ruffrecords said:
I am uncomfortable with this. If such an output cannot drive both a 1K and 10K loads it seems a bit of a retrospective step. If it can, the implication is it can adjust its output impedance depending on the load in which case measuring its output impedance seems rather pointless.
My point was, more generally speaking, that changing significantly the load changes the behaviour, in a way that may not be subtle.
That is a well-known fact; the basic method for simulating open-loop gain and all other relevant parameters was traditonally to alter the NFB network with a large inductor and a large capacitor that would allow proper DC analysis before doing the AC analysis. It has been shown that it modified the operation of the circuit, because the impedances of the nodes are not anymore what they are in normal operation. As Ricardo mentioned a few posts earlier, what the circuit is connected to matters.
Many tube amps need to see a load that's pretty close to nominal to operate correctly; it does not make them particularly quirky.
Just think of atube amp that's designed for a nominal 4 ohms load; when you load it with 16 ohms, it's close-loop gain suddenly jumps by about 6 dB, but global NFB maintains gain at almost the same, which results in actually halving the output Z! That's why an impedance measurement cannot be taken in ignorance of the load it is supposed to be attached to.
Now you may find that a McIntosh is much less sensitive to its environment than a Dynaco, but it doesn't make the latter a product to reject shamefully.
Indeed, negative-impedance drive and distortion cancellation schemes that rely on too much NFB can create havoc, however, there are many existing products that testify to the fact that, when well-designed, they can significanty improve performance without any drawbacks.
Many of my commercial products used a tertiary winding NFB scheme that improved performance in many respects:
LF response: -0.1dB @20Hz instead of -1 for the bare xfmr
Output Z: 22r instead of 44
LF THD: 0.04% @ 22Hz +20 dBu instead of nearly 1%!
All measured into 600r
Susceptibility to external magnetic fields reduced by 30 dB (big saving here since it saves the costs of a magnetic shield)
And not a single issue as to stability.