The β22 Stereo Amplifier

The β22 Stereo Amplifier

Wiring & ground

Be sure to read through this section completely before you proceed, in order to determine which of the configurations and options best apply to you. Some of these will affect your choice of chassis case, grounding scheme, connectors and other parts.

Please also see the Other options section for additional β22 options.

Power supply configurations & wiring

There are several options regarding the power supply.

β22 boards in a separate chassis from the power supply (including power transformer(s) and regulated PSU board(s)).
β22 boards in a common chassis with the regulated PSU board(s), but the power transformer(s) are in a separate chassis.
β22 boards in a common chassis with the power supply (including power transformer(s) and regulated PSU board(s)).

The first two options provide some important benefits. The power transformer, which is a source of electromagnectic interference (EMI), is in a separate chassis and could be positioned away from the amplifier chassis to eliminating potential hum and noise. These options also provide more optimal grounding, allowing the chassis with the IEC power entry receptacle and power transformer (which carries AC mains voltage) to be connected to AC earth for safety, while the amplifier chassis is tied to signal ground for RFI shielding. No "ground loop breaker" is required, and there is no problem with AC earth ground leaking noise into signal ground.

Option 3 provides the convenience of all-in-one packaging and straightforward wiring, but a "ground loop breaker" (see below) should be installed. Also, there is potential for leakage of noise from AC earth ground into signal ground. The power transformer(s) should be located as far away from the amplifier boards as possible to minimize EMI from inducing hum and noise.

If you choose to go with the separate chassis solution, option 2 may be simpler in some situations, because there are only three or four wires to connect from the power transformer to the regulator board. Option 1 requires a minimum of 3 or 4 wire (V+, G, V- in an 3-channel active ground configuration, or V+, G, V-, load ground return for 2-channel passive ground headphone amp). If you want to run separate V+, G and V- wires from the power supply board to each β22 board, then the number of wires go up dramatically. These are assuming that only one PSU regulator board is used. If you use multiple transformers and regulator boards, then the number of wires increases for both options. These wires are to be a part of the umbilical cable between the amplifier and power supply chassis, the more wires there are, the more complex and costly the connector(s) need to be.

For option 1, heavy duty wiring (18 gauge or better) should be used in the umbilical cable, and its length should not exceed 3 feet (1 meter) or so, in order to not to compromise the low output impedace of the regulated PSU.

The following illustration shows each power supply option and the recommended grounding scheme. A 3-channel β22 is shown but other configurations are similar. It is assumed that the IEC power-entry receptacle contains a built-in fuse. If not, then an external fuse holder should be installed. See the σ22 website for details.

Ground loop breaker

For option 3 above, the ground loop breaker is a 10Ω 5W resistor in parallel with a 0.1µF capacitor rated at least 250VAC. For safety this capacitor should be rated for class X or Y (good for across-the-line use) with flame retardant casing. The ground loop breaker should be connected between the signal ground and the chassis (which is in turn connected to AC earth ground via the IEC power entry receptacle). Mount the resistor and capacitor in a secure manner so that it will not come loose and come into contact with other circuitry. A good way to do this is to use a terminal strip.

Audio input/output wiring

The input and output wiring depends on whether your β22 is built for a 2-channel "passive ground" 3-channel "active ground" or 4-channel "fully balanced" configuration. The merits of each of these is discussed in the Tech highlights section.

All illustrations below are shown in the Option 1 "external PSU" configuration. Other options are similar.

2-channel passive ground

The wiring scheme for the 2-channel passive-ground configuration is shown in the illustration above. Of particular note is the ground-return wiring from the headphone jack. It should be connected directly to ground at the power supply (e.g., one of the σ22's "G" terminals). Do not connect the ground-return from the jack back to the β22 board. If the PSU is located in a separate chassis (as shown), then the ground return wire should be part of the inter-chassis umbilical cable.

3-channel active ground

The wiring scheme for the 3-channel active ground wiring is shown above. It is similar to that of the 2-channel passive ground, except the headphone jack's ground return wire should be connected to the output of the ground channel board. It should not be connected to signal ground or power supply ground. Also, the ground channel board's input "GND" pad should be connected to signal ground as they are for the left and right channel boards, but leave its "Input" pads unconnected.

4-channel fully balanced

The above figure shows the wiring for a 4-channel fully balanced configuration. A four-gang potentiometer (or stepped attenuator) is used to control the volume of all channels simultaneously.

The line input and the headphone output jacks are all XLR female connectors (the female XLR headphone outputs follows the convention established by HeadRoom. even though the industry-standard for XLR output is male.

Headphones will need to be re-terminated (to dual 3-pin male XLR connectors) and may also need to be re-cabled (to separate the left and right ground returns) in order to be used with a fully-balanced amplifier.

The XLR output jacks' pin 1 is shown connected to signal ground in the illustration. While balanced headphones do not require this connection, having these pins tied to ground allows the balanced β22 to drive the XLR input of another device, such as a power amplifier, through a male-to-male XLR interconnect cable.

Please note that a balanced headphone "sees" the difference voltage across the positive and negative amplifiers, thus the effective voltage gain is twice that of each amplifier alone. For example, if each β22 amplifier is configured for a gain of 5, then the effective gain will be 10.

Zobel network

In each of the three configurations above, if the amplifier is intended to drive speakers, the Zobel networks as shown (22Ω 2W metal oxide power resistor and 0.047µF 100V film capacitor in series) should be connected across the output binding posts of each channel to compensate for the inductive speaker load. Without the Zobel network, the amplifier may become unstable under certain conditions. If your β22 will only serve headphone duty, then the Zobel network could be omitted.

Chassis assembly

After you finish assembling the circuit boards, it's time to work on the chassis case. When ready, mount the β22 circuit boards into the case with standoffs or spacers. Make sure none of the leads and traces on the bottom of the boards touch the metal chassis. If your standoffs or spacers are short, you should glue an insulating plastic sheet beneath the board to prevent board flexture from causing short circuits. Then, mount the input and output jacks, power indicator LED, volume control potentiometer, and other panel-mount items as required.

Next, add all power supply and audio input/output wiring based on the appropriate illustrations above. Inspect your work and compare against the diagrams carefully.

The following are some important notes:

If your β22 is built as a headphone amplifier, be sure to use an isolated headphone jack (i.e., the sleeve or "ground" connection is electrically isolated from the amplifier chassis when mounted.
If your β22 boards will reside in the same chassis as the power transformer, then the RCA phono input jacks (or XLR connector for 4-channel "fully balanced" setup) should be isolated from the amplifier chassis. Use isolation mounting washers if required.
If your power supply regulator board is in a separate enclosure, then make sure none of the power supply leads (V+, G, V-) would short circuit to the amplifier chassis via the connector.
There are two sets of input terminals on the β22 board, they are connected together on the board. Use the set that is physically closest to your volume control potentiometer to minimize hookup wire length.
There are two sets of LED pads, which could be used for the power on-indicator LED. One set is powered from the positive rail, the other from the negative rail. You may use either one, or both of them. Pay attention to the polarity of the LEDs.
All wiring could be done with simple hookup wire. If the wiring from the audio input jacks to the volume control, and from the volume control to the input terminals of the board is over a few inches long, consider using shielded cables.
Keep all wiring neat. The DC power V+/G/V- wiring should be twisted together to improve common-mode noise immunity. Similarly, twist together the input wiring pairs, as well as the output wiring to the jacks. Use heat-shrink tubing to insulate soldered connections on the jacks.
If your amplifier chassis or front panel is non-conductive (e.g., plastic), then you should connect a short length of wire to the volume control potentiometer body and solder it to a (signal ground) pad on the board. There is one of these adjacent to each of the input pads. For the Alps RK27 blue velvet potentiometer, you can simply loosen one of its screws, wrap the wire around it and re-tighten.

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