The α20 Line Stage Amplifier


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.

Circuit board assembly instructions

The output transistors Q5 and Q6 may have a metallic mounting surface which is connected internally to its collector pin. If your heatsink will be in close proximity to the top metal cover when the board is installed in your chassis case, then you should mount the output transistors to the heatsinks using TO-220 mounting kits. Otherwise, you may mount them directly to the heatsinks using #4-40 or M3 machine screws and hex nuts, after applying some heatsink thermal paste to the mating surfaces.

Solder the components to the board, starting with the lowest profile parts. The order of items to work on are the small resistors, the small capacitors, small transistors, large resistors, trimpots, polyester film capacitor, followed by the electrolytic capacitors and output transistors.

Since the α20 circuit board holes are plated through, 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. Pay attention to the polarity of electrolytic capacitors, transistors, and the orientation of the trimpots.

Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush.

Install the output transistor heatsinks.

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 proceeding 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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