grace design m201
mk2
owner’s manual
8
1. With the power off, remove the 14 phillips head screws from the top cover and remove it.
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 (0.077vrms) @1KHz. If the generator
has an unbalanced output, refer to
figure
2
below for termination information.
1
2
3
SHIELD
HOT
XLR FEMALE
figure 2
- Unbalanced Output
5. Connect the level meter or DVM to the output of channel 1.
6. If the level meter has an unbalanced input, refer to
figure 3
(p. 9) for unbalanced output termi-
nation information.
7. Apply power to the generator, preamplifier, then the level meter.
8. Adjust the generator output level so that the preamplifier output level is at +16dBu
(4.88VRMS) or another desired red threshold level. If the proper level can’t be reached, adjust
the preamplifier gain control.
9. Locate VR2 (see
figure 3
, p.9) on the channel 1 audio circuit board and adjust until the peak
LED is between green and red. Repeat this procedure for channels 2 through 8.
10. If it is desired to calibrate the peak indicator to a specific AD converter input level, simply con-
nect the output of the preamplifier to the converter. Adjust the signal generator so that the
desired peak level is indicated on the converter and then adjust VR2 until the peak LED is be-
tween green and red.
INPUT OFFSET ADJUSTMENT
The input amplifiers in the m201mk2 are ultra-precision laser trimmed devices which 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. This procedure nulls the input offset and minimizes the clicking sound with no sig-
nal 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
