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rylangrayston 4th December 2012 05:21 AM

Are sound card line outs usualy DC-Coupled?
I have started a project in which I use a computers sound card to control lights motors valves etc.

I under stand that some sound systems out puts are AC-coupled
and there for if I hold one channel hi for more than a second it
will start drifting back to zero ( this would be a big problem for my project)

Where as a DC-Coupled out put would simply do as expected.
For example if I were to play a sound file with a 0.2 HZ square wave I would get the same square wave on the line out (no rounded corners or drifting to zero)

Here are my questions

1 how common is it for a sound cards line out/ head phone jack
to be DC-Coupled?

2. Dose any one know of a CHEAP usb sound card that is DC-Coupled..
Cheap like this one: Link

3.It seems to me that it would make seance for speaker output to be AC-coupled and line outs to be DC-Coupled, as holding a speaker output hi for a long time could get its voice coil hotter than any ac signal of the same voltage.(think rms watts vs dc watts) am I on the right track?

My end goal would be to have an audio file that holds a motor running for an hour straight simply by playing an audio file that holds the sound cards line out hi and .... connecting that line out to the gate of a mosfet power transistor to drive the motor.

If you donít know how your sound card behaves feel free to download the attached audio file it has some very very low frequency square waves in it. Im curious to know how your sound cards respond to this file... what voltages do your multimeter read
and how accurately do your sound cards reproduce these supper slow square waves?

Lavcat 4th December 2012 04:54 PM

I have a sound card that is DC coupled. I'm trying to think of the name of it but my back is out and I cannot go looking on the shelf. It is an ISA bus card however and I don't have any motherboards in service that have ISA slots.

Edit: It was *not* cheap.

jcx 4th December 2012 07:54 PM

very few will be DC coupled - many audio DAC chip running from 5V have near 1/2 Vsupply offset, are aiming for "desktop consumer" line levels of ~2 Vrms

more DAP today are DC coupled - still not universal - DC coupled outputs with sw C inveter/splitters or actrive gnd are built into some of these low V portable audio DAC chips to save the size of the output C and because they are used with low Z, sensitive headpnones that are loud with < 1 Vrms drive

jcx 4th December 2012 08:31 PM

parallel printer ports or usb to parallel printer port adapters are still available cheap bit banging

some DAQ are fairly low cost ~$200 gets 12 bits analog I/O with industrial specs, DC response

but really cheap is some of the "USB dongle" style uC developement tools like the EZ430 for $20

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