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
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
Initial setup & adjustments
- 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.
- 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
- Make sure your power supply outputs the correct DC voltage. See
the Power supply section.
- Install the ground channel opamp OPAMPG into its socket. Be sure
the orientation is correct.
- 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.
- 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.
- 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.
- 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.
- 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+.
- Turn off the power and wait at least 30 seconds for voltages to
- Repeat steps 4 through 10 for the left channel, except referring to
OPAMPL, R5L, OL, BIASOPL, R9L+/R9L-, and BIASL.
- Repeat steps 4 through 9 for the right channel, except referring to
OPAMPR, R5R, OR, BIASOPR, R9R+/R9R-, and BIASR.
- 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.
- 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.
- Optional: If you have an oscilloscope, you may wish to connect it to
the amplfier outputs to check for oscillation.
- 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.
- If all is well, you can now connect the input source as well
as headphones, and turn the volume up for a listen.
- Never connect headphones to the output if the DC offset voltage
is not within the specified limit. Otherwise damage to your headphones
- 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.
||Maximum recommended quiescent current|
- 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
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