Maximum bass boost
+11.3 dB at 33 Hz
3 ohms at 1 kHz
S/N ratio (1 mW, A-weighted)
Total harmonic distortion (1 mW/1 kHz)
0.099% (at minimum boost)
For the frequency response measurement, I measured the Personal Subwoofer in its least bassy mode, its most bassy mode, and at an intermediate setting. True to the manufacturer’s claims, the Personal Subwoofer doesn’t deliver just a simple bass boost; the curves do seem to be tailored to different degrees. The maximum bass boost (red trace, also red indicator light on the device) produced a substantial boost of +11.3 dB at 33 Hz. The minimum boost mode (green trace, also green indicator on the device) actually cuts bass substantially; it’s down -4.1 dB at 20 Hz. The middle setting (yellow trace, yellow indicator on the device) moves the center of the bass boost up by nearly 1/3rd octave, to +9.3 dB at 42 Hz.
Output impedance at 1 kHz measured 3 ohms, so the Personal Subwoofer should do a decent job of driving a typical set of headphones. — Brent Butterworth
With so many reasonably priced headphone amps on the market these days, if your favorite cans are struggling with your smartphone of choice there's no reason not to seek help — especially at these low, low prices.
The FiiO E02i's form factor is hard to beat for convenience, and it provides enough oomph to make edge-case portable cans like the HiFiMan HE-400 real traveling companions. And it adds smartphone controls to boot, a thoughtful inclusion. Clip it to a coat pocket, set it and forget it.
The DigiZoid ZO2 is a whole different animal. While those interested in a "natural" sonic signature might want to run screaming, it does offer significant tone-shaping controls in a tiny package, and good sounding ones at that, bettering what I've heard from onboard software algorithms and other hardware devices. If you can live with the somewhat cryptic controls, it should be on your list if you like to EQ your headphones.
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