The α24 fully-differential line amplifier
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Overview

α24 ("alpha 24") is a high performance, small form-factor audio line amplifier featuring ultra low noise, ultra low distortion, high-CMRR (common-mode rejection ratio), wide bandwidth, high input impedance and low output impedance characteristics. It provides the following input and outputs:
  • Fully-differential input (can be used with balanced or unbalanced source)
  • Fully-differential output (balanced)
  • Unbalanced output
The α24 board contains provisions to implement an optional fully-differential 3rd-order (18dB/octave) analog low-pass filter. Such a filter may be useful in certain applications.

The small size (2.5"x1.4"), simplicity and high performance attributes makes it ideal for a variety of audio applications. The following are some examples:
  • Balanced pre-amplifier line stage or buffer
  • Voltage DAC analog output stage with low-pass filter
  • Balanced-to-unbalanced converter
  • Unbalanced-to-balanced converter
  • Pro-audio applications
The AMB γ3 high resolution DAC uses a pair of α24 boards as its analog output stage and low-pass filter.

The α24 circuit employs three premium audio opamps from Texas Instruments -- OPA1612, OPA1632 and OPA1611, to form the basis of a modified instrumentation amplifier. Please see the technical highlights section and the α24 development forum thread for further details. These opamps are all in the easy-to-solder SOIC-8 SMD package. All other parts are through-hole for ease of build.

α24 is direct-coupled. There are no signal-degrading capacitors in the signal path. The voltage gain is configurable at build-time, and it can drive a 600Ω load with ease. Each α24 board contains one channel. For stereo, two boards are needed. This is true whether you're building a balanced or unbalanced system.

A regulated dual-rail power supply is needed to power the α24. The recommended supply voltage range is ±10V. The σ22 is the recommended supply for the utmost performance. For a lower cost, simpler, and more compact solution, twin σ25 boards configured as a dual-rail power supply may be used. Each α24 board consumes around 25mA. The power supply should be able to supply that much current multiplied by the number of boards, plus some additional headroom.

WARNING: Building the power supply for the α24 would require that you work with AC line voltage, which is a lethal shock hazard. If you don't know what you're doing, please ask someone who is experienced to help you.

α24 was designed by AMB as an open source project with community participation.

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.

α24 is itself an easy board to build and set up. Please be sure to read every section of this site carefully before attempting to build the α24. 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 α24 project

The α24 project is primarily centered around the α24 amplifier board. To build a working unit, you will also need a power supply of the appropriate specifications, as well as select and/or build an enclosure chassis to house everything. The power supply can be located in the same enclosure as the α24 board(s), or in a separate case for maximum isolation from magnetic field interference.

A populated α24 board, configured as a balanced line amplifier (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 (with a fine tip) and accessories, screw drivers, needle nose plier, diagonal cutter, a sharp tweezer (for positioning the SOIC-8 opamps), 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.

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 the AMB DIY audio forum. Search the forum for an answer before asking a question. Your question may have already been asked and answered.


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