■ FIGURE 4.
detect a 3 db difference in sound level. We could also
debate whether a simple balance control would provide a
better, inexpensive solution for the channel tracking errors
than a step attenuator. The choice is yours. However, at
least consider the Alpha potentiometer as an alternative to
a garden-variety potentiometer.
The layout of this project is straightforward. Simply
follow the fully documented component layout layer on
the PCB layout file. The file format is ExpressPCB, and the
application for reading and editing the file is available
for free download. If you elect to use an external power
supply or a supply of your own design, then about 70% of
the board design can be deleted.
Figure 6 shows the component side of the board, and
Figure 7 shows a close-up of the amplifier section of the
populated board. Note the 20-pin TPA6120A2 near the
upper third of Figure 7.
■ FIGURE 5. Relative tracking, 50K stereo pots,
and step attenuator.
Figure 8 shows the ground plane configuration and
thermal vias for the TPA6120A2, taken from the
ExpressPCB file. The TPA6120A2 is susceptible to
oscillations, and removing the ground plane under key
leads reduces inter-lead capacitance and tendency to
oscillate. The ground plane is shown in green.
Working with the 20-pin surface-mount TPA6120A2
presents a modest challenge in that the belly of the IC
must be soldered to thermal vias on the PCB. A hot air
pencil is useful but not necessary for this step.
The cables, connectors, and other peripheral
components should be wired directly to the relevant
terminal blocks on the PCB. Keep the AC input lines
separate from the audio input cables and output cables.
Figure 9 shows the completed amplifier with toroidal
transformer attached, ready for mounting, and Figure 10
shows a close-up of the instrument input jack and step
Carefully inspect your work — especially the solder
joints of the three ICs. Use a low-current ohmmeter to
check for obvious shorts. Use the test points to compare
values from left and right channels.
Before you apply AC power, remove the two 1A fuses.
Apply AC power to the transformer and verify that the
power supply produces both + 12 VDC and - 12 VDC.
Disconnect the AC power, replace the fuses, and reapply
power. Apply a signal to the input jack and monitor the
output with a pair of headphones. Because the amplifier
circuit is symmetrical, voltage and signal levels should be
symmetrical, as well. Therefore, if one side of your
amplifier isn’t working, use voltage and signal
measurements from the other side as a reference.
Although any aluminum box of appropriate
dimensions will do, I’m fond of the Hamond extruded
■ FIGURE 6. Printed Circuit Board, component side.