Nuts & Volts

The Sound Lab Mini-Synth Sound Synthesizer
cut-off frequency of the VCF. S8
switches the LFO control voltage on
or off. When on, the level of the LFO
output pot determines how much the
LFO controls the cut-off frequency of
the VCF. R37 controls the initial
cut-off frequency of the VCF. When S7
or S8 is on, it is best to turn R37 off or
nearly off.
Voltage-Controlled
Oscillators
The oscillators are the heart of
any synthesizer and they provide the
main sound generating capability
(see Figure 5). Just about every sound
you hear has some pitch to it — a
voice, a trumpet, an airplane, etc. The
pitch may be modulated or modified,
but it is there a great majority of the
time.
The Sound Lab uses two voltage
controlled ramp oscillators to provide
pitch sources. IC5-B and IC5-A and
associated transistors and components
comprise a linear voltage to
logarithmic current converter. The
control voltages that are summed by
IC5-B range from -8 to 8 volts. The
resulting current ranges from around
1 µA to 1 mA and, since the oscillators
go up approximately one octave every
time the current doubles, this gives
the oscillator a nice range from a subaudible
0.6 Hz to about 8.3 kHz.
Oscillation occurs because as the
current is pulled out of the input of
integrator IC5-D, its output goes high
until IC5-C (comparator with hysteresis)
pin 8 goes high and shorts the
integrating capacitor (C8), which
causes the cycle to begin anew and
subsequently repeat. IC6 is used in a
8.1 in.
similar configuration.
The output of IC6-D (point ZZ) is
fed into comparator IC2-A in order to
provide a square/pulse wave shaper
for Oscillator 2. The control voltage
fed into IC2-A pin 2 via resistors R86
and R87 determines the comparator
threshold and thus the point at which
the output (pin 1) goes high and low.
Varying R85 will change the pulse
width and vary the timbre of
Oscillator 2 when Rect Wave is
selected. S15 permits the LFO output
voltage to modulate the pulse width,
which can cause the output of
Oscillator 2 to sound like two
oscillators tuned very close together
(when approximately 1 Hz LFO
frequency is used).
S11, S13, S12, and S14 are used
■ FIGURE 6. Sound Lab Mini-Synth
Suggested Front Panel Layout.
Oscillator 1
Frequency
4
3
5 6
7
8
Oscillator 2
Frequency
5 6
40
4
7
30
3
8
Pulse Width %
50
Mixer
Oscillator 1
Sound Lab Mini-Synth by RJWSoft
Oscillator 2
White Noise
5 6
5 6
5 6
60
4
7 4
7 4
7
70
3
8 3
8 3
8
2
9
2
9
20
80
2
9
2
9
2
9
1
10 1
10 10 90
1
AR-Gen LFO Wave Sync AR-Gen LFO PWM
On On Ramp On On On On
10 1
VC State Var. Filter
AR-Gen
On
LFO
On
Mode
BP
4
5 6
10 1
Cut-Off Frequency
7
4
10
Resonance
5 6
7
3
8
3
8
5.94 in.
Off Off Rect Off Off Off Off Off Off LP
1 10 1 10
Low Frequency Oscillator Attack-Release Env Generator
Frequency Output Attack Time Release Time Output Main Output Level
5 6
5 6
5 6
5 6
5 6
5 6
4
7 4
7 4
7 4
7 4
7 4
7
2
9
2
9
3
8 3
8
3
8
3
8
3
8
3
8
2
9
2
9
2
9
2
9
2
9
2
9
1
10
Range
High
1
Waveform
Ramp Ramps
10
1
Manual
Gate
10
Mode
Gate
1
Repeat
On
10
VCA
4
1
Initial
5 6
7
10
AR-Gen
On
1
LFO
On
10
Power
On
Tri
3
8
Low
Saw
Rect
Trig
Off
2
1
10
9
Off
Off
Off
March 2006 49