The α24 fully-differential line amplifier

Wiring and ground

The α24 can be used in many applications and configurations, it's not possible to document every case. This section will cover the most common usages, namely, as a pre-amplifier line stage, a voltage DAC analog output stage, a balanced-to-unbalanced converter and an unbalanced-to-balanced converter.

Pre-amplifier line stage

The diagram below illustrates a typical example application of the α24 as a pre-amplifier line stage. Only one channel is shown in the diagram, the other channel is identical.

As can be seen, the pre-amplifier has an input selector switch to select amongst multiple input sources (two balanced XLR and one unbalanced RCA), and a potentiometer for volume control. This configuration provides both a balanced XLR output and an unbalanced RCA output.

A four-pole input selector switch is needed to simultaneously switch both stereo channels, and a four-gang audio taper potentiometer should be used for the volume control. More deluxe builds could use the δ2 relay-based input/output selector and δ1 relay-based stereo attenuator (two boards each, all controlled by a LCDuino-1 display I/O processor).

If you plan to connect only one source, the input selector switch may be omitted.

Voltage DAC analog output stage

A digital-to-analog converter (DAC) chip either has current-output or voltage-output. Current-output DACs require an I/V stage to convert the output current into voltage, because audio signal transmission between components are voltage-based. Voltage-output DACs, on the other hand, need no such conversion, and some designs connect the DAC chip's output directly to the output jack. But DACs produce ultrasonic arfifacts that, if allowed to enter the next stage (i.e., a pre-amplifier, headphone amplifier, etc.), could adversely affect performance. A 1st order passive filter is inadequate for this purpose, hence a higher-order, active low-pass filter (LPF) is needed.

Higher performance DAC chips, both current-output and voltage-output, usually have differential outputs. While it's possible to simply use only one of the two differential signals (and ground) to get an unbalanced output, it is far from optimal. Thus a balanced-to-unbalanced converter is usually employed to do so.

The α24 satisfies all the above requirements and is specifically suited for use with voltage-output DACs. It supports both differential or single-ended DAC chips, provides a fully-differential buffer with a 3rd order LPF, and has both balanced and unbalanced outputs.

This diagram illustrates the use of a DAC with differential outputs. If used with a DAC with single-ended output, simply short the α24 In- input to G. Note that in this configuration, the α24 will act as an unbalanced-to-balanced converter and provide a fully-differential signal at the balanced outputs.

Balanced-to-unbalanced converter

The α24 can serve as a balanced-to-unbalanced converter. The gain is configurable at build-time. You could also (optionally) populate the LPF as you see fit. See the parts list page for details.

If you connect the Out+/G/Out- terminals to an XLR jack, then you also get a balanced output.

Unbalanced-to-balanced converter

The α24 can serve as a unbalanced-to-balanced converter. The gain is configurable at build-time. You could also (optionally) populate the LPF as you see fit. See the parts list page for details.

If you connect the Out/G terminals to an RCA jack, then you also get an unbalanced output.

Wiring and casing considerations

The input wiring (including those from the input jacks to the input selector, volume control, and to the α24 boards) should be kept as short as possible to avoid interference, capacitive coupling, and ground loops. If the input wiring will be more than a few inches long, consider using shielded cables.

The power supply transformer should be located as far as possible from the input wiring and the α24 boards. Ideally, you should locate the transformer in a separate enclosure to eliminate magnetically-induced hum and noise.

If you use a separate enclosure for the transformer, assuming that the chassis is metallic, the AC ground on the IEC power entry module should be connected to the transformer chassis for safety. The amplifier chassis should be connected to signal ground as shown by the ground symbols in the diagram above. Do not let the amplifier chassis touch the transformer chassis. Note that it is up to you whether to locate the voltage regulator board in the transformer chassis or the amplifier chassis. Each has its merits.

If you use a single enclosure to house everything, then a magnetically-shielded transformer is recommended. Also, the signal ground should be isolated from the chassis. Connect the AC ground directly to the chassis, and then install a "ground loop breaker" between the signal ground and the chassis. The recommended ground loop breaker is a parallel-connected resistor and capacitor combination, the resistor is a 10Ω 5W wirewound type, and the capacitor is a 0.1µF 250V (or higher) suppression-type, X or Y rated for across-the-line use.

Note that the illustration below shows the use of an angle-bracket, shaft extensions and panel bearings to mount the input selector switch and volume potentiometer close to the input jacks.

Balanced XLR pin assignment

The industry standard for balanced line-level interconnect is female 3-pin XLR jacks for inputs and male 3-pin XLR jacks for outputs. The pin assignment is:
  1. Ground
  2. Hot signal (+)
  3. Cold signal (-)

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