Building: Power Supply Assembly and Test

Before we get started, it would be a good idea for you to unpack the chassis components. Even though we won't be assembling it yet, it's a good idea to get a sense of how they go together. Essentially, the whole thing is held together by the rails that you affix to the heat sinks. The front and back panels attach to those, as well as the chassis base plate. Those items essentially provide the rigid structure, with the ventilated top and bottom panels going on last. The screws holding the bottom panel will also do double duty for holding on the four plastic feet. 

Step #1: Chassis bottom plate

The perforated chassis base plate has a top and bottom. The bent edge lip around the edge should be facing downward.

First, drill out the mounting hole for the transformer mounting screw. You will need a hand drill with a 3/8" drill bit for this. Make sure the drill bit is rated for metal (they usually are). Clean up any metal shards with a file or larger drill bit. 

Next, mount the bridge rectifiers. These will be between the transformer and the front panel (where we will mount the power supply board). You will need a M4-20 machine screw, as well as a matching flat washer, split washer, and nut, for each bridge. Feed the screw up through the base plate, so that the nut ans washers are on top of the bridges. As this connection will see lots of heating and cooling, be sure that you've got it nice and snug, fully compressing the split washers. The baseplate will also act as the heat sink for the bridge rectifiers. 

On the opposite side of the transformer hole, nearer the back panel, mount the 4-position terminal block that will connect the AC power to the transformer, through thermistors and a safety capacitor. You will need two M4-16 machine screws and matching flat washers for this. I also fed the screws up through the baseplate, putting the nuts and washers on top. 

To mount the transformer, feed the mounting screw up through the newly expanded hole in the chassis base plate. Next comes one of the rubber gaskets, then the transformer itself, then another rubber gasket, then the round metal top plate, and finally the washer, split washer, and nut. For what it's worth, I used  9/16" and 14mm sockets to tighten the mounting bolt and nut that came with my Antek transformer, but I have no idea whether that's always the size they use. 

Step #2: Back Panel

Now let's mount the back panel hardware from the backpanel kit. This includes the Schurter AC inlet module, the speaker binding posts, and the input connectors (I'm using Neutrik XLR connectors, but the RCA jacks included in the back panel kit share the same "D series" mounting configuration). 

The back panel has a "perfect" and "working" side, so be sure you've got the nicer side facing to the outside. You can tighten down the AC inlet module, but do all the others only finger tight for now. 

Speaker binding posts: The speaker binding posts are "keyed" so that they won't rotate once installed. The key is molded into one of the colored plastic rings. The ring can be pulled free and turned 180 degrees if you have unscrewed all the other hardware. The key position will determine whether the binding post opening for spades/bare wire is facing upward or downward. (If you only use bananas, this won't make any difference, but it looks nicer if the openings all face the same direction.) 

Mounting these is pretty straightforward: the nut, split washer, flat washer, and plastic ring go on the inside, and the rest of the binding post is on the outside. The plastic parts insulate the post from the back panel itself. 

AC power entry module: Not much to say about that. Make sure you've got the right side up, insert the black mounting screws from the outside, use the toothed locking nuts on the inside. 

Input jacks: The back panel opening are designed to work with "D series" jacks. The back panel kit includes two RCA jacks mounted in a D sized unit. I recommend using a balanced XLR input jack, instead, because it's easy to use an adapter to accommodate RCA connections (I've included Neutrik XLR jacks and adapters in the Mouser cart, so you should already have them if you used the pre-populated cart). In contrast, if you install RCA inputs and later want to use XLR, you'll need to replace the input connector and change the internal wiring. Whichever you choose, they are mounted using the black screws and toothed lock nuts. DON'T TIGHTEN YET (we will need to unmount them to solder later). 

Step #3: Front Panel

Mount the power supply board to the front panel using the standoffs from the back panel kit. You will want the Euroblock connectors on the top, the Faston blades at the bottom. 

Step #4: Wiring up the power supply. 

A word about connectors: We will be using 16 gauge hookup wire to connect the power supply components, which will require using ring connectors and female quick disconnect connectors. I've opted for three connectors made by Panduit: 0.250 female disconnect, 0.205 female disconnect, and #6 ring connector. These are all fully insulated and intended for crimping (rather than soldering). My wire stripper has a crimping feature, so I figured I would go this way, rather than soldering each individual connector. (For a basic primer on crimping, see this YouTube video.)

Some are skeptical of crimp connections, preferring to solder the connectors to the wire. It's more time consuming, but potentially more secure. If you prefer to solder your connectors, you can get connectors intended for soldering (like this TE Connectivity 0.250 female quick disconnect, plus matching insulation sleeves). Particularly for the ring connectors, if you opt to solder, make sure that your solder joint doesn't prevent a screw terminal from tightening completely flat against the connector. 

Lay the front panel, chassis base, and back panel next to each other: Doing the power supply hookup and testing BEFORE assembling the chassis gives you more room to make and adjust the connections and see where all the wires are going, as well as for troubleshooting if things don't go as planned. Here's a preview of the whole thing, all hooked up, to give you an idea of what goes to where and how long to make the wires. 

In the photo, I went ahead and plugged an AC cable into the back panel IEC inlet (but not into power!), just because it props the back panel at a convenient 45 degree angle. 

Transformer to Rectifier Bridges: The blue and green leads on the transformer are called the "secondaries" -- they connect to the secondary windings of the transformer. These carry the AC output voltage as specified for your transformer (either 18 or 20 volt). 

You need to identify which of these blue and green secondary leads share continuity, using your multimeter's continuity function (it will beep if there's continuity between the two leads you are touching with the probes). Twist together each pair that has continuity -- one pair will go to each rectifier bridge. Trim each pair of leads to be a little longer than necessary to reach the rectifier bridges, in case you need to rotate the transformer later to reduce noise. Strip the wire ends and attach your female quick disconnects (Panduit 0.250). 

The blue and green secondary leads connect to the AC input connectors on the rectifier bridges, which are marked with a little sine wave symbol on the side of the housing.

Also, you can tell by orienting relative to the notched corner, per the photo below. 

Rectifier Bridges to power supply board: Using appropriate lengths of your 16 gauge hookup wire and Panduit 0.250 female disconnects, connect the DC + and DC - outputs from the rectifier bridges to the four Faston blade connectors on power supply board. I used red and blue for +, white and black for DC -. Make sure you are connecting DC + and DC - to the corresponding positive and negative terminals on the board. Once again, a reminder that DC + is the notched corner terminal. 

AC power entry module to terminal block: The photo below shows how to wire the Schurter AC power inlet module that comes with the back panel kit. 

Use your 16 gauge hookup wire and the smaller Panduit 0.205 female disconnects, which are a better fit than the larger 0.250 disconnects (although those will work, in a pinch, as well). You'll need the Live and Neutral wires to be long enough to reach the 4-position terminal block that should already be mounted on your chassis base plate.  

For safety earth, use your 16 gauge hookup wire. (I used a piece of the 14 gauge green discard wire trimmed from the transformer, which was a bit too large for the ring connector that I needed to fit to the other end. So you probably should use the 16 gauge hookup wire, instead.) The Panduit 0.205 female disconnect works here, and the wire needs to be long enough to reach your chassis ground screw. 

Here's where we're heading next:

4-Position Terminal Block: The 4-position terminal block comes between the AC inlet module and the transformer primaries (red and black leads), providing current in-rush protection (that's the two black CL60 thermistors) and an additional safety measure (the blue safety capacitor). 

You'll be using the Panduit #6 ring connectors for the wire connections. For the thermistors and safety capacitor, I trimmed and bent their leads into a loop and hooked those around the terminal screw. You'll see that I used the discard insulation from the blue transformer leads to add a little extra insulation to the safety capacitor leads to reduce the risk of a short.  

Here's the wiring diagram that I followed, provided by 6L6 in this post over in the "Aleph J illustrated build guide" thread at DIYAudio: 

Per 6L6 in that same post: "The important thing to understand in this photo is Red 1 and Black 2 is the first primary winding (they will have continuity to each other if you use your meter's beep mode) and Red 3 Black 4 is the second primary winding, they will also have continuity to each other." So, just as you did to identify the paired secondary leads, use your multimeter's continuity test to identify the first and second primary windings, twisting each pair that share continuity together, and then hook up per 6L6's wiring diagram and instructions. 

Power supply board to thermistor to chassis ground: The power supply board requires a connection through a CL60 thermistor to the chassis ground. I used a ceramic 2-position terminal block for this connection. 

Figure out where on the chassis base you intend to mount the ceramic terminal, BUT DON'T affix it to the chassis base just yet. You will eventually attach it using M3-18 machine screws, along with a matching nut and split washer. But for now, it's easier to attach everything to the terminal if it isn't screwed in. 

You will need one length of 16 gauge hookup wire to run from the power supply board to the ceramic terminal block. Strip each end of the wire and twist the stripped end, but do not tin (solder flows over time, making terminal screw connections to tinned wires loosen over time). Connect one end to any of the 6 GND ports available on the Euroblocks on the top of the power supply board. Connect the other to the ceramic terminal block. 

You will need another length to run from the ceramic terminal block to the chassis ground screw. Strip one end of the wire and connect to the ceramic terminal block. On the other end, put a Panduit #6 ring connector that will go to the chassis ground screw. 

Trim the leads of the CL60 thermistor and connect to the remaining ports on the ceramic terminal block.   

Once you have completed the connections to the ceramic terminal block and have decided on a position for it on the chassis base plate, secure it to the base plate with two M3-18 machine screws, using a nut and split washer (nut and washer go under the baseplate). 

Chassis ground screw: Three wires should be secured to the chassis by a ground screw: (1) the purple ground wire from the transformer, (2) the safety earth from the AC power inlet module, and (3) the GND-thermistor wire from the power supply board. All three wires should be terminated with a #6 ring connector (note, I had to use a different ring connector because the 14 gauge scrap lead from the transformer didn't fit in the ring connector -- don't do that!).  I used an M4-10 machine screw and lock nut (the one with the teeth). Tighten this one down good and tight. 

Fuses: You will find two fuses in the back panel kit. One is a 10A, the other 2A, both 250V fast blow types, both 5mm x 20mm. The 10A fuse is fine to use, but the 2A fuse is insufficient for the Aleph J (which is to say, I blew three of them). Instead, I used a 3A, 250V, slow blow (aka time delay) fuse. 

Fuses must be inserted in the fuse tray just below the power switch on the AC inlet module. 

The 10A fuse should go on the Neutral side, and the 3A on the Live side. This is meant to ensure that if there is a short to the chassis, the smaller Live fuse will blow first, and the chassis won't be a live shock risk.

For what it's worth, I found the fuse tray to be a real pain to reinsert fully after the fuses were seated. I actually shattered one of the fuses while trying to get it to push in flush. Might be I just had a tight fitting one, but be patient and gentle with it. 

Step #5: Testing the power supply. 

OK, here we are, at the Big Event: applying real AC power to your build for the first time! If all goes well, you should get +26 volts (give or take a little, depending on your transformer rating) across V+ to GND and -26 volts across V- to GND, measured from the Euroblock connectors on the top side of the power supply board. 

Let's do this step-by-step. Remember AC power can kill you, so be patient and methodical. 

First, recheck all of your connections. Are your female disconnects all firmly seated? Are the screws on both the terminal blocks and the Euroblock GND tight? All ring connectors snug and flat? No stray wire strands sticking out anywhere? (A magnifying glass or phone cam zoom can help check for stray wire strands.) Double-check the wiring of the bridge rectifiers to be sure you've got it hooked up properly. Double-check the wiring of the terminal blocks. 

Second, bring out your dim bulb tester. The dim bulb tester is a crude but effective tool to keep really bad things from happening if there is a short or some error to make the circuit pull a ton of current. Because the bulb is wired in series with the load, it will glow when a bunch of current is drawn past it, and because it's a resistor, the voltage will be lower on the far end (where the amp is), where the lower voltage hopefully will keep things from burning up.

I used a 100 watt light bulb in mine for this test. Make sure the power switch on the back panel is OFF. If your dim bulb tester has a switch, make sure it is also OFF. 

Plug your amp into the dim bulb tester using a proper AC cable with an IEC connector on one end and 3-prong plug on the other. Now plug your dim bulb tester into the wall. 

Third, time to throw the switches and power it up. You should see the bulb briefly light and then dim (indicating that the filter capacitors have charged) and the two LEDs on the power supply board should light up. Keep your hand on the power switch, and if the bulb does not immediately begin to dim, cut the power. If the bulb dims immediately, then you can grab your multimeter to measure the DC voltages at the Euroblock outputs at the top of the power supply board. 

 

V+ to GND should be positive 26 volts (or thereabouts). It'll be a bit lower if you used a transformer with 18V secondaries.

V- to GND should be negative 26 volts (or thereabouts). It'll be a bit lower if you used a transformer with 18V secondaries.

Fourth, assuming all went well, turn off the amp power switch. WAIT A MINUTE OR TWO. (It is important to allow 20 seconds or so for the CL-60s acting as the in-rush current protection to cool after each power down. If they are not close to their cool temperature resistance, it is possible to blow a fuse. This advice applies even after the amp is complete -- no quick off-then-on-again cycles!) 

Make sure your back panel switch is in the off position. Remove the dim bulb tester and plug the amp directly into the wall socket. Turn on the amp power switch and re-measure the DC voltages at the Euroblock connectors on the power supply board. 

Assuming you get the same (or very nearly the same) voltage readings, congratulations! 

NOW TURN OFF THE AMP, REMOVE THE POWER CORD, AND LET IT SIT OVERNIGHT. The filter capacitors on the power supply board store a dangerous amount of electricity. Without a load connected, it will take the bleeder resistors several hours to drain off the charged capacitors (you'll see the LEDs stay lit after switching off power, demonstrating that the caps are still charged, and you can use your multimeter to measure the remaining voltage across V+ and GND or V- and GND). Exactly how long it takes to drain the capacitors depends on the value of the bleeder resistors. So be patient. Enjoy your success and wait until tomorrow before tinkering further.