The Sound Lab Mini-Synth Sound Synthesizer
from the first LF444 and feed both
inputs of a second LF444 through two
1M resistors. We hang a capacitor off
of the inverting input and — voila —
the inverting input always lags the
non-inverting one. As the noise
voltage is taking its time trying to go
up and down (due to the cap) on the
inverting input, it is racing up and
down on the non-inverting input.
This results in the voltage on the
non-inverting input randomly being
higher or lower than the voltage
on the inverting input as the noise
voltage randomly changes. Since the
op-amp is wired as a full blast
comparator, its output is swinging up
and down between the voltage limits
of the LF444 in time to the noise
fluctuations. Varying the cap and
varying which input you hang the cap
on will change the characteristics of
the noise at the output of the second
stage.
The LFO is a simple inverting
integrator with positive feedback. IC7-
A ramps up when its input is held low
and then ramps down when the input
is brought high. IC7-B is a comparator
that senses the output of IC7-A and
goes high when the voltage out of
IC7-A goes above its positive threshold. This high voltage is fed back to
the input of IC7-A, which goes low in
response until IC7-B goes low and
then the cycle continues.
Switch S16 and Diodes D9 and
D10 control the shape of the LFO’s
waveform (centered for triangle either
pole gives ramp and sawtooth). In low
range (S17 on), a 2 µF cap (C13) is
placed in parallel with the integrator
capacitor (C14) to reduce the range of
frequency provided by the Frequency
pot R90.
Two nine-volt batteries power the
Sound Lab and the two by-pass
caps absorb any large current spikes
generated during operation. All of the
circuits in the Sound Lab together
draw less than 6 mA.
Voltage-Controlled
Amplifier and Voltage-Controlled Filter
The VCA and VCF are sound modifiers (see Figure 4). The VCA allows
you to modulate the amplitude of the
unit’s sound sources with the AR and
the LFO. It can produce tremolo
effects, ring modulation, and
amplitude envelopes. The VCF lets
■ FIGURE 4. Voltage-Controlled
Amplifier and Voltage-Controlled
Filter Schematic.
+9V
R18 R19 1
2 1 2 VCA-Initial
470K 100K 3
R16 1 S4 2
AR
2 1 470K SPST
R17
BP -9V 2 1 150K
Voltage Controlled Amplifier
2 S6 1
3 SPDT
R21
21
33K
R22
21
1K
3+
2 IC3-A
LM13700
4-
1
5
7
-9V
11
R20 1 S5 2
LFO
2 1 470K SPST
C22 1/4" Jack
2 Main Output
J1 1
R23
21
1K
LP
R24
21
100K
12
1uF NPO
+9V
R25
2 1 +9V
620K
8
8
2
R26
10K
1
1 R27
2
3 100K
R28 1 S7 2
AR
2 20 1 0K SPST
+9V
O1
1
-9V
R29
2
3 100K
R30
21
150K
R31 1 S8 2
LFO
2 20 1 0K SPST
Voltage Controlled State Variable Filter (Band-pass and Low-pass)
2
R32
10K
1
R34
21
1K
R35
21
1K
O2
1 R38
2
3 100K
R33
21
150K
R39
21
150K
3+
2 IC4-A
LM13700
4-
Cut-Off
Frequency
R36 R37 1
2 12
100K 100K 3
1
5
7
11
BP
R40
8
82 1
20K
14 +
16
15 IC4-B
LM13700
13
-
10
12
11
2
R45
1K
1
1 560pF
C4 +9V
2
2
R42
20K
1
1 560pF
C5 +9V
82
LP R41
9 4.7K
1
NS
1
R44
2
3 100K
R46
21
20K
R49
21
47K
R47 R48 1
2 12
20K 100K
2
2
R43
20K
-9V
1
-9V
3
Resonance
-9V
LM13700 Power
Pin 11 to +12V
Pin 6 to -12V
March 2006 47
