FiiO E17 Headphone Amp and DAC

July 09, 2013

The FiiO E17 Headphone Amp and Dac is a versatile very compact portable audio headphone amplifier and DAC. It has sufficient capability to properly drive headphones in the impedance range of 16 ohm to 300 ohm. The noise and distortion properties of the E17 are very good and will improve the headphone audio output experience of many computer soundcards. With a very low headphone output impedance of ~ 0.1ohm, the E17 can drive very low impedance headphones without any tonal coloration. Also, many personal music players and smartphones will benefit from the higher quality audio of the S17 (by connecting the headphone output of the player to the analog line-in of the E17). The stereo input jack connects to the analog audio part of the FiiO E17. The DAC part of the E17 enables decoding of both USB audio (from a computer) and also optical and coaxial S/PDIF digital audio input signals. This note provides some information not contained in the specifications of the E17.

The input impedance of the line-in connection is ~ 12 kohm (measured)
The headphone output impedance is 0.10 - 0.15 ohm (measured)
The volume +/- increments are approximately 0.5dB
For a full digital level optical S/PDIF input signal, and for the default gain setting of 6 dB
- at the maximum volume of 60, the headphone output level (into open load) is 3.6Vpeak (2.5Vrms)
- at the default volume of 20, the headphone output level is 55mVpeak
- at a volume level of 40, the headphone output level is 1130 mVpeak
For a line-level analog input signal (bottom 1/8" jack), the default Gain setting of 6dB means a voltage input gain to the headphone output is ~ 2x. At a volume setting of 45, the headphone output voltage is approximately the same as the line-in voltage level (0 dB gain).

The image below shows using the optical S/PDIF output connection from a Western Digital Media player. This player also has HDMI and standard analog line-out connections but no headphone output. In the configuration shown below, the player is used simply to organize, navigate and read large databases of audio files on USB attached drives (at left) and stream the digital music over a S/PDIF stream to the E17. This makes use of the higher quality DAC and amplifier of the E17 to provide a superior headphone audio output experience:

The image below shows an analog audio line-in connection from the headphone output of the Samsung Galaxy S4 phone:

The Benefits of an External Headphone Amplifier

To demonstrate the sonic benefits of using a high-quality external headphone amplifier such as the E17, the spectra below demonstrate the distortion and background noise for a 1 kHz pure tone. A Samsung Galaxy S4 phone was used for playback of the pure wav file using the default Samsung Music player app at full volume (15/15) with all EQ and effects disabled. The first two spectra below show the headphone output of the S4 alone into an open (infinite) load and into 33 ohm loads which represents a typical impedance of earbuds commonly provided with these types of smartphones. As expected, under a typical headphone load, the harmonic distortion products appear. Furthermore, his distortion will increase for lower impedance headphone loads:

The spectra below show the output of the E17 with the S4 headphone output connected to the E17 line-in for loads of 33 ohms and 12 ohms respectively. The result for the 33 ohm loads is comparable to the unloaded (open) S4 alone. Further, even though the E17 is only specified down to headphone loads of 16 ohm, the distortion is still quite low even with very low headphone impedances at these moderate drive levels (~ 0.85Vpeak input). Also the noise floor is comparable with and without the E17 headphone amp connected:

The test setup is shown below. A Creative Technologies X-Fi Elite Pro sound card was used along with RMAA to acquire the audio spectra above:

Detailed RMAA Audio Analysis

An RMAA audio analysis conducted at 16bit/44.1 kHz and with the E17 headphone output loaded with 33 ohm resistors, a typical headphone load within the specifications, demonstrates that the best performance of the FiiO E17 is achieved using the line-in analog connection which doesn't use the DAC. For RMAA measurements, the volume settings were adjusted, using the default 6dB gain setting, in order to achieve a nominal headphone output level of ~ 2.5Vpeak which is below the 3.6Vpeak maximum at 33 ohm loading. The optical SPDIF input has the next best performance but with slightly higher THD but significantly degraded IMD distortion. Finally and even more surprising, USB playback has the poorest audio performance with much higher THD and IMD distortions. Effects due to ground loops in the measurement however could explain some of the poor performance for USB input. The frequency response for the SPDIF and USB playback modes are very similar and both fall off by ~ -3.5 dB at 20 kHz whereas the frequency response for the line-in mode is essentially flat up to 20 kHz.

RMAA Comparison of E17 Line-In at 16bit/44.1kHz and 24bit/96kHz sampling rates
RMAA Comparison of E17 Alpen and E12 Mont Blanc Amplifiers

Frequency Response Plots for E17 Alpen and E12 Mont Blanc Amplifiers