Saturday, January 26, 2013

High Quality Vacuum Tube Headphone Amplifier

Theres no transformer in output, it only need coupling capacitor in the signal path is the output cap. A relay circuit was used to provide muting and a time delay was used to keep the output muted during warm-up in order to protect the headphones from the high voltages that would otherwise be present at the output of the amplifier.

A separate chassis was used for the power supply to keep noise levels as low as possible. Selection of input tubes (6SN7s) for low noise is important in order to realize the full capability of this circuit. Since I already have a regulated source of 12 volts DC, I intend to modify this amplifier in the future to use 12SX7s.

The power supply is a fairly conventional design using a 6550a as a series pass regulator and a zener string to set the operating voltage.

The amplifier circuit is a 6SN7 run in grounded cathode mode and is self-biased using an un-bypassed cathode resistor. The input stage is direct coupled to a 6CA7 that is used as a cathode follower. The output from the 6CA7 is coupled to the headphones using a 47uF capacitor, providing extended bass response. The output capacitors are bypassed with 1uf and .01uF polystyrene capacitors. An Alps 'Black Beauty' volume control is used to help preserver sonics and easily allows adjustment at low gain levels with adequate tracking between the two channels.

A switchable cross-feed circuit is present at the input of the amplifier to be used to help correct imaging problems, if desired.

Schematic of Hadphone Amplifier

Power Supply of Headphone Amplifier

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