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Faulty USB sound card (U71-SW)

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 approximately

4.75V DC with reference to ground. 
--
M.Joshi
Reply to
M.Joshi
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In my experience, no - at least, not if you have any sort of resistive load present on that pin.

These sorts of devices normally place a DC-blocking capacitor between the output driver and the output jack, so that the output device "sees" a signal which swings on both sides of ground and has no DC bias.

Even if the DC blocking cap is present, you may measure a significant voltage on the output jack if you measure it with a DVM after plugging the device in and you don't have headphones plugged into it. With no resistive load after the DC-blocking cap, the output voltage can "read high" initially. If you try it with headphones plugged in, you should see a zero DC voltage. A well-designed device would include a bleeder resistor to ground after the cap, to keep this from happening... but inexpensively-built devices may omit this.

It's even possible that the blocking caps themselves were omitted from the device you bought... either not designed in at all, or "not stuffed" on the board and replaced by jumpers. Or, the caps might be there but might be defective, or installed "backwards" if electrolytic.

Once again, this probably shouldn't occur in a well-built device, but the measurement may not be reliable if you don't have something plugged in to provide a load. Having both sides of the mic jack above ground really does seem rather strange.

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Dave Platt                                    AE6EO 
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Reply to
Dave Platt

...ly voltage is apppearing across each phones channel output?

Why would you expect a cheap knock-off no-name USB sound card to work well?

John :-#(#

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Reply to
John Robertson

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