� 2004  H. Davis


Have you ever hooked up your guitar or bass directly to a home stereo system? Pretty disappointing compared to the sound of your favorite amp, right? Many theory-oriented design engineers not experienced with music equipment think it should be the other way around. Unless overdriven, the distortion of a high fidelity amp is so low it is sometimes hard to measure, and with state-of-the-art speakers it can take a live band right off a CD and put it in your living room. So why can't it do justice to an actual instrument'?


To an engineer in high fidelity, less distortion means better sound. The goal is to reproduce the incoming signal with perfect accuracy; what goes in electrically is what comes out acoustically, no more and no less. This is fine for music in its final recorded form, when the musician's work is done. What the engineer may not realize is something musicians instinctively know - that the guitar or bass alone is not the whole instrument; the effects, amp, speakers, and acoustic environment are all a part of it, and without them, little would he heard. The resonance and distortion of the high power but lo-fi guitar amp, the very antithesis of high fidelity, are effectively a part of the instrument itself.


Good distortion is any signal alteration that adds to musical quality or produces a desired effect. Such subjective evaluations as "good" and "bad" are not easily defined in the technical terms an engineer uses, and as with acoustic instruments, the design of amps and effect pedals is as much of an art as a science. But a familiarity with a few technical terms will help you get the most out of your equipment, so let's examine some basics.


A pure tone, as produced by a sine-wave oscillator, has little more sonic character than silence. It is one frequency alone, a fundamental pitch without overtones, or as engineers call them, harmonics. Harmonics are multiples of the fundamental frequency; the even harmonics of 100 Hz (Hz is short for Hertz, the unit of frequency measurement formerly called cycles per second) are 200 Hz, 400 Hz, 600 Hz, and so on; the odd harmonics are 300 Hz, 500 Hz, 700Hz, etc. The presence of harmonics in varying proportions gives an instrument the unique timbre that distinguishes it from all others. If recorded music is to be faithfully reproduced the fundamentals and harmonics must be reproduced in proper balance, with nothing added or taken away. As distortion generates harmonics not present in the original signal, it is intolerable in high-fidelity reproduction.


The signals from guitar pickups are not very rich in high-order harmonics, with practically nothing coming through above 4 KHz (4000 Hz). Normal human hearing extends over two octaves above this to 20 KHz, and some acoustic instruments, particularly percussion, have substantial output in this region. On the low end, the open E string of a normally tuned guitar sounds at about 80 Hz, two full octaves above the 20 Hz low frequency hearing limit. The standard 4-string bass has a range one octave below the guitar, with its low E at about 40 Hz. The parts of your instrument at the other end of the cable - the effects pedals, amp, and its speakers - have the tasks of filling in the sonic gaps and increasing the guitar's versatility by modifying and adding to its inherent harmonic structure. They endow the electric guitar with its unique musical expressiveness and a variety of sounds no other instrument can match.


Distortion is the result of nonlinearity in signal amplification. What does this mean? If a linear (high fidelity) amplifier with a gain of 10 is driven with a 0.1 volt signal, the amp will deliver exactly 1.0 volt at its output. Should the input go to 0.2 volt, the output becomes 2.0 volts; 0.3 volts input results in 3.0 volts output, and so on. This straight-line relationship continues to the maximum output level the amp can deliver, which is determined by its power supply voltage and circuitry limitations. At this point the amplifier abruptly becomes non-linear and clips (chops off the top and bottom of the signal waveform), as it is not physically capable of delivering more output voltage.


In a nonlinear amplifier, the gain (ratio between output and input voltages) does not stay constant, but changes as the signal level increases. The manner in which it does so determines the nature of the distortion produced. With proper design, low distortion can be achieved with many amplification devices - tubes, bipolar transistors, FET's (field-effect transistors), and op-amps. When overdriven though, the nature of the distortion produced by each of these differs, and when distortion is desired for musical effect the device employed makes a difference.


As with bypass, there are ambiguities in the terminology of distortion. Overdrive, fuzz, grunge, crunch, tube sound, and other descriptive terms are all basically the same thing � harmonic distortion: the generation of harmonics through nonlinear amplification. The various forms have a position or positions on a subjective spectrum of distortion, which I like to classify as going from soft to hard.


Through a practically distortion-free audiophile sound system, a direct input guitar sounds clean and thin. Multiple notes played simultaneously retain their individual character so that chords are well defined. Overdrive the amp, or turn its gain up far enough, and you get clipping - a harsh fuzz-like distortion, but you'll only get it for as long as the speakers and amp can take it. As they are not made for this, they can easily be damaged by such abuse.


We can thus see that a harsh or "hard" distortion, as from the raspy-edged fuzzbox, is a clipping effect. Filtering the signal can moderate the harshness, but it still retains some hard quality. Fuzzboxes using op-amp and diode circuitry tend to sound hard and grungy; some utilizing transistors have a sweeter sound due to the different distortion characteristics of these devices. The dynamic range compression and long sustain of the fuzzbox are due to its very high gain, because a string can vibrate for quite a while before the pickup's output drops so low that the clipping ceases. The harmonics generated are mostly odd and high-order - 3rd, 5th, 7th, 9th, and so on, harsh and biting. A tweeter array can really bring out their edge, but if loud, ear protection is a must!


How much distortion is too much, and what is bad distortion? Home stereo amps often have less than 0.1% distortion, and their preamps less still, but ten times this is low for even a "clean" guitar amp. Speakers driven at high levels generate more distortion than a clean power amp below clipping does, and while audiophile speakers are designed to keep distortion low, guitar amp speakers generally do not. Mechanical and magnetic nonlinearities in speakers cause harmonic distortion, and when a single cone is driven with both high and low frequencies simultaneously, the lows modulate the highs due to the Doppler effect. The Doppler effect is probably most familiar as the rising of a siren's pitch as a fire truck comes toward you, and the lowering of the pitch as it moves away. While the cone is moving outward towards you, driven by a low-frequency signal, any high frequencies also going through the speaker are increased slightly in pitch. Then as the cone moves back in response to the low frequency, the high pitches drop a bit. The result is an unpleasant effect known as intermodulation distortion.


Intermodulation distortion in large amounts produces the discordant, chime-like effect of the ring modulator. This is due to an interaction between two or more frequencies in which many signal byproducts are generated that have no direct harmonic relationship to the fundamentals. When two notes are played together, frequencies that are the sum and difference of the two are heard, creating beat notes (similar to two strings out of tune with each other), and the characteristic ring modulator sound results. It can sound hard, or muted and bell-like, depending on how it is filtered by tone controls and/or equalizers.


Fortunately, intermodulation distortion is not serious when a speaker handles a single instrument, and unless badly overdriven, a speaker's distortion is of the low-order harmonic type more acceptable in instrument amplification. For really low distortion one can use separate low, midrange, and high-frequency speakers, with the signal either biamped (filtered and amplified separately according to frequency range) or routed through an electrical crossover network to the appropriate speakers - woofer, midrange, and tweeter. Audiophile speaker systems use this design approach.


The fullness and musical character of the sound of an electric guitar or bass depend on the nature and amount of distortion generated, and tube amps are sometimes preferred over solid-state amps for this reason. Tubes, and the massive output transformers used with them, tend to produce the low-order, largely even harmonics of soft distortion. These add a warmth and fullness to single notes, and a soft, mushy distortion to chords. Overdrive a good tube amp hard and the sound gets very muddy, where with a typical solid-state amp it becomes more fuzzlike. Some well-designed solid-state amps overdrive like good tube amps do, and even if not pedals such as the Pigtronix Disnortion and Polysaturator produce "tubier than tubes" overdrive sound without the large size, weight, fragility, cost, and unreliability of tube equipment. This is due to the "soft" nonlinear distortion characteristics of FET or CMOS circuitry, easily as good as tubes when properly designed.


What is this soft distortion? Where hard distortion puts sound on edge like electronic amphetamines, soft distortion blurs it, kind of mushes it, like a few stiff drinks. Soft distortion in an amp begins at levels well below clipping, rounding off the signal peaks rather than razor-slicing them off like hard clipping does. Compression and sustain are somewhat less pronounced than with hard clipping, and the increase in distortion with the rise in drive level is more gradual. As the overdrive level is increased the distortion becomes harder due to the generation of the odd harmonics characteristic of clipping.


Frequency distortion is the engineer's term for unevenness in frequency response, and it is present to a perceptible degree even in most audiophile speakers. Speakers and room acoustics play major roles in response flatness, and correct speaker placement and the use of equalizers can be good remedies. Frequency response is of less concern to the musician; if you want a dull and heavy bass, or a biting treble, you use your tone controls or an equalizer to tailor the response to your taste. Much of the unique character of some amps lies in their uneven frequency response due to speaker cabinet resonance as well as their harmonic distortion characteristics.


Distortion affects not only your basic guitar sound, but also the sound of some effect pedals used with it. In phasing and flanging for example, a complex peak-and-null or comb filter response is generated and swept up and down the frequency spectrum, yielding these unique effects. Harmonic distortion introduced prior to phasing or flanging enhances them greatly by providing a richer harmonic presence for them to operate upon. Try your fuzzbox after your phaser or flanger, then before it, and you'll be amazed at the difference.


Bottom octave deficiency is a form of frequency distortion in which the lowest bass tones disappear or are weakened. Both speakers and amps can cause this. Speakers have a low frequency resonance below which their response drops off rapidly and distortion rises astronomically. In an experiment, I once fed an ordinary 4 x 12 ported speaker cabinet with a moderate power sine wave and observed the acoustic spectrum produced. As the frequency was brought below the system resonance (about 50 Hz), the fundamental became greatly attenuated and the second and some higher harmonics - all distortion products - boomed forth. The uneducated ear would still be duly impressed, as we tend to mentally dub in the missing fundamental, a psychoacoustics phenomenon known as synthetic bass. Strong fundamentals in the 16 Hz to 40 Hz region though are unmistakable when experienced and seismic in effect. Loud and prolonged enough, they can get you dizzy or even empty your stomach!


What can be done to strengthen a weak low end, especially those two octaves between the guitar's range limit and the lowest audible frequency? Subharmonic generating effects create strong musical content in this region. These pedals divide the fundamental frequency by two or four, for a one-octave or two-octave transposition, respectively. The low frequency signal generation here is easier than its acoustic production and projection. Large, extended bass range speakers (subwoofers) in rigid infinite-baffle cabinets (airtight enclosures designed to absorb the energy from the back of the speaker cone) are a must, as are high-power solid-state amps. Like inferior speakers, tube amp output transformers distort the low bass, muddying it and cutting the fundamental. Direct-coupled solid-state amps without such transformers are very clean and have no low frequency limit. With solid-state muscle direct coupled to really good bass speakers, an electric bass, a guitar or keyboard-driven synth, or a guitar with a suboctave-generating effect pedal can have astounding impact!


To the audiophile and many audio engineers, distortion is a dirty word. To the musician, it is part of his artist's palette.