2BAMP

2BAMP is a vacuum tube headphone amplifier, based on a design originally by Bruce Heran, utilizing a dual triode 12AU7 vacuum tube. 

While the original design was used with a car battery, 2BAMP can be powered by any wall wart between 15 and 35 volts. I use a 15V transformer. Alternatively, it has a screw terminal to use any 12V battery or other power supply. There is an on/off switch on the back, so it can be left plugged in without being on.

It has a 3.5mm TRS jack for audio input. The output is a 1/4" TRS jack. The circuit board has 1/4" mounting holes on all four corners. 


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The audio signals enter the circuit through a dual-gang 10 kΩ potentiometer, allowing the listener to alter the amplitude of the input signal and change the volume. Each output signal enters a coupling capacitor (C1 and C2), which blocks any DC signal from the source. There are also DC-blocking capacitors (C5 and C6) immediately before the output of the amplifier. 

Each channel (left and right) of the amplifier has two stages, a voltage gain stage and then a current gain stage. The first is a common cathode amplifier, which amplifies the voltage. The 12AU7 is a dual triode vacuum tube, so one tube is capable of handling both of the channels. The second stage is a MOSFET source follower, for current gain. I used the IRF630 N-channel MOSFET transistor (Q1 and Q2). The transistor is biased with a potentiometer (RB1 and RB2). The potentiometer is adjusted such that the source of the MOSFET is at 6V (one-half of the supply voltage), forcing the device into the saturation region. An LM317 voltage regulator (U1 and U2), is configured as a constant current source (125 mA). The transistors and voltage regulators operate at a hot temperature, but heatsinks are not required for operation.  

The circuit can be powered two different ways. The first is by the barrel jack (CON1), which immediately connects to a 12V regulator (U3), whose input and output are connected to the ground through decoupling capacitors (C7 and C8), which block any high frequency noise in the supply voltage. There is a power switch between the ground and the V- of the barrel jack connector. The circuit can also be powered through a screw terminal (J3). It requires a 12V battery or other low-noise DC source.

The circuit board was laid out in Kicad, and ordered from dirtypcbs.com. The board layout is shown. There is a copper pour ground plane on the back of the board. There were several layout iterations, with the goal of minimizing trace distances, and limiting the number of through-hole vias. 

Also shown is KiCad's 3D view option, which displays a 3D render of the circuit board, with models for various standard-package components. It did not have models for the jacks, potentiometer, or tube, but it still gave a good sense of the 3D layout of the board. 

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