Noise and Grounding 387point on this track as the audio-ground starpoint, as it carries heavy charging
pulses and will induce ripple into the signal. Instead, take a thick tee from the
center of this track (through which the charging pulses will not fl ow) and use the
end of this as the starpoint.● Low-value resistors in the output stage are likely to get very hot in operation—
possibly up to 200°C. They must be spaced out as much as possible and kept
from contact with components such as electrolytic capacitors. Keep them away
from sensitive devices such as the driver transistors and the bias-generator
transistor.
● Vertical power resistors. The use of these in power amplifi ers appears attractive at
fi rst because of the small amount of PCB area they take up. However, the vertical
construction means that any impact on the component, such as might be received
in normal handling, puts a very great strain on the PCB pads, which are likely to
be forced off the board. This may result in it being scrapped. Single-sided boards
are particularly vulnerable, having much lower pad adhesion due to the absence
of vias.
● Solderable metal clips to strengthen the vertical resistors are available in some
ranges (e.g., Vitrohm) but this is not a complete solution, and the conclusion must
be that horizontal-format power resistors are preferable.
● Rail decoupler capacitors must have a separate ground return to the reservoir
ground. This ground mustnot share any part of the audio ground system, and
mustnot be returned to the starpoint.
● The exact layout of the feedback takeoff point is criticial for proper operation.
Usually the output stage has anoutput rail that connects the emitter power
resistors together. This carries the full output current and must be substantial.
Take a tee from this track for the output connection and attach the feedback
takeoff point to somewhere along this tee. Do not attach it to the track joining
the emitter resistors.
● The input stages (usually a differential pair) should be at the other end of the
circuitry from the output stage. Never run input tracks close to the output stage.
Input stage ground and the ground at the bottom of the feedback network must
be the same track running back to the starpoint. No decoupling capacitors may