# Analog Equaliser

Posted on Feb 7, 2014

The time constant of R1C1 determines the low-cutoff frequency while the time constant of R2C2 determines the high cutoff frequency. The pass-band gain, Avpass=R2/R1. For a low-cut filter, that is a high-pass filter, Za must be capacitors and Zb resistors. By substituting Za=1/sC and Zb=R into above equation, the following equation is obtained: For

a high-cut filter, that is a low-pass filter, Za must be resistors and Zb capacitors. By substituting Za=R and Zb=1/sC into above equation, the following equation is obtained: The best choice for the Q-factor is 0. 707. This results in the sharpest curve, without any overshoot. If this Q-factor is selected, the value of K must be 1. 586. Note that this is also the pass-band gain. The signal should therefore be attenuated if constant signal level is desired. If a slope steeper than 12db/oct is desired, a higher order filter can be used. It is best to use the Butterworth response for audio, because of its flat response. The simplest type of equaliser is the low and high shelving equalisers. This is usually called Bass and Treble on Hi-fi sets. Figure 3 shows the basic topology for such an equaliser. The potentiometer, Rp, sets the boost cut ratio. The impedance of Z(s) must be high at the frequencies to boost/cut and low at other frequencies, effectively shorting out Rp. If a capacitor is substituted for Z(s), a low shelving equaliser is realised, while an inductor is needed for a high shelving equaliser. Figure 4 shows a circuit for a combined Lo and Hi shelving equaliser as used in conventional hi-fi sets and mixing consoles. As can be seen, the inductor in parallel with Rtreble is omitted, C2 is instead inserted. At the high frequencies, Rbass is shorted out by C1. The higher the frequency becomes, the lower C2 s...

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