I recently purchased a cheap USB sound card off E-bay (U71-SW). It is white in colour and has a short cable with a USB connector attached to one end and the electronics and phones/mic sockets housed in a tiny enclosure at the other.
I noticed that the sound from the sound card was very 'tinny'?
Also, after using the sound card with some earphones for a few minutes, I noticed the earphones getting very warm. I disconnected the earphones immediately and found that they were indeed very warm as a result of being plugged into the sound card?
The sound card is built around a C-Media all-in-one chipset. I suspected that there may be a DC voltage across the phones output causing heating of the earphone drivers. Also, there's always a click in both earphones when connecting the jack into the socket.
I have opened up the enclosure and measured 2.25V DC across the ground and right & left pins on the phones jack. I assume that half the USB rail supply voltage is apppearing across each phones channel output?
The seller on Ebay is sending me another one but I wonder, is this normal to expect a DC voltage on each phones channel pin in a USB sound card?
Incidentally, each pin on the Microphone jack measured approximately4.75V DC with reference to ground.