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 = (Vunloaded – Vloaded) / Vunloaded * Rload =
= (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).
First revision: 2016-11-10.
Last revision: 2016-11-10.