Audio connection on PC side

A project log for USB CDC Robosapien V1

Robosapien's actions controlled through a virtual USB port. Speaker switchable between Robosapien & external audio jack

Christoph TackChristoph Tack 02/23/2018 at 19:590 Comments

In the final application, a BeagleBone Black (BBB) will be used to control the Robosapien.  Having one in my drawer is the only reason for using a BBB.  It unfortunately doesn't have an analog audio interface.

Adding an external USB-Audio interface is a cheap (<€1) and easy solution.  It's supported by default in Ubuntu Linux.  The audio chipset is from Generalplus.  There's no more specific data.  I think it's either the GPD8102B or GPD8106B under the blob.

AliExpress USB Audio interface

Unluckily, in an attempt to cut down costs, the manufacturer has also cut down on specifications.  The audio output lacks low frequencies, which makes it very annoying to listen to.  Opening up the unit reveals that the audio is capacitively coupled.  This coupling capacitor forms a high pass filter with the input impedance of the headphones.  This device is cheap.  The capacitor is too small.  The cutoff frequency is too high, cutting off lower frequencies from the headphones.  Luckily, there's an easy fix:

  1. Open up the USB-audio interface.  Easiest way: drop it from 1m on a concrete floor.  For whatever reason (probably cost again) the housing is not glued.
  2. On the bottom you'l find C8 & C9.  These two 0805 caps are 10µF.
  3. Replace these caps with 22µF or higher value.  Higher cap values in small values are relatively expensive.  It's clear why they tried to save cost here.
  4. Normally, you should be able to assemble it without problems.