Dear RH, What you did was such a tease... not even a hint on the M2's sound quality?
I have a few comments:
- You mentioned that the M2 does not use a digital filter. I think it does or has to since the device uses upsampling and noise shaping just like a regular DAC before the last step of PCM to PWM conversion. I believe it has to use a low pass filter to remove aliasing. In one of the documents published by Zetex, it stresses the importance of the digital filter in a digital amplifier. Zetex has its own proprietary upsampling filter algorithm called the ZTA-filter algorithm. NAD does not mention the type of filter they use. This is one of the key things that I am wondering about. With all the recent developments in the digital filtering technology, is their digital filter up to the current state of the art?
- Regarding the power supply, one of the claims Zetex makes about its invention is that it eases the constraints on power supply design compared to an open loop switching amplifier. As you mentioned, the feedback scheme is meant to compensate for power supply noise as well. The fact the M2 has beefier power supply than other digital amplifiers you have seen is interesting.
- Regarding whether this is the future or not, Mark Levison's latest flagship amplifier is a switching design ( as I am sure you are aware). They claim it was the best in blind tests. I have read their published patent on a switiching amplifier (filed under Harman) which was very sketchy but it uses feedback as well without saying how it is used. The real difference in their implementation is that they interleave the operation of the switching transistors (described in a different patent) to reach a combined switching rate of 2Mhz. For comparison, the M2's switching frequency is about 844khz and that of Tact/Lygndorf is a multiple of the input signal which is less than 400khz.
Now when will the Dec 09 issue of TAS become available on line? Do we get to get to sneak a peek on the sound quality part beforehand?
It is always great to read your reviews.