I thought I would shed a little light on the m902. The m902 replaces the 901 in our lineup (although we still have a few 901s left!). It has the same headphone output amplifier as the 901 which is based on a high current, high speed transimpedance amplifier (AD815). This is where the similarities end though.
The first important difference is in the level attenuator. The m902 uses a digitally controlled analog attenuator which provides 95dB of attenuation with precision .5dB steps. I think there are several advantages to this type of control. First off there are no contacts to become contaminated, there is no switch capacitance that varies from setting to setting, and there is no wire or connectors to color the sound. Also, the digitally controlled attenuator has a very wide volume range which makes the m902 handle a variety of headphones from super sensitive types like the Shure E5 to the not-so-sensitive types like the HD600 or K1000. Anywhere in the entire range you can find the exact listening level with .5dB steps. (final firmware might switch to 1dB steps at attenuation levels of –85dB and below). The attenuator IC that had the best transparency and musicality also happened to have four channels so we hooked up the extra two channels to a line output so that you could use the m902 to control speakers or use it as a DAC.
Speaking of DACs, the m902 contains our “second generation” DACs which have some notable improvements over the DACs in the 901. While the 901 used a very high performance chip (CS43122) we chose the Burr Brown PCM1730 for the m902. This DAC is one of the few that has current outputs and allows us to use our own current to voltage converter. The current to voltage converter uses very high speed transimpedance amplifiers which assures that reconstruction of the analog signal is done without the non-linear slew rate limiting of regular op amps. While you won’t see the benefits of this type of topology in the measured performance I think it makes a significant improvement in the clarity and resolving power of the DAC. The m902 also employs a second PLL which we call s-Lock. The s-Lock PLL re-clocks the recovered clock from the digital inputs (including the USB) using super stable crystal oscillators and delivers a sample clock to the DAC with very low intrinsic (self generated) jitter and well over 100 times attenuation of incoming jitter from external sources. Call me old fashioned but I’m just not a believer in sample rate converters. While the 901 DAC is capable of fine performance with a low jitter source the clock recovery circuitry is not capable of attenuating incoming jitter so its performance degrades with long cables and jittery sources.
Like the 901, there are no electrolytic capacitors in the signal path. The inputs to the volume control are capacitor coupled with metalized polymer capacitors while the output amplifier is controlled with a DC servo. I think that a coupling capacitor sounds better than a servo since, when using a servo, you are not only listening to the integrator capacitor but you are also listening to the servo amplifier as its output is summed into the audio signal. However, in the case of the output amplifier we would need a very large value output capacitor to protect the headphones from DC offset so a servo is necessary here. (to keep the low frequency roll off below 10Hz while using 32 Ohm phones we would need around 5000uF which would only be available in an electrolytic type.)
Other enhancements to the m902 include:
All audio signal path resistors are 0.5% tolerance thin film surface mount types. Thin film resistors use the same type of metalic material used in thruough hole metal film types but have no lead inductance.
Improved capacitors in the output amplifier compensation network.
Larger power transformer with separate winding for digital circuitry. (the 901 derived digital supplies from the +analog supply)
Improved headphone jacks
4 layer pcb (this allows a continuous, low impedance ground plane and low inductance power supply distribution)
Separate power supply regulators for analog amplifiers, digital reciever PLL, DAC digital supply, DAC analog supply. 7 regulators in the m902!
Passive cross feed circuit.
Optional IR remote control. I wanted to be able to sit on my couch and have the m902 in my stereo cabinet so I could have it hooked into my loudspeaker system as a DAC…
Yup, there is more stuff in the m902 than in the 901…and it costs more. No cut corners. Just nice rounded ones on the outside.
PS. The little I-river IHP player is a nice low jitter device for playing wav files. No laser pickup error correction…