Moog Satellite Service Manual

The transistor pair to the right of the low pass filter
controls t \e amplitude of the audio waveform by variable trans-
conductanc "J. The current which determines this transconductance
is in turr determined by the voltage of pin 12 of IC20, and the
resistance between pin 13 and ground. The voltage applied to
pin 12 is the amplitude contour voltage, and the resistor, from
' B19 to. grc and is the 100k volume control potentiometer. IC22 is
a differential amplifier, the output of which is the final
audio wave form.
* . • •
Of the two contour generators, the amplitude contour
generator is the simplest, so it will be described first. This
contour gsaerator consists of Q35, Q36, IC18, transistor pairs
026-27 and Q23-29, Q30, Q31, and the associated circuitry. When
the trigger voltage goes on, Q35 partially discharges C25 so
that the eaitter of Q35 remains at 5 volts or so. If Q36 is
saturated, Q34 does not go on at all. Row if 3 in the resistor
matrix det srraines whether or not Q36 is turned on. If Q34 is not
turned on, the C25 is free to charge again through Q26. The
charging carrent from Q26 is determined by the voltage control
which the current from the resistor matrix develop across R189.
Thus, the /oltage at the emitter of Q35 is a decaying curve if
Q36 is on, and a step followed by a decaying curve if*Q36 is off.
The rise time of the voltage at the emitter of Q35 is determined
only by th inability of Q35 to discharge C25. Typically, this
rise time Ls less then 1 millisecond. The decay time of the
amplitude contour is determined by the voltage difference
between the bases of Q26 and Q27. The voltage across R189 results
from the a nolitude contour decay time control currents coming
from row #2 of the resistor matrix, the keyboard voltage applied
to R1S9, and the shaping current from R169 and R171 (described
below). R190 corrects for transistor offsets and other normal
component variations. A voltage increase of 18.5 rav at the
base of Q25 cuts the decay time in half.
IC18 and Q32 comprise a voltage follower whose slew rate
is propora cional to the bias current of IC13. The bias current
comes from the collector of Q28 and is determined by the voltage
difference between the bases of Q2? and Q29. Thus, since.the
decay time of an envelope is generally longer than the attack
time, the /oltage appearing at the source of Q32 has an attack
time inversely proportional to the collector current of Q26.
The contributions to attack time control are similar to those
of decay time control. The quick-set current comes from row #1
of the resistor matrix. - ‘ .
Sincj Q26 is a nearly ideal current source, the decay slope
at the source -of Q32 would be a straight line, were it not for
the action of Q30. At the beginning of the decay slope, the
voltage at the base of Q30 is more positive than the emitter,
and Q30 do as not conduct. When the base of Q30 goes below (
-0.6 volts, Q30 acts as an emitter follower. The current through
R169 slows down the decay slope. The more negative the base of
Q30 goes, the higher is its control current, and the more the
decay slop* gees down. This gives the decay slope an extended
tail and t \erefore sounds like a more natural exponential decay.
When the voltage at B21 is +9, Q31 is saturated and there
is very li :tle current flowing through R171. When the voltage

Moog Satellite Specifications

General

Brand	Moog
Model	Satellite
Category	Synthesizer
Language	English