Ashly GQX-3102 Betriebshandbuch - Seite 8

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Operating Manual - GQX 3102, GQX 3101, and GQX 1502 Graphic Equalizer

DESIGN THEORY

While most graphic equalizers look very much the

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.

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 non-polar

types or electrolytic capacitors with poor audio perfor-

mance. Also, when this filter type is combined with a

potentiometer to adjust the equalization, the resistance of

this pot affects the "Q" of the filter so that a little equal-

ization produces a much broader curve than a lot of equal-

ization.

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 Wein-bridge filter. Because it is a "sym-

metrical" design using matched tuning components, the

"Q" is easily set and is very stable.

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Figure 7.1: Passive RLC Filter Design

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Figure 7.2: Simulated Inductor Filter Design

Figure 7.3: Wein-Bridge Filter Design

OUT