I probably missed that, sorry. I wouldn't worry much about the 8 ohms impedance unless you're using 16 ohms headphones and you aim for the perfect dumping factor. I did some measurements recently and found out the following:
- max. volume 1 KHz @600 ohms = (7.5V RMS x 7.5V RMS)/600 = 93.75 mW
- max. volume 1 KHz @300 ohms = 187.5 mW
- max. volume 1 KHz @60 ohms = 925 mW
- max. volume 1 KHz @30 ohms = 1.825 W
I was using dummy resistors to do the above tests and 10MHz scope.
From the above seems that output resistance for PLAY's amplifier is about
0.5 ohms for a load resistance
> 32 ohms. For 16 ohms headphone most likely that PLAY's internal output resistance is around 8 ohms, like BURSON states, but I don't really care about this because I don't have 16 ohms IEMs and this amplifier was not designed with 16 ohms in mind, although it supports 16 ohms headphones (other amplifiers could get damaged or start oscillating the output stage when using low-impedance headphones, which is not the case with PLAY).
Explains how to measure output resistance of an amplifier
Below there are few scope graphs of a 1KHz sine-wave test signal (16 bit/48KHz).
7.5V RMS for loads of 600 ohms up to infinit
7.45V RMS for 60 ohms load
7.4V RMS for 30 ohms load
Seems a very potent headphone amplifier with an internal gain of
3.75X (7.5V RMS on outputs when 2V RMS applied) driven by latest generation of
XMOS USB transport followed by an
ES9018K2M DAC, a very good addition inside a PC case.