Leach Amp Power Supply

The power supply is really very simple, but still takes some time to assemble. The first thing you probably noticed is that the transformer I've called for weighs about 14 pounds and the capacitors I'm using add up to 4 times the amount of capacitance that Dr. Leach says is a "minimum". The reasons are a really two fold. Firstly, I wanted this amp to be able to be connected to very low impedance (as low as 2 ohm) loads and I didn't want to worry about it running out of current. Secondly, the difference in price between the "minimum" power supply and the one I've laid out was about $20 and I felt it was a fair trade. Lets take a look at the power supply circuit, the schematic below is a modified version of one created by Dr. Leach.

The basic idea is that you take 120VAC as input into our transformers primary windings. 40VAC show up on both of the secondary windings with reference to one ground. Both secondary windings are then used as input for the bridge rectifier. One is rectified to +40VDC and one is rectified to -40VDC. Both the positive and negative side of the power supply (called "rails") has about 24,000uF capacitors connected to it which help to smooth the DC current as well as act like a reservoir for power. The schematic is color coded to match the color of the wires I used when constructing my power supply. The purpose of this is so you can look at the power supply schematic and then reference the pictures I've provided of my completed power supply to get an idea of how everything is connected. Lines shown in the schematic that alternate from one color to another represent two wiring twisted together. The "central ground" is where all of the ground wires come together, when a wire in the schematic ends in a central ground symbol that means that it will connect to all of the other wires that end in a central ground symbol. All of the wire I used with the power supply is 18 gauge with the exception of the green wire that connected to AC Ground, that wire is 20 gauge. I would advise you to use heat shrink tubing or electrical tape to cover all of the solder joints that don't need to be exposed. If you choose not to you need to take extra caution to make sure you don't touch any of these exposed joints will there is current in the system and also to make sure none of these joints touch each other. Both would have very unfortunate results.

 

Assembly

First step is to wire the transformer. Your transformer should have a diagram similar (or exactly) like this on the side. You need to wire the primary windings in parallel, this means twisting the blue wire with the violet wire and the brown wire with the gray wire. Solder the purple/blue pair of wires to the single fuse holder (the fuse holder for F1). Solder about 10 inches of white 18 gauge wire to the other side of the single fuse holder and solder about 6 inches of blue 18 gauge wire to the brown/gray pair of wires. The blue wire will go into the power switch and then to the AC receptacle, the white wire will go straight to the AC receptacle. Twist together the red/orange wires and then solder to them about 10 inches of 20 gauge green wire, this solder joint will become your central ground where all of the ground wires come together, the green wire will go straight to the AC receptacle. To see a picture of what the joint looks like with all of the ground wires together take a look at the Assembly section under "Final Wiring".

Next step is to place the bridge rectifier and the quadruple fuse holder onto the transformer mounting disc. As you can see from the picture at the beginning of this section I decided to paint my transformer mounting disc black with some spray paint, this was purely for aesthetic reasons. I used Liquid Nails (general adhesive) to affix the bridge rectifier and the quadruple fuse holder to the transformer mounting discs. Once the glue has dried you can begin connecting the bridge rectifier. They yellow and dark gray wire from the transformer each connect to one of the bridge rectifier pins marked with a ~ (this symbol represents an AC waveform). It doesn't matter which wire is connected to which ~ pin. When the AC current enters these pins it will cause DC current (not very clean yet) to appear on the + and - pins. Then you can solder the 0.1uF 250V capacitor (C3P) to the ~ terminals of the bridge rectifier. This is a non-polar capacitor so you don't need to worry about it's orientation. Once you've got the bridge rectifier wired we can connect the filtering capacitors to each rail of the power supply.

The first thing I did was link together a few "zip ties" and strap the filter capacitors to the transformer. Then with 18 gauge wire I connected the positive pairs and negative pairs of terminals on each set of capacitors together, respectively. For the "positive" rail capacitors I used blue wire for the + terminals and white wire on the - terminals and on the "negative" rail capacitors I used blue wire for the - terminals and white wire for the + terminals. Then I ran blue 18 gauge wire from the + pin of the bridge rectifier to the + terminals of the "positive" rail capacitors and from the - pin of the bridge rectifier to the - terminals of the "negative" rail capacitors. Afterwards I connected an additional blue wire to one of the + terminals of the "positive" rail capacitors and another blue wire to one of the - terminals of the "negative" rail capacitors. I then connected another white wire to the - terminals of the "positive" rail capacitors and another white wire to the + terminals of the "negative" rail capacitors. It is important during this process that you not cover up the terminals of the filter capacitors, you'll need to access the terminals to discharge these capacitors during testing. Now the first and second (F2 & F3) fuse holder on the quadruple fuse holder need to be connected together as well as the third and forth (F4 & F5) fuse holder, with 18 gauge blue wire. Once that is done you can connect the blue wire that you soldered to the + terminal of the "positive" rail capacitors and connect it to the first fuse holder. Also connect the blue wire that you soldered to the - terminal of the "negative" rail capacitors and connect it to the forth fuse holder. The two white wires, one that's connected to the - terminal of the "positive" rail capacitors and one that's connected to the + terminal of the "negative" rail capacitors", now need to be connected to central ground (which should currently include the red and organ ge transformer wires, a green 20 gauge wire for connecting it to the AC receptacle, and two white wires from the filter capacitors).

 

Testing

Your result should resemble that power supply shown in the picture at the very top of this page. Don't forget to make sure C3P is in place. Take a moment before moving forward to look at the power supply schematic and carefully inspect all of your wiring. Testing the power supply requires a volt-amp meter (or "multi-meter"), if you don't already own one I would recommend this one because it's inexpensive. Great care is required while performing these tests, never touch any of the components with your bare hands until AC power is removed and the filtering capacitors have been discharged.

Begin by connecting a standard IEC (computer) power cord that is NOT currently plugged in to the wall to the AC receptacle. Now, assuming you've used the same colored wire as I did, you should temporarily connect the blue AC wire (which is connected to the brown/gray transformer pair) and the white AC wire (which is connected to the single fuse holder) to the AC pins on the AC receptacle and the green wire (connected to central ground) to the AC safety ground pin. Now you can plug the power cord into the wall. Do not touch any of the components with your bare hands or with non-insulated metal objects until after the power supply capacitors have been discharged. Set your multi-meter to measure DC voltage (which is usually denoted by a V and a solid line with 3 dots underneath it) in the range of up to 60V (this range is usually denoted by a "200"). Place the red lead of the multi-meter on one of the fuses connected to the "positive" rail and the black lead to central ground. The multi-meter should read about 58V (within 1 or 2 volts). Now place the red lead on one of the fuses connected to the "negative" rail and the black lead to central ground. The multi-meter should read about -58V (within 1 or 2 volts). Now you can unplug the power cord from the wall, but don't get grabby yet, these components are still dangerous. Take one of those 100ohm 2W resistors and hold it in the center with a pair of needle nose pliers with insulated handles. Now touch one of the leads to the positive terminal of one of the filter capacitors on the "positive" rail and the other lead to the negative terminal of the one of the filter capacitors on the "positive" rail. Hold it in that position for about 20 seconds. Now do the same, but with one of the filter capacitors on the "negative" rail. Now, just to make sure you really did get all of the charge out of the filter caps, check the voltage on the positive and negative rail again, both should now show zero volts (or very close, within 0.1V or so).

Assuming all of these tests went well you have yourself a working power supply.