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I honestly was not sure where to start with my research on the necessary circuitry for driving headphones, however, here is what I could find. For reference, the headphones I possess are 80 Ohm impedance, so that’s what I’ll be basing it off of. Common earbud and headphone impedances I see are 16 and 32 Ohm.

Operational Amplifier Selection

For the preamplifier and the output amplifier/buffer, I will be using operational amplifiers, or op-amps in a non-inverting amplifier configuration with adjustable gain through the use of physical potentiometers.

For comparison, the venerable NE5532 has the following characteristics under typical conditions:

  • Price: $0.43
  • Noise: 5nV / Hz^1/2^
  • GBWP: 12MHz
  • CMRR: 100dB
  • Slew Rate: 5V/us
  • Bias Current: 100nA
  • DC Voltage Gain: 100V/mV
  • Distortion not listed
  • PSRR: 100 dB

The recommended op-amp for the PCM1754 is the OPA2134

  • Price: $5.42
  • Noise: 8nV / Hz^1/2^
  • GBWP: 8MHz
  • CMRR: 100 dB
  • Slew Rate: 20V/us
  • Input Bias Current: 5pA
  • Distortion: 0.00008% (-122 dB)
  • PSRR: 106 dB

The much more reasonably priced OPA1678 has the following characteristics:

  • Price: $0.79
  • Noise: 4.5nV / Hz^1/2^
  • GBWP: 16MHz
  • CMRR: 110 dB
  • Slew Rate: 9V/us
  • Input Bias Current: 10pA
  • Distortion: 0.0001% (-120 dB)
  • PSRR: 110 dB

An op-amp that I already posess is the OPA189IDVR:

  • Price: $2.90
  • Noise: 5.2nV / Hz^1/2^
  • GBWP: 14 MHz
  • CMRR: 168 dB
  • Slew Rate: 20V/us
  • Input Bias Current: 70 pA
  • Distortion: 0.00006% (-124 dB)
  • PSRR: 166 dB (+-0.005 uV/V)

The OPA189 chopper amplifier has excellent characteristics, however, I will save it for a project requiring higher levels of precision.

Mute Circuit

In order to prevent pops and noise from being transmitted from disconnected inputs, power-on/off, and “hot-plugging” audio devices, a mute circuit can be implemented to turn off inputs/outputs. On the output side, the PCM1753 DAC has a soft muting function as well as 2-channel zero flag pins that indicate zero outputs and can be used to operate an external mute circuit. While a mute circuit is not strictly necessary, I intend on implementing a mute circuit.

For the mute circuit, I’ll be using a normally-open solid-state relay to shunt the amplifier’s output to ground when there is no audio playing or when input/outputs are disconnected. I’ve selected the $0.96 Texas Instruments

Input Pre Amplifier

To bring audio signals from my instruments to an appropriate level and to protect the ADC inputs, I will need a pre amplifier stage.

Output Amplifier/Buffer

Not sure if this is necessary, however, an output amplifier may be desired to bring signals to the correct level for driving headphones or to provide low output impedance. Further reading should inform me whether this will be necessary.

(Optional) Analog Audio Mixer

If I so choose to skip the digital mixing and opt for an analog audio mixer for possibly reduced latency, I will need a mixer circuit (AKA an analog adder circuit).

Citations

  1. “Muting” (https://sound-au.com/articles/muting.html)