A high quality mains powered graphic equaliser which can be built easily in an evening using a few common handtools.

★ Ten frequency bands
★ 12dB boost and cut
★ Mains powered
★ Bypass switch
★ Ready-built PCB assembly with detailed instructions

Many of our readers will already appreciate the great value of an equalisation device either in the home recording studio, on stage or even on the hi-fi. An equaliser allows the frequency response of a system to be modified so that an uneven or unnatural response can be 'ironed out', or indeed exaggerated for special effects. A graphic equaliser utilises several discrete frequency bands whose effect on the signal can be independently controlled by a 'graphic' array of slider controls. This graphic equaliser is also especially useful for eliminating any unevenness in room acoustics which can cause frequency response anomalies and, on stage, premature feedback.

The Amdek GEK-100 Graphic Equaliser Kit provides ten frequency bands spaced at one octave intervals to cover the range 31Hz to 16kHz. Each band is controlled by a slider pot such that the gain of each band can be set from -12 to +12dB. A bypass switch has been thoughtfully provided to allow A/B signal comparison, and to allow the response to be instantly 'normalised'. Another nice touch is the use of centre detent sliders, enabling the zero position to be quickly and accurately found. Being mains powered eliminates the worry and inconvenience of dud batteries.

The Kit

The only tools required in addition to the hexagonal wrench supplied with the kit are: a fine tipped 15-30W soldering iron, side cutters, wire strippers, pliers, screwdrivers, and a crimping tool for the mains terminations.

Firstly, lay out all the parts so that they can be identified and checked off easily. The step by step construction commences with preparing and soldering lengths of insulated wire to the ready-built PCB assembly as described in steps 1-4. After mounting the PCB assembly and the slider controls on the sub-chassis (steps 5-7), the LED is fitted, and all parts are wired up as shown in steps 8-10. The slider bypass switch, mains switch and mains transformer are now fixed to the sub-chassis and wired in accordance with steps 11-16.

The rubber slider pot mask and the slider switch mask are now fitted, and the sub-chassis mounted in the upper part of the case as in steps 17-19. Steps 20 and 21 describe fitting and wiring the jack sockets. After preparing and clamping the mains lead in position, it is terminated firstly to the two transformer primary leads, the joints of which are protected by crimped caps; and secondly to the subchassis earth by means of a solder lug, as shown in steps 23 and 24. Steps 25 and 26 see the completion of the project by fitting the case bottom, the rubber feet and the control knobs. The 2-pin plug moulded onto the mains lead could be changed for a 3-pin type if required.

Circuit

The circuit is an inductorless design, using op-amps IC1-IC5 as gyrators to give the equivalent of series LCR band-reject circuits, one for each frequency band. These can be either in the inverting input circuit of IC6 for boosting, or in the non-inverting input circuit for attenuating, according to the setting of the sliders. The values of the capacitors determine the centre frequencies of each band. The 16kHz band uses a CR series circuit, which gives boost or cut to all frequencies above 16kHz. The inputs and outputs are buffered by IC5 and IC6 for high input and low output impedances respectively. The power supply is quite conventional, using resistors to split a single rail unregulated rectified voltage.

Operation

Since a fully tested PCB assembly is supplied, there is no reason why the unit should not work first time. If you have any problems, ring the 'Amdek Hot Line' on (Contact Details). The best way to get to know the graphic equaliser is by experimentation, using the bypass switch to compare the equalised signal with the original. Unless you have access to a spectrum analyser, room acoustics equalisation will be largely by trial and error. The unit will accept signals directly from instruments such as guitars, which will benefit greatly from the wide range of tonal variation available.

Modifications

Amdek suggest a couple of possible modifications, both of which involve changing components on the PCB assembly. The centre frequencies of the bands can be altered by using the equations given in the instruction booklet. The range of the controls can also be lowered to +/-6dB for finer control by changing R29 and R30 from 2k7 to 680 ohms. It should be possible to increase the range of the controls slightly by increasing the values of R29 and R30, although stability problems may be encountered if you make them too high.

Since the on-off switch is on the secondary side of the transformer, it is advisable to unplug the mains when the unit is not in use.

E&MM's special offer price for the Amdek Graphic Equaliser Kit is £67.50 including VAT and P&P. Please order as GEK-100 and send to : E&MM, (Contact Details).