±35V power supply unit

Rohit Balkishan Dubla

Warning!!! This project requires working with mains or high voltages. Do not attempt it if you are unsure or scared of working with electricity.

The figure shows the power supply for the 56W IC amplifier based on LM3875. The transformer needs to be atleast 150VA for operation into a pair of LM3875 amplifiers. I have used a 225VA type. The diode bridge must be a 25A or 35A type, since the capacitance used is 6800µF and charging currents can be expected to be high due to the somewhat high VA rating of the transformer. With lesser VA ratings the bridge may be of smaller rating. The mains fuse must be a 2A rated type - a slow-blow would be preferable if the transformer is torroidal. Optionally, 100nF ceramic capacitors may be connected across the main filter capacitors to bypass them.

The filter capacitors used are sufficient for normal listening conditions. Adding another pair of 6800µF capacitors to these will give a very stiff power supply indeed. However, anything more than this is not required and the resulting cost will simply not have any added benefit. So, depending on budget, the following capacitor combinations per rail can be used (ordered according to increasing cost):

• 1. Single 4700µF/50V cap - Cheapest, but with somewhat higher ripple and reduced maximum power.

  2. Single 6800µF/50V cap - Cost slightly more than option 1. Filtering better than option 1.

  3. Two paralleled 4700µF/50V caps - Double the cost of option 1 (and perhaps more than option 2) but very good filtering, and lower ESR. Best option considering cost v/s performance.

  4. Single 10,000µF/50V cap - Cost may be about the same as option 3 or slightly more and filtering slightly worse than option 5. ESR can be higher than options 3 and 5.

  5. Two paralleled 6800µF/50V caps - Best and costliest option, i.e., double the cost of option 2 but the best filtering and lowered ESR.

That said, capacitor costs vary greatly across manufacturers and are not necessarily directly related to the capacitance (especially in the >4700µF area), so you may be able to get a different or better cost/performance compromise than what the above indicates.

Note: This power supply can be used for any stereo 50W/8Ω amplifier or mono (single) 100W/4Ω amplifier.