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.