USB Sound Adapter Modification Advice

Generally we do not recommend that users attempt to modify USB sound devices. We highly recommend the purchase of a completed USB Radio Adapter such as the URI or radio specific adapters complete with the target radio connector from the Xelatec commercial products catalog.

The basic USB sound adapter to be modified must contain the CMedia CM-108 integrated circuit. There are many USB sound adapters advertised to contain the CM-108 but unfortunately they turn out to be counterfeit products that use a substandard and incompatible chip instead. Those that do contain the CM-108 may have other flaws. The vendor qualification process is costly in terms of time and money.

Now having said that, Xelatec does offer commercial products based on carefully sourced USB sound adapters.

To begin, the unit must be opened and both 3.5mm jacks removed.

Note that the common connection on the USB sound adapter audio output 3.5mm jack is NOT ground. It is a low impedance DC reference point. Connecting this point to ground will damage the CM-108.

There is also a DC bias voltage present on the TX (headset left and right outputs) from the USB sound device. Refer to the schematic for the target radio. If there is a blocking capacitor in the radio and no DC bias applied, you do not need to add a DC blocking capacitor when modifying the USB adapter.

Locate and remove the resistor that applies bias to the USB adapter microphone input.

A single 100K Ohm resistor in series with the USB device microphone input from the radio RX audio output provides sufficient amplitude scaling and isolation for all applications we have encountered.

When making the modification we prefer to use a special transistor with built-in biasing resistors to simplify and improve the results of the modification.

We also typically route the small diameter insulated wire from the CM-108 to the PTT switching transistor under the other CM-108 pins as a strain relief and to hold the wire in place while making the delicate solder connection to the corner pin.

Other practices include using a strain relief wire strap around the interface cable and filing the empty 3.5mm jack opening with a hole plug.

Of course connecting to the radio receiver flat RX audio output and an unfiltered composite TX modulation input point eliminates the need for a COR connection and further simplifies (only ground, rx audio, tx audio and PTT) and improves the interface.

We wish you the best of luck if you set out down this path!