Combined HF-Boost and HF-Cut Equaliser

400  Equalisation

The component values shown in Figure 14.11 give a centre frequency of 1.26 kHz, which can be simply altered by scaling the values of C1 and C2 while keeping them in the same ratio. This apparently random frequency is a consequence of the fact that both potentiometers and capacitors

come in relatively few values. The maximum Q is 0.79, though this is only obtained at maximum boost or cut.At intermediate settings the curves are flatter and the Q considerably lower; with a boost of +8 dB the Q is 0.29. The boost/cut limits are ±15 dB, though this is hopefully a much greater range than will be required for equalisation in practice; the range can be reduced by increasing R1, R2.Note that

R4 and R5 are needed to maintain negative feedback for unity gain at DC and to keep the stage biased properly. They must be high in value compared with the impedances in the rest of the circuit.

It is not possible to obtain high values of Q with this configuration, and that is probably its major drawback. The capacitor ratio in Figure 14.11 gives the maximum possible Q. This equaliser is therefore mainly useful for dealing with large-scale trends in the frequency response.

As with the previous equalisers, the shunt-feedback configuration used here means there is no common-mode voltage on the opamp inputs, but with that comes an inconvenient phase inversion which must be taken into account in the system design.

Adjustable Peak/Dip Equalisers: Variable Centre

Frequency and Low Q

The fixed-frequency peak/dip equaliser we have just looked at gives excellent control over the amount of boost or cut applied, but the centre frequency can only be altered by changing the capacitors. In the development phase of crossover design it is extremely useful to be able to temporarily include an equaliser that also has continuously variable control of its centre frequency as well as the amount of peak or dip. When optimisation of the crossover is complete, it is replaced in the final design by a fixed equaliser like that in the previous section.

The circuit shown in Figure 14.13 also uses mixing console technology, where it is usually called a “sweep-middle EQ”. It is based on a modified Wien-bridge network of the sort sometimes used in oscillators; this acts as a low-Q bandpass filter, so that only a selected band of frequencies reach the non-inverting input of A1. When RV1 is in the boost position, the input signal passes through the Wien network and adds to the output, giving a peaking response. When RV1 is in the cut position, the signal through the Wien network constitutes extra negative feedback, and the gain is reduced, causing a dip in the response.

The variable load that the Wien network puts on the cut/boost pot RV1 and the variable source impedance from its wiper cause a small amount of interaction between boost/cut and centre frequency settings. This is not likely to cause any significant problem, but if necessary it could be eliminated by putting a unity-gain buffer stage between the RV1 wiper and the Wien bandpass network. There will also be minor inaccuracies due to imperfect matching of the two sections of the frequency control.

The combination of 100k pot sections and a 6k8 end-stop resistor gives a theoretical centre frequency range of 15.7 to 1, which is about as much as can be usefully employed when using reverse-log Law C pots. Greater ranges will give excessive cramping of the frequency calibrations at the high-frequency end of the scale. These calibrations should only be used as a guide; it is more accurate to measure

Equalisation  401

Figure 14.13: A peak/dip equaliser with variable centre frequency, intended for crossover development work.

Figure 14.14: Frequency response of the variable-frequency peak/dip equaliser, showing extremes of frequency setting.

the response of the circuit after you have completed the optimisation of the crossover system. The measured frequency responses at the control limits are shown in Figure 14.14. The frequency range is from 150 Hz to 2.3 kHz; the ratio is slightly adrift from theory due to component tolerances. To obtain different frequencies, scale C1 and C2, keeping the ratio between them the same. More information on this kind of equaliser can be found in Small Signal Audio Design. [5]