Now it’s time to zoom in and view the CCDA at a higher resolution. (Here's an analogy: if we were absolutely certain that the government was perfectly all-knowing and all-capable, we would all be socialists now.wait a miniute, I forgot that we all were socialists now.) Now, doesn’t a constant-current-draw operation by a line amplifier make more sense? Of course, if a perfect power supply were used, then both the constant-current-draw and the non-constant-current-draw line-stage amplifier would perform equally well. Now if the power supply presents a resistance, or worse an impedance - in other words a frequency-dependent resistance - then the input signal will be superimposed on the B+ voltage, frequency skewed and phase-shifted, where it will feed back into the first and second stages. Notice how the current draw varies with the signal. Imagine, in the above circuit, using a dissimilar-triode tube, such as the 12DW7 type tube, wherein a 12AX7 triode is wed to a 12AU7 triode. Now let’s imagine a non-constant-current-draw circuit. In the examples above, as far as the power supply is concerned, the entire CCDA circuit is nothing more than a 9.9k resistor that conducts a steady 20.2mA from its +200V B+ voltage. Thus, the net current draw from both stages remains unchanged, although the cathodes and plates might be swinging hugely. This downward swing is then relayed into the cathode follower's grid, whose cathode follows its grid, reducing the voltage across the cathode follower’s cathode resistor, which in turn causes the cathode follower to decrease its current conduction to the same degree that the previous stage's current increased, as they share the same load resistance. For example when the grounded-cathode amplifier sees a positive going input signal, its plate current increases, which increases the voltage developed across the plate resistor, which in turn swings the plate voltage down. Both the grounded-cathode amplifier and the cathode follower are in voltage phase, but not current phase. Now here is where the constant-current draw enters the picture. Thus, total current draw for the two stages equals twice that of either stage individually. In order to achieve constant-current draw, in the CCDA circuit each triode splits the B+ voltage, so each sees the same cathode-to-plate voltage and each works into the same load resistance with the same idle current.
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