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Reducing PC Noise floor

post #1 of 16
11/13/08 at 9:19am
Thread Starter 
I am trying to develop a touch screen jukebox as a front end for my 2CH setup. Bsaically its a HTPC using a M-Audio Audiophile 192 directly connected into my 2 tube monobloc amplifiers via RCA. I am trying to keep the chain as short as possible and was going to use the HTPC as a pre-amp.

I have run into a huge problem, the noise floor (static) is very noticable. I am assuming it has to due with the PC power supply but not 100% sure. I had an Antec Neo 500W so I went out an purchased a PC Power & Cooling 370W but that did very little to lower the noise floor (although it actually did do a little). I have tried the onboard audio as well but the noise floor is worse (to narrow down the problem source, so it isn't a sound card issue). The noise is there even when the PC is shutdown (I think because the mobo is getting 5V power still, because when I unplug the PC it dissappears). I have tried other sources connected in this manner and don't have any noise floor issue so I can defintely say its within the PC. I have tried different grades of power cables, RCA cables, connections etc and am at a loss. The las tthing to try is a Panamax Power conditioner is arriving today but that will only help upstream the PC and I think the issue is with EMI and the PC itself.

Another discrepancy is the M-audio RCA connections are set at 1.95Vrms and the input on the Amp's are set for .911V (911mV). Not sure if this would create as large of an issue as I am having since the CD player was modified to have a 1.25V output and doesnt have any noise floor issues with everyhting else the same.

The PC is a
Antec NSK2480
--Intel DG33TL
--Celeron 420
--2GB RAM
--M-Audio Audiophile 192
2 1TB Seagates in a RAID 1

Any ideas would be greatly appreciated.
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post #2 of 16
11/13/08 at 12:23pm
You are correct on the EMI. It's a common problem with PC sound cards. Ground loops and bad grounds in general are all too common with PC analog audio. It's one of the reasons SPDIF out is common on many motherboards nowadays.

Additional voltage on the output shouldn't harm anything (in fact it helps with a preout) unless you have an insufficient ground. I've heard of all kinds of crazy things people do to eliminate/reduce the ground on analog outputs, and I don't know that any of them work reliably.

However, after reading the part of your post about the PSU switch helping, are you SURE that the noise was lower with the PCP/C PSU compared to the Antec? It's possible that some EMI is being generated by the coils inside the PSU (do you hear them buzzing under operation?--probably not noticeable over fan noise) in which case the only option is to use an external PSU if you power draw is low enough. There is a device called a PicoPSU that connects to an external brick (the actual PSU) for use in low-powered systems. I'm not sure if that would solve your issues, but it's a possibility. Another option would be an external sound card, but that wouldn't solve the actual problem.
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post #3 of 16
11/14/08 at 9:42am
For this level of sophistication I would go with an external DAC.
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post #4 of 16
11/14/08 at 10:43am
This is one reason why I think my receiver doesn't have a 3-prong power cord.

I had a ground loop issue, too. These do not need to solved by chopping off the 3rd prong, or ground loop isolators (they kill sound quality). Noise conditioners likely won't help, either..

Instead check ALL your connections. My ground loop noise didn't go away until I grounded the cable TV jack to the power outlet for the rest of my equipment. That's right you heard me, CABLE. What did I do? I wrapped a length of speaker wire around the wall jacket the cable was going into, sliced off the female-end of a 3-prong power cable (I have dozens of these), soldered the speaker wire to the ground cable while CAREFULLY sealing off the negative and positive cables and plugging the power cable into a surge protector.

I imagine it's because the cable itself was being grounded over 25ft, possibly 100ft away.
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post #5 of 16
11/14/08 at 11:24am
If you are able, I'd test different motherboards.

Maybe more expensive ($180-250) motherboards with all that hooplah about multi-phase power cleaning will filter out more of that noise?

Not sure, but might be worth a shot if this is important to you. You could always return it if it doesn't fix anything for you.
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post #6 of 16
11/14/08 at 1:11pm
Too much gain with the soundcard. It will pick up everything. As a test try unplugging (momentarily) the internal USB connections and give it a listen.

The local ham radio shop sells braided shielding for ground straps -- I put it another use: I gave all internal USB and firewire cables another braided shield. Then put a cover of plastic mesh (used by PC modders) to cover the shield. Some help.
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post #7 of 16
11/14/08 at 1:17pm
This board should do the trick...more outputs than you need, but very clean audio...
THD+N out of the audio card is 0.01%. SNR is 112dB.

http://www.newegg.com/Product/Produc...82E16813130212
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post #8 of 16
11/14/08 at 2:09pm
Quote:
Originally Posted by HaLo6 View Post

I am trying to develop a touch screen jukebox as a front end for my 2CH setup. Bsaically its a HTPC using a M-Audio Audiophile 192 directly connected into my 2 tube monobloc amplifiers via RCA. I am trying to keep the chain as short as possible and was going to use the HTPC as a pre-amp.

Bad idea. You are making the whole system more sensitive to noise. When you use digital volume control you reduce the S/N ratio by the amount of volume reduction.
Although for commercial non-classical music it might be OK as they apply compression to reduce dynamic range so you might not have to use a lot of digital attenuation, if your power amps have a basic volume control themselves.

Also I wouldn't advise tube amps, they are power hungry, often not high fidelity, and you can do much better for less money and complexity. There is just much better technology nowadays.
Quote:


I have run into a huge problem, the noise floor (static) is very noticable. I am assuming it has to due with the PC power supply but not 100% sure. I had an Antec Neo 500W so I went out an purchased a PC Power & Cooling 370W but that did very little to lower the noise floor (although it actually did do a little). I have tried the onboard audio as well but the noise floor is worse (to narrow down the problem source, so it isn't a sound card issue). The noise is there even when the PC is shutdown (I think because the mobo is getting 5V power still, because when I unplug the PC it dissappears).

Must be a grounding issue of some sort. Are the PC and amp on different grounds? In that case you have to ground one to the other. Does the audiophile have it's own ground screw? You could always lift the amp ground but you risk your life .
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post #9 of 16
11/14/08 at 2:52pm
Alien DAC
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post #10 of 16
11/14/08 at 3:07pm
It sounds like you have a grounding problem. I would not place much hope in a power conditioner. IMHO, those things are never really necessary.

Are your PC chassis, mboard and ground electrically connected to the third prong on the power cord to the PC PS (check with an ohmmeter) ? Are you powering the PC and tube amp off the same house circuit? Same surge supressor? You need to keep the ground connection between the PC and amp as short and direct as possible. Are you using RCA cables or TRS balanced? Or converting between the two? You can introduce ground problems at the point where you go from single-ended to differential.

I would also listen to the guy who mentioned the cable television cable, those are also a potential source of ground noise. Do you have a tuner or antenna in the PC?

All the other comments about HTPC not suited for audio . . . wrong. They work great for many people, myself included. I use a 200W ADCOM GFA5500. Not a hint of hiss from the Xonar D2X soundcard.

You have a problem that needs fixing. Good luck.
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post #11 of 16
11/14/08 at 3:46pm
HaLo6, I would try to figure out if the noise is coming through the RCA cables or getting into the AC line through the pc's power cord. Remove the RCA cables but leave the PC powered up to check. If it's coming through RCA cable you might be kinda screwed, if it's leaking through the AC line then should be solvable by putting your amp on a line conditioner or maybe even just a ferrite on the pc's power cord.

Also, how close are the pc and the amp? It could possibly be RFI and moving them farther apart might solve it.
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post #12 of 16
11/14/08 at 3:47pm
Quote:
Originally Posted by CSMR View Post

Bad idea. You are making the whole system more sensitive to noise. When you use digital volume control you reduce the S/N ratio by the amount of volume reduction.
Although for commercial non-classical music it might be OK as they apply compression to reduce dynamic range so you might not have to use a lot of digital attenuation, if your power amps have a basic volume control themselves.

Also I wouldn't advise tube amps, they are power hungry, often not high fidelity, and you can do much better for less money and complexity. There is just much better technology nowadays.

Must be a grounding issue of some sort. Are the PC and amp on different grounds? In that case you have to ground one to the other. Does the audiophile have it's own ground screw? You could always lift the amp ground but you risk your life .

Use a Hum-x. No ground lifts needed. Can be bought at most online music retailers for about $70.00 (I think).
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post #13 of 16
11/15/08 at 1:39pm
Quote:
Originally Posted by HaLo6 View Post

I have run into a huge problem, the noise floor (static) is very noticable.

Hi there

Describing the "noise floor" as "static" sounds ominous. Usually the background noise is described as hum (120 and/or 60 Hz from the power supply or line leakage) or white noise (thermal noise, tube rush). Proper characterization of the noise can sometimes lead to the source.

I have a PC setup that has an audio connection to a stereo tube amp (PP EL84), and do not have any PC "noise floor" issues. The PC is a 2-year old HP that has a Realtek audio chip on an Asus motherboard. The on-board line-out is connected to the tube amp through a passive volume control (attentuator), so the PC's volume is typically set to the max. The PC and audio amp both have 3-prong AC plugs, and are powered from a single power strip.

Have you tried measuring the "noise" voltage at the amp outputs or speaker terminals with an AC voltmeter? Is it more than 0.05 VAC ? (Speaker sensitivity (SPL per watt) comes into play here.)

Good luck
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post #14 of 16
11/15/08 at 10:05pm
Is your amp's and PC pluged in at the same power outlet ??
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post #15 of 16
11/16/08 at 7:23am
What is the input sensitivity (in milli-volts) of your tube amps? Oh wait I see that. Sounds like a gain issue not EMI or RFI. But you said you connected a CD player with 1.2vrms and it's fine? The noise is an electrical buzz (static) or hum? An external DAC would be useful here; DAC1 would allow you to either use adjustable volume or calibrated potentiometers; Lavry DA-10 also has a volume control. Keep or cut out the M-Audio card depending on what your board has. Maybe run Toslink or Coax to a DAC from onboard S/PDIF; you'll end up with more versatility.

ah myths....

HTPC not for audio?.....well, let all us low-fidelity tube amp owners reclaim the acronym to Home Tubed PC, or HMPC which is much more accurate imo. (Home Media or Music...you get to choose : ) Plenty of silence here with these archaic tube amps hooked up. A simple tube audio circuit will best a transistor-based design of the same cost. All we have nowadays with transistors is LESS HEAT not better audio. I know a guy whose LaScalas are hooked up direct to his M-Audio 24/96 card. Do you know how senstive those speakers are; he's got no noise. (You occasionally have to watch out for an impedance issue i.e. connecting an old tube tuner into a tape-in which may then by outting to a PC for archiving-I've seen this cause major attenuation at speaker output when monitoring)

Digital attenuation is not an issue if used sparingly and within reason, especially with things like ReplayGain.

DC
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post #16 of 16
11/16/08 at 8:24pm
Quote:
Originally Posted by doctorcilantro View Post

Digital attenuation is not an issue if used sparingly and within reason, especially with things like ReplayGain.

Well said. This is true, and obvious from the math. It takes quite a bit of digital attenuation before you start degrading the signal to noise ratio of the analog circuits that follow the DACs.

Transistors and tubes both work very well when properly designed.
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