By now you’ve installed all parts but the DIY500 mkII side panels and knob. We will use these parts in the following procedures, before powering on and testing our completed preamp.

Fig. 101 Locate the (8) flat undercut 4-40 screws from your DIY500 mkII Minimal Kit hardware bag. Use them to attach the side panels as shown.

Fig. 102 A side view shows how the side panels wrap around to form the module enclosure.

Fig. 103 With the bottom side panel installed, the module is half covered.

Fig. 104 The second identical side panel is installed on top for full coverage.

Fig. 105 Space is left for ventilation.

Fig. 106 The edge connector is left accessible.

Fig. 107 Use a 1/16" hex wrench (aka allen key) to tighten the control knob over the gain pot shaft.

Fig. 108 The knob should rotate 300˚ without scraping against the front panel.

Fig. 109 Congratulations! Your module is complete.

Fig. 110 Lookin' good.

Fig. 111 Install your module in a 500 Series enclosure for testing. Note the neighboring DIY500 mkII module has been redecorated. There are many ways to do this, which we may explore in a future tutorial.

Fig. 112 Using a Purple Audio CANS headphone amp and Shure SM57 microphone, our preamp sounds OK!

FIN

Your DIY500 mkII Minimal Kit included a bag of hardware. From it, locate (4) 1″ hex standoffs, (4) 1/4″ hex standoffs, and (4) lock washers. They will connect through the four holes in the DIY500 mkII PCB, creating a framework on which to mount the module enclosure side panels. Proceed according to the following photographs.

Fig. 89 Locate and assemble these pieces of hardware

Fig. 90 The longer hex standoff goes through the lock washer and the top of the PCB, screwing into the shorter hex standoff on the bottom of the PCB. Do not over-tighten. Finger tight is adequate.

Fig. 91 A side view of the PCB with four sets of standoffs installed.

Fig. 92 Top view.

Fig. 93 Bottom view.

HARDWARE – FRONT PANEL

It’s time to install the front panel and accompanying control hardware. Locate a second bag containing black components and proceed as follows.

Fig. 94 Our DIY500 module is ready for its front panel and control hardware.

Fig. 95 Push the switch caps all the way back.

Fig. 96 Switch caps properly installed

Fig. 97 Two black screws, a black plastic nut, and a 1/4" metal nut are used to attach the front panel to the XLR connector, 1/4" connector, and gain pot.

Fig. 98 Carefully tighten the 1/8" gain pot panel nut without scraping the front panel finish.

Fig. 99 Our front panel has been successfully installed.

Fig. 100 A rear view of the front panel shows the connectors, gain pot, and PCB edge sitting flush against it.

Before applying power to our module it is a good idea to check for mistakes: certain accidental open or short circuits caused by bad solder joints or solder splash across joints. These simple problems could cause module malfunction and possible damage to your components or rack upon powering up. Obviously, it is easier to perform these PCB tests before you’ve installed your front and side panels. Aside from a thorough visual inspection and review of these instructions, we can also perform certain electrical tests. We will need an ohmmeter, or any DMM (Digital Multimeter) which can test for DC continuity.

Fig. 81 Check the resistance/continuity of the meter alone by touching the probes. It should read essentially 0 (ZERO) Ω, indicating a short.

Fig. 82 Verify 0 Ω from the "GND" pin of A1 (op amp/ sockets) to pin 5 of the PCB edge connector.

Fig. 83 Measure resistance continuity from the "-V" op amp pin/socket to pin 14 of the PCB edge connector. You should read less than 50 Ω.

Fig. 84 Perform the same test with the "+V" op amp pin/socket and pin 12 of the PCB edge connector. You should get the same reading.

Fig. 85 Very an open circuit (infinite resistance = no continuity) between "-V" and "GND", and the same should be true of "+V" and "GND".

Fig. 86 Verify an open circuit (infinite resistance = no continuity) between "+IN" and "GND".

Fig. 87 Verify the same between "-IN" and "GND".

Fig. 88 Lastly, verify an open circuit between "OUT" and "GND". When first touching the probes, your meter will momentarily read some resistance, but once any affected capacitors have charged up you should read infinite resistance, or something close to it.

With your output transformer prepped, it’s time to locate the mounting hardware and install it on our PCB. By now all other components should already have been installed. Proceed as follows.

Fig. 74 From the DIY500 mkII Minimal Kit hardware bag, locate the (2) 7/8" 4-40 machine screws, (2) flat washers, and (2) 4-40 kep nuts.

Fig. 75 Insert the machine screws through the two holes from the top of your prepped output transformer, and add the washers on the bottom.

Fig. 76 It is VERY important that the rounded edge of the washer face down towards the PCB surface, and that the sharp edge faces the transformer.

Fig. 77 A profile view shows how the transformer sits on the washer, with the kep nut holding the screw on the other side of the PCB. Please be careful not to over tighten, or you may damage your transformer/PCB. Just a turn or two past finger tight is enough to properly grip.

Fig. 78 Place each tinned transformer lead through the designated hole, according to color.

Fig. 79 Now solder the leads on the bottom of the PCB

Fig. 80 Your completed PCB now looks like this. Next we'll test for bad connections before adding the front panel and module enclosure.

If we want our preamp module to have a balanced output, with the possibility of 6 dBu additional headroom/max gain, then we will need to install an output transformer. Whichever output transformer you’ve purchased, it is bound to have extra long leads (connecting wires). They will need to be trimmed to length, and then have their tips stripped and tinned for soldering. This process is explained in the following photographs.

Fig. 68 Bend the transformer leads over to the opposite side, and trim them about 5/8" past the edge of the lam stack (stack of flat metal pieces).

Fig. 69 Leads are now cut to length. Discard the scraps.

Fig. 70 Use your wire strippers to remove a small portion of the colored plastic jacket at the end of each wire lead.

Fig. 71 The transformer leads are now stripped and ready to be tinned. Be careful not to fray the tiny individual strands of wire exiting each lead. Twist them with your fingertips together for rigidity.

Fig. 72 Apply solder to the exposed wire tips. This is called "tinning", and aids in soldering to the PCB later on.

Fig. 73 All transformer wire leads are now prepped, and we're ready to proceed.

CAPACITORS

Now it is time to install the last, and perhaps the tallest, of our PCB components – the capacitors. The radial capacitors we’re using already have their leads in position. Simply insert each part into the designated pair of holes on top of the PCB and bend its leads over on the bottom.

Fig. 62 Hold the electrolytic capacitor flat to the PCB surface while bending its leads outwards underneath. This will hold it in place for soldering.

Fig. 63 Film bypass capacitors, if used, are installed in the same manner.

Fig. 64 With all capacitors installed and leads bent, it's time to turn over the PCB for soldering..

Fig. 65 Apply heat to both the capacitor lead and surrounding plated hole, and flow solder to complete the joint.

Fig. 66 With all joints soldered and leads clipped, the bottom of the PCB should look similar to this.

Fig. 67 Congratulations! All of your through-hole circuit board components have now been soldered in place. Next, we'll add an output transformer before installing the module hardware and enclosure.

The last of our front panel PCB mounting components is the XLR connector. It is significantly taller than the rest, which is why we waited until now. You will notice that its solder tabs conveniently grip the PCB holes, holding the connector in place while we turn over the PCB. Still, care must be taken to insure that the connector is held perfectly still, and flush to the PCB surface while soldering.

Fig. 59 Mount the XLR connector at the PCB front edge as shown.

Fig. 60 Keep the connector straight and steady while soldering on the bottom of the PCB.

Fig. 61 With all of the axial and front panel components soldered in place, here is what our PCB now looks like. Capacitors come next!

Now it is time to install the front panel pushbutton switches. These parts are identical to one another. Place any one in the positions labeled “SW1″, “SW2″, and “SW3″, and proceed as follows.

Fig. 51 Locate the pushbutton switches and corresponding PCB positions.

Fig. 52 Flip the PCB over while holding the switches in place, and place it on a hard, flat surface. To keep the switches flat against the PCB it may be helpful to support the opposite end of the PCB with an object of equal height.

Fig. 53 Press down on the PCB either side of the switches to maintain a steady, flush mechanical contact while soldering just the middle row of pins.

Fig. 54 The switches will be held temporarily flat/straight with just the middle row of pins properly soldered.

Fig. 55 Now check that the switches are evenly straight from every angle.

Fig. 56 Notice the uniform orientation of all our front panel components, perpendicular to the PCB surface. This will guarantee that the front panel mounts correctly.

Fig. 57 One last check. Straight switches look like this.

Fig. 58 Finally, solder the rest of the switch PCB pins. Good joints will look like this. No clipping is necessary.

In this tutorial we are using an unusually small input transformer, the Lundahl LL1587. Granted, it happens to be the next shortest component after our 1/4″ connector and gain pot, and thus will be installed next. Most input transformers are taller, and so we would install the switches first. If this is true of your input transformer, then skip to the next step and return here later.

Some important precautions:

• Installing the input transformer in the correct position and orientation is crucial. Among the many overlapping transformer footprints on the DIY500 mkII PCB you will find an outline and set of holes which match your selected model. Using the transformers datasheet and labeling, locate its pin 1 or chassis pin and correlate with the PCB. Most of the PCB transformer footprints have a white circle printed around the hole for pin 1, perhaps with an arrow pointing to it and indicting the manufacturer. (e.g. “LUNDAHL_SM” for the smaller sized Lundahl footrpint)
• Unless your input transformer is particularly large and heavy (e.g. the Cinemag size “C” models, such as CMMI-2CPC), it is advisable to leave a small amount of space between the PCB surface and the transformer can. This will help to avoid accidental shorts from the transformer can to a PCB signal connection.
• Take extra care to make quick, light, and effective solder joints around your input transformer pins/wire leads. It it easy to damage an input transformer through excessive soldering and overheating, especially small and delicate packages such as our Lundahl LL1587.

Fig. 45 Locate your input transformer footprint and determine the correct pin orientation.

Fig. 46 Gently insert the input transformer.

Fig. 47 In the correct position, this is what our Lundahl LL1587 input transformer looks like.

Fig. 48 Turn over the PCB. Quickly and carefully solder the transformer pins, using no more solder than necessary.

Fig. 49 Nicely completed solder joints will look like this. There is no need to clip the leads.

Fig. 50 A side view of our successfully installed input transformer. Though there is no gap, we can see that the clear laminating wrap creates an insulating layer between the conductive transformer case and the PCB surface.

The DIY500 mkII Minimal Kit includes a specialized, high quality potentiometer (pot) for gain adjustment. Designated “RV1″, it has 3 interior pins carrying the electrical connections, and 4 exterior solder tabs supporting a mechanical brace (standoff). Since this pot is just slightly taller than the front panel 1/4″ connector, we will now install it in a similar manner.

Fig. 39 Solder the pot from the bottom of the PCB, pushing down to hold it flush and steady in place. The solder joints will look like this when complete.

Fig. 40 Carefully trim the leads and solder tabs while holding the PCB outwards, with bottom layer facing away from you.

Fig. 41 This is what it looks like from the other side.

Fig. 42 Successfully trimmed leads.

Fig. 43 Make sure the pot is installed straight, with its standoffs flush against the PCB surface.

Fig. 44 Also make sure it is straight from the side view as well. We want our front panel to mount perfectly perpendicular.

Ignore the capacitors, transformers, and XLR connector for now – they will come later. Lining up the rest of our components, we can see that the 1/4″ connector is the next shortest and will now be installed.

Fig. 32 The front panel 1/4" connector is our next shortest component.

Fig. 33 Trim the front panel ground tab as short as possible. Be VERY CAREFUL to point it away from your face while doing so! The trimmed pieces will be violently projected.

Fig. 34 Now we're ready to install this connector on the top side of the PCB.

Fig. 35 Turn over the PCB and bend over some of the solder tabs. This will help keep the mounted 1/4" connector mechanically rigid and flush against the PCB surface.

Fig. 36 Solder the tabs while the connector is held still against the PCB.

Fig. 37 The soldered 1/4" connector should now look like this.

Fig. 38 Make sure that the connector sits flush against the front edge of the PCB before proceeding. If necessary desolder, adjust, and resolder.

By default, the DIY500 mkII front panel has a small hole drilled next to the 48V phantom power ON/OFF switch. If present, we can use this hole to install an optional indicator LED to let us know when phantom power is ON and present. 

Remember that for ease of assembly components should be installed in ascending order of height. Granted, we are installing this LED now – after the resistors and before the switches – because it is the next shortest component, and resides on the bottom of the PCB.

Also, please keep in mind that this LED (standard 3mm diameter) is: 1) polarized (pay attention to orientation!), 2) will be the only component installed on the bottom of the PCB, and 3) requires the presence of resistor, R24. One method of installation is described in the following photographs.

Fig. 27 a) Determine the LED polarity: the square PCB pad is (+), which is the side of the LED with a longer leg and rounded base flange, and the round PCB pad is (-), which is the side of the LED with a shorted leg and flatted base flange. b) Determine the length at which to bend the leads (legs). You want the LED base flange to be flush with the edge of the PCB.

Fig. 28 Bend the LED leads to length at a right (90˚) angle.

Fig. 29 Insert the LED from the bottom PCB layer as shown, and solder its leads in place on the top PCB layer.

Fig. 30 Once soldered, trim the excess LED leads.

Fig. 30 Very good! Proceed to the next step.

If your op amp is new, then it should have sufficiently long leads (connecting pins), the surfaces of which are clean and smooth. If this is the case, then you may elect to install sockets, rather than soldering the op amp directly to the PCB. Sockets will allow for quick and easy replacement/substitution of the op amp, no tools required, should the need ever arise. One method of installing the sockets is described in the following photographs.

If your op amp has been removed from another piece of equipment and has cut pins, covered in solder residue, then it’s best just to solder it to the DIY500 PCB without sockets, and skip the following.

Fig. 19 Empty the DIY500 hardware bag to find the (7) op amp sockets.

Fig. 20 You only need 6 sockets. Use your discrete op amp to hold them in place.

Fig. 21 Gently ease the op amp sockets into their PCB holes.

Fig. 22 Hold the socketed op amp in place while flipping over the PCB.

Fig. 23 Try to hold the PCB flush, pushing down against the sockets, while you solder in the outermost ones. Remember that a good solder joint must be held perfectly still while hardening.

Fig 24. Now you can let go of the PCB and quickly (e.g. 4 seconds) solder the rest of the sockets. Remember not to overheat or you could damage your op amp!

Fig. 25 With all sockets soldered in place, the bottom will look like this.

Fig. 26 Turn over the PCB and remove your op amp. The installed sockets should appear as shown here.

Resistors

The DIY500 utilizes many resistors, and a few other axial components (e.g. diodes, ceramic capacitors) of similar size and shape. Since these are the smallest, shortest, and most plentiful components used, we will make assembly easier by installing our resistors first.

Fig. 4 A pile of resistors, ceramic capacitor, and PCB on our slip mat.

These so-called axial components come to us with their leads (wire ends) sticking out of each side like the axle of a motor vehicle. On the DIY500 PCB, installation locations are indicated by a small printed rectangle with a metal plated hole at either side, and a component designation (e.g. “R8″) printed adjacent to it. The closely spaced vertical holes, which the component leads are inserted into to, indicate that we must bend the axial leads at a right angle such that they are pointing down and align with these two holes.

All holes are uniformly spaced 0.4″ apart, and we can bend the resistor leads roughly to length using our fingers (Fig. 5), or precisely to length using a graded component bending tool (Fig. 6).

Fig. 5 Bend resistor leads by gently pushing with your finger.

 or…

Fig. 6 Form leads to precisely 0.4" at 90˚ using a bending tool.

Once we have all of our resistors bent and separated by value (Fig. 7) it will be simple to insert them into the appropriate PCB locations. If you have a vice grip, you can use it to hold your PCB. If not, just hold the PCB in one hand and insert the resistors with your other hand.

Fig. 7 Bent axial components, ready to be installed.

We insert our axial components into the top layer of the PCB, which is screen printed  with the component designations in white (Fig. 8), but we (usually) want to solder them in place from the opposite (bottom) layer. Also, due to the effects of gravity and the long wire leads now poking out the bottom of each resistor mounting hole, sitting components may become easily displaced when setting down or turning over our PCB. To overcome these obstacles, we shall slightly bend outwards each components leads on the bottom layer while holding it flush against the PCB surface on the top layer, as shown in figures 9 and 10 below.

Fig. 8 Zero ohm (0 Ω) jumper resistors (shorts) installed in place of the optional protection diodes; D1, D2, and D3.

Fig. 9 Resistors installed from the top layer can fall out unless...

Figure 10. Exposed leads on the bottom PCB layer are bent outwards to hold components in place.

Once we have installed all of our tiny axial components in this fashion, we can lay the PCB face down on our work surface (or in a vice grip) and get ready to solder.

Fig. 11 With all axial components installed and held in place, place the PCB face down on your work surface.

Recalling proper soldering technique, proceed to carefully solder each joint in place (Fig. 12).

Fig. 12 Heat each joint (where the lead exits the hole) and flow solder onto it. Two hands are required.

Good solder joints flow from the lead into the hole and onto the surrounding plated area, forming a smooth reflective slope as shown in figure 13.

Fig. 13 Properly completed solder joints.

When you’re done with this stage of soldering, your PCB will look like figure 14.

Fig. 14 All axial components have been soldered in place.

Now it’s time to clip (shear) off all of the excess component leads. Beware these sharp little pieces of rigid wire being launched into the air once clipped! Eisen Audio does not want you to injure yourself, and recommends the following safety practices:

1. Wear safety goggles, and perhaps even safety gloves.
2. Point the bottom of the PCB (solder side) away from your face while clipping leads.
3. Gently place your finger over the lead(s) to be clipped, as this will block it from being launched into the air, but beware skin penetration! (Figures 15, 16)

Fig. 15 Clip leads close to the PCB, while (gently) guarding the excess portion with a finger tip.

Fig. 16 Clip all excess leads while holding the PCB away from your face.

Now your PCB will look like this (Fig. 17). Verify that you’ve installed all of the axial components in their appropriate locations, correlating with the list of values for your particular preamp configuration. If everything is okay, then it’s time to move on to the next step.

Fig. 17 All leads have been clipped just above the solder joint.

Fig. 18 All resistors and similar axial components have now been installed.

With a minimum of proper tools (and experience!), successful assembly should be relatively easy. Without these tools, assembly is bound to be frustrating, and may result in damaged components and/or a non-functioning preamp.

Surely, your hands are your most important and skillful tool, but in this tutorial we are also using the following, as shown in Fig. 1 below:

• A  Digital Multi-Meter (DMM) [optional] – for measuring voltage and resistance/continuity, to check for mistakes
• B  thin diameter (e.g. 0.031″) rosin-core solder (e.g. 63/37 with %2 flux) – for soldering small electronic components to Printed Circuit Boards
• C  wire strippers – for removing (stripping) a portion of the plastic jacket covering hookup wire
• D  solder sucker (plunger type removal tool) [optional] – for removing solder, to remove/replace a component or clean up a mistake
• E  temperature-controlled soldering iron (e.g. 700˚ F  20W) – regulated soldering iron (not soldering gun!), with cleaning sponge, for delicate electronics
• F  adjustable wrench – for tightening hex nuts
• G  component lead clippers – for close and accurate trimming of excess component leads once they’ve been soldered to the PCB
• H  #1 phillips screwdriver – for tightening screws used to affix the DIY500 module enclosure
• I  1/16″ hex wrench (aka “allen key”) – for affixing the gain pot control knob
• J  needle-nose pliers [optional] – for handling and bending component leads
• K   component lead forming tool (e.g. Mouser part # 5166-801) [optional] – bends component leads to accurate 0.1″ spacing for ease of assembly

Fig. 1  Typical tools.

Components

Next to your tools, unpack the contents of your DIY500 mkII minimal kit and the additional required components which you’ve gathered to complete the kit. Make sure you have everything you need before beginning assembly. Ease of assembly relies on all components being installed sequentially, in order of size. It will be more difficult to go back and add/replace a component later if you don’t have it now.

Work Area

When working with tiny components which are easy to loose and hard to see, you want a well-lit and uncluttered work surface. To avoid components rolling off the edge of your surface, we recommend keeping everything in the middle, and employing some sort of slip mat if the work surface is smooth. Also, beware that solder drip, sharp tools, and component leads may damage your work surface finish. Make sure to put down some sort of (non-flammable) protective cover if this is a concern.