WhitakerAudio – ExpressPCB Customer Spotlight
The 40 W XL Stereo Power Amplifier is an all vacuum tube design intended for moderate power output levels. A tube-based amplifier imposes some unusual requirements on circuit board design, notably high operating voltages and considerable heat from the active devices. The typical operating voltage of this amplifier (plate voltage) is 425 Vdc. To accommodate these levels, ample space must be provided between adjacent circuit board traces. The other design challenge is heat. Large vacuum tubes, such as those used in this amplifier, generate a considerable amount of heat.
In the 1960s and early 1970s, when vacuum tubes were still in general use and printed circuit boards were beginning to enter widespread use, tube sockets were routinely mounted directly on the PCB. This placed the heat-producing tubes just above the board. The result was that the boards tended to deteriorate over time as heat from the active devices took its toll. It is fair to note that 1960s era PCBs were far less robust than current boards, both from the standpoint of materials and construction. Armed with lessons learned from the past, a different mounting approach was taken for this amplifier, as shown below.
The tube sockets are mounted on standoffs that keep the heat-producing vacuum tubes away from the PCB. The twisted green wires provide filament voltage (6.3 Vac) to the tubes. These wires are dressed away from the board and against the metal chassis to minimize noise. The filament connections could have been implemented on the board, but at the risk of introducing hum in the output circuit.
The amplifier is built around three main printed circuit boards and five special purpose boards. One board is a 4-layer design; the rest are 2-layer boards. A ground plane covers the component side of the 2-layer boards. Component placement and board traces have been organized to minimize hum and noise. The PCBs include top and bottom solder masks and top-side silk-screened legends. The PCBs are designed to minimize off-board connections, thereby simplifying layout and providing for controlled characteristics from one unit to the next.
The bottom-view of the chassis is shown above. The board in the center rear of the chassis is inverted as a space-saving measure. An important part of any project is documentation. Schematic drawings of the amplifier circuits were done in ExpressSCH. For this particular project, the parts list (bill of materials or BOM) was compiled manually, a time-consuming process. Now, however, the tool xCheck makes generation of the BOM an automated step, which greatly simplifies the work and ensures accuracy.
The schematic diagram of the amplifier is shown above. Note that multiple “sheets” were used, one for each major functional block.
The xCheck application performs a number of functions, including developing a BOM, as illustrated above.
Each amplifier channel is implemented on a separate board. The right channel PCB is shown above with the ground plane hidden. The same board with the ground plane visible is shown below.
Shown above is the 4-layer board. All other PCBs in the amplifier are 2-layer. The 4-layer board operates at low voltage (24 Vdc), other than the 120 V ac input to the transformer.
The ExpressSCH and ExpressPCB tools have been indispensable in producing the 40 W Stereo Amplifier. With the addition of xCheck, the development process is further simplified.