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output of IC2-B is fed to R15, the AR Envelope output level adjustment pot. The circuit point “AR” is the wiper of R15, which is fed to the AR-Gen switches of the modules. While contemplating this circuit, remember that the inverters are Schmidt triggers and that their inputs must go lower than a third of the supply voltage before their output goes high, and then the input must go to greater than two-thirds of the supply voltage before the output goes low. This characteristic is known as hysteresis. The zener diode on the external gate is meant to protect against gate signals greater than nine volts. When the external gate is high, Q8 will discharge C2 the same way the switch S1 does. NOTE: If you don’t plan to use the ■ FIGURE 3. LFO, Noise Source, and ±9V Battery Power Supply Schematic. external gate capability of the device, you can eliminate the following components: D7, R199, R1, D8, Q8. Low Frequency Oscillator, Noise Source, and ±9 Volt Battery Power Supply The low frequency oscillator is another modulator used to produce cyclic voltage changes with which the unit’s sound generators and sound modifiers can be controlled (see Figure 3). By applying the LFOs triangle wave to the control voltage input of a VCO, you will produce the sound of a siren. If you increase the frequency of the LFO enough, you will begin to hear bell-type tones as the oscillator being modulated starts to produce a characteristic ring-modulated sound. The noise generator is used to produce non-pitched sounds like rain, wind, or steam and, of course, the batteries power the unit. By using the recommended low power op-amps, the batteries will last a long time. Let’s discuss this part of the circuit. Two 2N3904s walk into a bar. One makes a lot of noise and gets thrown out while the other one sits and quietly drinks his stout. The punch line Some transistors are noisier than others. So how do we get noise out of any 2N3904 We do it with this circuit. Notice that we do the usual ... reverse bias a low V(ebo) emitter base junction, listen to the junction through C10, and a gain of about 1,000. If this transistor is whispering, it is still whispering into a lot of gain. This circuit does not care if this transistor is in a confessional — you are going to get at least 100 mV of noise from the first stage. We take whatever we are getting Low Frequency Oscillator 1 3 2 D9 Ramp a k 2 S16 R88 1 Tri 1 2 SPDT 3 1K k a on-off-on D10 Sawtooth R90 100K 2 Frequency R94 500 ohms R91 1 2 1Meg C13 1 2 2uF C14 1 2 .005uF 1 S17 SPST 2 - LF444 IC7-A 1 3 + Lo Range (closed) 2 R93 1 2 100K R89 1 2 200K 5 + LF444 7 IC7-B 6 - Triangle 2 3 Rect S18 1 SPDT LFS 1 3 -9V R92 100K 2 LFO +9V 1 C19 2 0.1uF 1 C20 2 0.1uF -9V 1 LF444 Power Pin 4 to +12V Pin 11 to -12V R96 1M b -9V +9V 2 1 e 2N3904 Q7 c Cut off collector .1uF 1 2 C15 2 1 2 R97 1M 1 R98 1K Noise Generator 10 + Selected for best noise Q7 can also be a 2N2712 9 IC7-C LF444 - 2 R100 1 4.7M 8 2 2 14 R95 12 + 1M 1 IC7-D LF444 R99 13 - R101 1 47K 1M C17 2 330pF 1 2 R102 10K 1 1 NS 2 Power Supply + B1 - 9V + B2 - 9V off 1 2 S19 on DPST 3 4 Power 1 + + C16 100uF 2 +9V 1 + + C18 100uF 2 -9V 46 March 2006
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 BP 2 3 LP O1 O2 NS 1 3 1 3 1 3 -9V S6 1 SPDT R29 2 100K R38 2 100K R44 2 100K 2 2 R21 33K R17 1 150K 2 2 1 1 150K 2 1 R30 R33 R39 2 1 150K 2 1 150K R45 1K Voltage Controlled Amplifier R22 1 1K 2 R24 1 100K 2 3 + 2 4 The Sound Lab Mini-Synth Sound Synthesizer 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 Voltage Controlled State Variable Filter (Band-pass and Low-pass) R34 1K 3 + 1 7 2 IC3-A LM13700 5 4 11 - 1 IC4-A LM13700 - +9V 1 2 2 2 1 560pF 2 C4 R25 1 620K +9V R46 20K R49 47K 1 1 7 +9V 5 8 11 8 BP 2 1 -9V 2 R42 20K 8 8 2 1 2 -9V R40 20K R26 10K 2 1 R18 1 2 470K 100K R47 1 3 R19 R27 2 100K R48 1 2 20K 100K 1 3 1 3 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 +9V VCA-Initial -9V Resonance 2 2 R28 1 S7 2 1 200K SPST R31 1 S8 2 1 200K SPST +9V R16 1 470K R20 1 470K 1 S4 2 2 SPST 1 S5 SPST 2 1 -9V 1/4" Jack 2 Main Output J1 1 AR LFO R43 20K 2 2 +9V 2 1 -9V AR LFO LM13700 Power Pin 11 to +12V Pin 6 to -12V R32 10K Cut-Off 1 R35 Frequency 2 1 R36 R37 1 1K 2 1 2 100K 14 100K 3 + 16 10 15 IC4-B 2 LM13700 12 8 LP R41 13 11 9 - 4.7K 1 2 ■ FIGURE 4. Voltage-Controlled Amplifier and Voltage-Controlled Filter Schematic. 2 560pF C5 R23 1K 1 C22 1 2 1uF NPO March 2006 47
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