The M³ stereo headphone amplifier

Power Supply

The M³ amplifier printed circuit board does not include a power supply. It needs a separate DC supply to provide power to the circuit.

The default recommendation is a regulated 24V DC power supply, that could support 0.5A of continuous output current. Ripple and noise should be as low as possible, 1mVp-p would be the maximum tolerable.

Only a single supply is needed. It is not necessary to use a dual (or "split") power supply with positive and negative rails. The single supply is internally split on the M³ circuit board into separate rails, providing a virtual ground centered at one-half of the supply voltage. This simplifies the power supply requirement, provides better common-mode rejection ratio (CMRR), and gives similar benefits of a dual-tracking power supply without the complexity.

The actual current draw of the M³ amplifier is approximately 0.3A, depending on its quiescent current adjustment settings. It is a good idea to use a power supply with some headroom above the maximum draw.

If you plan to drive speakers with the M³ amplifier (which it is capable of doing to reasonable volumes), use a power supply that is rated at 3A. If you are using a power supply based on the LM317 chip, you should upgrade it to a LM338, along with associated power transformer, rectifier diode and regulator heatsink upgrades for increased current capacity. See the Options section for more information about using the M³ as a speaker amplifier.

The recommended power supply is the σ11, featuring an ultra low noise, high current, wideband, all-discrete design. It offers the best performance possible. An alternative would be a σ25 with a σ78 ultra low noise voltage regulator.

For USA and other countries with 115V AC mains, an Elpac WM080-1950-760 wallwart (rated at 0.33A) will also work with most headphone loads, although it would be operating at its maximum current rating. Note that this power supply has been discontinued by the manufacturer, so it's a viable solution if you can find a used one, or a surplus stock, and you're on a low budget.

For European and UK 230V AC mains, the Mascot 2183 wallwart (rated 1A) is a suitable alternative. It is available from RS Components.

Do not use an unregulated power supply. Not only will an unregulated supply degrade performance (such as increased hum and noise), the lack of voltage regulation may cause damage to the opamps or other parts if the actual voltage exceeds the maximum ratings of the components.

Also, do not use a power supply that has its V- output connected to the "earth" pin of the AC plug. This is because V- is used as the negative DC rail on the M³ amplifier, and the signal ground is synthesized by an onboard TLE2426 rail splitter chip. If you use this kind of power supply, and any other component in your audio chain has its signal ground connected to the AC earth, then you will cause a short circuit from the V- to signal ground.

The power switch should always be on the AC mains, not between the PSU's DC output and the amplifier. This is due to the high capacitance across the M³'s power rails, which can appear similar to a dead short to the PSU momentarily while the PSU attempts to charge the capacitors. If the power switch is at the output of the PSU, when it is turned on, the sudden current surge could damage the PSU. This is especially important for the σ11 PSU, which has no current-limiting protection. Even LM317-based PSUs (with over-current protection) have been known to be damaged in this manner.

Always measure and verify the actual output voltage of the power supply before connecting it to the amplifier for the first time, to make sure its output voltage is as expected. Do not connect the PSU to the amplifier when the PSU power is already on.

The recommended configuration for the M³ is to use an outboard power supply. This allows the power transformer and other AC components to be located far away from the sensitive amplification circuitry for minimum induced hum and noise. You may instead use a large chassis and put both the M³ amplifier board and the power supply in it, but be sure to position the power transformer and other AC components as far from the input side of the amplifier board as possible. A toroidal power transformer is recommended in this setup for its lower magnetic field leakage.

With certain opamps, you may use a power supply with higher output voltage for more output swing capability. This may be desirable for high impedance headphones with low efficiency. The maximum power supply voltage is opamp-specific, see the table below. Also, be sure that all capacitors on the M³ board are rated higher than the power supply voltage.

For example, using the OPA604AP opamps and a 40V power supply, over 22Vp-p output swing can be achieved without clipping into a high impedance load.

These maximum ratings have a small margin of error, if you exceed them the opamps may be damaged. If you will be "rolling opamps" (i.e., trying different models), keep the power supply voltage at 24V and do not use any opamps with lower ratings than that, or use an adjustable power supply.

Do not use a power supply over 24V if the amplifier board is mounted in an unventilated chassis case. A higher power supply voltage causes increased heat dissipation in the MOSFETs. Also, due to a limit with the TLE2426 chip onboard the M³, do not exceed 40V even if you use opamps that could handle more voltage.

OpampMaximum PSU voltageNotes
OPA637AP36V Use in L & R channels only, G channel use OPA627
OPA228PA36VUse in L & R channels only, G channel use OPA227
OPA604AP40VGood for large voltage swings
LT1122CCN840VGood for large voltage swings

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