2 MIXER
Card
2.1
Theory of Operation
The MIXER Card consists of eight identical Voltage Controlled Amplifier
(VCA) circuits followed by mixing circuitry. In addition, certain other
related functions are placed on this card.
The functions on this board include:
•
Eight channel demultiplexer for ADSR 2 voices 0-7
•
Demultiplexing for Volume L, R, Noise volume L, R, and slave
synthesizer Gate, CV
•
Noise generator with associated VCAs and Low Pass Filter
•
Headphone
Amplifier
2.2 VCA
Operation
The VCAs on the MIXER Card are made up of SSM 2024 Quad VCA ICs.
The audio inputs to these VCAs come from the VCF card audio outputs.
Whenever a key is pressed and a voice activated, an ADSR voltage is
generated at the appropriate CD4051 output and drives the VCA for that
voice. This causes the VCA to pass the VCF audio at its input to the
summing amp at its output. The gain is set by a trimmer in line with the
ADSR signal.
The VCA outputs are mixed in four groups of two such that VCA 0 & 1
come from one op amp output, VCA 2 & 3 from another op amp output,
and so on. When the keyboard WHOLE 8 Mode is selected, the output of
sub-mixer 0, 1 is hard panned to one side of the VOYETRA output, while
the output of sub-mixer 6, 7 is hard panned to the other side. The
outputs for 2,3 and 4,5 are panned to be slightly offset from the center of
the stereo spectrum. When the SPLIT or LAYER modes are selected,
the outputs of 0, 1, 2, 3 are hard panned to one output while the 4, 5, 6, 7
outputs are hard panned to the other side. Of course, the MONO output
always contains the sum of all eight voices.
The switching to accomplish this output panning is accomplished by
using a DG308 analog switch that is turned on by a 0, +8V control signal
called SPLIT/WHOLE 8 generated at the DAC CARD.
The L and R output mixers consist of two VCAs that are used to set the
output level and serve as noise gates. When no keys are pressed, the
CPU turns off these VCAs to quiet the system output. The output level is
set by the Master Volume Control on the POT PANEL and the Parameter
trimmers. The CPU digitizes the parameter trimmers and demultiplexes
