APOX-3

This a PGA2310 based volume controller, it accepts 1 fully differential input. ex.(L+, L-, LGND) It used to be two inputs, but due to the large number of parts I thought it was simpler and cleaner to break the board up. The only additonal cost is 1 microcontroller, but the layout is identical for both channels. So by physical distance, you can completely eliminate crosstalk between your left and right channels

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OPA134's in a voltage follower configuration, buffer each input to the PGA2310

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The PGA2310 is completely opto-isolated from the microcontroller. This should enhance the quality of the sound by greatly reducing the digital noise generated from the microcontrollers clock.

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The board has its own on board regulators for the PGA2310 chip. By having linear regulators very close to the PGA2310 chip as opposed to an external regulated power supply with long wires feeding the board. The power supplies should be very stable.

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An external +/-18V and +5V supply is required. (These external supplies should ideally be regulated if you want to acheive very high PSRR (power supply ripple rejection)) The external 5 volt supply feeds the microcontroller. The +18V is regulated to +15V and +5V (for the PGA2310). The -18 is regulated to -15V

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A relay can swap the output to either straight out, or to an onboard headphone amplifier (This allows for a seperate output connector for you headphones)

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The headphone amplifier consists of a OPA134 configured as a differential amplifier, feeding a BUF634 high current buffer for crystal clear sound. (I Hope!)
Microchip microcontroller controls all digital communication to PGA2310 to accomplish equally spaced decibel output

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Microcontroller acts as I2C Slave for simple wiring requirements to APOX-IR board

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Up to eight of these boards may be stacked up with 3 jumpers for address offsets.

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Preamplifier is not need with this design, with capability to drive 600 ohm load. (Direct out.)
MUTE function

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+31.5dB to -95.5dB with 0.5dB steps