OTL single-ended  headphone amplifier using
3 output triodes and a choke load

OTL Push-Pull  Output Stages
     Class A push-pull amplifiers offer the advantage of delivering twice the idle current into the load. The single-ended amplifiers can only deliver the idle current. Understand that the relative efficiency remains the same: 50% tops. The disadvantage of the push-pull totem pole arrangement is that it necessitates twice the B+ voltage, which means that two 72 volts DC-DC converters will be needed. While this seems to constitute a huge liability, but do not forget that tubes thrive on voltage. Doubling the B+ in this example will not increase the output tubes Vp voltage, but it does increase the voltage available to the input and phase splitter tubes.

    The choke, on the other hand, theoretically displaces no voltage, but in actual practice the DCR of the choke must be factored into our calculations. If the value is too great to allow the simple biasing arrangement of one capacitor and one resistor at the grid, then a second resistor can be added to pull up the bias voltage from ground potential. Conversely, if the DCR is too low, a resistor, or even a forward bias diode, can be placed in series with the choke to correctly bias the output tubes. In other words, the choke replaces the cathode resistor in a grounded cathode amplifier.

push-pull  headphone amplifier



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