Tvad, what you are missing is the output impedance is not the spec. For example, the Modright might have a 100 ohm output impedance- if the output winding is designed to drive 600 ohms then it would work fine. Ditto Roland and Wadia. The output impedance is something very different from what load the circuit will drive.
Its a good idea for an electronic circuit to have about 1/10th the output impedance vs the load it has to drive, however there are some exceptions where the output impedance can appear to be much higher, yet it will drive the load just fine. For example, my Neumann microphones are set up to drive 150 ohms. What this means is that if you don't load them at 150 ohms (if instead you have a load of 1000 ohms or higher), the output transformer will express the inter-winding capacitance rather than the turns ratio, and you will get coloration and no bass.
If we take the example of the Modwright, a similar situation exists- its measured output impedance is one thing, the load it drives (and is optimized for) is another. I suspect it has that load built-in, much like the old Ampex tape machines did, so that their output transformers would be properly loaded.
The Cary has sufficiently low enough output impedance to drive 600 ohms, but if it employs a coupling cap, its likely that you will get a low frequency roll-off if you try to do it. IOW the only tube units that will drive 600 ohms properly will have:
1) a low output impedance (well withing the range of several of the units already mentioned) and
2) will either employ
a) an output transformer, or
b) a very large coupling cap, or
c) be direct-coupled.
It turns out a large coupling cap is impractical, so you can see how the realities of trying to do this limits the field.
In the world of transistors, its quite easy to get semiconductors to drive 600 ohms, so there should be lots of the examples there.
Its a good idea for an electronic circuit to have about 1/10th the output impedance vs the load it has to drive, however there are some exceptions where the output impedance can appear to be much higher, yet it will drive the load just fine. For example, my Neumann microphones are set up to drive 150 ohms. What this means is that if you don't load them at 150 ohms (if instead you have a load of 1000 ohms or higher), the output transformer will express the inter-winding capacitance rather than the turns ratio, and you will get coloration and no bass.
If we take the example of the Modwright, a similar situation exists- its measured output impedance is one thing, the load it drives (and is optimized for) is another. I suspect it has that load built-in, much like the old Ampex tape machines did, so that their output transformers would be properly loaded.
The Cary has sufficiently low enough output impedance to drive 600 ohms, but if it employs a coupling cap, its likely that you will get a low frequency roll-off if you try to do it. IOW the only tube units that will drive 600 ohms properly will have:
1) a low output impedance (well withing the range of several of the units already mentioned) and
2) will either employ
a) an output transformer, or
b) a very large coupling cap, or
c) be direct-coupled.
It turns out a large coupling cap is impractical, so you can see how the realities of trying to do this limits the field.
In the world of transistors, its quite easy to get semiconductors to drive 600 ohms, so there should be lots of the examples there.
