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Headphone attenuator

Headphones, the attenuator and my phone.

My new Nokia 2710 has a radio. However, the sound is way too loud, even with the headphones that came with the phone. It's also not the only device with this problem: often the volume control doesn't go low enough so that you can listen comfortably in a silent room.

Schematic

PCB for the attenuator

Basic attenuation is simple to achieve with a series resistor on each channel. I chose to go a bit further, adding passive crossfeed. There are also 1nF bypass capacitors so that the attenuator does not affect the radio reception.

Schematics for the attenuator

Casing

Circuit potted in epoxy

I potted the whole thing in epoxy glue. The mold I used was just a piece of baking paper held together with tape.

Frequency response

And here is the mandatory bode plot. I used the actual headphones as the load when I measured it. I assume that explains the peak around 200 Hz. Also notice the rise above 100kHz, caused by the radio frequency bypass.

Solid lines are simulated values (using Qucs, schematics). Dots are measured values.

Frequency-gain-graph. Time delay graph.

The response is far from flat, but I'm not a hifist.

– Petteri Aimonen on 1.3.2011

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