Make: Electronics by Charles Platt (classic books to read TXT) 📗

can create your own waveform as an input for your audio amplifier with the basic astable 555 timer circuit shown in Figure 5-51. You have to be careful, though, not to overload the amplifier input. Note the 680K series resistor on the output pin of the timer. Also note the 500Ω potentiometer.

Figure 5-51. A 555 timer is wired in astable mode using the component values shown here to generate a wide range of audible frequencies when the 100K potentiometer is adjusted. After the output is reduced in power, it can feed into the amplifier chip that was used previously.

Disconnect your music player and connect the output from the 555 circuit to the input point (the 33K resistor) in the amplifier circuit shown earlier in Figure 5-41. You don’t have to worry about a separate connection on the negative side as long as the 555 timer shares the same breadboard and the negative side of its power supply.

Make sure that the 500Ω potentiometer is turned all the way to short the output from the timer to the negative side of the power supply. This functions as your volume control. Also make sure the 100K potentiometer is in the middle of its range. Switch on the power and slowly turn up the 500Ω potentiometer until you hear a tone.

Now adjust the 100K potentiometer to create a low-pitched note. You’ll find that it doesn’t have a “pure” sound. There are some buzzing overtones. This is because the 555 timer is generating square waves such as those shown in Figure 5-52, not sine waves, and a square wave is actually a sum of many different sine waves, some of which have a high frequency. Your ear hears these harmonics, even though they are not obvious when you look at a square-shaped waveform.

Route one of the connections to your loudspeaker through your spool of hookup wire, and now you should hear a much purer tone, as the buzzing high frequencies are blocked by the self-inductance of the coil. Remove the coil and substitute the 10 μF capacitor, and now you hear more buzzing and less bass.

You’ve just taken a small step toward sound synthesis. If this subject interests you, you can go online and search for oscillator circuits. For a thorough understanding of the relationship between waveforms and the sounds you hear, you’ll really need an oscilloscope, which will show you the shape of each waveform that you generate and modify.

Figure 5-52. The output from a 555 timer is either “on” or “off,” with a very fast transition between those two states. The result is an almost perfect square wave. Theoretically, this can be disassembled into a complex set of sine waves that have many different frequencies. The human ear hears the high frequencies as harsh overtones.

Experiment 30: Fuzz

Let’s try one more variation on the circuit in Experiment 28. This will demonstrate another fundamental audio attribute: distortion.

You will need:

One more 100K potentiometer.

Generic NPN transistors: 2N2222 or similar. Quantity: 2.

Various resistors and capacitors.

Background

Clipping

In the early days of “hi-fi” sound, engineers labored mightily to perfect the process of sound reproduction. They wanted the waveform at the output end of the amplifier to look identical with the waveform at the input end, the only difference being that it should be bigger, so that it would be powerful enough to drive loudspeakers. Even a very slight distortion of the waveform was unacceptable.

Little did they realize that their beautifully designed tube amplifiers would be abused by a new generation of rock guitarists whose intention was to create as much distortion as possible.

The most common form of waveform abuse is technically known as “clipping.” If you push a vacuum tube or a transistor to amplify a sine wave beyond the component’s capabilities, it “clips” the top and bottom of the curve. This makes it look more like a square wave, and as I explained in the section on waveforms, a square wave has a harsh, buzzing quality. For rock guitarists trying to add an edge to their music, the harshness is actually a desirable feature.

Figure 5-53. This Vox Wow-Fuzz pedal was one of the early stomp boxes, which deliberately induced the kind of distortion that audio engineers had been trying to get rid of for decades.

The first gadget to offer this on a commercial basis was known as a “fuzz box,” which deliberately clipped the input signal. An early fuzz box is shown in Figure 5-53. The clipping of a sine wave is shown in Figure 5-54.

Figure 5-54. When a sinewave (top) is passed through an amplifier which is turned up beyond the limit of its components (shown as dashed lines, center), the amplifier chops the wave (bottom) in a process known as “clipping.” The result is close to a square wave and is the basic principle of a fuzz box commonly used to create a harsh guitar sound.

Schematic

The output from the 555 timer is a square wave, so it already sounds quite “fuzzy,” but we can make it more intense to demonstrate the clipping principle. I’ve redrawn the whole circuit in Figure 5-55, as several components have changed. The principal alteration is the addition of two NPN transistors.

If you assemble this circuit on your breadboard, note carefully that the 33K and 10K resistors at the bottom of the amplifier chip have been removed, and there’s now just an 820Ω resistor in that location. The bottom of the adjacent 0.22 μF capacitor is still the input point for the amplifier, and if you follow the connection around to the middle of the schematic, you’ll find it leading to a 100K potentiometer. This is your “fuzz adjuster.”

The two NPN transistors are arranged so that the one on the left receives output from the 555 timer. This signal controls the flow of electricity through the transistor from a 33K resistor. This flow, in turn, controls the base of the righthand transistor, and the flow of current through that is what ultimately controls the