Operating Manual -
9. DESIGN THEORY
Graphic Equalizers: The Basics
While most graphic equalizers look very much the same, there are several important differences in the circuitry used to implement various designs.
Perhaps the major differences are in the filters. Some equalizers use a filter made of a capacitor, an in- ductor, and a resistor, or “RLC” filter. The advantage here is simplicity, but the real disadvantage is the induc- tor itself. An inductor is a coil of wire with a core of some sort. Inductors are susceptible to hum fields and they are large and expensive.
Other equalizers use the same basic approach, but replace the inductor with a “simulated inductor”, which is actually a circuit comprised of an amplifier, a capaci- tor, and a couple of resistors. This adds parts but is less expensive than a real inductor. The problem with this approach is that simulation is less than ideal; it produces an inductor with high resistive loss resulting in poor curve shape when used in a filter.
Another problem with “RLC” designs is that large capacitors must be used for the lower frequency filters, limiting the choice to large, expensive
The other filter approach is a true bandpass fil- ter. This can be made with no inductors and more practi- cal sized capacitors; the “Q” is easily set and remains constant, and the parts count is reasonable. there are sev- eral types of bandpass filters suitable for this job. Ashly uses a “Q” enhanced
In designing a graphic equalizer, a selection of filter sharpness must be made. More sharpness (higher
Q)produces less filter overlap and tighter control over an individual band, but also causes ripple in the frequency response when many filters are boost or cut together to produce a flat response. We feel that the graphic equalizer’s primary use is for “voicing” and tone control, and have set our filter sharpness to produce a maximum of 1dB ripple.
The summing system in a graphic equalizer is also important. Since there are a number of filters which combine to produce the overall response, it is important that the filters not interact (they WILL overlap, but the response of one filter should not modify the response of another). Ashly uses an “interleaved” summing system where every other filter uses the same summing amplifier so that adjacent filters never share the same drive and feedback signals. This allows the filters to maintain their natural response.
Compressor/Limiters: The Need For Gain Control
The human ear excels in its ability to detect an extremely wide range of loudness levels, from the quiet- est whisper to roar of a jumbo jet. When we attempt to reproduce this dynamic range, by means of amplifiers, tape recorders, CD players, or radio transmitters, we run into one of the fundamental limitations of these electronic media: limited dynamic range. Amplifier dynamic range is quite good, and is adequate for most musical program material. However, some types of audio equipment, such as cassette tape recorders, have a very narrow useful dy- namic range.
What is it that compromises the dynamic range of this equipment? The useful operating region of a piece of audio equipment is squeezed in between noise and dis- tortion. As program level decreases, it approaches what is known as the “noise floor”, and if the volume of the program material goes lower still, it is engulfed by the noise. The noise floor, or minimum constant noise level, will consist of hiss, hum, transistor noise, tape hiss, buzz and whatever noises are inherent in the medium. When the program level is considerably higher than the noise floor, our hearing masks the noise, and it is not a prob- lem. However, when listening to very quiet sections of a program for example, a pause between movements of a string quartet the noise can become very bothersome.
At the other end of the loudness spectrum, the limitation on dynamic range is usually distortion, either in the form of amplifier overload, tape saturation, or A to D clipping. In most transistorized equipment, the transi- tion from clean, undistorted operation to severe distor- tion is very abrupt. Therefore, it is common practice to operate a piece of equipment at a level that is somewhat below the distortion point, leaving a margin of safety for unexpected, transient volume peaks in the music. This safety margin is known as headroom, and may range from 10 to 25 dB. Lowering our standard operating level to leave ourselves some headroom helps prevent distortion, but at the same time it moves our average program level closer to the noise floor, thereby compromising signal-
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