By using this DC-DC converter, a 12 volt power source, whether it be a battery or a wall wart, can feed the heaters directly and power one or two DC-to-DC converters. The output is isolated on these devices, so placing two outputs in series or parallel is possible. As is adding the input voltage to the output voltage to yield 84 volts of B+ or using the battery voltage for a negative power supply.
    Now 84 volts may not seem like much, but it is enough to make a tube headphone amplifier possible. This low B+ voltage dictates that the output tubes must be able to draw a fairly high current at low plate voltages, which requires low rp, high perveance tubes, such as the 12B4, 6DJ8, 7111 or 6N1P. Yet the tubes must have a low heater current consumption: a catch 22 in the making. Thus fewer stages, thus fewer tubes  are a better bet to lowering power consumption, which will limit the choices in circuit topology somewhat. In addition, the choice of output stage mode of operation must be addressed: Class B or A or AB?. Fortunately, here we get a break, as headphones require so little power that we can tolerate less efficiency; if we are repaid by better sound that is.  Still, we should know how much efficiency we are giving up.

Modes of Operation
   Class A output stages, whether push-pull or single-ended, strive to meat a theoretical limit of efficiency of 50%, where efficiency is defined as power delivered into the load divided by the power dissipated by the output stage. For example, if an amplifier's output stage consumes 100 watts and puts out 50 watts into a load, its efficiency would be 50%. Solid-state Class A amplifiers come close with efficiencies of up to 48%, pentodes usually fare worse, with efficiencies of only 25 to 40%, and triodes usually fare the worst, with efficiencies of only 10 to 35%. If the stipulation that the grid never enter grid current is removed (Class A2), then the efficiency of tube output stages can move closer to the 50% theoretical limit, but at the cost of a much more robust, higher-current driver stage.

    Still, this last option is worth pursing, if you have the skill and time and patience. Fortunately, a maker of DC-DC converters, Newport Components, has come out with a two devices for phone equipment that offers 80% efficiency, 72 volts output from a 5 or a 12 volt source, 42 mA of current output, and a oscillator frequency of 85 kHz. The NMT1272SZ uses a 12 volt input and the NMT0572SZ, 5 volts. Both are packaged in an 8 pin SIP case and are wonderfully cheap, $15.00. If this sound too good to be true, it's because it is. The devices do not offer a tight regulation of the output voltage or a very low output noise voltage. The 12 volt version offer much better specs than the 5 volt version does, but it is still not close enough to what we need in a headphone amplifier. 
   Fortunately, the same tricks that work in a conventional power amplifier also work in a portable headphone amplifier, i.e. chokes and pi filters.  The noise voltage equals 1.2 volts, which an output capacitor and choke and final capacitor would shrink greatly.


pg. 10


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