The Mini³ portable stereo headphone amplifier

Project history & acknowledgements

After the successful launch of the M³ amplifier, morsel and I talked about developing a small, portable, inexpensive, and easy-to-build amplifier using a similar active-ground 3-channel topology found in the M³. Actually, such an amp had been on morsel's mind for some time, but the size contraints means that this would have to be a design where the opamps would drive the headphones directly without a separate output buffer stage. A suitable opamp had not been found.

The arrival of Analog Devices' AD8397 changed all that, especially the AD8397ARDZ version with EPAD. Here is an audiophile-quality dual-opamp with an abundant output current capability, where it is not a compromise to eliminate a separate output buffer stage. It also boasts superb performance in many other areas, which are confirmed by bench measurements and listening tests. Thus, morsel and I announced the Mini³ project on the headwize forum in February 2006.

The original concept, now called Mini³ v1, was housed in the Serpac H65 plastic case, and used twin 9V batteries. It had two AD8397 opamps, one for the left and right channels, and another as the rail splitter and ground output channel. We did extensive development and testing, and built a couple of prototypes.

Even though the AD8397 is rated for up to 24V, we found that the circuit was sometimes unstable when powered with such high voltages while driving low impedance loads. The result was oscillation, rapid heating and blown opamps. The lack of built-in output current limiting or thermal protection compounded the problem. Ironically, one of the reasons why the AD8397 sounds good is because it has no current limiting. Such circuitry often results in undesirable sonic consequences. Tangent's PINT amplifier (which was originally called "Mint 2.0", and very similar to Mini³ v1), also experienced a high problem rate. He eventually discontinued it.

Due to other priorities, the Mini³ project stalled, and nothing was released. However, it was decided that a change to single 9V battery power was a key solution, which also provided an opportunity to use a smaller, more robust and more pocket-friendly case. The heat dissipation is also halved by this change.

Changing the ground channel opamp was another important solution. We determined that the AD8397 did not serve that role well, and two of these opamps is not good for battery run-time due to the high quiescent current. The AD8397 with its rail-to-rail output and high current capability is still attractive as the choice for the left and right channels.

While the Mini³ v1 project sat in hiatus, I worked to release the β22 amplifier, σ22 and σ11 power supplies, and the CK²III headphone amplifier.

Fast forward to April 2007. After the San Jose headfest meet, I finally found time to restart the project, and the result is Mini³ v2 that you see in these pages. The revised Mini³ was announced on the headwize forum in June 2007, solving all the technical difficulties of v1, and was reborn in the smaller, more robust Hammond 1455C80x casing, as well as an improved power supply section. In addition to retaining the AD8397 for the left and right channels, the ground channel now uses the excellent OPA690 opamp (thanks to a tip from KurtW). This was chosen in favor of the AD8010, which consumes much more current without any benefit over the more miserly OPA690. The rail splitter was changed to the well-proven TLE2426, instead of using one half of a dual opamp. This also helped improve overall stability and paid dividends in reduced battery drain.

A low-power, extended runtime edition was also added, using the LMH6643 and LMH6642 opamps. These reduced battery draw to only 40% that of the standard "high performance" edition. Due to the reduced supply voltage, the left and right channel opamps were specifically chosen for their rail-to-rail swing capabilities with an eye for robust output current capability. With single 9V battery power, the AD8397 and LMH6643 opamps could both swing to within 0.5V of each rail, which is as much voltage as many other opamps running on two 9V batteries.

Mini³ v2 stays true to the original goal, to make a small, highly-integrated, high quality portable amplifier that's low in cost and easy-to-build while offering excellent performance. A fast track development and testing phase led to the general release of production circuit boards in July 2007.

I would like to thank morsel for all her help during the Mini³ v1 development and prototyping phase, and to everyone who participated in the forums.

Look for more exciting DIY projects from AMB in the future!

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