Click image to see enlargement.

Design Goals
The argument was made in September's Circuit of the Month that most line stages offered far too much gain; a piece of the supporting evidence was the observation that many audiophiles prefer to use  passive switching boxes and attenuators, which have zero gain. However, there are circumstances, such as the need to drive a high capacitance load (such as long stretches of interconnect) or the need to drive a solid-state amplifier with a very low input impedance (say, 10k) that demand the use of an active line stage. But if we examine these cases more closely, we find that in some instances the added gain did prove necessary and that in all cases the greater current delivery ability was absolutely essential. Both the high capacitance and the low impedance needed a heavy current flow to develop drive voltage at high frequencies in the first example and high volume settings in the last. Tubes can yield current into high capacitance loads and low impedance loads, if the tube is beefy and the idle current robust enough. So here is the design goal: no voltage gain, but a healthy output current delivery.

Which circuit to use?
No gain, a fairly low output impedance, and a robust output current ability--all are design objectives. The Cathode Follower and the White Cathode Follower have low output impedances, and no gain. The Plate Follower has a low output impedance, but is awkward to use because of its fairly low input impedance, which will shunt the volume potentiometer resistance.

All of this leaves the Cathode Follower and White Cathode Follower as the only contenders. The White Cathode Follower is actually a push-pull amplifier that uses feedback from the plate resistor to drive the bottom triode and lower the output impedance far below what a straight Cathode Follower would using the same triode. The question is not which circuit has the lowest output impedance, but just how low an output impedance is needed? The answer is that if loads as low as 10k ohms are to be

Email sales@glass-ware.com
Phone: 810-714-3424