Microplane 201 - Manual (Page 8)

iv. Apply power to the generator, preamplifier, then the level meter.
v. Adjust the generator output level so that the preamplifier output level is at the
desired red threshold. If the proper level can't be reached, adjust the
preamplifier gain control.
vi. Locate VR1 on the channel 1 audio circuit board. (See figure 7) Adjust VR1
until the peak LED is between green and red. The color should appear amber
when the setting is correct.
vii. Repeat this procedure for channel 2.
viii.If it is desired to calibrate the peak indicator to a specific tape recorder input
level, simply replace the level meter with the recorder. Adjust the signal
generator so that the desired peak level is indicated on the recorder and then
adjust VR1 until the peak LED is between green and red.
Figure 7. PEAK LED ADJ. VR1 AND OFFSET ADJ. VR2 LOCATIONS
C. INPUT OFFSET ADJUSTMENT: The input amplifiers in the 201 are ultra precision
laser trimmed devices that have a very low input offset. However, even the smallest
differential offset at the input can result in an audible "click" when the gain control is
turned. It should be noted that this type of click will be audible when no signal is
present. With no signal present there is no signal amplitude to be changed unless
input offset is present. The DC servo circuit in the 201 corrects any DC errors that
appear at the output of the instrumentation amplifier stage. It can not correct the
very small offset at the inputs. This procedure nulls the input offset and minimizes
the clicking sound with no signal present. In contrast, there will almost always be an
audible click if turning the gain control while a signal is present. This is because the
gain control, being a stepped control, will make an instantaneous change in
amplitude when turned, which creates a transient signal.
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