Owner`s manual
grace design m201
owner’s manual
13
2. Set all of the gain controls to the 30dB position.
3. Connect the audio generator to the input of channel 1.
4. Set the generator output level to approximately -20dBu @1KHz.
5. If the generator has an unbalanced output, refer to gure 2 below for termination information
6. Connect the level meter to the output of channel 1.
7. If the level meter has an unbalanced input, refer to <gure 1> for unbalanced output termination
information.
8. Apply power to the generator, preamplier, then the level meter.
9. Adjust the generator output level so that the preamplier output level is at the desired red thresh-
old. If the proper level can’t be reached, adjust the preamplier gain control.
10. Locate VR2A (VR2B for channel 2) (see <gure 3>) on the channel 1 audio circuit board and adjust
until the peak LED is between green and red. The color should appear amber when the setting is
correct. Repeat this procedure for channel 2.
11. If it is desired to calibrate the peak indicator to a specic converter/recorder input level, simply
connect the output of the preamplifer to the converter/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.
INPUT OFFSET ADJUSTMENT
The input ampliers in the m201 are ultra-precision laser trimmed devices which have a very low
input oset. However, even the smallest dierential oset 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 o-
set is present. This procedure nulls the input oset 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 instanta-
neous change in amplitude when turned, which creates a transient signal.
EQUIPMENT NEEDED
• DC Volt meter with at least 1mV sensitivity
• Plastic alignment tool or a small screwdriver
• #2 phillips head screwdriver
PROCEDURE
1. Remove the 8 phillips head screws from the top lid but leave the lid in place.
2. Set all of the preamplier gain controls to maximum.
3. Turn on the m201 preamplier and allow to warm up for at least 45 minutes.
4. Remove the top lid and locate the DC oset test points TP1A and TP2A (see gure 4 below). Place
the Volt meter probes into these holes.
5. Locate DC oset potentiometer VR1A (VR1B for channel 2) and rotate it for a reading of 0.000VDC
+/- 0.003V.
6. The input oset does drift with temperature and having the lid o will allow the circuitry to cool. It
is advisable to replace the lid and let the preamplier warm up again (about 20 minutes) and then
re-check your adjustments.