The impedance of an electrostatic headphone varies with frequency, because it partly resembles a capacitor, although not completely, because, unlike a capacitor, the headphone makes sound. In the case of the 007 and 009, they have a capacitance close to 100 pf, so at 1 kHz the 007 has an impedance of about 170 kilohms, and the 009 has an impedance around 145 kilohms, both very high. The impedance varies as the inverse of frequency, so at 10 kHz the impedance is 10-fold lower, at 100 Hz the impedance is 10-fold higher. However, since the headphone makes sound, there is another term to the impedance which can be approximated by a high value resistor, say around 100-200k. In combination, they still result in a high value impedance.

The output stage active device (tube or transistor) sees this high impedance in parallel with the output stage load, whether it is a resistor, choke or constant current load. In order to have sufficient current drive, the resistor must be significantly lower impedance than the headphone, however, this also means that the majority of the signal current is sucked up by the plate resistor due to Ohm's law. In the case of an output choke, the impedance of the choke varies directly with frequency, which means at low frequencies where much of the power demands in music lie, again most of the signal current goes to driving the choke. Thus, both resistor and choke loads are very inefficient in supplying current to the headphone, which is the point of the exercise. By comparison, a good constant current load requires essentially none of the signal current, hence allowing all the signal current (minus that needed to drive the feedback resistors) to drive the headphones. Not only is this more efficient, but it decreases distortion in the output device because it is not as heavily loaded.

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