Faulty USB sound card (U71-SW)

[M.Joshi]

I recently purchased a cheap USB sound card off E-bay (U71-SW). It i
white in colour and has a short cable with a USB connector attached t
one end and the electronics and phones/mic sockets housed in a tin
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 earphone
immediately and found that they were indeed very warm as a result o
being plugged into the sound card

The sound card is built around a C-Media all-in-one chipset.
suspected that there may be a DC voltage across the phones outpu
causing heating of the earphone drivers. Also, there's always a clic
in both earphones when connecting the jack into the socket

I have opened up the enclosure and measured 2.25V DC across the groun
and right & left pins on the phones jack. I assume that half the US
rail supply voltage is apppearing across each phones channel output

The seller on Ebay is sending me another one but I wonder, is thi
normal to expect a DC voltage on each phones channel pin in a USB soun
card

Incidentally, each pin on the Microphone jack measured approximatel
4.75V DC with reference to ground


--
M.Joshi

 

[Dave Platt]

In article <M.Joshi.bbb2609@diybanter.com>,
M.Joshi <M.Joshi.bbb2609@diybanter.com> wrote:

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?

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.

Incidentally, each pin on the Microphone jack measured approximately
4.75V DC with reference to ground.

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.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Arfa Daily]

"Dave Platt" <dplatt@radagast.org> wrote in message
news:gccg4a-0cp.ln1@radagast.org...

In article <M.Joshi.bbb2609@diybanter.com>,
M.Joshi <M.Joshi.bbb2609@diybanter.com> wrote:

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?

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.

Incidentally, each pin on the Microphone jack measured approximately
4.75V DC with reference to ground.

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.

It's probably intended to work with a condenser mic, so the voltage on that
socket is likely correct, but as you say, I would not expect to find any DC
across the phones jack

Arfa

--
Dave Platt <dplatt@radagast.org> AE6EO

 

[John Robertson]

On 04/22/2013 9:36 AM, M.Joshi wrote:

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'?

....ly 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.

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

John :-#(#

--
(Please post followups or tech enquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."