Tube Headphone Amplifier
     A smaller scale use of this distortion canceling technique might be a designing a headphone amplifier. The principle remains the same, but the output wattage is greatly reduced.
     The two most popular high-end headphone brands are Sennheiser and Grado. Both of these companies make excellent headphones in the $100 to $600 range. I own a pair of Sennheiser HD-580 headphones and I can attest to their clean, smooth, tube-compatible sound. The HD-580 has been replaced by the HD-600, which sounds even better. Both Sennheiser headphones share a 300-ohm impedance. While this impedance is ten times greater than the Grado's, it is still quite a severe load for a vacuum tube. Used as a plate resistor value, 300 ohms would yield only a gain of 2.75 and a distorted output from a 6DJ8-based grounded-cathode amplifier. The same tube and load impedance in a cathode follower yields a gain of 0.73 and a still distorted output. How do we drive the 300-ohm load without using an output transformer?
      The circuit below shows an complementary inverse pre-distortion cancellation headphone amplifier. As configured, this headphone amplifier will deliver slightly higher than unity gain and a fairly low output distortion. However, there is plenty of room for improvement.

      Increasing the number of output triode to three increases the gain and lowers the output impedance and distortion.

      The use of three output tubes effectively increases the 300 ohm load by threefold for each output tube, which in turn demands the use of a 900 ohm plate resistor for the input stage. The gain of this revised tube headphone amplifier is 3.6; the output impedance, 42 ohms; the maximum output voltage swing +/-8.5 volts. All in all, a very buffed headphone amplifier, but how good is it as a linestage amplifier?
       The problem with the inverse pre-distortion technique is that the circuit is optimized for only one specific load impedance, thus limiting its universality. Unlike conventional line amplifiers, whose distortion decreases with unloading, somewhat paradoxically, the unloading of the circuit increases its distortion. One solution is to switch into place a 300-ohm load, when the headphones are removed. This would be easy to implement, as jacks with built in switches are readily available. The alternate approach is to switch out the shunting capacitor at the junction of the pr and the 9.1k voltage-dropping resistor. The 900 and 9.1k resistors combine to form a 10k resistor, which is three times the value of the cathode follower load resistor (3.3k). In other words, we now have a higher gain and lower distortion linestage.

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