WebEE

Set the stop on SW1 to position six. Fit the pots and switches to the front panel, and then position the PCB and transformer in the base of the case. The PCB should be mounted with three M3 screws, with extra nuts to act as spacers. The transformer is also mounted with M3 hardware, which should ideally be nylon to insulate the mounting frame from the outside world. A 10VA torroidal transformer was used in the prototype because it was to hand, however a 250mA conventional type is suitable.

The rear panel needs to be drilled for the audio input and output sockets, and the mains flex. The hole for the latter should be fitted with a cable gland or grommet, and the cable must be secured to prevent it being pulled out.

The interwiring is shown in figure *, the pots are shown viewed from the rear. Audio coax may be used for the connections to the rear panel sockets if desired, although the prototype worked fine with ribbon cable. The remaining connections may be made with ribbon cable or whatever is available.

The mains flex should be connected to the flying leads on the recommended transformer with a chock-block type connector. No mains switch or fuse were fitted on the prototype for simplicity. A 3A (or lower) fuse must be fitted in the mains plug.

Testing and Using
Ensure that the internal mains connections are adequately insulated. Connect the unit to the mains and switch on. If a test meter is available, check the voltage outputs from IC11 and IC12 on the appropriate pins. The voltages required should be 12V +/-0.5V and 5V +/-0.25V.

Unless an audio pulse generator and oscilloscope are available, the remainder of the unit is probably best tested by connecting it to suitable audio equipment and trying it. The effects can be heard readily on male speech.

The audio input signal should be between 0.5 and 1V RMS for optimum performance. Lower levels will give greatly increased distortion on the echo signals due to the lower number of sampling points available. Larger signals (over 2.5V pk-pk) will be distorted due to clipping at the A-D convertor. If the signal level is likely to vary significantly, this unit should be preceded by an automatic level control or compressor circuit.

Initially set both delay controls fully clockwise. As the repeat control is advanced, the echo should be heard more times. You may notice that if a sound is repeated several times it becomes a little more distorted each time. With a more normal decay, the distortion will tend to occur as the sounds are getting quieter, and will generally pass unnoticed. When the control is at maximum, all the signal is recycled which can cause some awful sounds!

By operating the delay controls, the period between each echo can be altered. You may notice an apparent pitch shift as the Fine Delay control is operated, this is due to sounds being replayed at a different rate to the one they were recorded at. Momentary bursts of noise may also be heard as the Coarse Delay control is set to longer periods, this is due to old data stored in the RAM being replayed. Neither of these effects is relevant in practice, since the controls would not be altered when the unit is being used.

The best way to understand the unit's capabilities is to play with it for a while. Have fun . . . fun . . . fun!