User Guide
24
The Zobel network R44/C48 is only fitted when a transformer is incorporated.
ST7 allows the optional insert unit to be linked into the matrix output chain. When
the insert option is not used a jumper is inserted linking pins 1&2.
NOTE: Pin 1 of the input and output XLRs are connected together and to the
other channels but not directly to the mixers 0 volt rail.
Channel monitoring.
Both audio and visual monitoring is provided for each channel. The meter changeover
switch [SW3] selects whether the direct channel input or group output is to be
audibly monitored pre fade or the post fade level monitored on the channel mod-
ules output meter.
Output meter
IC7a and b receive, rectify, and calibrate the signal selected by the meter changeover
switch SW3. The signal, now a varying positive voltage, is fed to IC7C and d;
IC7c has 30dB of gain, which enables D21-D30 to turn on 30dB ahead of D31-
D40.
Q2 provides a regulated power source for the LM3915s; Q3 and Q4 are constant
current sources for the LED chains.
To calibrate the meter, a signal of 0dB is applied at pin 1 of ST6 [the level can
be externally verified at whichever output is driving the meter, selected by the
meter changeover switch SW3], and the cathode of D15 is monitored by an os-
cilloscope. When a symmetrical trace is obtained by adjusting VR4, VR5 is
adjusted so that D33 [the 0dB yellow LED] just illuminates. Then the signal is
reduced to -30dB, when the LED chain should have only the first three LEDs
[D21-D23] illuminated. If the calibration appears to be wrong, the symmetry
should be rechecked.
Pre fade routing.
The required pre-fade signal as selected by SW3 is fed via ST6 Pin 2 to the matrix
level board. Here it is presented to a DG211 quad analogue switch [IC1] configured
as a T switch for maximum off isolation.
As with the matrix group mix buss input, the channel is assigned to a pfl channel
corresponding to the module location in the main frame. This is accomplished by
inserting the appropriate jumper in ST5/6.