The M³ stereo headphone amplifier

Before you start

Use the step-by-step procedure below to set up your amplifier after you have completed assembling all parts on the circuit board and wired up all its input and output connections. Read through all steps first and familiarize yourself with what you're about to do before proceeding. Do each step in order, and don't proceed to the next step until you have successfully completed each step.

Do not connect the power supply or turn on the power until called for.

You will need a multimeter for this procedure (a digital multimeter is preferable). If at any point your measured voltages do not match what is described, immediately remove power and check the board and wiring for errors.

Several steps of this procedure requires you to be measuring the voltage across a resistor while adjusting a trimpot at the same time. Unless you have three hands this is tricky to do, and a slipped meter probe could cause a short circuit and lead to damage. Thus, please use mini-grabber adapters for your multimeter's probes (e.g., Pomona 4826) so that they can be securely clipped to the resistor. This also prevents a sharp meter probe from scratching through the board's soldermask and shorting to the ground plane.

Initial setup & adjustments

  1. Pre-set the BIASL, BIASR and BIASG trimpots to their minimum positions (fully counter-clockwise). You should feel a slight click when the stop is reached. Pre-set the BIASOPL, BIASOPR and BIASOPG trimpots to approximately the center of their control range. For example, if you use a 20-turn trimpot then rotate counter-clockwise to its stop, then rotate 10 turns clockwise.
  2. Measure the resistance between the V+ and V- rails. It should not be short-circuited. Similarly, measure the resistance between pins 7 and 4 of each of the opamp sockets, as well as between these pins and the virtual ground (the IG contact) at the input pads. There should be no shorts.
  3. Make sure your power supply outputs the correct DC voltage. See the Power supply section.
  4. Install the ground channel opamp OPAMPG into its socket. Be sure the orientation is correct.
  5. Connect the power supply to the amplifier, set the volume and bass boost controls to their minimum position (fully counter-clockwise), and set the bass boost switch to "off", if equipped.
  6. Turn on the power. The power LED should glow. Measure the DC voltage across pins 7 and 4 of the opamp. You should get approximately the power supply voltage minus 1.5V to 3V. Also, check the DC voltage between pin 7 of the opamp to IG, as well as between pin 4 and IG. These should be half of the first measurement.
  7. Measure the DC voltage across the R5G resistor. Adjust the BIASOPG trimpot until you get around 0.4V to 0.5V. This corresponds to 4mA to 5mA through the JFET cascode current source. This is an optimal amount of current, high enough to provide good MOSFET drive and keeps the opamp output stage deep in class A operation, but not too high as to strain the opamp.
  8. Set the multimeter to DC mV range. Measure the DC offset voltage between IG and OG. At this point the MOSFETs are running with very little bias so the DC offset may be higher than desired, but should still be lower than 100mV.
  9. Measure the DC voltage across the R9G+ resistor, and adjust the BIASG trimpot clockwise until you read 80mV. This corresponds to 80mA quiescent current through the MOSFETs (See note below). Check the voltage across R9G-, it should be within 5% of that across R9G+.
  10. Turn off the power and wait at least 30 seconds for voltages to drop.
  11. Repeat steps 4 through 10 for the left channel, except referring to OPAMPL, R5L, OL, BIASOPL, R9L+/R9L-, and BIASL.
  12. Repeat steps 4 through 9 for the right channel, except referring to OPAMPR, R5R, OR, BIASOPR, R9R+/R9R-, and BIASR.
  13. Leaving the amplifier powered up, re-check the DC offset voltage between IG and OL, IG and OR, IG and OG respectively. Each should now be less than 2mV, but tolerable at 5mV. Also, measure at the same points with the multimeter in AC mV range. The readings should be less than 0.1mV. Note that many DMMs cannot measure such low voltages accurately. Also, if your amp isn't yet mounted in an enclosure and grounded properly with good wire routing, then you may not get the expected measurement result.
  14. Let the amplifier warm up for a half hour. Re-check the DC voltage across the R9 resistors of each channel, and adjust the BIAS trimpots for 80mV if necessary (See note below). The quiescent current will change a bit over time. When first powered up, the quiescent current will be higher, but as it warms up, the negative thermal coefficient of the MOSFETs will automatically cause the current to drop and stabilize at a lower current. For this reason, you may want to set the quiescent current 10mA to 15mA higher than the target, so that after you close the chassis cover, the elevated temperature will cause the current will drop to where you want it.
  15. Optional: If you have an oscilloscope, you may wish to connect it to the amplfier outputs to check for oscillation.
  16. The initial setup is done. Before you connect any input source to the amplifier, measure the source to make sure it has no DC offset at its output. Any such DC offset will be multiplied by the gain of the M³ amplifier.
  17. If all is well, you can now connect the input source as well as headphones, and turn the volume up for a listen.

Notes

  • Never connect headphones to the output if the DC offset voltage is not within the specified limit. Otherwise damage to your headphones may occur.
  • 80mA quiescent current should be adequate for most applications. However depending on your power supply voltage, you may increase the quiescent current somewhat. This will allow the amplifier to drive a very low impedance load without dropping out of class A operation at extremely high volume levels. Also, the MOSFETs operate in a more linear region (less distortion) when running with more quiescent current, but doing so will cause the MOSFETs to dissipate more heat. The following is a table of maximum recommended quiescent currents based on power supply voltage, assuming that you are using a 1.5" tall extruded heatsink on each MOSFET and you have adequate ventilation in your chassis case. Observing the recommendations of this table, the power dissipation on each MOSFET will be kept below 2W.

    PSU voltage Maximum recommended quiescent current
    24V160mA
    26V140mA
    28V130mA
    30V120mA
    32V115mA
    34V110mA
    36V105mA
    38V100mA
    40V95mA

  • If you are using an unventilated case, you should restrict the quiescent current to no more than 50mA per channel.
  • If you are going to be moving the board, desolder parts or otherwise work on the board after turning off the power, be sure to wait enough time to let all capacitors discharge completely. Stored charge in the capacitors could damage parts if board traces are short-circuited to each other by contacting other objects or the soldering iron.

Share your joy

When you are all done and the amp is installed in its case, show your pride by submitting photos of your M³ for inclusion in the M³ gallery!


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