SRPP vs. White Cathode Follower
A test to determine which circuit is better suited to drive low impedance loads is presented here. The arbitrators are the oscilloscope, the distortion analyzer, and the ears. Two circuits need to be built: one using a Common Cathode amplifier driving an SRPP and another using a Common Cathode amplifier driving a White Cathode Follower. In both circuits, the output is not inverting and global feedback is used to set the gain to a fixed amount, say times ten (+20 dB). The load could any high quality headphones with a low impedance, say 32 to 600 ohms.
The SRPP circuit is a push-pull amplifier that uses the current flowing through the bottom triode to define an inverted signal to drive the top triode. It works best at one load impedance and can yield twice the idle current into that load impedance. It is often misused as a line stage amplifier (where the load impedance is not predefined) or as a driver stage (where the load impedance is far too high).
Preloading the output overcomes much of the mismatching problems. Preloading means adding a fixed resistor to the output a value slightly higher than optimal, so that when an external high impedance load is added, the combined paralleled impedance will prove optimal. Still the question remains: if the load to be driven is a high impedance one, why bother with a push-pull output stage? If high gain and a good PSRR figure are needed, use a current source loaded Grounded Cathode amplifier instead. If low output impedance is needed, use a White Cathode Follower, Plate Follower, Cathode Follower, or even a lower rp triode instead. On the other hand, if only one tube envelope can be used per channel and the power supply cannot be greatly taxed and some gain is needed and a somewhat low output impedance is needed to drive a fairly low impedance load, then the SRPP is the best choice.
Are we done with the SRPP? Not likely. Expect to see a few e-mails and new articles on this circuit. And if anyone has access to the patent, please send it our way so that we can post it in this journal.
MJ Stereo Technic, 1987/2, page 206
"Vacuum Tube Amplifiers," Valley & Wallman,
pages 456-458, Dover 1965 (originally,