Eliminating Ground Loops.

Even if you were to use your own generator it, and its wiring, would cause AC magnetic fields. Anytime an AC field cuts through a loop there is a current induced. if any part of the loop has resistance which is not zero there will be a voltage drop across it. Copper wire at room temperature has nonzero resistance and so the magnetic induced current causes a voltage drop across the shields of the audio cables placing a source of hum in series with the audio signal.

What can be done?

In the case of Figure 1.

You might also try bringing the line cords close together and close to the audio cable but most likely all you will accomplish will be to increase the hum. You will place AC carrying conductors into close proximity with 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 cable rarely works with unbalanced inputs. There will always be a potential difference between the two chassis' even though they are grounded to the power line ground. The reason is that the grounding wire gets a current induced in it because it is in such close proximity to the current carrying conductors of the line cord. If this were a real situation, the only solution would be to somehow disconnect the power line ground from one of the chassis'. This is not approved by electrical inspectors and fortunately doesn't need to be done in home stereo equipment.

Consumer grade sound components are made on the same principle as many power tools and small kitchen appliances. They are doubly insulated which eliminates the UL requirement for a third wire safety ground. Professional and semiprofessional audio gear is very likely to come with a 3 prong plug because of the more difficult environment in which it is used. Such equipment also has balanced inputs and outputs which gives an alternative method of eliminating ground loops as will be discussed below.

In the case of Figure 2.

What I had in addition to the computer was a HMM (home made mixer), a MSM (MIDI sound module), a reverb unit, and a DTR (digital tape recorder). It should be noted that the DTR had (and has) both unbalanced and balanced ins and outs. Only the computer and DTR had three wire plugs. In addition to that the DTR, MSM, and reverb were mounted in a steal equipment rack.

I had a choice of "floating" the computer or the rack with the DTR in it. I decided to keep the computer grounded since it was connected to the cable TV system through a cable modem. The modem provides isolation or there would have been another giant ground loop involving the cable. I felt that best lightening protection would be afforded if the computer remained grounded.

The stuff in the rack was a problem. The ground loop involved the rack itself and the shields of the audio cable. I tried cutting the shields of 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' of the MSM and Reverb from the rack. That reduced the hum to such a low level that it was barely audible with the volume on the monitor amp at full. That's as close as you are likely to get to eliminating the hum.

Some professional fear mongers would say that I was in mortal danger by operating a rack full of electronics without a safety ground. In fact the rack equipment was grounded to the computer which was in turn grounded to the power ground. As long as the audio cables remained connected, a fault would have blown fuses or popped circuit breakers and I would have known something was wrong. The odds of a fault occurring at the exact moment that all cables happened to be disconnected are so small as to only be of interest to professional worriers.

Balanced Inputs and Outputs.

If an unbalanced output is connected to a balanced input it will work but it needs to be wired in a special way to get cancellation of the unwanted hum. More about this later.

In a balanced system the equipment is always grounded to the power line and often is in an equipment rack which ensures that it all gets grounded through the chassis'. If there are multiple racks you must make sure that all racks are bonded together.

The audio cable that is used to connect everything together is called two conductor shielded. It actually has three conductors. One is the shield which surrounds the other two which are twisted together. The twisting tends to cancel magnetically induced currents which are not effected by the copper wire that makes up the shield. The two signal carrying wires are connected to the two balanced output connections and to the two signal inputs at the other end. The shield is only connected at one end usually where the wire connects to the output. The shield is not involved with carrying signal currents, unlike the unbalanced lines, but serves only as a shield against electro static fields.

In general two types of connectors are used in balanced systems. They are called XLR and TRS. XLR connectors are usually used on microphones. They have Three pins in a triangular configuration. TRS stands for tip, ring, and sleeve. They are the standard one quarter inch phone plugs that you are accustomed to finding on stereo headphones. The tip and ring instead of carrying left and right channels carry the two balanced signal lines. The sleeve of the plug connects to the shield of the cable but only at one end.

I am no longer using the floating rack system. I bought a mixer which had a built in reverb unit eliminating the need for the one which did not have balanced ins and outs. The DTR used a special plug for balanced in/out so I had to buy a cable for it. I made up the rest of the cables. Then I constructed a balanced to unbalanced and unbalanced to balanced converter to interface the whole thing to my computer sound card. The schematic is shown below.

Adjustment with test instruments.

The converter operates from a wall transformer so is not grounded to the power line ground. It is grounded to the computer through the unbalanced lines that connect between the converter box and sound card. That is it's only ground. The balanced lines go from the converter box output to the mixer input and from the mixer output to the converter box input.

The monitor amplifier is a conventional consumer grade integrated stereo unit. One of it's inputs comes from the tape output (unbalanced) on the mixer. Another input comes from the computer's sound card output. The shields had to be cut on this line to avoid forming a ground loop. An output can feed more than one input.

Connecting the MIDI sound module.

Conclusion.

Ground only one chassis to the cable ground and keep the inter connecting cables tied together to keep the loops small. If you have three chassis' with audio connections that form a triangle you are going to have to cut some shields on one set of audio cables. Try the cable with the cut shields in each of the three positions to find which placement gives the least amount of hum.

If you have a home recording studio, obtain, or make, the proper XLR or TRS patch cables and keep everything grounded as the manufacturer intended.