The M³ stereo headphone amplifier

Other options

M³ as a speaker amplifier

The M³'s high current, low impedance MOSFET output stage is robust enough to drive speakers. The maximum output power is directly related to the power supply voltage and current capability. For primarily speaker use, a 36V supply rated at 3A or higher, and opamps that could handle the voltage (see the Power supply section) are recommended. The σ11 power supply is an excellent choice.

It is also recommended to increase the total C7 rail capacitance to 6000µF and upgrade D1 to a 1N5401. This diode is too long for horizontal board mounting, but you could mount it vertically. You could also use a diode in a two-pin TO-220 package, such as the MUR820. No heat sinking of the diode is necessary. The minimum heatsink recommended for the MOSFETs is the 1.5" version, and be sure that the case has good ventilation. Due to increased current draw, the 0.200" version of the screw-down terminal strip is recommended for the M³ board V+ and V- DC power input (if you choose to use one). See the Parts list section for details.

This configuration should result in a high quality 6Wrms per channel (into 8Ω) power amplifier, and can drive efficient speakers to satisfactory volumes in a small to medium sized room.

The left and right channel outputs are connected to the plus (+) terminal of each speaker, respectively, and the ground channel output should be connected to the minus (-) terminals of both speakers.

For even more power, two M³ boards can be used in a balanced configuration (see below). This is commonly known as "bridged" outputs and will give about 20Wrms per channel (into 8Ω) when used with a 36V, 5A power supply (and appropriate opamps, see the power supply section for details). 2" heatsinks (or taller) for the MOSFETs are recommended for this setup due to the increased power dissipation. See the Aavid-Thermalloy 531202B00000 or 531302B00000.

M³ as a pre-amplifier

The low-noise performance and moderate voltage gain of the M³ also makes it an excellent line-stage pre-amplifier. The ultra-low output impedance allows the M³ to drive a very long interconnect cable without loss of quality. Since a pre-amplifier's primary function is to switch among multiple input sources, just add multiple input jacks and a selector switch. To minimize noise and crosstalk, mount the selector switch as close to the rear panel input jacks as possible, and use a shaft extension kit (with couplers and panel bearings) for the front panel knob.

If the M³ will serve strictly as a pre-amplifier (which would be used to drive an outboard speaker power amplifier), then the output ground channel can be left unpopulated (i.e., all parts with designations ending with a G-suffix). The pre-amplifier output jacks should have its ground connected to the IG pad of the M³ board, not the OG pad. The easiest way to do this is to wire all the input jack grounds and preamp-out jack grounds together and run a single wire to the IG pad of the M³ board.

If the M³ pre-amplifier will also serve as a headphone amplifier, then the ground channel should be fully populated. The headphone jack should be connected as described in the wiring & ground section.

The following diagram illustrates a pre-amplifier configuration with three-inputs, a loop-out, and independently switchable pre-amplifier and headphone outputs. Use a double-pole, 3-position rotary switch for the input selector, and DPST switches for the loop-out enable and pre-out enable switches.

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Balanced configurations

A "balanced" amplifier topology is one which has two amplifiers per channel, each driving the + and - leads of the headphone transducers respectively. The + amplifier and the - amplifier outputs the same signal, except reversed in phase. Thus the transducer "sees" the output difference between the two amplifiers.

In a balanced amplifier configuration, the maximum output voltage is doubled, which represents a theoretical quadrupling of the output power. For speaker amplifiers, this is known as "bridging", and is a common method to increase output power. Lack of power is seldom an issue in a good headphone amplifier, however.

Standard headphones have a three-contact plug, with a common "ground" return for the two channels. Modifications must be made to the headphone wiring (to separate the left and right ground leads) in order to use a balanced output amplifier. The plug also has to be changed to a non-standard configuration, such as two mono plugs, or a four- or five-pin XLR connector.

Another advantage of balanced output, when compared to conventional amplifiers, is the fact that the transducers see the same transfer function at both leads. Nonlinearities in the amplifier are therefore cancelled and distortions reduced.

The M³ is not a conventional amplifier. Its active ground channel, in conjunction with isolated power rails, virtual signal ground, and class A biasing provide similar benefits to a balanced amplifier while requiring one less amplifier channel and avoids the need to rewire the headphone.

Despite this, some people have expressed an interest to convert an M³ amplifier to operate in a balanced topology. To do so would require two M³ boards to provide four amplifier channels for stereo operation. The ground channel on each board will be unused. In fact, all parts for the ground channel (whose designations end with a G-suffix) can be left unpopulated.

Balanced outputs and balanced inputs

One scenario where it makes sense to run an M³ amplifier in balanced topology is when it has fully balanced XLR inputs. This assumes, of course, that your source also has balanced XLR outputs. With such a source, there are already two separate signals per channel, 180° out-of-phase with each other. These two signals can then feed two amplifiers to drive each transducer, without a conversion to unbalanced mode.

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For the first M³ board, the + signal line from the left input XLR connector can be routed to the left channel input (IL), and the - signal to the right channel input (IR). The left output (OL) should then be wired to the + lead of the headphone transducer, and the right output (OR) to the - lead. Do the same for the right input XLR connector for the second M³ board.

Note that with this setup, the volume and bass boost controls are separate for each stereo channel. To control both channels simultaneously, you should use four-gang potentiometers for these, wired off-board. Four-gang log taper panel-mount potentiometers are difficult to find for the DIY hobbyist, but stepped attenuators in this configuration are available from several vendors, albeit quite expensive.

Balanced outputs and unbalanced inputs

This configuration really doesn't make a lot of sense, especially in the M³ amplifier, and is thus not recommended. But if you really want to drive your headphones in a balanced fashion even with unbalanced inputs, the following is the simplest way to do it (you will need to hack the circuit just a bit). Specifically, the second channel in each M³ board is modified to derive an inverted phase output from the first channel. It requires only the addition of two resistors and does not use any extra active circuitry or transformer.

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Leave the R2R resistor position unpopulated, and install a 1KΩ resistor in place of R1R. Then, connect an additional resistor Rx between the OL trace and pin 2 of OPAMPR (you need to do this on the bottom of the board). The value of the Rx resistor should be the same as R4L and R4R.

For the first M³ board, connect the left channel input to the IL pad, leave the IR pad unconnected, and connect the outputs the same way as in the balanced output with balanced input scheme above. Do the same with the right channel input for the second M³ board.

If you want bass boost, you should use a two-gang potentiometer and wire it up such that each gang is connected to each board's first channel only, and install a wire jumper in the Sbb position of the second channel. This is because the second channel of each board acts only as a phase inverter in this configuration, and must not provide additional bass boost.

A two-gang stereo volume control pot should be wired off-board in this configuration.

Balanced and unbalanced inputs and outputs

The following illustrates a topology which allows two M³ boards to operate as one fully-balanced stereo amplifier (with balanced inputs and outputs), or as two unbalanced stereo amplifiers (with unbalanced inputs and outputs). The two boards must be fully populated (including the ground channels) for this configuration. There is a single pair of XLR balanced input jacks and two pairs of unbalanced RCA phono input jacks (A and B), plus connections to one balanced headphone and two unbalanced headphones (A and B).

When the amplifier is to be used in fully balanced mode (which is equivalent to the "Balanced outputs and balanced inputs" topology described above), use the XLR balanced inputs and connect a balanced headphone to the amplifier (shown as the middle headphone in the diagram).

For unbalanced operation, there are two independent amplifiers A and B. Use the unbalanced inputs "A" to drive an unbalanced headphone "A", and likewise use the "B" inputs to drive headphone "B". Each unbalanced amplifier A and B operate with an active ground channel, just like a standard M³.

If four-gang volume and bass boost potentiometers or stepped attenuators are used, then both boards are controlled simultaneously.

This topology makes a versatile amplifier that can be used in either balanced or unbalanced mode as required by the available source and headphones.

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To save panel space, two Neutrik NCJ6FI-S combo jacks (each containing a 3-pin female XLR and one 1/4" stereo phone jack) would be ideal for this application.


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