A single channel, modular equalizer with variable centre frequency. Design by Keith Simon.

A parametric equalizer has a wide range of uses. However, the design presented here should appeal to musicians that would like to 'individualise' the sound of their instruments and tapes.

The equalizer features three controls; the first sets the frequency at which equalization is performed (infinitely variable between about 150Hz and 1600Hz), while the second determines the amount of boost (or cut) the selected frequency band will receive. The last control governs the range of frequencies on either side of the centre frequency on which the unit will have an effect. This last control is referred to as the Q pot (the effect of varying the Q is shown in Fig 1).

Circuit Outline

The circuit can be split into three distinct sections — two of which are very similar. In order that the circuit operation will not alter if it is used with different items of equipment, it is necessary to 'isolate' the filter from the effects of any variations in the input impedance 'seen' by the equaliser, by providing an input buffer amplifier. For similar reasons the output of the equaliser is buffered by a similar amplifier.

Buffers

Both buffer amplifiers are formed by low noise amplifiers. Fig. 2 shows just how straightforward the stages are. The amplifiers are configured as unity gain, inverting types and their characteristics are completely governed by just two resistors.

In operation, a positive signal at the input will cause an exact copy of the signal to appear at the amplifiers output but, as the amplifier is inverting, it will be in the negative direction. A small part of this negative output signal will find its way back to the input via Rb of Fig. 2. This will tend to oppose the input signal that started the chain of events. Rb is known as the feedback resistor and the higher its value, the less the signal fed back will be and the higher the gain of the stage will become. A lower Rb will, in a similar fashion, decrease the amplifier's gain.

In the case of the parametric equaliser, resistors have an equal value and the gain of the amplifier is one. If it seems pointless to have an amplifier with a gain of unity, remember the purpose of this stage of the equaliser is, in fact, not to amplify, but to provide isolation between the unit and any external equipment it is to be used with.

Filters

The heart of the equaliser is formed by the four operational amplifiers in the quad op-amp package. The section around IC2a is a familiar inverting op-amp stage, this time with a gain of about two.

Operation of the other three amplifiers is quite complex and would require a page of formulas and a BSc in Applied Maths to get through. Very basically though, two of the stages, IC2c and IC2d can be recognised as being similar to the standard inverting op-amp stage but with a capacitor replacing the feedback resistor. Since capacitors change their 'resistance' as frequency changes, it's a fair assumption that the gain of the amp would depend on the input frequency. The ganged frequency pot replaces Ra and determines the amount of amplification.

IC2b is again a straightforward amplifier, but both its inverting and non-inverting inputs are used. In this case the amplifier acts as a summing stage. The Q control varies the characteristic of IC2b to provide the variable bandwidth function.

The op-amps require both positive and negative supplies and these are best provided by two 9V batteries.

Construction

The building of any project is greatly simplified if a PCB is used. Following the pattern shown, it is only necessary to place all the components in the correct position to build the unit.

Take care that the ICs and electronic capacitors are fitted the right way round — it is a good idea to use sockets for the ICs in case of any problems, as it is very difficult to remove an 1C once it's soldered into place.

When the board is complete it can be fitted into a case along with the batteries and suitable input/output sockets.

In Use

To test the unit, connect up a suitable input signal — music with a wide frequency content is OK — and, with the Q control set to minimum and the other pots set to their centre position turn the unit on. Gradually turn the level control towards the boost position. It should be possible to note the effect on the input signal.