Designing a portable tube headphone amplifier requires making many compromises. Large chokes and potted output transformers are out of consideration, as are large output tubes such as the 6C33 or 6AS7. In fact, even the 6BX7 or 5687 are out of the running because of their heavy heater current draw. Realistically, we must limit the size and power consumption of the amplifier, if we hope to make the amplifier portable and use a battery power source. Four 6DJ8s require almost 9 watts just to power the heaters; four 5687s, almost 23 watts. A strong AA type battery yields about 1.4 watts for one hour. Thus at least 7 cells would be needed just to heat four 6DJ8s for one hour. Additional cells would be needed to supply the B+ voltage.

Battery Concerns
    Several options present themselves when considering how to battery power the amplifier. We can use only batteries and no DC-DC converters. Impractical, yet elegant, what could be more hum free than a battery? This option would require four C or AA cells for the heaters and eight 9 volt cells in series to establish the B+ voltage. This assumes normal non-rechargeable batteries. Rechargeable batteries yield fewer volts: 1.2 for the single cells and 7.2 volts for the 9 volt package. Thus using rechargeable batteries would require five C or AA cells and ten "9 volt" batteries. Alternatively, we could use only "9 volt" batteries and use a DC-DC converter to step down the B+ to 6 volts for the heater supply.
   The obstruction in this path is the losses in the DC conversion, since the heaters are the biggest power hogs in the amplifier. Well then, why not low voltage batteries for the heaters and step their voltage up to the B+ value with a DC-DC converter? This is probably the best path to follow, as it allows for the use of a low voltage lead-acid battery. Still new problem arise. Finding a DC-DC converters can be a headache. Off the shelf units are amazingly expensive and few in voltage increments. Designing a switching boost regulator is difficult and the results almost always require much tweaking.

   In the last issue's letter section, I promised that in this issue we would begin covering the designing of a portable tube headphone amplifier for driving dynamic headphones. As always, this will be a more of a broad design example than a how-to-construction article, although in this case it will be a short a jump to finished amplifier by the next installment. First we need a goal and this time the goal is an amplifier that is small enough to fit in a backpack or briefcase, that is powered by either a wall wart transformer or a 12 volt battery, that is robust enough to drive headphones as low 32 ohms and as high as 300 ohms. Different goals are certainly possible, such as limiting the amplifier's size to that which would comfortably fit in a shirt pocket or driving electrostatic headphones. Different goals limit choices and results, but having a goal is as important as having a destination before packing for a trip, as it helps free us from ill-considered whimsies, such as the amplifier specifying that must use tube rectifiers and 300B output tubes.