Headphone output - what is the function of these resistors going to ground between the decoupling capacitors and headphones

Question

The Korg Monotron Duo synth uses the TPA6111A2 headphone amp. The circuit they use is largely the same as the one from the datasheet - except for R78, the 470ohm resistor going to ground right after the decoupling capacitors. I'm wondering if anyone can tell me what the purpose of that resistor is. I've built the circuit on a breadboard, but under my testing conditions I can't see any change in the output.

@StainlessSteelRat I tested with signals from 1hz to 2khz and didn't notice any difference — Kenny Winker, Mar 30 '22 at 22:13

score 9 · Accepted Answer · answered Mar 30 '22 at 21:05

The 470 ohm resistor to ground keeps the headphone output DC level biased to 0V.

The amplifier chip output is biased to VDD/2 so if there resistor is removed, the both capacitor ends would measure VDD/2 volts, and plugging in headphones would charge the cap voltage only via headphone resistance, causing a large snap and could even damage the headphones with the surge of current.

Headphone output - what is the function of these resistors going to ground between the decoupling capacitors and headphones

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