The Mini³ portable stereo headphone amplifier

Technical Highlights

Ultra-portable, battery-powered headphone amplifier

  • The Mini³ is almost the same size as a credit card, and about 0.9" (23mm) thick. The diminutive dimensions makes the amplifier very pocket-friendly.
  • The extruded aluminum Hammond 1455C80x case, available in silver, black or blue anodized finishes, is attractive and sturdy.
  • Big, high-end sonics from a tiny package -- the Mini³ is carefully designed for excellent performance, rivaling amplifiers many times its size and cost.

Two battery options

  • Standard configuration: Rechargeable NiMH 9V battery gives good runtimes, while allowing a small case form factor.
  • LiPo upgrade: The optional χ1 battery management board adds support for the use of LiPo battery packs for much longer runtimes and short recharge time, with no increase in overall size.

3-channel active ground topology

  • Similar in concept to the celebrated and the reference class β22 (3-channel version), the Mini³ is also a 3-channel active ground design.
  • In addition to the left and right channels, the "ground" wire of the headphone is actively driven by a third channel. 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 power supply rails, thus removing the primary source of signal ground contamination. The headphone transducers "see" active amplifiers on both sides, rather than an amplifier on one side and a capacitor bank of the power supply ground on the other.

Precision virtual ground reference

  • Also similar to the M³, rather than using a conventional dual rail power supply with positive, ground and negative outputs (which, in the Mini³, would require two batteries and resulting in a larger, less portable case), a single 9V battery is employed, and a virtual signal ground is synthesized by using a TLE2426 precision rail splitter chip. This simplifies the power supply, yet provide the advantage of a dual-tracking split supply without associated complexity. The result is improved common-mode rejection ratio (CMRR).
  • The TLE2426 ensures that as the battery drains down, the power supply rails to the opamps remain evenly-split.
  • The signal ground quality is not affected by the return current from the headphone transducers due to the 3-channel active ground topology.

Two Mini³ variants to suit all needs

  • There are two variants of this amplifier, the "high performance" edition and the "extended runtime" edition.
  • The two editions are identical except for the choice of opamps.
  • The high performance edition utilizes an Analog Devices AD8397 dual opamp for the left and right channels, and a Texas Instruments/Burr-Brown OPA690 opamp for the ground channel. These opamps have very high output current capability (~190-250mA), and could drive low and high impedance headphones with authority.
  • The extended runtime edition use a Texas Instruments LMH6643 dual opamp for the left and right channels, and a LMH6642 opamp for the ground channel. This edition has respectable output current capability (75mA), but draws only 40% of the quiescent current compared to the high performance edition. This gives approximately twice the runtime between battery charges (up to 20+ hours).
  • The high output current characteristics of the opamps allows them to drive headphones directly without the use of additional output buffer stages, simplifying the circuit and allows the amp to be made smaller.
  • In both editions, the left and right channel opamps are rail-to-rail, capable of swinging output voltage to within 0.5V of each supply rail. This makes maximum use of the available battery voltage, providing improved output clipping/overload headroom.
  • The opamps in all variants have very high slew rate, wide bandwidth, low output impedance and low distortion.
  • All resistor values are chosen to reduce DC offset, minimize stray capacitance and inductance (which may cause opamp instability), and to maintain low noise.
  • Low value resistors at the output of the left and right channel opamps offer short circuit protection. These resistors are wrapped within the global feedback loop so that they do not cause the effective output impedance to be compromised.
  • Ferrite beads at the output of each opamp isolate headphone cable capacitance, and keeps the opamps stable.

Fully direct-coupled

  • There are no signal-degrading coupling capacitors at the input, output or negative feedback loop.
    This is a true DC amplifier. Care should be taken to ensure that the input source does not have DC offset at its output.

Suitable for a wide variety of headphones

  • Can be used with low-impedance and most high-impedance headphones.

Versatile power supply/battery charging circuit

  • Both 7-cell ("8.4V") and 8-cell ("9.6V") rechargeable NiMH battery types are supported. A battery capacity of 230mAH or greater is recommended for long run times.
  • A 7812 voltage regulator allows the use of an AC-to-DC adapter ("wallwart") that outputs anywhere between 15V DC to 24V DC (actual voltage) with a minimum current rating of 300mA for battery charging and for running the amplifier. Onboard voltage regulation allows a low cost unregulated linear AC-to-DC adapter to be used, while protecting the opamps from being damaged by over-voltage.
  • The charging circuit, utilizing a LM317L voltage regulator wired as a constant current source, safely charges the battery at 16mA without risk of damage. A depleted battery can be charged back to capacity overnight.
  • A zener diode protects the amplifier from damage if the battery becomes disconnected while the AC-to-DC adapter is plugged in.
  • A power-on indicator LED is on the front panel, and a battery charging LED is on the rear panel.
  • NiCd batteries are not recommended due to the poorer charge capacity and memory effect. Specified LiPo battery packs are supported when the optional χ1 battery management board is added. Any other battery types, including non-rechargeable types, should not be used.
  • Two 470µF reservoir capacitors (for a total of 940µF) provide ample charge to feed the current demands from the amplifier. These capacitors are before the power switch, so they are always charged as long as the battery is connected. This avoids a large surge current during power-on, preserving the integrity of the switch contacts. It also ensures that the opamps' supply rails turn on and off instantaneously, eliminating noise.

Designed to be easy to build

  • A specific target case, DC power jack, input and output jacks, volume control, battery contacts and other parts make this a highly integrated design.
  • All parts are through-hole except for the two SOIC-8 opamps, making the amplifier very easy to solder and assemble.
  • All parts are board-mounted, there are no wires to connect.
  • There are no trimpots to adjust after assembly.

High quality circuit board

  • Glass epoxy 3.1375"x1.9875" 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.
  • A low-impedance ground plane covers the entire surface of the bottom side. The top side has a partial V- plane that doubles as the heatsink for the 7812 voltage regulator.
  • Exposed copper islands beneath the SOIC-8 opamps allows improved heat dissipation when a small amount of heatsink compound is applied, especially for the AD8397ARDZ opamp which has a thermal "E-pad" at the bottom.
  • A strip of the ground plane is exposed along each bottom side, allowing the amplifier's signal ground to be electrically connected to the case (via the board slots) without the need to add any wires. The grounded case improves RFI shielding without additional wiring.
  • The board is designed to slide perfectly into a Hammond 1455C80x series extruded enclosure, making the casing of this amplifier very easy.

A 3D rendering of a partially-populated Mini³ circuit board.

Main: Mini³ Main | Prev: Overview | Next: Schematic diagram