Now moving to the bottom triode, if its grid were grounded, then the only opposition to the 1 volt plate voltage increase would be the rp of the triode. Connecting the grid to the Split Load phase splitter allows the triode actively to oppose the voltage increase. Moving the plate voltage up one volt will also move the phase splitter's plate resistor up one volt and this pulse is given to the bottom triode's grid. Now the triode's current will increase by the transconductance of the triode times the 1 volt pulse.  So the effect transconductance of this triode is the sum of the rp's contribution to the increase current flow and the grid's transconductance, which ends up equaling the top triode's effective transconductance: (mu + 1) / rp. What we have then is a doubling of this transconductance figure: 2(mu + 1) / rp.   
   Quickly applying the battery's voltage to the output will give rise to a current flow into the battery equal to 2(mu + 1) / rp. If we divide the battery's 1 volt by this amount of current, we will have the output impedance as a result. 
   This output impedance is referenced from ground to only one output, but the electrostatic headphones have stators that bridge both outputs, which means the effective output impedance the headphone sees is twice our previous figure or rp / (mu + 1), the value of just one triode alone. Given the 5965's rp of  7800 ohms and mu of 47, the output impedance is 196 ohms from output to output.

Non-Balanced Output
   The problem with presenting any circuit within the context of serving some desired goal (in this case, driving electrostatic headphones) is that the circuit will be seen as only being good for that purpose. This output stage configured as a buffer is a good example. It can be used for many more tasks or purposes than just driving headphones. In fact, the principles outlined here apply to the design of any OTL amplifier whether it be a line stage or a 100 watt power amplifier.

   Since line stages do not usually require 100 volt voltage swings, the power supply can be scaled down by as much as three fold: +/- 100 volts. And since line stages do require a low output impedance, the 5965 can be replaced with a 6BL7, 6BX7, 12B4, 5687, 6922, or 7111. As drawn above, the circuit's gain is almost unity and the output impedance is a low 50 ohms. The maximum output voltage swing is limited to about 30 volts peak in either direction. The maximum Class A peak output current is twice the idle current of the output tubes. In this example, as the idle current is 10 mA, 20 mA would be the maximum output current. If we are willing to abandon Class A for Class AB1, the peak output current will equal 100 volts divided by the sum of the rp of the output tube used and the load impedance being driven:
       Ipeak = Vb / (rp + Rload).
Given a 6DJ8 and 600 ohm load, the maximum peak output current is about 25 mA's. If we are willing to drive it into Class AB2 operation, the 6DJ8 is capable of even greater peak output current.
   The input to this buffer is capacitor coupled to allow dividing the voltage potentials correctly for the Split Load phase splitter.

pg. 11

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