The α24 fully-differential line amplifier

Instructions

Before you start

You can print out an image of the circuit board silkscreen layer (PNG format | PDF format), to use as a guide for installing components.

While you look at the board layout, please also take the time to look at the schematic diagram and associate each part with their location in the circuit. While this is not normally required to build a working amplifier, one of the opportunities of DIY is to learn about how the circuit works. Try to determine what each part does and why the particular part or value is chosen. There are many web resources to help you with this, including the AMB DIY audio forum. You will find the overall DIY experience more rewarding as a result.

Do not remove the opamps from their sealed packaging until you're ready to solder them on the board. This is to protect them from electrostatic discharge and moisture.

Due to α24's small size and close approximity of parts, as well as the three surface-mount (SMT) opamps, you should have the following tools and supplies to help you work on the board and case:
  • A good soldering iron with fine tip, preferably with adjustable temperature. For example, a Weller WLC100 with ST6 tip. A more deluxe soldering station such as the Weller Weller WES51 or Hakko FX888 is nice, but not necessary.
  • Liquid flux or flux pen for electronics soldering/rework
  • Thin gauge solder, such as 0.025". Your choice of 60/40 or 63/37 tin/lead. Avoid silver solder as it requires high heat. Lead-free solders also require higher heat and their durability is still in question.
  • Fine tweezer with sharp points, such as those from Wiha
  • Desoldering braid
  • Chip Quik® SMD desoldering kit (if needed).
  • Vision aid, such as magnifier lamp, loupe, magnifying eyewear (e.g., Fisherman Eyewear Flip-n-focus), etc.
  • Needle nose pliers
  • Diagonal cutter
  • Cotton Q-tips
  • 99% isopropyl alcohol or specialty electronics flux remover spray
  • Multimeter with sharp probes

Circuit board assembly instructions

Clean both sides of the blank α24 board with paper towel and isopropyl alcohol or electronics flux remover, then solder the components to the board, beginning with the the opamps for maximum access.

If you're new at soldering surface-mount devices, please view the following video, an excellent tutorial. The opamps on α24 are easier than the examples in the video.

Surface Mount Soldering 101
by Curious Inventor

Do the U1 opamp first, as described below.

Apply some liquid flux to the opamp pads. Place the opamp over the pads, use the tweezers to pick and nudge the opamp until it is centered and aligned perfectly over the pads. Be sure that the pin orientation is correct. The side of the opamp with pin 1 is beveled. Viewing the board with the "α24" logo proper side up, the pin 1 pad of all opamps are at the bottom left side.

Press the tweezer tip on the top of the opamp to keep it from shifting while soldering. Apply only a tiny amount of solder to the tip of your iron, and tack down one corner pin of the opamp. I find that it helps to use a "wiping" motion of the tip on the pin and pad. If necessary, make small adjustments while heating that pad and pin again. If all is well, do the pin on the diagonally-opposite side. Then, do the remaining pins one at a time, reflow any pin that needs a bit of touch-up. If necessary, use the desoldering braid to remove any excess solder, and be sure there are no solder bridges between the pins.

Use your multimeter and check the resistance between each pin of the opamp and the pad to make sure you don't have a cold solder joint (i.e., your meter should read close to zero ohms). Also check continuity between adjacent pins for solder bridges (meter should read infinity).

Repeat the above procedure for the U2 and U3 opamps. When you are done, use a Q-tip and isopropyl alcohol, or specialized electronics flux remover to clean off the flux.

Now mount and solder the rest of the parts, starting with the lowest profile parts and work up, in the following order (see details below):
  • Resistors
  • Multilayer ceramic capacitors
  • Polypropylene film capacitors (if used)
  • Conductive/organic polymer electrolytic capacitors
  • Connectors
Since the α24 circuit board holes are plated through, with the exception of the SMT opamps, you only need to solder the parts from the bottom of the board. Do not drill or enlarge the holes because that would damage the through-plating.

Make sure the correct part goes into each position on the circuit board. Measure each resistor with your multimeter to ensure it's the proper value before installing it. Pay attention to the polarity of the polymer electrolytic capacitors, the positive lead is usually the longer one.

Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush. Using your diagonal cutter, trim all protruding pins, leads, solder tabs, etc., on the bottom side.

Inspect all solder connections carefully, using a magnifying glass, to make sure there are no solder bridges or cold solder joints. Use a multimeter in ohms scale to check for short circuits. Correct any mistakes before moving on to the next phase.

Next, proceed to the Wiring & ground section for details about wiring up the connections to the board.


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