WebEE

Construction
All the components, except the transformer and pots, are mounted on a single sided PCB, 113mm * 51mm. The component overlay is shown in figure #.

There are two wire links that should be fitted first, along with the resistors and diodes. The remaining components may then be fitted. Sockets may be used for the IC's, but this is not really necessary providing they are fitted last and soldered carefully.

The non-polarised capacitors should be reasonable quality types, dipped polyester or mylar types are suitable. Try to avoid the cheap ceramic disk types, for this project. The PCB holes for these components are on a 0.4" pitch, which is suitable for the suggested types.

Two PCBs are required for a stereo unit. Do not fit D1-D4, C12 and C13, on one PCB.

The prototype had rotary pots, and was constructed in a plastic case that matched my other projects. Many constructors may prefer a more orthodox layout, with slider controls and an instrument or desk case.

Whatever case is used, the PCB's should be mounted as close as possible to the control pots. Use short lengths of wire to connect the pot tags directly to the PCB pads. On the PCB, the left pad for each pot is the minimum or anti-clockwise end of the track, the centre pad is the wiper, and the right pad is the maximum or clockwise end.

Connect the input socket to the SK1 pads, and the output socket to SK2. In both cases the centre core of the screened cable goes to the left pin. The IN/OUT switch connects to the SW1 pads, with the wiper to the centre pad.

The transformer may now be connected to the X1 pads on the PCB, with the centre tap to the middle pad. For a stereo unit, connect the transformer to the PCB with the diodes fitted, and link the SK3 pads on the two boards together. The mains input flex may be joined to the transformer primary wires with a choc-block connector or similar.

Testing and Operation
There is nothing to set up on this unit, it will either work or fail depending on how well it was put together! These few checks with a test meter will confirm that everything is reasonably OK, before connecting the unit to your audio equipment.

First set the test meter to the 20V range, and check the supplies on one of the IC's. There should be about +12V on pin 4 and -12V on pin 13. Now connect the meter to the output (pin 1 of IC1), the voltage should be 0V or thereabouts. Turn each of the five pots to both ends, and check the output remains at almost 0V. If the output is not at 0V (+/- 0.1V) something is wrong, which should be investigated. Now connect the unit to your audio equipment, cross your fingers, and try it.

If the output of this unit is connected to a video recorder or some other piece of equipment with an automatic level control, you may find that the sound level drops if the 100Hz or 300Hz controls are turned up too far. Most automatic level circuits respond more to the bass frequencies.

Bearing this in mind, faint and grotty recordings can sometimes be avoided by turning the 100Hz and 300Hz bands down a bit to reduce the effect of the level control. Turn the 1KHz and 3KHz up a bit to improve the clarity, and turn the 10KHz down to get rid of any hiss. The result may sound a bit thin, but it is better than a quiet muffled sound with tape hiss.

There's not much else I can say about using this unit. Most people know what a graphic equalizer does, and a little trial and error will show the effect of each frequency band.