Along the top of the Explorer are three white LEDs that indicate whether the sample rate is 44.1/48, 88.2/96, or 176.4/192 kHz — the Explorer is one of two models I tested that support this last rate. Meridian includes a short USB cable and a carrying pouch.
High Resolution Technologies MicroStreamer
High Resolution Technologies (HRT) made a splash in the hi-fi DAC market a few years back with inexpensive designs that nevertheless offered high-quality USB-audio inputs at a time when many DACs included this feature only as a toss-in afterthought or an expensive add-on. The company's latest model, the $190 MicroStreamer, is its most portable.
The MicroStreamer is a plain, but well-made aluminum box, slightly larger than the DragonFly, with a Mini-USB input on one end and two 3.5mm audio outputs on the other end. As with the Explorer, one of these is a line-level output for connecting to an audio system, while the other's level is software controlled for use with headphones. Along the side are a series of LEDs to indicate the incoming sample rate (44.1, 48, 88.2, or 96 kHz), and whether or not the unit is muted (via software). HRT includes a USB cable and carrying pouch.
The MicroStreamer's firmware can be updated over a USB connection to a Windows PC; a recent update brought a cool new feature: the capability to use the MicroStreamer with iOS devices. The iPad — and, as of iOS 7, the iPhone and iPod touch — unofficially support USB DACs via Apple's Camera Connection Kit USB adapter or Lightning to USB Camera Adapter, but only if the device's power draw is 100 mA or less. The MicroStreamer's version 1.2 firmware reduces power draw to 98 mA, making it the only bus-powered DAC I've found that works with a direct connection to an iOS device (more on iOS compatibility in a bit). This change in power draw also benefits people with Android devices that support USB On The Go.
Audioengine D3 Premium 24-bit DAC
Audioengine has made a name for itself with its excellent powered monitor speakers, which have become popular as alternatives to computer speakers, iPhone/iPod docks, and even small home stereo systems. The company also offers a line of high-quality electronics, including the N22 desktop amplifier and the brand-new, $189 D3 Premium 24-bit DAC headphone amplifier and DAC.
The D3 mimics the DragonFly's thumb-drive form factor. The D3's attractive, brushed-aluminum case sports two LEDs: One indicates power, and the other indicates a high (88.2 or 96 kHz) sample rate. The D3 comes with a carrying pouch and a 1/4-inch-to3.5-mm headphone adapter. (The company also makes the older, $169 D1 Premium 24-bit DAC, which is larger but offers an optical-digital input, RCA outputs, a hardware volume knob, and 176.4 and 192 kHz capability).
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