Other optionsβ22 as a pre-amplifierFor best performance, the δ1, δ2 and LCDuino-1 system is recommended as an upgrade from using a conventional potentiometer for volume control and mechanical selector switch for source selection. See their repective websites for details.The low-noise performance and moderate voltage gain of the β22 also makes it an excellent line-stage pre-amplifier. The ultra-low output impedance allows the β22 to drive a very long interconnect cable without loss of quality. Since a pre-amplifier's primary function is to switch among multiple input sources, just add multiple input jacks and a selector switch. To minimize noise and crosstalk, mount the selector switch as close to the rear panel input jacks as possible, and use a shaft extension kit (with couplers and panel bearings) for the front panel knob. If the β22 will serve strictly as a pre-amplifier (which would be used to drive an outboard speaker power amplifier), then the 2-channel passive ground configuration should be used (see the Tech highlights and Wiring & ground sections for details). If the β22 pre-amplifier will also serve as a headphone amplifier, then you may build the 3-channel active ground configuration. The headphone jack should be connected as described in the Wiring & ground section, but the preamp output RCA jack's ground should be connected to the signal ground. The following diagram illustrates a pre-amplifier configuration with three-inputs, a loop-out, and independently switchable pre-amplifier and headphone outputs. Use a double-pole, 3-position rotary switch for the input selector, and DPST switches for the loop-out enable and pre-out enable switches. 
Combined balanced / unbalanced β22 headphone amplifierThe following illustrates a scheme which allows multiple β22 boards to operate as one fully-balanced stereo amplifier (with balanced inputs and outputs), or as two unbalanced stereo amplifiers (with unbalanced inputs and outputs).There is a single pair of XLR balanced input jacks and two pairs of unbalanced RCA phono input jacks (A and B), plus connections to one balanced headphone and two unbalanced headphones (A and B). This topology make a versatile amplifier that can be used in either balanced or unbalanced mode as required by the available source and headphones. First variation - Using four β22 boards:For unbalanced operation, there are two independent amplifiers A (boards 1-2) and B (boards 3-4). Use the unbalanced inputs "A" to drive an unbalanced headphone "A", and likewise use the "B" inputs to drive headphone "B".This variation, when used with unbalanced headphones, is the same as two unbalanced 2-channel passive ground amps. 
Second variation - Using six β22 boards:This is similar to the first variation, except the two unbalanced amplifiers A (boards 1-3) and B (boards 4-6) each have an added active ground output channel. The unbalanced headphones are therefore driven with independent active grounds, just like a standard unbalanced 3-channenl active-ground β22 amp. The β22 boards labeled "G" should be built as a ground channel amplifier (see the parts list for details). The ground channels are unused when operating the amp in balanced mode.
In both of the above variations, if a four-gang potentiometer or stepped attenuator is used, then all boards are controlled simultaneously via a single volume control knob. To save panel space, two Neutrik NCJ6FI-S combo jacks (each containing a 3-pin female XLR and one 1/4" stereo phone jack) would be ideal for this application. Switch-selectable balanced/unbalanced inputsIn both the four-board and six-board combined balanced/unbalanced configurations shown above, you may connect a balanced input source or an unbalanced input source, but not both at the same time. This is because the XLR and RCA input jacks are wired in parallel. If both types of sources are connected, one source will "see" the output of the other, which will cause the two to "fight" and potentially be damaged.The following diagram shows an alternative input scheme, allowing both types of sources to be connected simultaneously, and selected by a switch. You may use a single 4PDT switch, or two DPDT switches. Another bonus of this scheme is that it allows the use of an unbalanced source with balanced headphones without the need of an unbalanced-to-balanced converter. The output wiring to balanced and unbalanced headphones is the same as shown in the variants above. 
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