Modern PC DAC vs "Audiophile DAC"

[boris81]

Don't motherboards already have very intricate circuits to keep the voltages steady? A fluctuating ground will be pretty bad in a computer, right?

Can somebody post a link to a comprehensive study on this topic? I'm interested on seeing measurements of how much a switching power supply can affect the sound.

[kevinahcc20]

Originally Posted by boris81

Can somebody post a link to a comprehensive study on this topic? I'm interested on seeing measurements of how much a switching power supply can affect the sound.

It's a very easy experiment to do on your own...hook up the analog output of a laptop to your sound system and listen to music while running on battery power. Then plug it into the SMPS power brick. You will usually hear an added overlay of squealy hash from the SMPS supply that is not at all subtle. Go back to battery power and the noise will be greatly reduced.

[MarcelvdG]

On computers with cheap internal sound cards, I've often heard very audible sounds coming out of the headphone as soon as I moved the mouse. Depending on the volume setting, my laptop with audio chips on the motherboard frequently produces soft high-pitched sounds when it is supposed to be silent. When you try recording with internal sound cards using a programme with a good level meter, you rarely see it dropping below
-50 dB quasi-peak while the CODEC chips are specified for 85 dB(A) or higher. There is always a 12 dB or so difference between quasi-peak and A-weighted RMS, but 35 dB difference clearly indicates that there is some interference problem.

So yes, the internal interference can definitely mess up the audio performance.

About spurious signals in the megahertz range: these become problematic when they produce mixing products in the audible range. For example, if your DAC is a sigma-delta modulator, and it almost certainly is, interference on its reference voltage at just about any frequency will mix back out-of-band quantisation noise into the audio band.

[boris81]

Originally Posted by kevinahcc20

It's a very easy experiment to do on your own...hook up the analog output of a laptop to your sound system and listen to music while running on battery power. Then plug it into the SMPS power brick. You will usually hear an added overlay of squealy hash from the SMPS supply that is not at all subtle. Go back to battery power and the noise will be greatly reduced.

Thanks, Kevin!

[TheLaw117]

Ah well...I see what you guys are saying about the bad enivornment. I guess it makes sense. I think I'll wait on it for a while...

But...if I do build one somewhere down the line...

I like the USB interface because although I have an optical SPDIF port, I'll then need some sort of transciever amongst other things. USB is just more versatile anyway.

I know there are a ton of USB DAC chips. Surface mount is obviously fine. I'll either just make a 100% SMD board, try to make a dual sided board, or use a SMD adapter to get through hole.

Again, I'm not going to build it now, but just to quench my curiosity. Is something like a PCM1704 any good? Or are there better. Simplicity would really be a plus.

Thanks.

[DrDyna]

Maybe someday they'll come up with a sound card that has a discrete output stage, but I don't think that's going to happen any time soon.

Of course now that I've said that, Audio Research is going to come out with a PCI express version of their DAC8 that I can pine for. Oh, if only it were true.

[SoNic_real_one]

Originally Posted by kevinahcc20

It's a very easy experiment to do on your own...hook up the analog output of a laptop to your sound system and listen to music while running on battery power. Then plug it into the SMPS power brick. You will usually hear an added overlay of squealy hash from the SMPS supply that is not at all subtle. Go back to battery power and the noise will be greatly reduced.

For your information, inside laptops are lots of SMPS that convert the battery voltage (12-19V) to the multitude of voltages necessary (0.8-1.2V variable for CPU, 1.8V for RAM, 3.3V for PCI bus, 5.0V and 12.0V for USB and HDD). So no, your "experiment" shows nothing.

[kevinahcc20]

Originally Posted by SoNic_real_one

For your information, inside laptops are lots of SMPS that convert the battery voltage (16-19V) to the multitude of voltages necessary (0.8-1.2V variable for CPU, 1.8V for RAM, 3.3V for PCI bus, 5.0V and 12.0V for USB and HDD). So no, your "experiment" shows nothing.

Shows that laptops running off wall-power SMPS are a lot noiser than when not...hardly nothing!

[eeka chu]

I have a plan! Go with me on this one....

Remove the sound card from the PC. Power it up externally from a linear supply. See if one can hear any difference? Or try shrouding the card with foil? It'd still be connected to the possible noise sources inside the computer however since it'd still have a bunch of pins attached to the motherboard.

I'm guessing most of you are worried about noise in terms of it causing corrupted and delayed packages? Has anyone done some measuring of this and seen if the switching is interfering?

There must be something in the EMI regulations that states the switching noise shouldn't be allowed to expand that far out of the supply, so it doesn't interfere with the buses and so on?

Is it really just the noise people are trying to get away from or are the external DAC noticeably clearer sounding?

My soundcard is an on board one, in the cheapest possible packard bell I could find (being a student at the time).

The cable on my HD650's just went, so that needs swapping. The amplifier I was using is gone, so I'm now looking at making something with the LME49xxx's. In for a penny in for a pound I guess... the DAC :P

[abraxalito]

Originally Posted by Boscoe

Why would you be bothered about switching at MHz and GHz?

I'm not bothered by it at all. I'm trained as a digital designer, its my bread and butter. However the noise gets into audio kit and that is bothered by it.

Don't say ahh well you can hear a difference...

OK so then I'll say I sometimes notice I'm inclined to turn the system off.