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IC2 (MN3101) is the clock for the delay line, and produces two anti-phase square-wave signals, with a frequency of 16kHz, which gives a delay sampling frequency of 8kHz. The IC also generates a reference voltage for the delay line, which is decoupled by C7. IC3 (MN3004) is the delay line itself, and the two outputs from it are summed by R15 and R16.

This summed signal then passes through another filter built around IC1:C. This is similar to before, but with the addition of R20 and C11, which reduce the signal by about 6dB above 300Hz, effectively giving a little bass boost. The signal is then buffered by IC1:D, and fed to the volume control via C12. The output from the volume control will then be connected to your power amplifier as required.

Please note that the frequency responses of the filter circuits have been calculated using analysis (Spice) software, based on the op-amp specified. Using a different device will give incorrect results, and may result in instability.

The mains enters via SW1, and passes to the transformer, T1. The output from the transformer is rectified by D2 and D3, and smoothed by C13, to give about 23V. The power supply voltage required is 15V, which is regulated by IC4.

PCB Construction
The component overlay for the PCB is shown in Fig 2. Double check the position, value and orientation of each component, against the Overlay and Parts List, before soldering it into place. Take care when soldering, to ensure reliable joints. In particular, take care where tracks pass close between pads, to avoid solder bridges.

Start by fitting the resistors, these components can be fitted either way round. The colour codes are given in the parts list. If the resistors used have a tolerance better than 5%, the Gold band will be Brown (1%) or Red (2%).

Next fit the diodes. These must be fitted the correct way round, as shown on the Overlay.

The IC sockets should then be fitted. Again orientation is important, the notch in the centre of one end should be positioned as shown on the Overlay. Do not plug the IC's into the sockets yet.

The terminal pins (if used) should be inserted now, in the positions marked P1 to P9. These will be a fairly tight fit and should be pressed firmly but carefully into place with the soldering iron. Support the board whilst doing this, and check the pins are properly in place and straight before finally soldering.

The ceramic capacitors can be fitted next, these can be fitted either way round. The values of these may be shown in a variety of ways, the most likely markings are shown in the parts list.

The electrolytic capacitors can be fitted now. Polarity is important, the positive connection is shown on the overlay with a +. On most electrolytic capacitors, the negative connection is marked with a -, and the positive lead is the longer one. Fit with the negative marking away from the + on the overlay.

The voltage regulator, IC4, is fitted next. Orientate as shown on the Overlay, and gently push down until the base of the body is about 5mm above the PCB before soldering.

Finally fit the potentiometers. Before fitting, trim the shafts to suit the knobs being used. Grip the end of the shaft (not the body of the pot) in a vice, and cut to the length required with a junior hacksaw, then remove any burrs with a fine file. The two pots are different types, so check the markings before fitting into the PCB. Push the pots well down and make sure they are correctly aligned before soldering.

Now give the board a thorough visual examination, checking in particular for incorrectly placed components, badly soldered joints and solder bridges.