Given that this is Erik’s thread, I presume Guido and the others posting above won’t deem it inappropriate if I comment on the disagreement between Erik and Ralph. Apologies if that is an incorrect presumption.
Ralph, it appears to me that what underlies much of the disagreement between you and Erik is that he is viewing the impedance of an output coupling capacitor, if present, as contributing to and being part of the component’s output impedance. While you are not, possibly because you are considering "output impedance" as corresponding to "specified output impedance," which as we all agree is often based on a mid-range frequency such as 1 kHz. The capacitor’s impedance of course being unlikely to be a major contributor to the 1 kHz output impedance in just about any reasonable design.
Regarding that terminological difference, I would have to agree with Erik, and consider "output impedance" to be whatever impedance is seen "looking back" into the output of a component, at a frequency that should be and hopefully is stated. Also, as he pointed out, measurements of **many** popular high quality components that have been presented over the years by John Atkinson in Stereophile, that have specified output impedances of perhaps a few hundred ohms, show impedance rises to the vicinity of 3 or 4K or even more at 20 Hz. Obviously the sonics of such a design would be compromised to some degree when driving a low impedance, such as 10K, or even 30K or more in some cases.
On the other hand, regarding this part of Erik’s statement that you quoted above:
I say high output impedance causes frequency response changes which vary based on the load. This is an irrefutable fact based on simple serial circuit analysis.
As I assume he would agree, high output impedance will not affect frequency response when driving a resistive load if the output impedance itself is resistive, and does not vary significantly at relevant frequencies. As I said in an earlier post, it is the **variation** in output impedance, as a function of frequency, that may be significant in that regard. Although of course under extreme circumstances high output impedance can in itself affect frequency response in the treble region, even if there is no variation as a function of frequency, due to its interaction with cable capacitance. Capacitance not being a resistive load, of course, by definition.
Hopefully this post will result in the divergent opinions expressed in the foregoing posts becoming less divergent.
Best regards,
-- Al
