intermittent digital noise, ground loop?

I

innovine

New member
I recently changed PCs and have run into some digital noise issues. When the PC does stuff (resize windows, mouse over buttons, etc) I can hear a burst of digital noise. If i don't move the mouse over stuff and there's not much cpu use happening, the noise vanishes. It's either occurring, or not, and it's quite loud when it does.

I've reduced the problem to my PC, a firepod soundcard connected via firewire to the PC, a yamaha mg124cx mixer, and a NAD C326BEE amplifier. By plugging headphones into all the outputs on the soundcard, I do not hear any noise, despite the fact that I can still hear it in the mains. I don't hear the problem in the mixers headphones out either. I can turn all the mixer sliders to zero and the problem persists. If I disconnect the last patch cable from the soundcard to the mixer, the problem goes away. It doesn't matter what this cable is connected to, it can run from an out on the soundcard to an aux out on the mixer, or in to a strip which is not getting routed and with the slider down. If I leave the soundcard connected to the mixer, and disconnect the firewire cable then the problem also goes away.

All devices are connected to grounded electrical strips, apart from the soundcard which is only connected to the mixer and pc via firewire. I thought ground loops were more of a constant deep buzz, not this intermittent high pitched bzzz of digital noise. I've removed unnecessary pci cards, usb hubs, etc. If the problem IS a ground loop near the soundcard, why is the noise not heard on the mixer headphone outs, but IS heard on the amp once the amp is connected to the mixer (with the master fully down)? I also took the cables runnign from the mixer master outs to the amp inputs, and connected from from a stereo IN on the mixer to the amp IN. The problem re-occured. This leads me to think it is NOT an issue with the audio path at all.

Worth mentioning perhaps is that the amplifier is also connected to some home theater gear, wifi routers and other crap, but I haven't disconnected that yet since it's very difficult to reach. However, those devices have always been there. The problem has only appeared since I installed the new PC.

I'd appreciate some tips on what to try next!
 
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Needless to say, what you have described is difficult to imagine. Perhaps posting a block diagram might help, but it soulds like you already know where the problem is occuring, the question is how to address that problem. My suggestion with out seeing this setup is to connect to your mixer using a DI box and try using the ground lift.
 
Lets start here. A ground loop is usually a constant sound. I have had similar issues with sound interfaces that are firewire powered. the computer noise dumps through the fire wire. Adding a good ground to the interface sometimes helps. I usually try to power off an AC adaptor if the unit has that option as well.
Brad
 
"The problem has only appeared since I installed the new PC."
I have heard of certain MOBOs that have a sort of ground loop issue with FW interfaces and it is very difficult to fix without some very deep knowledge of PCs. Basically it is a hack into the BIOS.

Try the usual isolating suggestions and it would be worth ordering a couple of the very cheap, £5.00 two channel isolators you see on Amazon. These do in fact have surprisingly good audio performance but are at least cheap enough to use as diagnostic tools, i.e. if you find where to break the "loop" you can replace the cheapy with an Art Cleanbox e.g.

But I hold out little hope. This IS a ground loop of sorts giving rise to "zipper noise". Ask over at Sound On Sound | Recording Techniques | Audio Technology | Music Production | Computer Music | Video Media I am sure Pete from Scan will do his best to help you.

Dave.
 
Does your new PC have hardwired keyboard and mouse, or are you using a wireless KB and mouse?
 
studio loops.jpg

Thanks for the replies everyone. Here is a block diagram which illustrates the setup. Note that my initial text description had an error, I mentioned the soundcard was only connected by firewire, but there is a wall wart power supply too (two prong). I grounded the chassis, no difference.

The noise problem goes away if I do any of these:
- remove the firewire connection
- remove the connection between firewire card and mixer
- remove connection from mixer to amp

The problem is NOT heard on any of the headphone outputs, and it is still heard even if I turn the mixer volume fully down. It is also heard when I connect the audio paths incorrectly (ie, soundcard INPUT connects to a mixer INPUT).

I am guessing that the problem is getting to the amp via the ground, not via any audio path. Is that possible?

I will try the mentioned isolators. I am using a wired usb keyboard and mouse. Problem is still there if I disconnect them.

I've also tried with a different firewire card, and got the exact same issue. My current workaround is to keep the amp volume low, and provide a strong signal to it from the mixer. This gives reasonable snr but it's still audible during quiet passages or detailed work.
 
innovine said:
I recently changed PCs and have run into some digital noise issues. When the PC does stuff (resize windows, mouse over buttons, etc) I can hear a burst of digital noise. If i don't move the mouse over stuff and there's not much cpu use happening, the noise vanishes. It's either occurring, or not, and it's quite loud when it does.

I've reduced the problem to my PC, a firepod soundcard connected via firewire to the PC, a yamaha mg124cx mixer, and a NAD C326BEE amplifier. By plugging headphones into all the outputs on the soundcard, I do not hear any noise, despite the fact that I can still hear it in the mains. I don't hear the problem in the mixers headphones out either. I can turn all the mixer sliders to zero and the problem persists. If I disconnect the last patch cable from the soundcard to the mixer, the problem goes away. It doesn't matter what this cable is connected to, it can run from an out on the soundcard to an aux out on the mixer, or in to a strip which is not getting routed and with the slider down. If I leave the soundcard connected to the mixer, and disconnect the firewire cable then the problem also goes away.

All devices are connected to grounded electrical strips, apart from the soundcard which is only connected to the mixer and pc via firewire. I thought ground loops were more of a constant deep buzz, not this intermittent high pitched bzzz of digital noise. I've removed unnecessary pci cards, usb hubs, etc. If the problem IS a ground loop near the soundcard, why is the noise not heard on the mixer headphone outs, but IS heard on the amp once the amp is connected to the mixer (with the master fully down)? I also took the cables runnign from the mixer master outs to the amp inputs, and connected from from a stereo IN on the mixer to the amp IN. The problem re-occured. This leads me to think it is NOT an issue with the audio path at all.

Worth mentioning perhaps is that the amplifier is also connected to some home theater gear, wifi routers and other crap, but I haven't disconnected that yet since it's very difficult to reach. However, those devices have always been there. The problem has only appeared since I installed the new PC.

I'd appreciate some tips on what to try next!
Click to expand...

sometimes changing the power supply will fix this . but since the grounds are shared you can break the loop with using a 6 pin to 4pin firewire cable 6 pin side to the interface and 4 pin to the pc. if you don't have a 4 pin connection on your pc you can use an adapter to go from 4 back to 6. this way you will not induce any power base noise into the interface via electromagnetic radiation from the power and ground ( since a four pin is just the data lines and not power) the interface would essentially be isolated. now there is isolation transformers for the firewire connectors they are used in more industrial environments but they are pricey (firewire isolators for PLC applications start around $200)
 
Maybe its time to swap out the amp or use a pair of powered speakers to eliminate the amplifier. Since the amp is the last thing in your chain, and you do not indicate any other connections, perhaps the old Pioneer is needing a new filter cap? : )
 
"The noise problem goes away if I do any of these:
-------------------------------------------------
remove the connection between firewire card and mixer"
So, fit one of those cheapo isolators in that path. If that fixes it you can either buy a better one or, best answer, make up a special earth free lead. I can post a schematic.
Filter caps rarely fail inside 40 years. There WAS a case of some MOBOs that had crap caps and now they all seem to have been tarred with the same brush!
Dave.
 
cecerre said:
Maybe its time to swap out the amp or use a pair of powered speakers to eliminate the amplifier. Since the amp is the last thing in your chain, and you do not indicate any other connections, perhaps the old Pioneer is needing a new filter cap? : )
Click to expand...

This is not actually likely to fix anything. The amps inside the powered speakers will still be the last thing in the chain. And, as it happens, I have the same problem, and I have powered speakers. My old PC was noiseless, the new PC is spurting out digital noise like crazy. I have yet to find an adequate cure for this, though I have been able to reduce it by decoupling devices from ground.

However, I have been a bit casual about the whole thing, and I'm still using unbalanced lines from interface to speakers and headphone amp.
 
Did you get a new monitor with your new computer? Sounds like it could be RFI from your video card. What happens if you disconnect your video cable from the back of the computer? If this makes it go away you probably forgot to install the ferrite beads on your video cable.

Glen
 
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