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Post by mahons on Nov 11, 2008 17:44:40 GMT -7
So, i'm looking to set up some sort of a primative "home studio," using a firewire interface to record directly onto a computer. I've heard of the various hum issues that a computer can cause with a guitar amp, and i'm just wondering what it is exactly that causes this, and how I can avoid this hum to get nice clean, quiet recordings. From the little reading i've done i've read that hum can be caused by computer monitors, cooling fans, and other random grounding issues (something about ground loops ) I've also read that you can sometimes eliminate hum by isolating certain parts in a different outlet/circuit (?) or by buying some sort of a filter to eliminate the buzz. Can you avoid the problem by keeping the computer in a different room? How far away would it need to be? Would the room need to be sealed off fairly tightly? Any input would be greatly appreciated...
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Post by Lefty on Nov 11, 2008 18:42:09 GMT -7
Distance and a separate circuit are a great start. Some of the biggest noise issues are with the CRT monitors.
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Post by benttop (Steve) on Nov 11, 2008 18:57:09 GMT -7
Never had any trouble with that. I do hear the computer's fan if I'm too close to it when I set up the mics though.
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Post by mahons on Nov 11, 2008 19:07:44 GMT -7
Thanks for the replies...i'm probably just over- complicating the matter and/or being too paranoid ;D
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Post by Phil (aka Phil) on Nov 12, 2008 5:58:15 GMT -7
Well, outside of the hum you pick up through your pickups (no pun intended) there's always the ground loop issue.
Ground loops can be really hard to avoid and troubleshoot sometimes but here's what I always do.
When I wire a home theater system I always like to run everything through the same power distribution device- one that has protection for the cable TV and/or satellite coax cables also built in. I plug the power cord of each component (TV, DVD player, CD player, tuner/amplifier, satellite box, etc.) and the incoming cable TV and/or satellite cables into the same power strip/distribution point. Good power distribution devices protect the ground/neutral and line/neutral from spikes as well as reference all the grounds to the same point.
The problems start arising when you plug, say, your TV into one outlet, and your satellite box or cable TV into another outlet. If you get a surge on your cable TV or satellite from say, a lightning strike, the fact that the coaxial cable's shield and your TV's ground are referenced to ground at different places (and therefore have different impedances) can easily cause equipment damage.
When I play my multi Z amp rigs I always plug both amps and all effects into the same power strip/conditioner. If you plug each amp into a different outlet you will probably hear the result- 60 cycle (or more accurately, 120 Hz, the second harmonic of 60 Hz) hum.
Here's where things get more complicated. If you're lucky, you'll plug in your computer and whatever mic pre's/mixers/USB recording devices/interfaces/guitar amps/effects you're using and get no hum. The problem with PC's is that their internal power supplies are nonlinear and actually generate a lot of harmonics, mostly the third (180 Hz). This noisy current has to go out on the neutral, which eventually will be interconnected with your ground. This can sometimes get back into your recording and/or monitors.
One "solution" to this is to use an isolated ground outlet on your PC. This is a special outlet (orange in color instead of the usual white/cream/etc.) that also involves running a separate ground all the way back to your main distribution panel.
Regarding filters: I used to troubleshoot high end car audio ground loops. Try figuring out what's causing car audio noise when you've got a radio, multiple amps, crossovers, eq's, etc. all interconnected with signal cables, and all grounded to the car's frame, which is the same place that lots of noise producing things like the alternator, turn signals, wipers, etc are grounded! Draw that system schematic out and it looks like spaghetti!! What I learned back then is that filters will reduce the hum/noise, but it's kind of like putting a band aid on a really deep wound that needs stitches. It's better to fix the original problem that to put a filter on it and reduce the noise IMO.
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Post by bluzman on Nov 12, 2008 8:15:35 GMT -7
Like all above info, keep it all in mind. Then go at it. Don't fix problems you may not have... Set it all up then see. Make small simple corrections first if you have issues. Some solutions you aren't familiar with may over-complicate things. If it hums and it bothers you, teach it the lyrics!!
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Post by mahons on Nov 12, 2008 13:21:25 GMT -7
Thanks again for the replies. To be an amp enthusiast, one must also be a "weekend warrior" electrician! ;D (Tubes, biasing, grounding/hum...)
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