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Graphic Equaliser

Graphic Equaliser

A problem I frequently encounter when copying video tapes is the deterioration in the sound quality. The five band graphic equalizer described here was designed to connect between two video recorders, so that the frequency response can be corrected somewhat.
Its use is by no means limited to video recording however, it is a general purpose design that will prove useful for many audio applications.

The five controls each have a range of +/- 10dB at centre frequencies of 100Hz, 300Hz, 1KHz, 3KHz and 10KHz. The 3dB points on each band are at half and twice the centre frequencies. Thus, the 3dB points on the 100Hz control are at 50Hz and 200Hz. With all controls at maximum the unit has a total gain of 15dB. The unit will accept an input of up to about 1V RMS (3V pk-pk) before distortion occurs with all controls at maximum.

I do not possess suitable test equipment to measure noise and distortion, although none was apparent on the oscilloscope trace. I would describe the unit as suitable for good quality stereo equipment, but not true hi-fi.

Although the design is mono, a stereo version could be built using two PCB’s and stereo pots. More details on this are given later.

The Works

The complete circuit diagram is shown in figure #. This basic circuit principle has been used in several graphic equalizer designs, so I am making no great claims about its originality!

The input is buffered by the first section of IC1, which has unity gain and a consistent output impedance. If any overall gain or attenuation is required, it may be achieved by altering the values of R1 to R4.

To make the explanation of the second stage clearer, assume that all five frequency selective sections have disappeared, as well as four of the control pots. The wiper of the remaining pot is connected to ground via a 1K0 resistor.

If the pot is in the upper position (fully clockwise), the 1K0 resistor appears between the inverting input of the op-amp and ground, giving the stage a gain of ten. If the pot track resistance is 10K (five 50K pots in parallel), the signal at the non-inverting input is halved, giving a total gain of five.

With the pot in the lower position (anti-clockwise), the input to the non-inverting input is reduced to a tenth, and the gain of the op-amp circuit is two, giving a total gain of a fifth.

With the pot in the centre the gain of the whole stage is unity, since the attenuation of the input signal is cancelled by the gain of the op-amp. If our imaginary 1K0 resistor is replaced with a tuned circuit, the effects described above will only occur around its centre frequency. In this circuit we have five tuned circuits giving the five bands.

Traditionally the tuned circuits would consist of a capacitor and inductor in series. Due to the lack of availability of suitable inductors, modern designs use a gyrator circuit to simulate an inductor. This uses an op-amp to reverse the phase relationship of a capacitor, to make it appear like an inductor.

Taking the first stage, C4 is the real capacitor and the op-amp and remaining components form the gyrator. The R7 controls the reactance of our “inductor”, and therefore the Q of the tuned circuit. In this case we do not want a particularly sharp response so the Q is fairly low. R7, R8 and C3 all affect the “inductance”, and I have yet to find the correct formula for calculating this!

The final output of the circuit is buffered by a unity gain op-amp stage. SW1 selects whether the equalizer is in the audio path.

The circuit requires a supply of +/-12 to 15V, at less than 10mA. This does not need to be regulated but must be smooth and have minimal ripple. The output of a 9-0-9 transformer is full wave rectified and smoothed giving approximately +/-13V across the 220uF capacitors. About a volt is dropped by the 100R decoupling resistors, leaving around 12V to power the circuit.

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