In most cases it is not necessary to use an ultra low ripple PSU for driving a synthesizer. A normal basic PSU with low ripple is sufficient in most cases. Those basic PSU has the advantage off better thermal and power efficiency. My implementation here is the same as my ultra low ripple PSU without the input low pass filter. Leaving this filter out lowers the power consumption at about 50%. And avoids the additional internal heating of the housing. It is possible to use a lower rated transformer with 15V/50VA. The output current is increased. One more benefit is more room on the PCB which is used to integrate a fuse on the PCB. Those changes lowers the overall parts count and cuts the cost down as well. The PSU is build around the well known LM317 and follows mostly a straight forward design as in the data sheet. I have tested this design with a current load of 1.25A for some hours. No problems whatsoever. For a +/- 15V PSU you need two of those. You have to daisy chain them and use the connection as ground
Specs and features
This design is straight forward like in the data sheet of the LM317. After the rectifier diodes C5 and C6 are the reservoir capacitors. LED1 indicates the presence of the unregulated voltage before the LM317. R1 limits the current for the LED. The LED is a low current 2mA type D1 serves as protection diode against reverse biasing the voltage regulator. R3, R4, TR2 serves as voltage divider to adjust the output voltage to15V. The 1.2V reference voltage between ADJ and out is amplified about 12-times to make a 15V output. So is the noise and ripple. C7 between ADJ pin and GND reduces noise and ripple considerably. Protective diode D5 is needed to safely discharge C7 if the output is short-circuited. D3 protects against short-circuited when you daisy chain two PSU for a +/- PSU if the other output is short circuited. With the above measures the ripple is quite low. With a 15V/50VA transformer (for two PSU) this PSU is good for output current of about 1.3A (both). Above this value the voltage drop over the regulator drops under the necessary 3V. To stay on the save side the recommended fuse is rated for 1.25A/slow-blow.
- Adjust the output with TR2 to 15V