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Clean Jazz Guitar Amp Builder Notes — Part 1: DC Power Supply

Clean Jazz Guitar Amp Builder Notes — Part 1:  DC Power Supply

I’ll go through the circuit starting with the Mains AC supply.

Years ago — for anything I make that for uses AC mains electricity. I switched to only using an IEC320 C14 receptacle — plus a cord containing an IEC320 C13 appliance plug on 1 end, and a grounded NEMA 5-15P polarized plug at the other Since they’re commonly used on personal computers in North America, you seemingly can always find a cord if you forget or misplace one.

Above — Another view of the power supply with the cover off. The IEC320 C14 receptacle contains a 10 amp DPDT switch. The 2-sided copper board is well soldered on both sides so it’s technically RF tight around the transformer and mechanically solid to strongly support the AC receptacle and fuse.

Above — The IEC320 C14 receptacle with DPDT switch & the toroidal transformer mounting hardware.

D. In Rush Current

When you turn on and energize a power transformer, an instantaneous surge of current gets drawn to charge up the reservoir caps and to magnetize the transformer. The magnitude of the in rush current is not related to the amplifier load, rather it’s dependent on the point in the Mains AC wave cycle when the transformer is switched on.

On the (low resistance) primary coil side this puts potential strain on your switch and fuse — some larger toroidal transformers will even snap off a house circuit breaker the odd time. Thus ensure you use a slow blow panel fuse, plus a well rated switch. For example, I used a 10 amp @120 VAC rated switch. Perhaps overkill for a small guitar amp, but you get the picture.


On the transformer secondary coil side, your rectifier diodes and reservoir caps get strained during any start up surge. Any rectifier diodes must be tough. I initially purchased a commercial 25 Amp rated diode ring that featured a heat sink and mounting hole. Sadly, this part tested defective. I looked in my rectifier parts drawer and saw about 40 Vishay brand 1N5822 3A Schottky barrier rectifier diodes and then studied their datasheet online. These should well handle my voltage, current needs and any brief inrush surge. They work well for my particular guitar amp. I have no idea when, or who I bought them from however.

I put a slow-blow fuse on each AC secondary to keep the transformer alive should the diode ring ever go shunt to ground. Currently, I’ve got a 2 Amp slow blow fuse in each slot. The X capacitors are to bypass any HF noise created by the switching diodes from going into the house wiring.

I used hulky capacitors because in the presence of AC bias, some small size ceramic caps may not hold their capacitance value. These 1 kV caps are cheap and easy to find. Carefully check the datasheets for any parts you place in your power supply and consider avoiding ‘no – name’ parts. It’s really sad when you fry a $50 transformer because you decided to save 50 cents by plying a, cheap no-name, part.

Above — The improvised diode ring. I’ll shorten the wires even more. Sadly, I misplaced my 0.01 µF/ 1kV X capacitors and had to order more. No doubt, I’ll locate them the day the fresh capacitors arrive. I’ll shorten the diode ring wires when I install those X capacitors.

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