Make: Electronics
Page 35
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 amplifier.
When you power up the circuit, use the 100K potentiometer attached to the 555 timer to adjust the frequency (as before) and crank the “fuzz adjuster” potentiometer to hear how it adds increasing “bite” to the sound until ultimately it becomes pure noise.
The two transistors act as amplifiers. Of course, we didn’t need them for that purpose—the input level for the amplifier chip was already more than adequate. The purpose of the lefthand transistor is simply to overload the righthand transistor, to create the “fuzz” effect. And when you turn up the output from the transistors with the “fuzz adjuster,” eventually they overload the input of the amplifier chip, creating even more distortion.
If you want to tweak the output, try substituting different values for the 1K resistor and the 1 μF capacitor that are positioned between the emitter of the righthand transistor and the negative side of the power supply. A larger resistor should overload the transistor less. Different capacitor values should make the sound more or less harsh.
You can find literally thousands of schematics online for gadgets to modify guitar sound. The circuit I’ve included here is one of the most primitive. If you want something more versatile, you should search for “stomp box schematics” and see what you can find.
Figure 5-55. For a quick demo of clipping, insert a couple of transistors between the output from the 555 timer and the input of the amplifier chip. One transistor overdrives the other, so that when you adjust the potentiometer at the center of the circuit, you hear an increasingly harsh, distorted sound.
Background
Stomp-box origins
The Ventures recorded the first single to use a fuzz box, titled “The 2,000 Pound Bee,” in 1962. Truly one of the most awful instrumentals ever made, it used distortion merely as a gimmick and must have discouraged other musicians from taking the concept seriously.
Ray Davies of the Kinks was the first to embody distortion as an integral part of his music. Davies did it initially by plugging the output from one amp into the input of another, supposedly during the recording of his hit “You Really Got Me.” This overloaded the input and created clipping—the basic fuzz concept. From there it was a short step to Keith Richards using a Gibson Maestro Fuzz-Tone when the Rolling Stones recorded “(I Can’t Get No) Satisfaction” in 1965.
Today, you can find thousands of advocates promoting as many different mythologies about “ideal” distortion. In Figure 5-56, I’ve included a schematic from Flavio Dellepiane, a circuit designer in Italy who gives away his work (with a little help from Google AdSense) at http://www.redcircuits.com. Dellepiane is self-taught, having gained much of his knowledge from electronics magazines such as the British Wireless World. In his fuzz circuit, he uses a very high-gain amplifier consisting of three field-effect transistors (FETs), which closely imitate the rounded square-wave typical of an overdriven tube amp.
Figure 5-56. This circuit designed by Flavio Dellepiane uses three transistors to simulate the kind of distortion that used to be created by overloading the input of a tube amplifier.
Dellepiane offers dozens more schematics on his site, developed and tested with a dual-trace oscilloscope, low-distortion sinewave oscillator (so that he can give audio devices a “clean” input), distortion meter, and precision audio volt meter. This last item, and the oscillator, were built from his own designs, and he gives away their schematics, too. Thus his site provides one-stop shopping for home-audio electronics hobbyists in search of a self-administered education.
Before fuzz, there was tremolo. A lot of people confuse this with vibrato, so let’s clarify that distinction right now:
Vibrato applied to a note makes the frequency waver up and down, as if a guitarist is bending the strings.
Tremolo applied to a note makes its volume fluctuate, as if someone is turning the volume control of a guitar up and down very quickly.
Background
Stomp-box origins (continued)
; Harry DeArmond sold the first tremolo box, which he named the Trem-Trol. It looked like an antique portable radio, with two dials on the front and a carrying handle on top. Perhaps in an effort to cut costs, DeArmond didn’t use any electronic components. His steam-punkish Trem-Trol contained a motor fitted with a tapered shaft, with a rubber wheel pressing against it. The speed of the wheel varied when you turned a knob to reposition the wheel up and down the shaft. The wheel, in turn, cranked a little capsule of “hydro-fluid,” in which two wires were immersed, carrying the audio signal. As the capsule rocked to and fro, the fluid sloshed from side to side, and the resistance between the electrodes fluctuated. This modulated the audio output.
Today, Trem-Trols are an antique collectible. When industrial designer Dan Formosa acquired one, he put pictures online at http://www.danformosa.com/dearmond.html. And Johann Burkard has posted an MP3 of his DeArmond Trem-Trol so you can actually hear it: http://johannburkard.de/blog/music/effects/DeArmond-Tremolo-Control-clip.html.
The idea of a mechanical source for electronic sound mods didn’t end there. The original Hammond organs derived their unique, rich sound from a set of toothed wheels turned by a motor. Each wheel created a fluctuating inductance in a sensor like the record head from a cassette player.
It’s easy to think of other possibilities for motor-driven stomp boxes. Going back to tremolo: imagine a transparent disc masked with black paint, except for a circular stripe that tapers at each end. While the disc rotates, if you shine a bright LED through the transparent stripe toward a light-dependent resistor, you would have the basis for a tremolo device. You could even create never-before-heard tremolo effects by swapping discs with different stripe patterns. Figures 5-57 and 5-58 show the kind of thing I have in mind. For a real fabrication challenge, how about an automatic disc changer?
Figure 5-57. Although electromechanical audio devices are obsolete now, some unexplored possibilities still exist. This design could create various tremolo effects, if anyone had the patience to build it.
Figure 5-58. Different stripe patterns could be used in conjunction with the imaginary electromechanical device in Figure 5-57 to create various tremolo effects.
Today’s guitarists can choose from a smorgasbord of effects, all of which can be home-built using plans available online. For reference, try these special-interest books:
Analog Man’s Guide to Vintage Effects by Tom Hughes (For Musicians Only Publishing, 2004). This is a guide to every vintage stomp box and pedal you can imagine.
How to Modify Effect Pedals for Guitar and Bass by Brian Wampler (Custom Books Publishing, 2007). This is an extremely detailed guide for beginners with little or no prior knowledge. Currently it is available only by download, from sites such as http://www.openlibrary.org, but you may be able to find the previous printed edition from secondhand sellers, if you search for the title and the author.
Of course, you can always take a shortcut by laying down a couple hundred dollars for an off-the-shelf item such as a Boss ME-20, which uses digital processing to emulate distortion, metal, fuzz, chorus, phaser, flanger, tremolo, delay, reverb, and several more, all in one convenient multi-pedal package. Purists, of course, will claim that it “doesn’t sound the same,” but maybe that’s not the point. Some of us simply can’t get no satisfaction until we build our own stomp box and then tweak it, in search of a sound that doesn’t come off-the-shelf and is wholly our own.
Experiment 31: One Radio, No Solder,
No Power
Time now to go back one more time to inductance and capacitance, and demonstrate an application which also makes use of the way that waveforms can be added to each other. I want to show you how a simple circuit with no power supply at all can receive AM radio signals and make them audible. This is often known as a crystal radio, because the circuit includes a germanium diode, which has a crystal inside it. The idea dates back to the dawn of radio, but if you’ve never tried it, you’ve missed an experience that is truly magical.
You will need:
Rigid cylindrical object, such as a vitamin bottle. Quantity: 1.
22-gauge hookup wire, solid-core. Quantity: 60 feet.
16-gauge wire, stranded. Quantity: 100 feet.
Polypropylene rope (“poly rope”) or nylon rope. Quantity: 10 feet.
Germanium diode. Quantity: 1.
High-impedance headphone. Quantity: 1.
The diode and headphone can be ordered from http://www.scitoyscatalog.com. You cannot use a modern headphone of the type you wear with an MP3 player.
Some of these items are shown in Figure 5-59.
Figure 5-59. Just add wire and a coil, and this is all you need to receive AM radio signals. The black disc becomes the tuning dial, after it is screwed onto the variable capacitor (right). This is actually an optional extra. A germanium diode (center) rectifies the radio signal. The high-impedance earphone (top) creates a barely audible sound.
First, you need to make a coil. It should be about 3 inches in diameter, and you can wind it around any empty glass or plastic container of that size, so long as it’s rigid. A soda bottle or water bottle isn’t suitable, because the cumulative squeezing force of the turns of wire can deform the bottle so that it isn’t circular anymore.
I chose a vitamin bottle that just happened to be exactly the right size. To remove the label, I softened its adhesive with a heat gun (lightly, to avoid melting the bottle) and then just peeled it off. The adhesive left a residue, which I removed with Xylol (also known as Xylene). This is a handy solvent to have around, as it can remove “permanent” marker stains as well as sticky residues, but you should always use latex or nitrile gloves to avoid getting it on your skin, and minimize your exposure the fumes. Because Xylol will dissolve some plastics, clearly it’s not good for your lungs.
After you prepare a clean, rigid bottle, drill two pairs of holes in it, as shown in Figure 5-60. You’ll use them to anchor the ends of the coil.
Figure 5-60. A large, 3-inch diameter empty vitamin bottle makes an ideal core for a crystal radio coil. The drilled holes will anchor wire wrapped around the bottle.
Now you need about 60 feet of 22-gauge solid-core wire. If you use magnet wire, its thin insulation will allow the turns of the coil to be more closely spaced, and the coil may be slightly more efficient. But everyday vinyl-insulated wire will do the job, and is much easier to work with.
Begin by stripping the first 6 inches of insulation from the end of the wire. Now measure 50 inches along the insulated remainder and apply your wire strippers at that point, just enough to cut the insulation without cutting the wire. Use your two thumb nails to pull the insulation apart, revealing about a half-inch of bare wire, as shown in Figure 5-61. Bend it at the center point and twist it into a loop, as shown in Figure 5-62.
Figure 5-61. Wire strippers expose the solid conductor at intervals along a 22-gauge wire.
You just made a “tap,” meaning a point where you will be able to tap into the coil after you wind it. You’ll need another 11 of these taps, all of them spaced 50 inches apart. (If the diameter of the bottle that you’ll be using as the core of your coil is not 3 inches, multiply its diameter by 16 to get the approximate desired spacing of taps.)
After you have made 12 taps, cut the wire and strip 6 inches off that end. Now bend the end into a U shape about a half-inch in diameter, so that you can hook it through the pair of holes that you drilled at one end of the bottle. Pull the wire through, then loop it around again to make a secure anchor point.
Now wind the rest of the wire around the bottle, pulling it tightly so that the coils stay close together. When you get to the end of the wire, thread it through the remaining pair of holes to anchor it as shown in Figure 5-63. The completed coil is shown in Figure 5-64.
Figure 5-62. Each exposed section of wire is twisted
into a loop using sharp-nosed pliers.
Figure 5-63. The stripped end of the wire is secured through the holes drilled in the bottle.
Figure 5-64. The completed coil, wrapped tightly around the bottle.
Your next step is to set up an antenna. If you live in a house with a yard outside, this is easy: just open a window, toss out a reel of 16-gauge wire while holding the free end, then go outside and string up your antenna by using polypropylene rope (“poly rope”) or nylon rope, available from any hardware store, to hang the wire from any available trees, gutters, or poles. The total length of the wire should be about 100 feet. Where it comes in through the window, suspend it on another length of poly rope. The idea is to keep your antenna wire as far away from the ground or from any grounded objects as possible.
High Voltage!
The world around us is full of electricity. Normally we’re unaware of it, but a thunderstorm is a sudden reminder that there’s a huge electrical potential between the ground below and the clouds above.
If you put up an outdoor antenna, never use it if there is any chance of a lightning strike. This can be extremely dangerous. Disconnect the indoor end of your antenna, drag it outside, and push the end of the wire into the ground to make it safe.
If you live in an apartment where you don’t have access to a yard outside, you can try stringing your antenna around the room, hanging the wire from more pieces of poly rope. The antenna should still be about 100 feet long, but obviously it won’t be in a straight line.