Circuits/grounding/noise/hum

One potential issue.  Even with one AC power line for everything a "ground loop" hum problem can appear.  I've seen this many times.  Fixes for ground loop hum problems include lifting the AC power ground on one device, using a device such as the Ebtech hum eliminator; or if its because of two units both mounted to the same rack rails, isolating the device causing it from the rack rails (commercial "hum frees" are sold for this purpose).
 
Sorry to hijack the thread but thought this needed brought up.
 
If you are going to run a special AC power line for recording, consider a 20 amp circuit and an "isolated ground" line with a Hubbel 5262 (orange) receptacle.  With an isolated ground circuit the ground wire is connected directly to the ground bus in the AC power panel and directly to the AC power receptacle, not to a conduit ground or just grounded to the outlet box.  Thus the "isolated" ground.  When I worked as a LAN/WAN Network Manager, all our computer equipment (including servers, printers, workstations, etc) was connected to isolated ground circuits, which minimized noise on the AC power line.  I don't know how much that would add to the cost of a "standard" AC circuit but ultimately may be worth it.  
 
 

I'm not really picking on your Fireberd, your post was good, but I'm afraid you offered some dangerous advice.
 

fireberd
One potential issue.  Even with one AC power line for everything a "ground loop" hum problem can appear.  I've seen this many times. 

 
This is true - ground noise comes from many places. and even a properly designed technical power system can still have ground loops.

However, ground loops aren't really the problem, it is current flowing in the ground loops that is the problem, and more to the point, poorly designed equipment that allows the ground loop current to interfere with the audio.
 

fireberdFixes for ground loop hum problems include lifting the AC power ground on one device, using a device such as the Ebtech hum eliminator; or if its because of two units both mounted to the same rack rails, isolating the device causing it from the rack rails (commercial "hum frees" are sold for this purpose).

Not true!
 
You should NEVER lift the SAFETY ground! This is an old wives tale that, sadly can reduce noise problems, but it can also create a lethal power problem where you become the low impedance path between an energized conductor and ground. NEVER life the SAFETY ground - it is called that for a reason.

So how do you build a noise free audio system? Turns out we've know for a very long time, we just forget every now and again.

#1 - clean power! You don't have complete control over the power that enters the building, but there are numerous ways to clean it up once it gets there. Probably the most cost effective solution (in most situations) is called a separately derived panel, fed from an isolation transformer. If that sounds like greek then I'd strongly suggest hiring a qualified electrician to install one.
 
#2 - properly designed balanced gozintas and gozoutas - if you have these then no ground current can affect your audio. It's really that simple. Any properly designed balanced input (active or transformer) will have sufficient ability to reject common mode noise.
 
#3 - twisted pairs - shields do nothing for power line noise,they are effective against radio frequencies, but at 60 Hz they are invisible.
 
#4 - proper termination of grounds - this gets tricky because (a) if everything else is good it becomes less of an issue, and (b) there are two perfectly acceptable solutions - "star ground" and "mesh ground" - and neither is particularly easy to implement. Good thing it isn't critical.
 
If it were me, and I were the curious type (which I am) I'd read Philip Giddings "Audio Systems Design and Installation" it is still considered the gold standard. It was out of print for a long time, and used copies used to command very high prices. PostToronto has recently reprinted it at a very reasonable cost of about $70 CDN.
 
After reading that I'd find everything I can on the topic of "The Pin-1 Problem". The late Neil Muncy first published his paper on this in 1996 and it was a real eye opener. Be prepared to diagnose and solve problems with existing gear.

I'd also check out most of the Rane Notes. There is a wealth of information there.
 
It is neither difficult nor expensive to build an electrically quiet system. If you can find your way around Sonar you can do this. Do you want this becomes the problem, and if you don't there are plenty of folks that can. I'd strongly suggest finding a local resource, I've tried helping people build studios remotely and it can become a wee bit frustrating for all concerned.

fireberd
Usually, as long as the device its attached to is properly grounded the attached device will be too.

 
Still not good advice. Lifting the earth connection on any equipment is just downright dangerous. You cannot rely on the audio earth connection between an earthed and non earthed equipment either to provide a solid ground. It might be quiet but in the event of the live mains wire coming into contact with the metal chassis of one of the items concerned then the audio earth may not be enough to save you.
 
A much better and safer way is to leave all items grounded, and if a ground loop appears then solve it by lifting the earth connections in some of the audio connections, not the power.
 
Using balanced audio connections wherever possible is a great start. (especially between your active monitors and what is feeding them e.g. interface monitor outs.) A lot of stuff has balanced connections these days and many don't know it and are using unbalanced leads to connect stuff.  Not only do you get great noise rejection with balanced connections but a 6 dB volume boost too for free.  Many synths have balanced outputs and many line inputs on mixers are also balanced as well. Use em!
 
Sometimes when you connect things like iPads or laptop outputs you can pick up hum, noise or other sounds. There are also ways to connect non balanced outputs into balanced inputs. e.g. by wiring into the balanced input using the balanced connections only and not the balanced inputs earth. This can also solve many hum and noise problems too.
 
Plugging your entire studio into a single power circuit is also a good thing to do. It puts all the earths in the system at the same potential and will minimise ground loops. Your entire studio will not draw enough power to trip the breaker either.
 

Afraid I can't agree completely, but your first point is spot on.

Jeff Evans

fireberd
Usually, as long as the device its attached to is properly grounded the attached device will be too.

Still not good advice. Lifting the earth connection on any equipment is just downright dangerous. You cannot rely on the audio earth connection between an earthed and non earthed equipment either to provide a solid ground.

 
Here's the thing, and neither Jeff nor I seem to be saying it well. It is NEVER good advice to suggest bypassing a safety ground. If two pieces of gear are connected, via shield, when only one is connected to earth ground then it is true that both are grounded. But you are now depending on that shield to carry a lethal current to ground, and more than likely it will fail and you will carry that current to ground.
 
It seems that everyone who understands grounding and shielding assumes everyone else does too. That's why I will often jump in when posts suggest defeating safety ground. To someone that is more musician than electrical engineer that might sound great. But they don't understand the safety implications.

That's why I get so insistent on the subject. I'm not trying to start - or sustain - a pi$$ing contest!
 
Part of the confusion stems from our mis-use of the terms ground and common. Ground is considered this mystical point where no amount of current flowing in or out can change the potential - but wait, that definition is missing a word - "difference". A potential difference can not exist at a single point, so we need to know with respect to what (or where.)

All of which is digging deeper than necessary. Always connect safety grounds and you don't have to think about it.
 

Jeff EvansA much better and safer way is to leave all items grounded, and if a ground loop appears then solve it by lifting the earth connections in some of the audio connections, not the power.

 
Also true, although I'd go one step further, consider "Aggressor" and "Victim" and treat accordingly.
 
Embarrassing tale I've told too many times to care any more - back in the bad old days I had this lovely Tangent console that I was rebuilding. For grins I decided to try the "huge copper bar" approach that I knew some of my heroes were using to quiet their consoles. It was expensive, tedious, time consuming, and dang, it worked. It worked really well!
 
Turns out we were all fixing the right problem with the wrong solution! As part of the huge copper bar solution we were terminating the shields outside the chassis. Yes, I felt kind of dumb. And yes, I was able to re-sell that copper bar, for a tad more than I paid for it, but I'll never get the hours spent drilling and threading!
 

Jeff EvansUsing balanced audio connections wherever possible is a great start.

 
And herein lies maybe the biggest source of confusion. Every source can be treated as balanced. And all you need to connect a balanced source to a balanced input is two wires, although it helps tremendously if they are twisted.
 
That leaves the input as the potential (excuse the pun) problem. If it is a single ended input you will need to do something to make it differential - you can add a transformer (ideal, but expensive) or you can add an active device. That's it! Meaning there really is no reason not to use balanced interconnections everywhere.
 

Jeff EvansNot only do you get great noise rejection with balanced connections but a 6 dB volume boost too for free. 

 
Part 2 of the great audio myth - a balanced connection is not dependent on signal symmetry, nor does it require that both input pins be driven at all. The ONLY requirement for a balanced connection is that both input pins see a equal impedance to ground. That's the definition of a balanced input. And that means that  the only requirement for the source is that it presents an equal impedance to ground on both legs.
 

Jeff EvansThere are also ways to connect non balanced outputs into balanced inputs. e.g. by wiring into the balanced input using the balanced connections only and not the balanced inputs earth. This can also solve many hum and noise problems too.

 
I think you are saying that correctly, but without a picture it could be confusing. I usually suggest folks read several Rane Notes to see the pretty pictures. Here are my go-to references:
Sound System Interconnections
Grounding and Shielding
Why Not Wye
Pin 1 Revisited
(these next two get a little deep, but are still worth skimming)
Shield Current Induced Noise
Considerations in Grounding and Shielding Computer-Controlled Audio Devices
 

Jeff EvansPlugging your entire studio into a single power circuit is also a good thing to do. It puts all the earths in the system at the same potential and will minimise ground loops. Your entire studio will not draw enough power to trip the breaker either.

This is great for really small systems, but it can cause as many problems as it causes, especially since most folks will stop there, assuming this is the cure-all.

All of this goes back to my previous assertion about safety - those of us with experience and education need to do a better job separating fact from fiction. We need to stop assuming everyone understands this stuff as well as we do. That's no different than assuming everyone has studied harmony, counterpoint, and orchestration when discussing sample libraries!
 
So we need to choose our words carefully, and sometimes we need to type a lot.
 
And now our regularly scheduled aside:
 

fireberdMy biggest audio challenge was shielding (RF Shielding) a studio in Kansas City, Mo (Big K Records) that was 1 block from an AM radio station transmitter site.

Wow does that bring back bad memories - in a good way! My biggest challenge was a combo (AM/FM) co-located with the AM transmitter. I was still a wee lad (might have still been in college) and in completely over my head! I had two things going for me:
1) the previous engineer was a genius, and the AM control room was quiet, so I studied what he did, and adopted a telescoped shields solution that copied his approach. And it worked!
2) while a radio station needs to be quiet, it doesn't need to be recording studio quiet, so my goals were somewhat less demanding. You had your hands full!

fireberdBTW, ex W5DVO, K3FFZ, ZD8JES

Why "ex"