# Output Impedance of Nexus 5X

## Estimation of output impedance of the final audio stage

In general, audio headphones should have an impedance of at least 6-8 times the output impedance of the final stage of the audio amplifier used to power them. Since my Nexus 5X is known to have a limited output power (up to 0.286 Vac) and my KNS-8400 have an input impedance of 36 ohm, I decided to measure the actual output impedance of the Nexus 5X.

I tried to load the Nexus jack output with one or two fixed 35.9 ohm resistors, about equivalent to the nominal and measured impedance of my headphones (KNS-8400). The impedance may vary with the frequency, but not in my case (check the link).

I used a Victor VC9808+ to measure the amplitude of the output in AC mode while playing a 1 kHz sine wave at the various output levels. I used the left channel but I also did one measurement with two channels loaded. The results are:

Volume | Unloaded [mV] | SX [mV] | SX+DX [mV] |
---|---|---|---|

0 | 0 | ||

1 | 0 | ||

2 | 0 | ||

3 | 0.8 | ||

4 | 3.2 | ||

5 | 6.2 | ||

6 | 10.3 | ||

7 | 15.6 | ||

8 | 24.9 | ||

9 | 39.9 | 36.4 | |

10 | 53.5 | 48.8 | |

11 | 75.4 | 68.9 | |

12 | 108 | 98.6 | |

13 | 145 | 132 | 130 |

14 | 203 | 186 | 182 |

15 | 286 | 261 | 258 |

I then calculated the output impedance using only the left channel:

impedance = (V_{unloaded} - V_{loaded}) / V_{unloaded} * R_{load} = (0.286-0.261)/0.286*35.9 = 3.1 ohm

Using the case with both channels loaded, the resulting impedance is 3.5 ohm.

The Nexus 5X has a weak output level and requires sensitive headphones, but its output impedance will not distort normal headphones with at least 24 ohm impedance (minimum value measured for the chosen headphones).

Author: Olaf Marzocchi

First revision: 2016-11-10.

Last revision: 2016-11-10.