Building: Chassis Assembly and Final Adjustments

Well, now that we have preliminarily adjusted bias and offset on both amp boards, it's time to put the whole thing together and (hopefully) make some music! 

I found it easier to take on chassis assembly after detaching the heat sinks from the front panel (disconnecting the amp board power wiring from the Euroblock connectors on the power supply board), the front panel from the base plate (by disconnecting the power supply board from the rectifier bridges), and the back panel from the base plate (by disconnecting the AC wires at the IEC inlet connectors). Having these chassis elements separated means not having to worry about accidentally yanking any connecting wires in the chassis assembly process. Be careful when disconnecting not to break anything, and take as many photos as you need to ensure that you can get the wiring all reconnected together again properly. REPEAT: TAKE PICTURES BEFORE YOU DISCONNECT THINGS, TO CHECK AGAINST WHEN YOU RECONNECT THINGS. 

Step #1: Mount base plate on heat sinks. 

The entire chassis is held together by the rails on the heat sinks. So let's start by making sure that the rails on the heat sinks are mounted flush with the heat sink edges. You may eventually need to loosen the rails in order to get things to all line up, but having the rails mounted flush, square, and tight is a good way to start. 

Mount the heat sinks to the base plate, using on each heat sink two M4 countersunk machine screws and matching nuts provided with the chassis hardware. The screws feed up from the bottom, through the heat sink rail and base plate. I also added flat and split washers (size: M4) for a more secure connection. 

Don't over tighten these screws, lest you bend the rails and bow the heat sinks outward. You'll see that my heat sinks bowed out a bit. A little bit seems fine, as you can push them together gently when securing the front, rear, and top panels. 

Step #2: Back panel to heat sink rails. 

Next, attach the back panel to the heat sink rails, top and bottom, using the black M4 machine screws. Here again I added M4 flat and split washers to the nuts that came with the chassis kit. You may need to gently push the heat sinks together to get the screws through the mounting holes on the rails. 

Step #3: Wiring up input connectors

Now is a good time to wire up your input connectors, before attaching the front panel, while you still have lots of room. 

I strongly recommend using XLR connectors for inputs (and I've included XLR connectors and XLR-to-RCA adapters in the saved Mouser cart, so you should already have them). 

Why? Well, because it's a lot easier to start with XLR and use an adapter to accommodate RCA, than it is to go the other way. And assuming you are using a properly wired adapter (like this one from Neutrik, or this XLR-to-RCA cable from Monoprice), using an adapter is electrically identical to what you would do if you had wired up RCA connectors (jumper between -IN and GND). 

How long should your input wires be? Well, you will want your input wires twisted together and routed against the heat sink, away from the transformer, as much as possible (I tucked it behind one corner of the amp board to hold it near the heat sink, see photo below). Plan your lengths accordingly. And remember that it's always easier to shorten than lengthen. 

Option 1: XLR (recommended)

To hook up XLR connectors, you will need to unscrew them from the back panel to solder your input wires to the appropriate pins (you will see that the pins have solder cups that face downward, so you can't solder them while they are mounted on the back panel). 

You will also see that the pins are numbered. The standard for XLR wiring is "pin 2 hot", which is to say, for purposes of our Aleph J build, the connector pins correspond to the amp board inputs as follows: 

• Pin 1: GND
• Pin 2: +IN
• Pin 3: -IN

The easiest way to solder your input wires to the XLR connector solder cups is to secure your connector with a third hand, then heat and melt solder into the cups. Then get your input wire, reheat the solder cup until the solder is molten, insert your wire into the molten solder, remove heat, and hold the wire in place until the solder hardens. If you like, you can then use your pliers to crimp the ends up the solder cups around the insulated section of your wire, so that the connectors provide a little strain relief (I didn't, but it's not a bad idea, especially if you're working with very narrow gauge wire!). 

I attached pin 1 to one of the connector mounting screws (since the chassis already provides a common ground), rather than route a GND wire all the way back to the amp board. This choice was inspired by this article. But you can instead use a third wire to connect pin 1 to the amp board. 

At the amp board end, you have a Euroblock connector for the input wires. Just in case you have forgotten which port on the Euroblock corresponds to which pad on the board, here are some photos to remind you. 

Here's what my connection looks like (recall that I connected pin 1 to the chassis via XLR mounting screw, rather than running a wire to GND here). 

Option 2: RCA connectors

RCA input connectors are included in the back panel kit. They are easy to mount because they come in the same "D" sized package that the Neutrik XLR connectors use, secured by the same two machine screws and locknuts (also included in your back panel kit). 

To hook up the RCA jacks, connect +IN to the center solder cup (see the XLR section above for tips on how to solder this connection), and GND to the outer sleeve connector. In the photo below, I used a four wire braid, using two wires for +IN and two for -IN. 

On the amp board side, if you opt for RCA inputs, you will need to put a jumper between GND and -IN. I'm afraid I don't have a photo to share, since I didn't actually hook up the RCA connectors. Just cut a small piece of your input signal wire, strip both ends, and run it between GND and -IN on the Euroblock connector. That means the GND port on your amp board Euroblock connector will have to accommodate both this jumper and your wire to the RCA jack sleeve. 

Step #4: Attach the front panel 

In order to complete the four walls of the chassis, you will need to attach the silver aluminum front panel. 

Before that, however...

Front Panel LEDs?

The easiest thing to do is to forgo front-panel LEDs. The internal LEDs are visible through the top panel, particularly if you opt for bright blue ones that you should already have received as part of the saved Mouser cart, in place of the red and green. 

That said, I finally succumbed to temptation and installed a single, blue, front-panel LED in my build. I didn't do this till late in the build, and I didn't take as many photos as I should have when I did it. So consider this an "advanced variation". I'll describe generally what I did, but you'll have to sort out the details. 

If you want a front-panel LED (or two), then you will need to drill out a hole (or two) in the front panel. Assuming that you opted for the silver front panel for the 4U Deluxe Chassis, it has several pilot holes already. There are five options: any of four inner pilot holes originally intended for the optional front panel handles or one pilot hole in the center, now hidden under the power supply board.

For my LED, to keep it simple, I drilled out the pilot hole closest to an amp board LED. A typical household drill with a metal rated drill bit will make short work of drilling out the hole. Just be sure that no resulting metal shards end up under your power supply board, shorting something to the front panel (I actually removed the power supply board while drilling, just to be extra cautious).  

In my case, I also had to desolder and replace one LED from the amp board and connect a new blue LED to a twisted pair of small gauge wire long enough to reach the front panel hole. Using heat shrink tubing, I insulated the LED leads to cover any exposed parts of the wire or leads. After I mounted the frontplate, I found that my LED fit pretty snuggly into the hole, but I also secured it in position by using the same twist tie that I eventually used to hold my DC power wires in position. 

If you want to try using the pilot hole under the power supply board, you might try doubling up stand-off spacers (there are four extra standoffs included with the back panel kit that you could use) to give more room under the board, and be sure to carefully insulate any exposed wire or leads to prevent any short to the board or the front panel. You might even consider desoldering one of the power supply board LEDs and bottom mounting it, rather than using one of the amp board LEDs. But all that seemed too fiddly to me, so if you go for the centered front panel LED, let us know in the comments how that went. 

To reiterate, you should feel free to omit a front panel LED. That certainly makes things easier. (In retrospect, I wish mine wasn't so bright. But I don't care so much that I want to desolder the dropping resistor that controls its brightness.)

Mounting the front panel

Because the front panel extends past the heat sinks on both top and bottom, you may find it easier to mount if you place the front panel flat and carefully tip the rest of the amp up onto it. 

The front panel mounts to the heat sink rails by means of four short machine screws included in the chassis kit. In retrospect, I probably could have used washers on these connections, but as I didn't have any extra handy, I went without. 

If the front panel mounting holes don't quite line up, you can loosen the rails on your heatsinks to nudge things into place a bit. 

Step #5: Attach the bottom cover

Before the bottom cover goes on, it's time to double check that all of the hardware secured on baseplate (transformer screw, screws securing the terminal blocks, chassis ground, and rectifier bridges) is snug and tight. 

this photo taken before attaching the front panel, but you get the idea

The bottom cover is meant to attach to the heat sink rails by means of four black M3 sheet metal screws included with the chassis kit. Using those, however, leaves you without an attachment point for the four plastic feet that also come in your chassis kit. I chose to solve this by using longer M3-16 machine screws to run through the plastic feet and the bottom cover, then into the heat sink rails. 

Step #6: Reconnect and route wires

Time to reconnect the wires that you disconnected to facilitate chassis assembly. Now is the time to take care to route and dress your wires to minimize noise, trimming things to the proper lengths. A few principles to keep in mind: 

• Keep signal wires away from AC power wires and the transformer, as much as possible. Why? As you may recall from basic electronics, when an alternating current passes through a wire, it creates a varying magnetic field that can induce a current in a nearby wire (this is the principle by which transformers work). If that magnetically induced current doesn't belong in that second wire, it is noise. 
• Twist signal wire pairs together. This reduces susceptibility to picking up magnetically induced noise, for reasons described here. 
• Twist AC power wire pairs together. This reduces the magnetic field created by the AC current flowing through those wires, which helps reduce induced noise. 

Here's a few photos of how I routed my wires, which resulted in a hum-free amp for me. I used twist ties to hold the DC power wires together in the upper corners of the chassis. 

 

Step #7: Powering up, plus final bias and offset adjustments

Check that you have everything connected correctly (you took photos before disconnecting, right? use them to double check) and that the connections are snug and secure. 

Hook up the dim bulb tester, just in case you've introduced a new short somewhere or made a wiring error while reassembling everything. 

Power up. As when you tested the amp boards, the dim bulb tester should light, dim, and then return to a less bright constant illumination (that's because this amp is quite hungry for current, but not as hungry as a dead short would be). Keep your finger on the switch and if the bulb stays bright, shut off immediately and trouble shoot. 

Assuming that the initial power up went as expected, remove the dim bulb tester, make sure the back panel switch is in the OFF position, and plug the amp directly into AC mains power.  

Power up and set the bias and offset for both channels exactly as you did when initially testing the amp boards (see Step #3 in Building: Amp Board Power Up and Test). You're aiming for bias at 400mV (0.4V) and DC offset as close to zero as you can get. 

Wait for 30 minutes and check bias and offset again. Re-adjust as necessary. Use an infrared thermometer to check the temperature of your heat sinks and MOSFETs. The rule of thumb for maximum temperature is 65C on the devices, 55C on the heatsinks. While you're at it, also check the temperature of the bridge rectifiers, transformer, and power supply capacitors (the bridge rectifiers are the hottest thing in my build).  

Now place the top cover on top (no need to use screws yet) and wait another 30 minutes. Re-adjust bias and offset as necessary to get back to your target values. I was surprised how much this simple change to the thermal environment altered the bias and offset, and how fast they change when you lift the top cover. 

Step #8: Making music

Well, assuming you've gotten to this point, it's time to connect a source and speakers and see if the amp makes music. Turn off the amp, connect speakers and source, and switch power on again. (NOTE: remember that the thermistors need a minute or two to cool, so no quick on-off cycling, lest you blow a fuse.) 

For initial testing, connect a pair of expendable speakers, just in case something goes wrong. For a source, I used my phone and an old iPod. Start with the volume low and gradually ramp up. 

Hopefully, you now have music! If not, or if you have hum or other unwelcome noises with the music, shut down the amp and visit the Aleph J thread DIYAudio to politely ask for assistance. 

Assuming things are working correctly, let the music play for an hour or two, then re-check heat sink and MOSFET temperatures, DC offset, and bias. Make any necessary final adjustments. On my build, I got the bias measurements to 401mV in one channel, 410mV in the other, and DC offset down below 0.5mV in both channels. 

A small turn-off thump is normal for Aleph amps. 

Step #9: Attach the top cover

Four black M3 sheet metal screws secure the top cover to the heat sink rails. You're done! Congratulations! 

My Aleph J build next to its famous ancestor, the Pass Labs Aleph 3