Combing two transformers not only serves to boast the rail voltage, but it also serves to flatten the aspect of the enclosure that will house the power supply. Two flat-pack style PC mount transformers are not nearly as tall a single horizontal mount power transformer that holds the equivalent windings. Furthermore, having the two lower voltage windings effectively center-tapped allows for easily establishing a +/- 12 volt rails about the ground. These rails will come in handy, if DC servos, slow start circuitry, or startup/faults muting circuitry are used, as these circuits will undoubtedly use ICs in their implementation. Of course this means the low voltage windings must be full-bridge rectified, which we should anyway to provide DC voltage for the heaters.

  Finding a 140 CT transformer is tough, as this is not a common value. Three fixes present themselves. The first is to have one custom winded. This will cost more much more money than the off-the-shelf option, but designing power transformers is not secret art like designing output transformers is. And a custom transformer quickly becomes cheaper if you can get a few of your friends to order one as well. The second fix is to obtain an over-specified 120 CT transformer, as 20% regulation on the transformer will easily bump the secondary voltage up to 140 under a light load. The final fix is to use two transformers in series. This is my recommendation.
   Since at least one extra winding will be needed for supplying the heaters, a second transformer would probably be needed anyway.  Wiring two transformers in series is easy, as a common transformer configuration is to have four windings on one core.   

Ancillary support circuitry: DC servo to correct for DC offsets at the output and an automatic muting circuit for protecting the headphones from a fault condition in the amplifier

pg. 15

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