In the schematic shown prior, two B+ voltages are available not only to allow better headroom for the input and driver stages but to distribute cathode-to-plate voltages fairly for the Cathode Followers. The top output tube's Cathode Follower's cathode resistor terminates into the output; the alternative layout would have this resistor connected straight to ground. The advantage to the first scheme lies in the unloading of some of the required current swing through this Cathode Follower so that it better matches the operating conditions of the bottom Cathode Follower. The bottom Cathode Follower sees only 33% of the output voltage swing, whereas the top Cathode Follower sees 66% of the output signal. By terminating the resistor into output, the top Cathode Follower effective sees only 33% of the signal. 
Headphone Amplifiers
  A tube amplifier for dynamic headphone serves two purposes. The first, obviously, is for driving high quality headphones, such as those from Grado and Sennheiser. The second is that a small headphone amplifier can be viewed as a test run for a large power amplifier for loudspeakers.

   If a headphone amplifier does not perform well at driving headphones, then a scaled up version probably will not work any better at driving your speakers. On the other hand, if the headphone amplifier proves stable, trouble free, quiet, low distorting, then the design concept warrants an expanded version. Basically, the lower the impedance of the load, the more output tubes will be needed. Even if you do not own a pair of quality headphones building a scaled-down version of an OTL amplifier is a good idea.
   For example, if the final OTL amplifier will comprise eight 6AS7s in the output stage, then build the same amplifier but with only two 6AS7s and test it with a 32 ohm load. What results will be indicative of what will result with eight output tubes and an 8 ohm load.
  The amplifier shown below inverts the phase of the input signal. The feedback is taken from the output and brought back directly to the grid via the 100k resistor. The advantage of this arrangement lies in the gain decreasing to unity when the amplifier is disconnected from its signal source. The disadvantage to this arrangement lies in the relatively low input impedance presented by the 48k resistor.

OTL ultra-linear headphone amplifier


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