Tube Active HPF/LPF

[amptramp]

Here's a thought experiment. You said about the circuit on the left:

The upper triode stage has a resistance more like Rp which is in the few thousand to tens of thousand ohms.  With the triode top section, it doesn't run into any worse dynamic range and clipping issues than an ordinary common-cathode stage.

You said about the SRPP on the right:

The signal will slam up against the rails, both positive and negative.

So with no load attached (infinite impedance), how does the circuit 'know' which one it is? How does it know whether to behave like an ordinary common-cathode stage, or whether to slam up against the rails?

You can still overload a half-µ stage when the bottom stage current goes to zero but it is hard to overload it when it pulls down because it is just a current source and the bottom stage will accommodate this current.

With SRPP, the upper stage pulls up as the lower stage current is reduced and the upper stage current is reduced as the lower stage current increases.  Since the currents are in series, there has to be a third circuit connected to the output that carries the difference current.  When the difference current exceeds the load the third circuit can take, you end up with the output going either to ground or to however high the upper stage allows.

[merlinb]

Since the currents are in series, there has to be a third circuit connected to the output that carries the difference current.

No third way is required, both the circuits I posted are the same. You are putting the cart before the horse. There is no current difference that requires a 'third circuit'; it is the load itself that creates the current difference. No load = no current difference = no problem.

[amptramp]

Since the currents are in series, there has to be a third circuit connected to the output that carries the difference current.

No third way is required, both the circuits I posted are the same. You are putting the cart before the horse. There is no current difference that requires a 'third circuit'; it is the load itself that creates the current difference. No load = no current difference = no problem.

While it is true that no load = no current difference, in an SRPP circuit, this equalization of current may happen when the output is either driven to the + or - rail where nonlinearities equalize the current.  Adding a load permits the current difference to exist within the linear range of the circuit.