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Output Impedance

Is the 100k resistor to ground considered a factor in calculating the output impedance of the preamp or is it considered a factor in calculating the input impedance of the amp? Or both??

I'm not very technically oriented but, as I understand it, the dominant factor when it comes to output impedance is tube's Rp in parallel with the value of the load resistor. A load resistor value that's 3x to 5x that of the Rp is common, sometimes higher.

The Rp of the 01A is ~10k which, in parallel with a 50k load resistor, results in a value of 8.33k. Although other resistances may combine with this the resulting value is even lower as long as those resistances are not lower than the Rp. If the 100k to ground is part of the preamp calculation, it only lowers the output impedance. The result of 10k, 50k and 100k in parallel is 7.69k.

Since tube amps generally have an input impedance of 100k there shouldn't be a problem with a preamp whose output impedance is 10k or less. If used with a SS or Class D amp a lower output impedance would be indicated.

Or is the 100k to ground part of the input impedance of the amp? In parallel with a 100k input impedance of a tube amp, it would lower the input impedance to 50k and may create issues. I'm not sure if they would be audible or not, but I imagine they could be measured.

If it contributes to the amp's input impedance it seems like it would be better to use a 33k with a 10k to ground. This would have the same effect on the output voltage and only lower the input impedance to ~9.1k. But this assumes that a tube amp is being used.

It seems like a 33k / 10k combo would also be better if it affects BOTH the output impedance of the preamp and the input impedance of the amp since this would also lower the output impedance of the preamp further. The result of 10k, 50k and 10k in parallel is 4.55k.

A SS / Class D amp typically has an input impedance of 10k to 20k. In which case the 330k / 100k combo would be best.

That's how I see it but perhaps I'm missing something.



Edits: 02/15/25
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