Line level

2.1 V RMS, centred on ground

At full scale the PCM5102A puts out 2.1 V RMS on each channel, which for a sine is about 2.97 V at the peaks (worked out). The chip's charge pump makes about −3.3 V for itself, so the output swings above and below 0 V rather than around a middle voltage. That is why the board has no capacitor in series with the jack.

A signal of that size is called line level. It carries the sound, not the power to move a speaker cone: whatever is plugged in has to bring its own amplifier, or be as light a load as a headphone.

The filter in front of the jack

Each output reaches the jack through a 470 Ω resistor, with a 2.2 nF capacitor from the jack side to ground. Together they are a low-pass filter, the one TI recommends in the datasheet, and the one TI's performance figures are measured with.

A filter's corner is where it has turned the signal down by 3 dB. For a resistor and a capacitor it is 1 / (2π × R × C): here 1 / (2π × 470 Ω × 2.2 nF), about 154 kHz. Across the whole audible band, up to 20 kHz, the loss is under a tenth of a decibel. Everything the filter does, it does far above hearing.

The 470 Ω does a second job

TI rates the output for loads of 1 kΩ and up, 10 kΩ typical, the kind of load a line input usually is. The 470 Ω sits in series with whatever is plugged in, so even a low-impedance headphone never loads the chip directly.

The price is that the resistor and the load share the voltage. Into a line input that costs almost nothing; into headphones it costs a lot, and the next article works out how much.