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Power Supply
For clarity the power connections to U2 are shown separately on the power supply circuit diagram. R37 is a biasing component for the VCA sections, and sets the output current.

The circuit requires a supply of +/-15V at about 50mA. This is derived from a 15V transformer with the usual rectification and smoothing components. The voltage is regulated by a pair of standard three-pin regulators (U4 and U5). Although the 78L15 and 79L15 TO92 cased 100mA devices would be adequate, it was felt that standard TO220 cased 1A devices would be better able to dissipate 600mW of heat. Indeed the power supply could power two circuits if separate compression and expansion arrangements were required for a record/playback system. No heatsinking is required.

Construction
The whole circuit with the exception of the transformer and switches is constructed on a single sided PCB. The dual track pot (RV1/RV3) should have PCB mounting pins otherwise it will need to be mounted with short pieces of stiff wire.

Due to the cost, a socket is recommended for U2. If you do not have a 22 pin socket, use a 14 pin socket and an 8 pin socket next to each other. Do not fit U2 into the socket until the power supply has been checked. The completed PCB is then fitted into a suitable case and wired up as shown. A single DPDT toggle switch is used for SW1 and SW3, and a single 4P3W rotary switch is used for SW2 and SW4.

Voltage Testing
Ensure the mains connections are adequately insulated - if necessary add some insulation tape while carrying out these tests. With U2 removed, connect the unit to the mains and switch on. Connect the negative lead of a meter to a suitable ground point such as the wire form J15. With the positive meter lead connected to pin 4 of U1 the reading should be 15V +/- 0.5V, and on pin 11 there should be -15V +/- 0.5V.

If this is OK, switch off and insert U2. Set presets RV2 and RV4 to the centre position. Switch on again and check the voltage at pins 1, 7, 8 and 14 of U1. These should all be between +0.5V and -0.5V. With RV1/RV3 set fully anti-clockwise the voltages on pins 1 and 7 of U3 should be within the same range. If these voltage checks are OK, the unit can be tested with an audio signal.