The M³ Stereo Headphone Amplifier

Technical Highlights

3-channel active ground topology

In addition to the left and right channels, in this amplifier the "ground" wire of the headphone is actively driven by a third channel of the same topology. The ground channel amplifier sources or sinks the return current from the transducers, which would otherwise have been dumped into signal ground or power supply ground. This shifts responsibility for the high current reactive load of the headphones from signal ground to the tightly regulated power supply rails, thus removing the primary source of signal ground contamination. The headphone transducer "sees" symmetrical output buffers with equal impedance and transfer characteristics on both sides, rather than an output buffer on one side and a capacitor bank of the power supply ground on the other. This results in lower output impedance, greater linearity and reduced stereo crosstalk.

High performance opamp voltage gain stage and high current discrete MOSFET output stage

A high performance IC opamp is used for voltage gain duty in the M³ amplifier, while the discrete power MOSFET output stage provides the current gain to drive the headphones. This separation of functions isolates the headphone load from the opamp and allows it to operate without strain. Both stages operate in class A for unparalleled linearity. The MOSFET output stage has enough output power capability to drive efficient speakers to reasonable volumes.

Capacitance multiplier for opamp power

The power supply to the opamps is further isolated and stabilized with a complementary capacitance multiplier. A capacitor is kept charged by two JFET current sources at each side, whose capacitance is then multiplied by the DC current gain of the complementary pass transistors to provide very low noise, low impedance DC rails to the opamps, unperturbed by the MOSFET supply rails.

Precision virtual ground reference

Rather than using a conventional split power supply with positive, ground and negative outputs, the M³ amplifier uses a single supply and internally synthesizes a virtual signal ground by using the TLE2426 precision rail splitter chip. This simplifies the power supply requirement, yet provide the advantage of a dual-tracking split supply without associated complexity. The result is improved common-mode rejection ratio (CMRR).

Adjustable JFET cascode current source

The opamp for each channel is biased to operate in class A by a pair of discrete cascoded JFETs. This prevents the opamp's output transistors from ever cutting off, eliminating crossover distortion. The amount of current flowing through the JFET cascode is adjustable via a trimpot.

Adjustable MOSFET bias

The MOSFET bias is controlled by a VBE multiplier transistor and the quiescent current is adjustable via a trimpot to suit individual needs.

Fully direct-coupled

There are no signal-degrading coupling capacitors at the input, output or negative feedback loop.

This is a true DC amplifier. The precision FET input opamps keep the DC offset very low, but care must be taken to ensure that the input source does not have DC offset at its output.

No input or output protection

The rugged and stable MOSFETs allows the simplest of output circuitry, providing maximum sonic transparency.

Optional bass boost feature

At the builder's option, a bass boost feature (either via an on/off switch or a continuously variable adjustment) can be installed to compensate for poor recordings or headphones without sufficient bass response.

Board-mounted premium potentiometers

The circuit board is designed to accommodate ALPS RK27 "blue velvet" or Noble AP25Y audio-grade dual-gang potentiometers for excellent inter-channel tracking. They are directly mounted onboard for the shortest signal paths to minimize crosstalk and interference.

Ground plane

Low impedance ground plane over low level signal areas provides shielding against interference, reduces crosstalk, and allows optimized component arrangement on the board.

High-current, superimposed power buses for MOSFET power

Wide "power plane" traces provide a low impedance path of DC power to the MOSFETs, with reservoir capacitors located immediately adjacent to the MOSFET banks.

Multiple input pads

The circuit board layout provides a set of input pads at the rear of the board as well as a set near the front. This minimizes the length of hand-wiring to the input jacks regardless of chassis case choice and board-mounting arrangement.

High quality printed circuit board

Glass epoxy 5"x7" printed circuit board, double-layer with plated-through holes, silkscreen and solder mask. The layout of all parts and traces have been carefully considered for maximum performance.

Suitable for a wide variety of headphones

Can be used with low-impedance and high-impedance headphones, and can be further optimized for each type.



Why a headphone amplifier

Most source components (CD player, tape deck, MP3 player, computer sound card, etc.) have a headphone output jack. However, due to cost, space, power, and other design compromises, they fall short measurably as well as audibly. The M³ amplifier is designed to deliver the power and the finesse to allow the headphones to perform at their best.

Why MOSFETs

The M³ amplifier is distinguished by its complementary push-pull power MOSFETs output stage. This offers advantages over the conventional BJTs (bipolar junction transistors) in several ways.
  • MOSFETs are more reliable. They do not have the characteristic "secondary breakdown" region of BJTs and thus no output current limiting protection circuitry is necessary (such circuitry have undesirable sonic consequences). The MOSFETs used in the M³ amplifier are rated at over 17 amperes and are thus nearly indestructable when serving duty in a headphone amplifier.
  • BJTs have a positive temperature coefficient. This means that the current gain increases as the device warms up, causing the temperature to rise further. This leads to "thermal runaway" conditions that must be dealt with. It usually involves mechanical coupling of biasing circuitry to the output devices for thermal feedback compensation. MOSFETs have a negative temperature coefficient and are thus inherently stable, requiring no more thermal management than simple heatsinking and ventilation.
  • MOSFETs are majority-carrier devices and are faster than BJTs of comparable ratings, because they do not exhibit the BJT's "hole storage and recombination delay" characteristics.
  • MOSFETs are voltage-to-current amplification devices (contrast to the BJT, which are current-to-current). The gate impedance of a MOSFET is very high and thus simplifies the needed drive circuit. They do have a capacitive gate characteristic but an opamp has very low output impedance and more than sufficient output current capability to provide the MOSFET gate charge requirements. In the M³, the output of the opamp is also biased deeply in class A with a JFET cascode constant current source, providing fast and abundant drive directly to the MOSFETs.
  • The 5th-generation HEXFET™ from International Rectifier specified in the M³ amplifier has very low voltage loss compared to earlier devices. It also has lower input capacitance than previous MOSFETs with similar current capacity.
  • The voltage drive characteristics of MOSFETs are similar to vacuum tubes. Amplifiers employing MOSFETs possess the best sonic qualities of tubes and transistors without the shortcomings of either.
Indeed, the low parts-count of the M³ owes much to the use of MOSFETs. In this amplifier, the simplicity is an advantage, keeping the cost modest while offering superior performance and reliability.

Why IC opamps

Early opamps had limited bandwidth and speed, high noise floor, inadequate output drive capability, or other characteristics that made them sonically inferior to fully-discrete amplifiers.

Things have changed dramatically as technology advances. Today there are a number of very high performance opamps that are quite appropriate for high performance audio.

An IC opamp has all its parts formed on one monolithic substrate, with excellent matching between internal transistors, resulting in excellent temperature stability, precision DC and AC behavior, small size and reduced parts count in the overall design.

The recommended modern FET-input opamps for the M³ amplfier all possess very low noise and distortion, wide bandwith, speed and low offset. When configured as a class A voltage gain driver stage for the discrete output MOSFETs (which presents a light load to the opamp), the result is an excellent combination of both worlds. Along with with other carefully designed details, this topology meets the simplicity and high price-performance ratio that are fundamental goals of this project.

The old belief that IC opamps cannot rival the performance of an all-discrete topology dies hard, and the M³ is an accomplished counterpoint, both in terms of measured characteristics and actual listening comparisons.

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