(This article originally appeared in the May 1977 issue of Contemporary Keyboard.)
In this column I'll talk about power amplifier and loudspeaker portions of synthesizer sound systems. Next column will deal with sound system front ends: tone controls, equalizers, mixers, and compressors.
Of all the commonly used electronic musical instruments, the synthesizer is capable of producing the widest variety of sound material. As a result of this versatility, synthesizers place severe demands on amplifier-speaker systems. Even the simplest synthesizers produce pitches from 31Hz (three octaves below middle C) to 4kHz (four octaves above middle C), with audible overtones that go beyond 15kHz. Synthesizer tones may be highly transient, lasting only a few hundredths of a second, or may sustain throughout an entire piece of music. A synthesizer timbre may be so thick and complex that the amplifier-speaker system has little effect on how it sounds, or so simple and static that the slightest speaker buzz stands out like a sore thumb. Put these extremes of sound character together with the range of loudness levels commonly required in live performance, and you have specified a sound system with "Rolls-Royce" frequency-response characteristics and "Mack Truck" power-handling capability!
First, let's talk about acoustic power output. What we want out of a speaker is sound (acoustic power). As converters of electrical power, loudspeakers are generally inefficient. Most of the high-price juice going into a speaker cabinet winds up as heat instead of sound. Good, wide-range speaker systems typically have an efficiency of from 1/2 to 5%. That is, 100 watts of amplifier power may yield 1/2 to 5 acoustic watts of sound power. How loud is one acoustic watt? Well, a premium home music bookshelf speaker will generally burn out before it produces one acoustic watt continuously. A typical full-size professional studio monitor will produce three acoustic watts at rated power. And a super group’s stadium sound reinforcement rig may produce a total of 100 to 1,000 acoustic watts, wide open. In a typical club environment, a speaker emitting one acoustic watt will produce a sound pressure of around 110dB onstage. Ten acoustic watts will produce an additional 10dB, which, as the textbooks say, is loud enough to hurt.
How about frequency response? The lowest E on a bass guitar is 42Hz, the C below that is 32Hz, and low A on an acoustic piano is 27.5Hz. Below 60Hz or so, every Hertz of response is a significant addition in speaker system size, weight, and price. One person can move a moderately efficient speaker cabinet with a low-frequency cutoff of 45Hz, but it will take two people to handle a 30Hz cabinet of the same moderate efficiency. To my ears, response to 45Hz is necessary for a good "commercial" sound, while a 30 or 32Hz low-frequency cutoff adds a fullness that is sure nice to have. At the high end, you can hear the difference between 12kHz and 15kHz. A 12kHz high-frequency cutoff (and flat response below) gives a smooth, sparkly quality to bright timbres; extending the response to 15kHz adds a touch of brilliance and tinkle that can be significant in recording, or in small clubs.
Distortion becomes important when more than one pitch is played through the sound system. If you're feeding two or more keyboard instruments through the same speakers, you will have to be concerned about distortion. Unfortunately, speaker distortion characteristics are not given on spec sheets. You'll have to listen for yourself. It's generally audible when loud, low notes are played. To test for distortion, play a loud bass note along with a midrange chord. Speaker distortion will produce a "muddiness" that arises from sum and difference frequencies generated by the distortion component. In general (but not always), high-efficiency speaker systems and large speakers distort less than low efficiency speaker systems and small speakers. The most distortion-free sound system is biamped—separate power amplifiers for low-frequency and midrange-high frequency speakers.
How do conventional musical instrument and public address amplifier-speaker systems meet our requirements for synthesizers? First, let's consider guitar amps. Without a doubt, a typical good 100-watt guitar amp has the efficiency and stamina to put out a few acoustic watts. However, its frequency response and distortion characteristics are optimized for guitar: no significant response below 100Hz, a broad spectral "hole" around 500Hz, and sharply rising response above 1kHz, with some "warm" (low order) distortion. Guitar amps, therefore, are generally not suited for synthesizer sound reproduction. Similarly, most P.A. systems are designed to make the human voice sound good. The P.A. frequency response (determined largely by the speakers) generally has a broad peak in the "presence" region of the spectrum (2-3kHz) and decidedly weak bass. Professional studio monitors, on the other hand, have more-than-adequate frequency response distortion characteristics, but often lack the stamina to produce loud, sustained, steady tones without over-heating. This is doubly true for some music speaker systems (although certain compact multiple-speaker systems such as the Bose 800 are attractive and convenient options for small-to-medium size environments).
Keyboard amplifiers come closest to meeting our power, frequency response, and distortion requirements. High-frequency response is sometimes a problem. Many keyboard amplifier-speaker systems are designed primarily for tone-wheel organs, electric pianos, and similar instruments with little harmonic content. Such systems rarely have adequate high-frequency response for synthesizers. However a keyboard amplifier-speaker system with good speaker response to 12kHz or so is likely to meet all of our requirements for synthesizer sound production. If the speaker system itself is efficient, a 50- to 100-watt power amplifier will produce 2-5 acoustic watts, which is plenty for rehearsal or club work, while a 200- to 400-watt power amplifier will produce upwards of 20-25 acoustic watts, which is adequate for 95% of indoor gigs. When selecting an amplifier-speaker system for your synthesizer, it is a good idea to pick a few speaker systems that are efficient (that is, they sound just plain loud when fed with the output of a modest power amplifier) and then select the speaker from that group that sounds the smoothest and fattest. Use spec sheets as a guide, but rely primarily on your ear.
For more articles by Bob Moog, please visit http://www.keyboardmag.com/analog/1318/moog-monday-hub-page/57761