No matter how you feel about Apple’s outrageously popular iPod portable music server, it has clearly affected how the world listens to music. From my audiophile point of view, one beneficial effect has been an increased interest in headphones and headphone amplifiers. Although a few audiophile-grade headphones have the low impedance and high sensitivity that would make it possible for an iPod to drive them directly, most need an external amplifier.
Headphone amps come in many sizes, shapes, and price ranges. From just over a hundred dollars to well into five figures, you can find a headphone amp that fits your financial and operational requirements. Some amps are not much bigger than the iPod itself and battery operated, providing a portable solution. Others are designed to be hi-fi components that sit on your rack alongside your preamp and CD player. Still others are designed to connect to a computer via a USB port, improving the computer’s sound. Whatever your needs may be, there’s a headphone amp to fill them.
South Korean manufacturer April Music makes two lines of components: Stello and Eximus. The Stello line consists of moderately priced, conventionally styled products, while the upscale Eximus line includes more ambitious components with advanced styling. The Stello line includes CD players, DACs, a combination DAC/preamp, an integrated amp, and power amps, all of which are very cleanly styled. While priced above the budget level, Stello components are still in the moderate price range and seem to provide great value for their cost. Let’s see if the $595 USD Stello HP100 fits this mold.
Weighing in at a solid eight pounds and measuring 8 3/8"W x 2 1/4"H x 11 1/2"D, the HP100 is definitely not portable, although it’s not unusually large either. On the rear panel you’ll find an IEC receptacle for AC power; the power supply is internal, not another wall-wart (hooray!). Two sets of sturdy gold-plated RCA input jacks and a set of RCA output jacks are also located on the rear panel. The presence of output jacks means that the HP100 can be used as a simple line-stage preamp or inserted into a tape-monitor loop. A toggle switch on the rear is labeled Filter and has two positions. Position A really means off, while position B rolls off frequencies above 20kHz. An on/off label would be more informative. (In use, I couldn’t tell any difference between the two settings.)
On the thick front panel are a power switch, a Neutrik headphone jack, an input selector switch (for the HP100's two inputs), a level switch that lets you select the amount of gain to match your headphones or power amp, and the knob for a smooth ALPS Blue Velvet volume control. A red LED tells you when the HP100 is powered up
Inside the unit you’ll see a neatly laid out circuit board with a hefty two-stage regulated power supply that's driven by a sizeable toroidal transformer. Although ICs are used in the power supply, the signal path uses discrete transistors in a class-A, push-pull circuit. Quality parts like Wima capacitors are used.
The HP100's input impedance is quoted as a whopping 1 million ohms, while the output impedance is a low 50 ohms. The gain through the output jacks is said to be 11.3dB with the gain switch in the Low position, and 16.3dB with the gain switch in the High position. All these specifications mean that the HP100 should be compatible with just about any imaginable headphones, source, or power amp. The all-black metalwork is impeccable. Overall construction is as good as for any headphone amp I’ve seen anywhere near the HP100’s price.
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