Just got back from AES in San Francisco and finally have a couple of minutes to myself. I thought I would tell a little about what is inside the m903. Basically there is no stone left unturned in the m903. Admittedly the specifications we published look a lot like the m902. I have yet to sit down at the Audio Precision and make official measurements for all of the performance parameters yet. The numbers you see are indicated as "preliminary" and are conservative. I will update them as we get closer to releasing production units.
Anyway, here is a list of some of the circuit changes from the m902 design. Sorry if some if this gets too technical.
We are using the PCM1798 as the DA converter. I think this is a brilliantly musical DAC. It may not have the same noise numbers as the 1792/4 but it doesn't draw 350mW either. This is important in the m903 since the DAC analog power supply is in the form of a shunt regulator.
Following the PCM1798 is a new current to voltage (I/V) converter based on an unlikely suspect: THAT1570. This is actually a low noise front end amplifier for microphones and utilizes a current feedback architecture. While the I/V converter in the m902 was a current feedback amplifier the THAT1570 has substantially lower noise and lower distortion.
The high pass filter section is now based on the OPA1612. Finally a low noise bipolar amplifier that sounds really good!
The DAC audio power supplies and the line input/output supplies have now been separated from the headphone amplifier power supplies. This reduces dynamic interaction between the headphone power amp and the DAC analog circuits when driving difficult headphone loads.
For bi-phase encoded digital inputs (toslink, spdif, and AES3) the m903 employs the same clocking architecture as the m902 with some significant improvements. (I'll get to the USB section in a minute).
The s-Lock PLL contains new lower phase noise (jitter) VCXOs. As well, the phase comparator in the PLL has been upgraded to a low noise charge pump type circuit. (The HC9046 datasheet gives a nice treatment of this type of phase comparator). The overall PLL jitter rejection performance has been increased with a higher order loop filter which is now 18dB/octave.
This all boils down to lower jitter at the DAC compared to the m902.
The m903 volume control is based on the Cirrus CS3318. This is a really advanced volume control! It has 7dB lower noise than the PGA4311 and it runs on +/-9V power supplies. The channel matching is really tight (within several hundredths of a dB).
This is, after all, the core of the m903. The headphone amplifier is the same basic topology as the m902 but the output short circuit protection circuit has been moved out of the signal path and into the power supply lines. As mentioned above, the headphone amp now has it's own set of dedicated power supply regulators.
The crossfeed curcuit, which is licensed from Jan Meier, is a more sophisticated design than the one in the m902. It has some passive compensation that reduces the perception of loss of bass in some program material while maintaining the excellent upper register tonal transparency of the original circuit.
The m903 has both balanced and unbalanced outputs. (the m902 had one or the other) Each ouput has independent level control so you can hook up multiple speaker systems or you can use one set of outputs to feed a sub and the other for the main speakers. These outputs have a firmware setting for +/- 9.5dB output level trim for matching speaker efficiencies. There are also individual balance adjustments for each line output and power on preset level settings.
the m903 now uses a completely galvanically isolated, asynchronous data transfer high speed USB interface. There is simply no PLL like *no* PLL.
Clocking data out of a buffer with a low noise fixed oscillator is the ideal digital audio playback architecture.
One thing we didn't do is change how the m903 looks for the most part. However, the white 7-segment leds are nice and clear and much easier to focus on from across the room. Oh, and the input selector switch is now a really nice Swiss made EAO rotary switch that has a very solid feel and will last for ever.
...one more thing, there is now a mono mode. This is probably a more practical feature in a music production environment but it will be fun for listening to Pet Sounds!