The M³ stereo headphone amplifier

Wiring & ground

After you finish assembling the circuit board, it's time to work on the chassis case. When ready, mount the M³ circuit board into the case with standoffs or spacers. Make sure none of the leads and traces on the bottom of the board touches the metal chassis. If your standoffs or spacers are short, you should glue an insulating plastic sheet beneath the board to prevent board flexture from causing short circuits. Then, mount the input and output jacks, power connectors and power indicator LED.

Connect the DC power input, audio input/output jacks, and power indicator LED to their respective pads on the board:
  • The V+ and V- pads on the board are for the DC power input.
  • The IL, IR and IG pads are for the left audio input, right audio input and the signal ground.
  • The OL, OR and OG pads are for the left channel, right channel and ground channel outputs.
  • The LED pads are for the power on-indicator LED. Pay attention to the polarity.
  • All wiring could be done with simple hookup wire. If the wiring from the audio input jacks to the board is over a few inches long, consider using shielded cables.
  • Keep all wiring neat. Each +/- wiring pair should be twisted together to improve common-mode noise immunity. Use heat-shrink tubing to insulate soldered connections on the jacks.
In all cases, the headphone jack(s) must be isolated (i.e., the "ground" contact must not be electrically connected to the mounting threads).

If your amplifier chassis is non-conductive (e.g., plastic), then you should connect a short length of wire to the volume control potentiometer body and solder it to the nearby IG (signal ground) pad on the board. For the Alps blue velvet potentiometer, you can simply loosen one of its screws, wrap the wire around it and re-tighten.

For use with external power supply

The DC power input jack must be isolated from the chassis (the contacts on the jack should not be connected to the mounting threads). Connect this to the V+ and V- pads on the board. You may optionally route one of the V+ or V- wires through a power switch.

If the power supply has a metal chassis that is connected to the "earth" pin of its AC power plug, then that chassis should not be allowed to touch the chassis of the M³ amplifier to avoid a ground loop. A ground loop causes hum and noise.

To allow the M³ amplifier chassis to act as a shield, the RCA input jacks' ground connections should be electrically connected to the chassis, which should also be mechanically and electrically tied to the housings of the volume and bass boost potentiometers.

For use with internal power supply

If your M³ amplifier chassis is metal, use the following wiring scheme to prevent a ground loop:

Isolate the RCA input jack ground connections from the chassis. Connect the AC "earth" pin to the chassis. In addition, connect a "ground loop breaker" between the IG pad on the M³ board and the chassis. A ground loop breaker is a 10Ω 5W resistor in parallel with a 0.1µF capacitor rated at more than 250VAC. For safety this capacitor should be rated for class X or Y with flame retardant casing. Finally, connect the output of the power supply to the V+ and V- pads on the board.

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