The β22 Stereo Amplifier

News

  • December 4, 2007: A limited quantity of ε22 v1.1 backplane boards are now available at AMB audio shop. Each ε22 supports three β22 boards in 3-channel active-ground configuration, and minimizes internal wiring while improving serviceability.
  • As of October 8, 2007, all new β22 boards from AMB audio shop are updated to version 1.02. Please see the Instructions section about changes to how Q1 through Q4 should be mounted.
  • December 2006: β22 circuit boards are now available.
  • Use this headwize.com forum thread for β22 build-related discussions. AMB and the community provides assistance to builders there.
  • Custom-built, ready-to-use β22 amplifiers are available from some professional builders.
  • Use this bbs.audiohall.net forum thread for β22 build-related discussions in traditional Chinese.
  • Reviews, opinions and comments:
    • "It doesn't sound solid-state, it sounds like music... takes you so close... doesn't get any better than this."  [pic]  - Ray Samuels, Ray Samuels Audio
    • "World class amplification."  - Tyll Hertsens, HeadRoom
    • Links to head-fi.org reviews have been removed for now, because head-fi's forum thread and message links have changed after their recovery from the big November 2007 crash.

Overview

β22 ("beta 22") is a high-end, reference-quality DIY stereo amplifier for dynamic headphones. It could also be built as a power amplifier for speakers (18Wrms into 8Ω unbalanced, or 50Wrms into 8Ω fully-balanced), or as a preamplifier.

The all-discrete (no integrated circuits), fully complementary and cascoded topology, high-current MOSFET output stage, full class A operation and carefully tuned operating points provide vanishingly low distortion, high speed, wide bandwidth, low output impedance, and high output power. All elements of the design were critically examined to create an amplifier that performs as well on the test bench as it does with music. Please see the Tech highlights section for a detailed description.

The β22 is the result of a community service project for the audiophile hobbyist. The schematic diagram is open source. The circuit design, as well as the printed circuit board layout are the work of AMB with participation from the DIY community. It is designed to be relatively easy to build, but it is not recommended as a first-time project for the beginner.

The History & acknowledgements section gives a summary of how β22 came to be, and gives credit to those whose work inspired its archtecture.

To obtain the blank printed circuit board and some related parts, visit the AMB audio shop. Other parts are available from online vendors. See the Parts list section.

Please be sure to read every section of this site carefully before attempting to build the β22 amplifier. There are many options to consider, some of which would affect your choice of chassis, heatsinks, various parts and wiring scheme. If you don't thoroughly understand everything, please consider having someone more experienced to build the amplifier for you, or build a simpler project.

The scope of the β22 project

The β22 project is primarily centered around the main amplifier board. Each board supports one channel. You may build the β22 in one of the following configurations:
  • Conventional 2-channel "passive ground" amplifier (2 boards needed)
  • 3-channel "active ground" amplifier (3 boards needed)
  • 4-channel "fully balanced" amplifier (4 boards needed)
The 3-channel configuration is recommended as the default for a stereo headphone amplifier. Please see the Wiring & ground section for full details.

To build a working amplifier, you will also need a regulated DC power supply to provide power to the board(s), as well as select and/or build an enclosure chassis to house the amplifier boards (and optionally, the power supply). For further information see the Power supply section and the Parts list sections. Also, read the DIY areas of the headwize.com or head-fi.org web forums to see what others are doing to build their β22 amplifiers.

An assembled β22 circuit board (click to enlarge)

What skills and tools are needed

You should know how to read a schematic diagram and correlate it to the circuit board layout, identify electronic parts and their pin-outs, have decent soldering skill, and be proficient with a multimeter. You need at least a basic set of tools for electronics work, such as a good soldering iron and accessories, screw drivers, needle nose plier, diagonal wire cutter, etc. For the chassis case, you will also need to be familiar with using the tools that are necessary, such as an electric drill, bits, center punch and others.

Why not offer a full kit

AMB is not prepared to stock all the needed parts and offer them as a full kit. This amplifier also provides a number of customizable options that makes it unwieldy for such a kit. Lastly, this amplifier is AMB's contribution to the DIY community rather than a for-profit product, AMB cannot assume the commercial support role that might be implicit with a full kit.

Why not offer completed amplifiers

This is intended to be a project for DIYers, and AMB is not set up to manufacture completed amplifiers.

Professional builders

Inquire with the following list of professional builders if they would build you a completed β22 amplifier, in case you cannot build one yourself. If you would like to become a professional builder of the β22 amplifier, please contact AMB.

Where to get help

Be sure you read all the information at this site. If you cannot find an answer to a question, or encounter a problem you cannot solve, the best place to get support is on the DIY forum at headwize.com. Search the forum for an answer before asking a question. Your question may have already been asked and answered.

AMB's own β22 and σ22 (click image to enlarge)
See more photos in the β22 gallery



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