Starting with the digital section. IC1 is the clock generator, which runs at four times the sampling frequency. The frequency is adjustable over a limited range by VR1, the Delay Fine control. This slightly unusual configuration gives an output with an approximately equal mark-space ratio.
The A-D convertor IC7, requires a negative bias on pin five. Since the current required in minimal, this negative voltage is obtained by rectifying the clock signal from IC1, giving approximately -4V.
IC2 produces the four timing pulses required, three of which are inverted by gates in IC3. When Q1 goes high, the A-D convertor IC7 starts a conversion. When Q2 is high, the data in the RAM is sent to the D-A convertor IC8, which produces the appropriate voltage. When Q3 is high, the data from the A-D is stored in RAM. Finally when Q4 pulses high, the address counters IC4 and IC5 are incremented.
SW2 (Delay Coarse) sets the count reached by the address counters before they are reset. This sets the amount of the RAM chip to be used.
Ideally I would have had just one continuously variable delay pot, on the 555 clock circuit. However, it was not possible to achieve the required range with a respectable frequency response due to the A-D conversion time.
Construction
The circuit is constructed on a single sided PCB. The track layout and component positioning are shown in figure *.
There are 30 links that should be made first, using thin (approx. 26SWG) tinned copper wire. The resistors, diodes and capacitors can then be fitted in size order. Sockets may be used for the IC's if required - since all the devices are static sensitive this may be a good idea. Do not insert the IC's into the sockets until the remainder of the PCB construction is complete.
IC11 and IC12 will become warm in operation and should be mounted on a small heatsink. The mounting tabs of both devices are connected to the 0V rail so no insulation washers are required. A small amount of silicone grease or heat transfer paste should be placed between the devices and the heatsink.
Terminal pins may be fitted in the holes for the off-board wiring, so that the connections can be made after the PCB is fitted into the case.
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.
Parts required for this project
Resistors (0.25W 5% or better)
R1,2,7,8,18,19,21 10K
R3,5,6,12 22K
R4,9 2K2
R10,14 390R
R11,13,17,20 100K
R15 68K
R16 47K
VR1 20K or 22K Lin Pot
VR2 47K or 50K Log Pot
Capacitors
C1 10n
C2 220p
C3,4,9,14,16,17,18 10u
C5,6,10,11 4n7
C7,8 2n2
C12,15 1u0
C13 47p
C19 2200u 25V
C20,21,23,24 100n
C22,25 100u 16V
Semiconductors
IC1 7555
IC2 4017
IC3 4001
IC4,5 4024
IC6 6264
IC7 ZN448E
IC8 ZN428E
IC9 LF347
IC10 LF351
IC11 7812
IC12 7805
D1,2 1N4148
D3,4 1N4001
Miscellaneous
SK1,2 Phono Socket
SW1 1 Pole 12 Way Rotary
X1 12-0-12V 250mA
PCB, Knobs, Case, Wire, 2 Core Mains Flex, 13A Plug with 3A Fuse, IC Sockets, M3 Screws and Nuts