Samsung Galaxy S4: Problem With Low-Impedance Headphones

July 25, 2013
[Update: The July 2013 firmware update for the S4 SGH-I337 I337MVLUAMG1 has fixed the "crackle/instability" problem discussed below. All audio characteristics have been confirmed to be unchanged except for the absence of the oscillatory instability. ]

This note provides audio measurements of the Samsung Galaxy S4, including dependence on headphone impedance. Using earbuds or headphones with a nominal impedance of over 32 ohm provides a superb music listening experience in my opinion. The outstanding audio quality of the S4 is supported by the measured extremely low background noise level and very low distortion levels reported elsewhere and verified by new measurements reported here. The measurements reported here, however, were motivated by reports from S4 users having audio quality issues using low-impedance headphones on the order of 12 ohm. The stock headphones provided with the S3 and S4 have impedances at 1 kHz of ~ 35 ohm (measured). Based on my measurements on a single S4 (SGH-I337), there is no problem with headphones with an impedance of 16 ohm or higher. The internal impedance of the S4 headphone output was determined to be ~ 1.2 ohm at 1 kHz by measuring the output voltage drop with a 10 ohm load and comparing to the output voltage with no load (open output).

To check out any audio problems quantitatively, the headphone output of the S4 and S3 smartphones were connected with various pure resistive loads with values ranging from 10 to 33 ohm. Pure sine wave audio tracks with format .wav and 16bit/44.1 kHz and with -0.8 dBFS (almost full digital amplitude) were synthesized and saved as audio tracks on the smartphones. The tracks were played back using Music player and with default settings (no EQ etc.). The output on the left/right channels were monitored using a Digilent Analog Discovery dual-channel USB oscilloscope. The sound quality was also determined using a parallel connection to an Edirol MA-15D micro monitor amplifier/speakers. Various cable interchanges and substitutions verified that the results below are real and are associated with the S4. The measurement configuration is shown below. The breakout box shows the load resistors on the left connected with banana plug binding posts, one for each audio channel:



The results indicate that there is definitely a strange problem with the S4 driving very low impedance headphones. DIRECT COMPARISON TESTING OF THE S3 WITH THE SAME SETUP SHOWED NO SUCH PROBLEM WITH THE S3. Specifically, it was determined that headphone loads with a resistive impedance below about 14 ohm show an oscillation problem in the negative output voltage swing. For these pure sine waves, the oscillation was seen only for the test frequencies 60, 250 and 1000 Hz but not for 10kHz or 20 kHz frequencies. and is amplitude dependent and with these tests files, for low impedance loads, the oscillation occurs at an S4 volume level of 11/15. Surprisingly the oscillation is ONLY present at this Player volume setting for these sine wave tracks (i.e. no oscillation for Player volume levels of 1-10 and 12-15). Audibly, the high-frequency oscillation is clearly audible as a "buzz" superimposed on the pure audio tone. This is NOT simply a current limit effect of the S4's output driver stage since there is no oscillation at the full volume setting of 15/15 as shown in the scope traces below. Measurements with load resistances from 15 - 33 ohm show no such oscillation at any frequency (measured down to 60 Hz) nor clipping distortion effects:

The first scope traces below shows results using a 10ohm load (on both channels) and for Player volume levels of 10, 11 and 15:



The graphs below show results for a 15ohm load (on both channels) for volume levels 11 and 15 (no oscillation was observed at any level or at any frequency):



Spectra

To monitor the audio-band spectrum during playback of the sine wave tracks above, the S4 headphone outputs were also connected to the LineIn of a Creative X-Fi Elite Pro sound card with very good input characteristics. The RMAA spectrum analyzer was used to view the audio spectrum. As above for the real-time oscilloscope case, the S4 Music player amplitude and headphone load were varied. The plots below show headphone output spectral content for the 1 kHz test tone track at different S4 Music player levels for headphone loads of 10 ohm and 33 ohm for pure sine wave playback as above. For the 10 ohm case (first 4 spectra), the instability is clearly seen at the 11/15 setting. Higher harmonic distortion levels appear at higher playback levels as expected but the overall noise level is lower than the unusual instability at 11/15. However since music is a superposition of many time-varying amplitudes and frequencies, music played with a Music player volume setting above 11/15 will certainly audibly manifest the instability observed for the pure sine wave tests at 11/15.

For comparison the results for 33 ohm headphone load show negligible harmonic distortion except for the highest volume levels and no sign of the instability noted for the 10 ohm case at any settings. :





For comparison, the spectral content using only the X-Fi Elite pro sound card with LineOut --> LineIn shows a very low noise floor demonstrating the measurement capability in this setup. The same wav file was used for playback and the level adjusted to be comparable to the 11/15 setting above for the S4 playback case:



It is difficult to understand the nature of the instability problem for the 10 ohm case but it seems to be related to the negative output voltage swing reaching a particular level for a sufficient length of time under a low-impedance load. Certainly during music playback with a continuously changing level, the audio effects will be noticeable WITH LOW IMPEDANCE HEADPHONES < 14 OHM. The fact that the Samsung Galaxy S3 unit that I tested does NOT show this oscillation instability points to an output headphone driver stage design artifact which in my opinion should be addressed in the next release of the Galaxy S series of smartphones.

Test Wav Files

For user testing purposes, JavaScience Consulting is providing a zip file containing several 16bit/44.1 kHz wav file audio tracks. The audio files contain programatically synthesized pure sine tones at frequencies: 60, 250, 1kHz, 10kHz, 20kHz. Track duration is 120 sec. Also included is a track with perfect digital silence. The file names in the archive distinguish the various tones. Please read the README.txt file included in the archive before using.

WARNING: JavaScience Consulting accepts no responsibility for any damage caused to headphones or connected powered amplifiers with these wave files. Since the wav files are almost full digital amplitude (-0.8 dBFS), exercise caution during playback on smartphones or computers and START WITH THE VOLUME TURNED DOWN COMPLETELY AND SLOWLY INCREASE THE VOLUME. PROTECT YOUR HEARING.

Download S4Waves.zip Audio Test Archive (618,786 bytes)

Headphone Output Levels

The chart below shows the headphone output voltage levels, under a resistive headphone load of 33 ohm (each channel). The levels were determined by playback of a 1 kHz pure tone 16bit/44.1kHz sine .wav file with digital amplitude of 30,000/32,767 (or -0.77 dBfs). Peak headphone output voltage levels (mV) were measured on a dual channel oscilloscope for each playback level using the Music player app. Corresponding dB values relative to 0 dB at full volume are shown:



RMAA Analysis and Dependence on Headphone Impedance

The summary below shows the impressive audio properties of the Samsung Galaxy S4. Tests were performed at the Music player app volume setting of 14/15 with (a) a pure resistive load of 33 ohms on each channel and (b) headphone output unloaded (open). As expected, the performance will degrade somewhat under a typical headphone load, particularly the THD and crosstalk. (Because of the problems discussed above with low-impedance headphones and the S4 , no RMAA measurements were performed at lower impedances values.) The background noise level is exceptional at -100dB with or without load and the IMD+Noise only degrades very slightly under load. The results below with open headphone load are very similar to those reported at Samsung Galaxy S4 review: Supernova and somewhat better for some parameters. However, under headphone load (presumably the stock 35 ohm headphones) the results reported by GSMArena show no significant change in THD or stereo crosstalk but the results reported here show a 2.4 factor degradation in THD (but still extremely low at 0.0075%) and significantly poorer crosstalk which is still low enough to be unnoticeable under all realistic listening conditions:



The frequency response and noise spectra are shown below for the 33 ohm headphone load case:



The chart below shows the degradation of audio performance, particularly the THD, as the headphone impedance is lowered. Resistors representing the typical range of common headphone impedance values were connected directly to the headphone output. The RMAA tests were performed using the default Music player app at full volume 15/15:



The final column above for R=3kohm represents the performance achievable with high impedance headphones with nominal impedance > 50ohm. Adding a high quality external headphone amplifier with an input impedance of 1kohm or greater to a smartphone will improve the audio quality, particularly if the headphones are in the lower impedance range. To demonstrate this, the chart below shows RMAA summary data for the S4 SGH-I337 phone. The Music player app was set at full volume 15/15. The 1st column shows the results with the S4 headphone output connected directly to 33 ohm load resistors. The 4th column shows the great improvement in distortion (particularly THD) and crosstalk when the S4 headphone output is connected to a high-quality external headphone amplifier terminated with the same 33 ohm load resistors. The 2nd and 3rd columns demonstrate the very low distortion measurement capability when the external headphone amplifier is tested directly with the sound card. The results in columns 2 and 3 represent the combined noise and distortion of the sound card line-out, the external headphone amplifier and the sound card line-in. The results in columns 1 and 4 represent the combined noise and distortion of the S4 headphone output, the external headphone amplifier and the sound card line-in. Note that the noise level of the S4 headphone output ~ -100 dB is lower than that of the sound-card lineout noise ~ -95dB: