Class-A Tube Headphone Amplifier

Posted on Sep 10, 2012

Pure Class-A triode OTL design and only one tube for amplification. The plate voltages should be low. The output impedance should be as low as possible and the maximum output current should be as high as possible. This amplifier is a stand alone Headphone amplifier without the property of input selection as mentioned in my previous project but could be expanded if desired. Preferring the Class-A OTL design I decided again to implement a long tailed pair for the input section and a parallel connected cathode follower for the output section. The voltages should be as low as possible. This condition was taken into account mainly because the risk of any high voltages at the output of the amplifier, if any damage occurs, should be kept as low as possible.

Class-A Tube Headphone Amplifier
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This could be the case if the output capacitor shortens and then the full cathode voltage would be connected direct to the output, damaging the headphone or there would be the risk of touching lethal voltages! In my opinion it's better that this voltage is 40V and not 150V or even higher. The audio input is connected to a volume potentiometer (47K log, ALPS, Noble or Panasonic) and then coupled with a 220nF MKP capacitor to a long tailed pair with the E88CC triodes . The long tailed pair delivers a signal which is not phase inverted and therefore the whole amplifier is not phase inverted. Pin 7 is connected to ground, because the amplifier is designed for asymmetrical input signals. If an upgrade to symmetrical signals is desired pin 7 could be connected to the second input signal. The 1M resistor connects the grid of the first triode to ground and the 100 Ohm resistor blocks RF oscillation of the circuit. The common cathode resistor with 180 Ohm sets the operating current of 5mA for each triode and the plate resistors 7k5 set the plate voltages of about 80V. The anode of the second triode is connected to the 220nF coupling capacitor. The output section is a parallel connected E288CC cathode follower. The 470K, 33 Ohm and 1K/3W resistors set the operating point of the tubes (about 76V plate voltage and 21mA plate current). The cathodes are connected to the output coupling capacitors 470µF/400V and 1µF/400V. The output...

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