A similar situation is found when trying to go capacitor-less. Just because no coupling capacitor can be seen does not necessarily mean that the signal does not flow through capacitors. In the illustration below, on the left, we see my favorite implementation of the a popular input stage and phase splitter, which has been, in this instance, optimized to lower power supply noise in the following push-pull gain stages. On the other hand, the circuit on the right seemingly does away with the first circuit’s internal coupling capacitor. Or did it?

 

Feedback-filled

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Feedback-free

 

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Only careful readers of this journal know for sure.

     First of all, the circuit on the left is not as capacitor-coupled as you might imagine, as the 1M resistor in parallel with the coupling capacitor allows a good bit of DC signal to flow across the capacitor (66%, in fact). Second, the circuit on the right is not as coupling-capacitor free as you might imagine, as the 20µF capacitor puts in place the same primary time constant as the .1µF capacitor does, but in an inverse way. Where the first circuit offered more gain at high frequencies than at DC, the second circuit offers more gain at DC and low  frequencies than at higher frequencies. Which is better? Which do you need? And for what application? One thing is for certain: electrolytic capacitors sound worse than just about any other type of capacitor. 

     Well, what if we do away with the second circuit’s 20µF capacitor altogether, wouldn’t that be the best way to eliminate unwanted coupling capacitors? The coupling capacitor would be gone, but the power supply noise would be back. Actually, the best solution might be to eliminate the first circuit’s 0.1µF capacitor! We lose 33% of the first stages gain, but we avoid capacitor problems, while retaining our improved PSRR figure.

      Once again, we are presented with two circuits, i.e. two choices. This first circuit is popular with the single-ended crowd, as it offers a good deal of gain and seemingly eliminates the need for a coupling capacitor (notice that the cathode-bypass capacitor must be used or the second ground-cathode amplifier will not provide any gain). The second circuit is more conventional and it too offers a good deal of gain.

      If the screen capture above looks unfamiliar, shame on you and doubly shame on me. (The original idea behind this website was that all the articles would illustrate how to use Tube CAD when designing tube circuits. In this respect, this webzine was meant to mimic the excellent Math Soft  Math CAD Journal. But I felt that promoting my software wasn’t as important as promoting a better understanding of tube circuitry. Math Soft was lucky, as math wasn’t in trouble; I wasn’t so lucky, as I felt that tube circuitry was. )

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