Operating Manual - GQX 3102, GQX 3101, and GQX 1502 Graphic Equalizer
7.DESIGN THEORY
While most graphic equalizers look very much the
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same, there are several important differences in the cir- cuitry 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.
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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 all these “RLC” designs is that large capacitors must be used for the lower frequency filters, limiting the choice to large, expensive
Figure 7.1: Passive RLC Filter Design
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Figure 7.2: Simulated Inductor Filter Design
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The other filter approach is a true bandpass filter. This can be made with no inductors and more practical sized capacitors; the “Q” is easily set and remains con- stant, and the parts count is reasonable. there are several types of bandpass filters suitable for this job. Ashly uses
a“Q” enhanced
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Figure 7.3: Wein-Bridge Filter Design
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