There are a handful of FM transmitters on the market with solid performance, including the Eznex EzCube, a tiny device that connects to any MP3 player and will set you back about $50.
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The EzCube is by far the smallest FM transmitter we've had our hands on, measuring just 1 inch cubed. Most MP3 players will dwarf the device, with the exception of the MobiBlu Cube, which strongly resembles it.
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The EzCube features a small, monochrome screen with a backlight; it displays radio frequency along with preset information.
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On the top edge of the device you'll find a power button and an M key, which stands for memory and allows you to set up to four presets.
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The left side of the transmitter features tuner buttons...
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...while the right has two ports: one for connecting the included cigarette lighter charger and another for attaching an MP3 player via the packaged line-in cable.
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The overall design is among the most compact for an FM transmitter, but it's also slightly messy, with the two cables and a lack of any console-mounting system. During testing, the whole setup was piled haphazardly into the ashtray of the car--not ideal.
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Also, because the EzCube is made for any MP3 player (not iPod-specific, for example), it won't charge your device while in use. These are both fairly minor gripes, however.
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The considerable plus is that the EzCube is a very solid little performer. We're pretty shocked that Eznex managed to cram such a strong transmitter into such a tiny package, actually. The unit pushed out to every open frequency in San Francisco fairly clearly