The M³ stereo headphone 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 MOSFET heatsinks must be tapped for imperial #4-40 or metric M3 machine screws, or you may use self-tapping screws in order to secure them to the board.

Since each MOSFET's mounting tab is internally connected to the drain pin, it will carry live voltages and should not be shorted to the chassis, ground or other parts of the circuit. If your heatsinks will be in close proximity to the top metal cover when the board is installed in your chassis case, then you should mount the MOSFETs to the heatsinks using TO-220 insulating kits. Otherwise, you may mount the MOSFETs directly to the heatsinks using #4-40 or M3 machine screws and hex nuts, after applying some heatsink thermal paste to the mating surfaces.

The MOSFETs are vulnerable to electrostatic discharge damage when you are handling them, so keep their pins plugged into anti-static foam while working with them until you're ready to solder them to the board. Set the heatsinks pre-mounted with the MOSFETs aside for now.

Clean both sides of the blank M³ board with paper towel and isopropyl alcohol or electronics flux remover, then solder the components to the board, starting with the lowest profile parts. This means the resistors, opamp sockets, and diode. Then solder the small capacitors, small transistors, TLE2426 rail splitter, trimpots, followed by the large capacitors, connectors and panel-mount potentiometers. Lastly, install the MOSFETs and secure the heatsinks to the board with screws, and solder their pins.

Since the M³ 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, diodes, transistors, the opamp sockets and the TLE2426 rail splitter, as well as the orientation of the trimpots. For electrolytic capacitors, the positive lead should always go into the pad with a square outline. Also, be sure not to mix up the N-channel and P-channel MOSFETs!

If you are not installing the bass boost feature, you must install a jumper wire across the Sbb positions for the left and right channels. If you are using a fixed bass boost switch, then you should install Rbb and Cbb for both channels. If you are using a variable bass boost control, then you should install Cbb for both channels and the bass boost potentiometer. See the Parts list section for details.

Do not install the opamps into their sockets at this time. They will be installed during the Initial setup phase for testing and adjustment.

The V+/V- DC power pads on the circuit board accommodates 0.100" or 0.200" headers or terminal blocks. The audio input and the LED pads accepts 0.100" headers or terminal blocks. These allow easy disconnection from the board. You may also simply solder wires directly into the pads. The next section describes the wiring and grounding considerations.

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

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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