Hi All,
I thought I would chime in here and describe some of what is new inside the m920.
As you can see the chassis is pretty much the same as the m903 but with a new color scheme and an additional LED to indicate the presence of a DSD stream. The technology inside however is the result of a person-year of design engineering. Some borrowed from our new m905 studio monitor controller and some new developments just for the m920.
The headphone amplifier topology remains unchanged from the m903 although we did change the cross-feed circuitry to a design based on the Ohman circuit. To me this design works better at presenting a cohesive sound stage from recordings that have hard left and right material while still remaining tonally neutral enough for critical recording/mastering application.
The unbalanced input topology is has improved noise/EMI protection. In a world of ever proliferating 1.9-2.5GHz radios (CDMA, WIFI, Bluetooth, etc.) unbalanced interfaces are more vulnerable to corruption.
Also, the low frequency response has been extended by nearly an octave.
The unbalanced outputs can be programmed with internal jumpers to act as a fixed DAC output. In this mode the DAC is connected directly to the output drivers and the volume control circuitry is bypassed.
The m920 uses a new USB audio controller IC. It is the XMOS XS1-SU1 operating at 500MHz. The significant improvement here is that this IC has an integrated USB PHY. This eliminates one chip and with it the pcb routing of the noisy high speed 8 bit buss that connects the two.
The software for the new XMOS IC was developed Caleb Roberts here at Grace Design. It includes highly optimized asynchronous USB audio stream management which now includes DSD stream reception up to 128x (VoP1.1) and DXD PCM (384kHz, 324bit). The XMOS is also capable of decoding DSD from the SPDIF and AES3 (64x only)
The entire clocking system in the m920 has been redesigned. When receiving SPDIF, AES3, or TOSLINK signals the m920 uses the s-Lock VCXO based PLL to recover a pristine clock to send to the DAC. This is the third generation of s-Lock PLL which was developed for the m905. It's ability to reduce low frequency jitter from the incoming clock stream is effective to almost two octaves lower than the previous design. As well the new s-Lock PLL has a fast lock mode which speeds up the lock up time. Once lock is achieve, the PLL automatically switches to the ultra low noise high jitter rejection mode.
If the incoming signal is outside of the lock range of the s-Lock PLL, as secondary digital PLL/clock synthesizer is used. This clock has very low jitter and a wide lock range. This can be advantageous when connected to Apple TV and other similar boxes which can have off-speed clocks.
The m920 has an s-Lock disable mode in the setup menu. This can be useful for the above mentioned devices which can have clock speed shifts that would cause the old s-Lock in the m903 to loose lock periodically.
The DAC in the m920 is the new ES9018-2M (not to be confused with the ES9018 which is the older 8 channel DAC). This is their state of the art 2 channel 32 bit DAC. We operate the ES9018-2M dac in synchronous mode so that the internal sample rate converter is bypassed. The internal ASRC is useful in a system that presents noisy clocks to the DAC but with the s-Lock PLL to remove incoming jitter it is not necessary.
The setup menu contains controls for setting the DAC filter response. For PCM audio streams the user can select between fast, slow, and minimum phase response.
For DSD audio there are four low pass filter corner frequencies:
47kHz, 50kHz, 60kHz, and 70kHz. The default setting here is 47kHz.
The 9018-2M is a current output DAC and, like in the m903, we use transimpedance current to voltage converters. These are the best at resolving complex harmonics while remaining linear in the presence of high slew rate noise from the DAC outputs. Regular op-amps, whether discrete or IC based, just don't compare in terms of clarity, dimensionality, and musicality.
In the end, the digital system in the m920 is much more sophisticated than in the m903. We wanted to be able to have the option to upgrade the internal code in the m920 without the user having to remove the cover and replace flash chips. Jamie Krapohl wrote the main control processor code for the m920. He and Caleb worked together to create a system that allows for the user to upgrade the firmware through the USB jack using the USB DFU (device firmware upgrade).
Jamie also wrote in code that allows the m920 to be controlled from the Apple IR remote. These are nice little remotes that are reasonably priced. The m920 can be "paired" to the Apple remote for exclusive operation.
I don't get on Head-Fi all that often but feel free to ask questions about any of this!
Cheers,
Michael
PS. third_eye, the m920 that you will be listening to does not have the minimum phase filter enabled. Sorry! It just wasn't quite ready when we had to ship it...
