in the late 80s, was one of the earliest companies specializing in high-end digital audio products. Founded by former 3M engineers, they came up with concepts way ahead of their time and in fact is the first company to come out with a stand alone DAC for hi-end audio. Their Digimaster algorithm was legendary. The Wadia 2000 was unmatched technologically and sonically at the time. Wadia was one of the first company to recognize the role of jitter in digital music reproduction, and was the first company to apply a digital volume (in modern audio application context), and lately the first company to extract a true digital output from the iPod with their Wadia 170i dock (others merely analog audio pass through using internal iPod internal conversion). One thing that Wadia has never done before, was producing an amplifier. the 151 is their first amp.
Design and Technology
My unit is in matt black, but there are also available in silver-white color. There are 5 rubber push button in the front panel: input, phase, mute, vol - , vol +. A two-line black lit bluish LCD display which can be turned off using the supplied remote which also controls all other functions. At the back there are two RCA SPDIF digital inputs, one USB, and one toslink as well as standard IEC power connector and standard plastic 5-way binding post. There are no analog inputs nor outputs - so you can't just use the DAC nor the amplifier (its PWM output amplifies directly, so difficult to implement line out, bad news for Headphone users). The Wadia 151's chassis is the same size and shape with the 170i and can be stacked over each other nicely. Cute!The 151 uses trickle down technology from higher end Wadia decoding computers (their fancy way of calling their DACs). Most notably is the asynchronous up-sampling and DigiMaster algorithm. If you use an imaginary microscope, all the audio analog sine waves are stair steps because of the sampling rate. Wadia 151 up-samples all inputs to 384kHz (from 44.1 - 192kHz) and apply an intelligent "forecasting" based on their model spline curve on what the analog sound curve should look like. This is done through a "very accurate" master clock which they claim to reduce jitter to astonishingly low levels and create an audio signal that is so close to real life analog sound waves. The volume control is done in the digital domain at 32-bit resolution, so even at -50db the resolution is never less than 16-bit.
The signal output from the previous stage are converted to pulse width modulations (PWM) which are amplified directly through a low-pass filter to drive the speakers. The amp is a variation of the class-D design and is rated at 25 watts into 8-ohm and 50 watts into 4-ohms. The 151 consumes only 4 watt when idle and is designed to be left on indefinitely, and my test shows that it sounds better after 30 minute after turning on.
The connectors are of good quality, except that of the coaxial digital inputs. My digital cable termination was quite tight fitting and I was trying to pull it off by turning but ended up unscrewing the SPDIF post off from its place. I would expect a higher quality connector from a company like Wadia. Fit and finish are good generally but I would like to see better screening of letters on the front panel. They are just screened painted and looks cheap (although higher models from Wadia also uses this method, but somehow managed to look "more expensive"). Well, what to complain at $1,195??
Listening
All my listenings were done after running signals through it non-stop for a week.
First thing I want to say is that I didn't expect a sound like this from a small little box! It sounded...well, BIG. It played louder than I would expected from a 25 watt amp. My reference desktop speakers, the Amphion Argon3 speakers, are not very sensitive (rated at 86dB) and are usually better off with more powerful amplifiers. However, there were never a sense of strain or stress on the 151. I turned the volume up, filling up a medium size listening room with no sign of the 151 running out of power. They felt as powerful as my Bel Canto s300 in terms of ability to drive. However, macro dynamics (changes from softest to loudest) were slightly lagging from the the more powerful amp. The slam emerging from quietest black background lacked the last bit of heft and immediacy, but I am nitpicking here. Bass was tight and solid, but if you like bass extension and slam, you may think the 151 is a bit lean sounding, but I find it is neutral and well balanced, neither thunderous nor thick. This is not a warm sounding DAC, it just doesn't have a character per se.
Micro dynamics, the small little details and nuances are among the best I have heard. The all-important mid range is clear as crystal, this make the 151 sounds sophisticated and is a great match with certain kinds of music such as jazz or acoustic recordings. The 151 is highly resolving and never edgy or grainy. The word "refined" sums the tonality of this unit very well.
Soundstage is very good. Sound field extended backwards compared to the Bel Canto DAC2.5. Vocals seems to move a foot backward. Width is good and instruments are well spaced out but soundstage seems to not go much beyond the speakers (Weiss DAC202 excels in this area).
USB implementation is adaptive isochronous, not asynchronous, but with well implemented super up-sampling and an accurate master clock this is probably benign. The more expensive Wadia 781i also uses this method. I tested the USB input and finds it to be as good as, if not slightly better than its SPDIF toslink input (driven from Mac mini optical output). I did not test the coax RCA input as I did not intend to use this DAC to decode anything but music from my computer, so can not comment there.
The only usability issue I have seems to be the inability to decode 88.2kHz. The 151 can only do everything else except 88.2 kHz, which is a shame because many of HDtracks' excellent downloadable hi-resolution music are at this sampling rate. The sample would be down sampled and locked at 44.1kHz, then up-sampled again, which is less than ideal. USB and toslink are both limited to 96kHz (former because of the DAC, latter because of the toslink implementation in the Mac hardware), so if you want to get 192kHz, you need to get it from a PC with capable sound cards, or newer 192 capable USB to coax SPDIF converters. See A survey of converters in the Blog archive.
(I was later informed that the 151's coaxial and toslink inputs, but not USB, can accept 88.2kHz. However the Mac OSX does not output 88.2 over toslink, so it didn't work for me.)
Conclusion
Since we can not really judge the 151 as a stand-alone DAC or integrated amplifier, I can only say that you can't get this sort of performance from a $500 DAC + $700 amp, you will have to spend closer to $2,000 to get this level of sonics. It's obvious competitor is the Peachtree Audio Nova (Hybrid tube integrated amp and DAC). I have not heard the Nova but heard good things about it, so please try to audition the 151 vs the Nova and see which one touches your soul. 
