The Izmo M1 DDC, DAC, and Headphone Amp
Izmo M1 ($499 expected price in US)
Size: 4 inches long, 2.3 inches wide, and less than 1 inch tall. Weight: 5 ounces. Front panel: 1/8" jacks for line-in and headphone-out. Sounds an awful lot like a portable amp to me. The Izmo M1 from Japan's Izo Co LTD is a lot of things but a portable amp isn't one of them. At least not in stock form. The unit has so much functionality stuffed into its tiny enclosure that there simply isn't room for a battery, though aftermarket batteries are easily obtainable and can add that feature to the M1 repertoire. Still not clear? Let me break down the core functionality first.
The M1 starts out as a 24-bit/192kHz capable USB DAC. It also has an integrated headphone amplifier, and will convert USB to SPDIF for folks using a non-USB (or inferior-USB) external DAC. For those keeping score so far that's 3 devices in one. Adding a USB battery pack such as the Sanyo Eneloop Mobile Booster enables the M1 to be completely portable for as long as the battery holds up (times vary from model to model). The M1 plus USB battery becomes slightly less compact and might be more accurately described as "transportable" rather than truly portable, but that's a matter of semantics.
The M1 has some relatively complex casework compared to your average amp of this size which tends to be a simple rectangle. This refined design extends to the interior as well, where the digital and analog sections each reside on a separate PCB (printed circuit board). Power is normally drawn from USB and filtered through a dual stage rectification system. One can also add an external power source ranging anywhere in the 5V to 9V range as long as it can provide 500mA or more. The AC-to-USB charger units that come with most recent smartphones should do the trick - I used the one from my iPad.
The USB input is tended to by an XMOS solution, a choice commonly seen in many recent high-end units from the likes of Simaudio, EMM Labs, Ayre, and many others. The unit is USB class 2.0 compliant, operating in asynchronous mode and accepting native 24-bit/96kHz signals without drivers. Mac OSX can go as high as 24-bit/192kHz native, while Windows machines demand installation of the included Thesycon driver. A coaxial SPDIF output is provided which allows the unit to function as a high quality DDC (Digital to Digital Converter). Separate fixed-frequency TCXO clocks are used for 44.1kHz and 48kHz (and their multiples) which results in lower jitter compared to the more common PLL solutions with variable frequencies.
On the DAC side of things, the M1 uses the relatively obscure Wolfson WM8718 DAC. It's not among the top of the line in the Wolfson family but still has nice specs such as a 111dB SNR rating. And it does appear to be the top Wolfson model available in the 20-pin SSOP package as opposed to the more common 28-pin SSOP which takes up more room. We're talking fractions of an inch here but when you see the guts of the M1 and consider all the functionality packed inside you start to sympathize with the designers and their lack of real estate.
The output stage of the DAC section is based around a National Semiconductor LME49723. Since the WM8718 produces a differential voltage output there is no need for extra circuitry handling I/V conversion. The completed signal then exits through a 3.5mm mini-jack - there's simply not enough room to fit standard RCA outputs on the rear panel. This requires the use of a 3.5mm to RCA cable if the M1 is to feed an external amp. I had a bit of bad luck with two different generic cables I tried, so I ended up using an Audioquest Evergreen cable ($30). It seemed more solidly built (as evidenced by the fact that it actually worked properly) and looked nicer than the generic cables too, though I make no claims about the sound. The output is set at a fixed 2 Vrms level so fiddling with the volume knob has no effect in DAC mode.
Finally we come to the third ingredient in the M1 portfolio which is the headphone amplifier. The amp can obviously play signals from the DAC section but it also has a front panel 3.5mm input like any typical portable amp would, with a front panel switch to select which one is active. Gain can be switched from +0 to +7 by a process that I still can't quite fully explain - turn the volume knob (which doubles as a power switch) on, then off for 2 seconds, then on again, and an LED will supposedly blink in a certain way indicating gain mode. In practice I usually took a few tries of monkeying randomly with the knob, but it always ended up working eventually. Low gain is perfect for sensitive in-ear monitors while high gain works best for full sized headphones. The amp section is based around a pair of Texas Instruments TPA6130 - one per channel in dual mono configuration. Those are buffered by a Linear Technology LT1364 dual channel opamp which is socketed and can be swapped out to tweak the sound. Output figures are given as 300mW/channel into a 16 ohm load and 32mW/channel into 300 ohms. Output impedance measures an impressively low 1.1 ohms meaning essentially zero chance of any impedance related mismatches.
As mentioned prior, an external power source can be added so the unit no longer relies on USB for power. This can come in the form of a portable USB battery for on-the-go use, or simply an AC power source to give some extra grunt when driving difficult full sized cans at home. My contact at Izo advised me that output level and dynamic range are both improved when using an AC connection but a high quality AC source is required if one wants to actually improve the sound aside from just making it louder. I happen to have a nice linear power supply from NuForce but it outputs 12V - too high for the required 5V through 9V range. As such I was only able to use a basic Apple wall adapter to power the M1. I can confirm what I was told - things did get noticeably louder which in some cases could be considered an improvement. But there wasn't anything like extra clarity or transparency to be gained aside from the volume increase. I suspect a better quality PSU operating closer to 9V might be an actual improvement but I don't know for sure.
Using a portable USB charger brings to mind the new ALO Pan Am with optional PassPort battery power supply in that it isn't the type of thing you would carry in your pocket, but could definitely bring to and from the office as needed.
All this functionality is great, but how does it actually sound? Read on to find out.