=20
For a verbal description click=20 here.
Eliminating Ground Loops.
One of the most = insidious=20 problems in an audio system is one or more ground loops. They not = only=20 cause hum pickup, they can inject interference from electric drills, = computer hardware and even AM radio stations. Eliminating them is = often not=20 a trivial procedure. Here are the causes of, and cures for, ground = loops.=20
What Is a Ground Loop?
A ground loop is set up when there is = more=20 than one ground path between two pieces of audio equipment. The = figure=20 below shows one way this can happen.
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For a verbal description click=20 here.
Why does this cause hum and=20 noise?
There are AC magnetic = fields=20 everywhere there are power lines. If you live in the country far = from any=20 power lines you don't have any AC fields but how do you run your = stereo and=20 computer?=20Even if you were to use your own generator it, and its wiring, = would=20 cause AC magnetic fields. Anytime an AC field cuts through a loop = there is=20 a current induced. if any part of the loop has resistance which is = not zero=20 there will be a voltage drop across it. Copper wire at room = temperature has=20 nonzero resistance and so the magnetic induced current causes a = voltage drop=20 across the shields of the audio cables placing a source of hum in = series=20 with the audio signal. =20
What can be done?
In the case of Figure 1.
You might try grounding the two = chassis'=20 together with very heavy wire in an attempt to short out the induced = voltage. The added wire will just create another loop unless you = place it=20 physically very close to the audio cable. Even if you do that all = you will=20 succeed in doing will be to reduce the volume of the hum but it will = still=20 be audible. =20You might also try bringing the line cords close together and = close to=20 the audio cable but most likely all you will accomplish will be to = increase=20 the hum. You will place AC carrying conductors into close proximity = with=20 the audio cables thus inducing more hum than you are getting rid of. =
The only sure way is to break the loop. Cutting the shields on = the audio=20 cable rarely works with unbalanced inputs. There will always be a = potential=20 difference between the two chassis' even though they are grounded to = the=20 power line ground. The reason is that the grounding wire gets a = current=20 induced in it because it is in such close proximity to the current = carrying=20 conductors of the line cord. If this were a real situation, the = only=20 solution would be to somehow disconnect the power line ground from = one of=20 the chassis'. This is not approved by electrical inspectors and = fortunately doesn't need to be done in home stereo equipment.=20
Consumer grade sound components are made on the same principle as = many=20 power tools and small kitchen appliances. They are doubly insulated = which=20 eliminates the UL requirement for a third wire safety ground. = Professional=20 and semiprofessional audio gear is very likely to come with a 3 = prong plug=20 because of the more difficult environment in which it is used. Such = equipment also has balanced inputs and outputs which gives an = alternative=20 method of eliminating ground loops as will be discussed below.=20
In the case of Figure 2.
In the case illustrated in Figure 2 = the=20 solution is to make the loop very small by bringing the two cables=20 physically close together. When you do this here is what happens to = the=20 ground loop.
For a verbal description click=20 here."
Figure 3 Tape deck with ground = loop=20 reduced to nearly zero.
An Example.
For a time I used several pieces of audio equipment in conjunction = with the=20 sound card in my computer. All of these had unbalanced inputs and = outputs. =20 An example of an unbalanced in/out is the standard RCA plug and jack = combination. This type of signal transfer is used almost = exclusively in=20 home audio equipment. =20What I had in addition to the computer was a HMM (home made = mixer), a MSM=20 (MIDI sound module), a reverb unit, and a DTR (digital tape = recorder). It=20 should be noted that the DTR had (and has) both unbalanced and = balanced ins=20 and outs. Only the computer and DTR had three wire plugs. In = addition to=20 that the DTR, MSM, and reverb were mounted in a steal equipment = rack. =20
I had a choice of "floating" the computer or the rack with the = DTR in it.=20 I decided to keep the computer grounded since it was connected to = the cable=20 TV system through a cable modem. The modem provides isolation or = there=20 would have been another giant ground loop involving the cable. I = felt that=20 best lightening protection would be afforded if the computer = remained=20 grounded. =20
The stuff in the rack was a problem. The ground loop involved = the rack=20 itself and the shields of the audio cable. I tried cutting the = shields of=20 the cable but that only reduced the hum without eliminating it. The = solution turned out to be using pieces of plastic to insulate the = chassis'=20 of the MSM and Reverb from the rack. That reduced the hum to such a = low=20 level that it was barely audible with the volume on the monitor amp = at full.=20 That's as close as you are likely to get to eliminating the hum.=20
Some professional fear mongers would say that I was in mortal = danger by=20 operating a rack full of electronics without a safety ground. In = fact the=20 rack equipment was grounded to the computer which was in turn = grounded to=20 the power ground. As long as the audio cables remained connected, a = fault=20 would have blown fuses or popped circuit breakers and I would have = known=20 something was wrong. The odds of a fault occurring at the exact = moment that=20 all cables happened to be disconnected are so small as to only be of = interest to professional worriers.=20
Balanced Inputs and Outputs.
The only practical and safe = solution to=20 ground loops is to use equipment with balanced in/out. Home audio = equipment=20 "gets away with" using unbalanced in/out because it never uses three = wire=20 power cords. I am beginning to hear from people about home theatre = systems=20 that are used in combination with cable and/or satellite systems = which are=20 running into ground loop problems. It probably won't be too long = before=20 high end systems will be using balanced audio lines. What are = Balanced=20 Inputs and Outputs? In a balanced output the signal comes out with = equal=20 voltage on two wires. The voltages are not in the same phase. When = one=20 wire goes positive the other is going negative and vice versa. On a = balanced input the signals applied to the two lines must be out of = phase in=20 this same way. If two signals of the same voltage and in phase are = applied=20 to the two wires of a balanced input It will be the same as if there = is no=20 input at all. This is called cancellation. The hum is always in = phase on=20 the two wires and so it gets cancelled out while the signal is not.=20If an unbalanced output is connected to a balanced input it will = work but=20 it needs to be wired in a special way to get cancellation of the = unwanted=20 hum. More about this later.=20
In a balanced system the equipment is always grounded to the = power line=20 and often is in an equipment rack which ensures that it all gets = grounded=20 through the chassis'. If there are multiple racks you must make = sure that=20 all racks are bonded together.=20
The audio cable that is used to connect everything together is = called two=20 conductor shielded. It actually has three conductors. One is the = shield=20 which surrounds the other two which are twisted together. The = twisting=20 tends to cancel magnetically induced currents which are not effected = by the=20 copper wire that makes up the shield. The two signal carrying wires = are=20 connected to the two balanced output connections and to the two = signal=20 inputs at the other end. The shield is only connected at one = end=20 usually where the wire connects to the output. The shield is not = involved=20 with carrying signal currents, unlike the unbalanced lines, but = serves only=20 as a shield against electro static fields.=20
In general two types of connectors are used in balanced systems. = They=20 are called XLR and TRS. XLR connectors are usually used on = microphones. =20 They have Three pins in a triangular configuration. TRS stands for = tip,=20 ring, and sleeve. They are the standard one quarter inch phone = plugs that=20 you are accustomed to finding on stereo headphones. The tip and = ring=20 instead of carrying left and right channels carry the two balanced = signal=20 lines. The sleeve of the plug connects to the shield of the cable = but only=20 at one end. =20
I am no longer using the floating rack system. I bought a mixer = which=20 had a built in reverb unit eliminating the need for the one which = did not=20 have balanced ins and outs. The DTR used a special plug for = balanced in/out=20 so I had to buy a cable for it. I made up the rest of the cables. = Then I=20 constructed a balanced to unbalanced and unbalanced to balanced = converter to=20 interface the whole thing to my computer sound card. The schematic = is shown=20 below.
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