Originally Posted by gnarlsagan
I'd like to know what source impedance refers to, the headphone, the usb input, or the output impedance of the amp itself. You think it's the output impedance of the amp? Maybe Nick can explain. What's his handle?
Source impedance refers to the output impedance of whatever is immediately upstream from the item being described.
From the perspective of the Leckerton's op-amp, this would be either the DAC's analog output stage or whatever device is connected to the amp's analog input (eg, an iPod through a LOD).
A somewhat misleading but not entirely wrong way of thinking about it: When you have a run of wire with something sending signal to something else, there are two impedance values: The output impedance of the thing upstream, and the input impedance of the thing downstream. So, eg, when your headphones are plugged into an amp, the amp has an output impedance value, and the headphones have an input impedance value. Keeping in mind that in most cases these are not fixed values: In particular, passive transducers (eg, the driver in your headphone) usually have an impedance that varies with frequency. It's not unusual and not necessarily bad for source components (DAC, turntable, tuner) to have output impedances in the hundreds or thousands of ohm when they feed into active stages (preamplifier, amp). To the extent damping factor is relevant at all, it's going to be relevant at the final output stage before the passive device, which can't compensate for the characteristics of the device upstream.