Power Supply Unit

To power the circuits described in this chapter a power supply capable of supplying + 12 volts and - 12 volts is required, and the supply must be well smoothed and stabilised. The maximum current required depends on which modules are used, and on how many of each one are incorporated into the instrument. For a simple single oscillator type the current consumption is likely to be less than 100 milliamps from both supplies, but with a two VCO type with a separate envelope shaper for the-filter and some of the extra modules to be described later, the current consumption could be well over 100 milliamps per supply rail.

Battery operation is not really a very practical method of powering a unit of this type, and a mains power supply is really needed. A suitable circuit is shown in Figure 18.

This is a very straightforward design using push-pull rectification to provide positive and negative supplies. C6 and C7 smooth the positive and negative supplies respectively. IC4 and IC5 then provide electronic smoothing and regulation of the two outputs. A mains transformer (T1) having a secondary rating of 500 milliamps should be adequate to supply even an instrument which uses all the circuits described in this chapter, including two VC0s and two envelope generators. For a simple instrument a type having a secondary current rating of 250 milliamps should be more than adequate. However, if you are likely to add to the basic synthesiser and gradually build it up into a fully fledged instrument it would be advisable to start with a .50OmA component. An alternative to using a 12V - OV - 12V transformer is to use a twin 12 volt type with the two secondaries wired in series. Mains transformers having twin secondary windings seem to be the more readily available these days.

PSU Components (Fig. 18)

Capacitors	
C6,7 1000uF 25V radial electrolytic	
C8,9, 10, 11	 100nF ceramic

Semiconductors	
1C4	7812 (+12V 1A regulator)	
1C5	7912 (- 12V 1 A regulator)	
D3,4,5,6	1N4002

Miscellaneous	
T 1	Mains primary, 12 - 0 - 12 volt 50OmA	
secondary	
S2	Rotary mains switch	
Printed circuit board	
Wire, control knob, solder, etc.