Here are a series of images of an amplifier built to drive my Sennheiser HD-600 headphones. Because of the relatively high impedance of these phones, I wanted plenty of voltage available to drive them. Because of the large dynamic range possible with these phones the system had to be very quiet. A high amount of resolution is available with these headphones, so no negative feedback was employed in the execution of an output transformerless design.
The only 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.
* NOTE - (In June 2004, the conversion to 12SX7s was performed and minor changes were made to the power supply. The schematics below reflect these changes.)
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.
