Spacey
I hope there is someone out there who can help me.
My gear is as follows:
Desktop pc
Alesis Photon X25 Midi controller with sound interface
Yamaha MG82cx Mixer
Yamaha is connected to soundcard via the record out and the rtn, so out from mixer goes to in for soundcard and out from soundcard goes to in for mixer. Main out of mixer goes to amp and speakers.
My problem is as follows:
When I plug my acoustic guitar into my mixer to record audio, I experience interference. I know Acoustic guitars give a certain hum at times, because they are all balanced, but I have noticed that this interference decreases considerably when I disconnect the sound card from the mixer. Logic told me that the sound card must have a problem, but I did a bit more investigation. Connected my sound card again and the interference is back. So I disconnected the usb from the sound card (this card draws eletricicty from an external power supply) and the interference disappeared. Decided to reconnect usb and turn of desktop. Interference is back. Unplugged the desktop from the wall, and the interference is gone. Disconnect my monitor, from everything, and plug desktop back into the wall, and interference is back, even though the desktop is switched of. This leads me to think that the interference comes from the ground of the desktop, but obviously we can't remove the ground from my cable. The Desktop is also the only component in my system that is earthed, so I doubt is can be a ground loop.
If anyone can help me, it would be much appreciated.
Bob-Dubery
Not some kind of earthing problem? Two devices that have different earths and when you plug the USB cable in an earth loop forms?
Spacey
I don't think so. The pc is the only component with an earth. The rest of my components all have normal "two point plugs".
Heath
sounds more like the usb cable is acting like an antenna , usb has a floating ground , so not directly connected to the earth . look for other source of the hum , like powerpacks , geysers , heaters , CRT monitors , microwaves .... along those lines . perhaps try a different usb cable
AlanRatcliffe
Sounds like a loop to me. Dirty power would always show up, not just when certain things are connected.
Does the PSU for the interface have a mains earth? Even if it only connects to the interface via a standard DC power plug, that can still be causing a loop - particularly with the modern switch-mode PSUs. If so, lifting the earth on the PSU will work - it will still be earthed when it is connected to the rest of the system.
FenderBender
Alan Ratcliffe wrote
Sounds like a loop to me. Dirty power would always show up, not just when certain things are connected.
Does the PSU for the interface have a mains earth? Even if it only connects to the interface via a standard DC power plug, that can still be causing a loop - particularly with the modern switch-mode PSUs. If so, lifting the earth on the PSU will work - it will still be earthed when it is connected to the rest of the system.
I second your opinion. The 0v on the output jack of most modern switch mode wall-wart type PSUs is connected to earth inside the supply. Lifting this ground at the plug of the sound card PSU (if there is an earth connection) and maintaining the ground on your desktop mains plug should do the trick as the PSU will be earthed via the USB socket shell on the desktop.
EDIT: I checked the specs on the X25. There are three power options with this unit: Mains PSU, Batteries and USB cable. Unplug the PSU from the X25 and try using only the USB cable.
Spacey
Thank you very much for everyone's input.
I have used various usb cables. Same problem. I have tried the usb without the PSU. Problem and symptoms persists. The PSU for the Sound card is a generic ac/dc adapter. Anything someone else can think of.
FenderBender
Are you sure there is no earth connection on your amp mains lead?
Spacey
Yes. I'm using pc speakers (don't laugh), but even when I unplug them and use my headphone, I have the same problem.
TomCat
Do you have another PC to connect the USB unit to to see if the noise/hum persists?
Spacey
Unfortunately not! What did you have in mind?
AlanRatcliffe
My next step would be to try some cables with the ground disconnected between interface and mixer. Break the ground on the source side (at the interface). There will still be a ground from the USB side. If you have access to a dual DI box, you can try inserting that between the interface and mixer and using the ground lift switch.
Spacey
If I don't have a dual DI, how would I break the ground?
AlanRatcliffe
Literally detach the wire from the plug on that side.
TomCat
What happens when you bypass the mixer and plug directly into the USB interface? Does the hum go away?
Neps
OK, I've put off long enough not to reply.. Cause I've been wrecking my brain, been lazy, been busy, just plain useless.. ?
1st of all, I guess all your gear is connected to the same wallwart? If not, get a mult, and plug your gear into that mult. Eventhough different wallwarts in the same room get power from the same main, you can have power issues.
2. Your desktop seems to be the culprit. You can have a floating earth in the box itself.. Open up the box, and check if the PSU has been earhted correctly. PC's tend to be flimsy.. And, yes, Mac's also can pose this problem due to moving them and bumps.
3. Try and move ALL the gear as FAR away as possible from the wallwart.
But the main cause here is your PC! Do you have other pieces of gear that runs from your USB, that was plugged in at the same time.. External HD's can also cause this!
Is your USB on a PCi card, or is it 'built-in'? If it's a PCi then take it out, slot it into another slot.. As far away as possible from youyr screen card. Screencards are power hungry buggers, and they tend to be the worst insultaed buggers for electrical interference .
Spacey
Hi Neps. Thank you for your post.
I will check my earth tonight. How must my PSU be earthed, or what should I look for?
I'm using a multi, with a surge protector. I have a mouse and flashdrives that run of my USB. My USB is not a PCi card. It is onboard.
TX
