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Old 28th September 2011, 02:54 AM   #1
spot is offline spot  Australia
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Default External sound card

Hi,

I'm looking for a cheap external sound card to use inside a 5.1 system Im designing. It would reside inside the subwoofer with the sub amp and have analog connections out to the active speakers.
I'm doing this to avoid any potential problems with earth loop humm and to stop people unplugging it accidentally and creating large clicks which destroy the tweeters - and it neat, only having one cable from the PC.

A USB device in ebay XD- MINI PCM2704 HI-FI USB DAC SOUND CARD BOARD | eBay
looks great except it is stereo only .

Other cheap 5.1 usb devices look OK, but I am concerned how to stop the loud clicks if the PC power gets cut suddenly, or the device is disconnected. I could do some tricks with a muting circuit to catch most cases, but I dont know if it would catch them all.

Any recommendations? How do you stop loud clicks when PC power is cut suddenly etc?
(I dont need amazing specs, but they would be nice if they're not $$$)
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Old 28th September 2011, 06:39 AM   #2
phofman is offline phofman  Czech Republic
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You have probably considered this as the first option, but how about monitoring the 5V power from USB, and hooking this to the muting circuit?
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Old 28th September 2011, 07:51 AM   #3
spot is offline spot  Australia
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I've looked at one of the 5.1 channel usb devices, and if I were using it I would probably use the power that it supplies to the spdif-out optics to power up the amp (this gets powered when the device is enabled), and then mute if the data led is not flashing.
This would catch most of the scenarios but not all - might be enough though. To get them all would probably require powering the sound chip from a separate psu (not the PC). Whether this even would catch all the scenarios would depend on some of the nuances of the IC which are not detailed in the datasheet.
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Old 28th September 2011, 09:14 AM   #4
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Cheapest:
Virtual 5.1 Channel USB Sound Card Adapter - Blue - Free Shipping - DealExtreme

Best:
Behringer U-Control UCA202 USB Audio Interface at zZounds
Specs:
BEHRINGER: UCA202

Doc
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Old 28th September 2011, 09:51 AM   #5
spot is offline spot  Australia
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Quote:
Originally Posted by thaumaturge View Post
This would be fine but that its only stereo, not 5.1.
They don't mention many specs - only 48khz. The chinese $12 5.1 unit I have here can do 48kHz.
I wonder too whether it is possible to make it output loud clicks by unplugging it or unplugging the pc etc. This is something they dont usually mention in the specs unfortunately as it is most important to me - dont want to damage my speakers.
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Old 28th September 2011, 11:19 AM   #6
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CompUSA.com | USB-SND8 | Sabrent USB-SND8 8-Channel Sound Box

CompUSA.com | XS71U | Diamond XS71U Xtreme Sound External Sound Card


Thump on power up is ALWAYS going to be a problem with USB device, as it is by definition a single sided supply. Therefore output will have to be cap coupled. On power up there will be a half rail offset on output cap.
Doc
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Last edited by thaumaturge; 28th September 2011 at 11:26 AM.
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Old 28th September 2011, 09:19 PM   #7
spot is offline spot  Australia
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Quote:
Originally Posted by thaumaturge View Post
CompUSA.com | USB-SND8 | Sabrent USB-SND8 8-Channel Sound Box

CompUSA.com | XS71U | Diamond XS71U Xtreme Sound External Sound Card


Thump on power up is ALWAYS going to be a problem with USB device, as it is by definition a single sided supply. Therefore output will have to be cap coupled. On power up there will be a half rail offset on output cap.
Doc
Looking at these they seem to have the same specs as the chip inside my chinese $12 one (CM106-L), I wonder if it uses the same IC.

The cap coupling is a problem, but a better design with relays shorting the outputs to ground until it has stabilised would fix the turn on thump problem (relays are the classic way of fixing this, used in power amps before ICs came along). I was hoping I could buy one with this feature but perhaps this is only in very $$$ models.

This (relays) is something I can add easily externally to the device though, and will fix the big thump issue, but there still may be smaller clicks from other events like PC power being unplugged while in playback, and the USB being unplugged while playing that I might not be able to catch in time. I would need more details about the IC (CM106-L) to fix this (if its possible) but the datasheet for this chip only provides very basic info .



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