The current which flows in the ground loop flows through audio cable shield. In balanced connections the current flowing in the shield should not effect the signal in the cable or this signal connections in equipments. If the equipments are well designed (cable shield grounding is done in the right way) then small amounts of currents do not cause any problems. In practice equipments are not that well designed and even very small current can induce humming to the system.
If you cut the audio cable shield the current will stops flowing but exposes this audio line to other kind of problems: if one of the equipments is not connected to electrical grounding, then the equipment don't have any common gound which then not to work as expected.
To avoid this kind of problems and still limiting the current passing in cable shield wire to value which does not cause problems an scheme called ground lift is introduced. Ground lift places a resistor (usually around 100 ohm) between the equipment ground and cable shield. This resitor limits the current passing in the ground loop situation, but still provides quite good ground connection. This system is unfortunately quite sensitive to radio interference, so the 100 ohm resistor is usually shunted with small capacitor (usually 4pF to 10 nF) which makes the impedance to be low at radio freqwncies but does not let too much 50 Hz current flowing.
Before trying to use ground lift circuit is is best to check that everything is otherwise wired correctly. It is stupid to use ground lift trick to fix other problems in the system because ground lift does has it's own problems. A good document how to do the audio wiring correctly is Rane Note 110: Sound System Interconnection.
Ground lift in balanced connections
Full ground lift circuits for balanced XLR connector
1 (not connected) 1 2 --------------- 2 3 --------------- 3This is the most basic ground lift circuit which works nicely when all equipments are grounded and have balanced inputs/outputs. Because the cable shield is cut the cable this arrangement makes the cable more prone to pick up radio frequency interference. This kind of ground lift circuits are built-in to some equipments (can be activated using switch) and they are available as ready made products (like GLX GROUND LIFTER and similar).
The shield of the cable is conneted to pin 1 only in one end of the cable. It's more common to tie the shield at the sending end and lift it at the receiving end. Either way will work, but tying the shield at the sending end has some advantages for reducing crosstalk.
Partial ground lift with RFI filtering for balanced XLR connectors
10 nF +---||---+ | ____ | 1 -+-|____|-+- 1 100 ohm 2 ------------ 2 3 ------------ 3This ground lift connection does not fully cut the gound carrying cable shield, but icreases the resistance so much that the currents which would flow in the shield in typical ground loop situation are limited to so low values that they have no effect in system performance. Because the ground is not completely cut then the circuit also works also when ungrounded consumer equipments with RCA->XLR adapters are connected to the system. The capacitor makes sure that the cable shield appears to be continuous to the radio frequency signals (radio frequency interference protection provided by shield is not lost). This circuit is quite universal and I have used this circuit for succesfully solving some balanced circuit ground loop problems.
Telescoping shield cables
In the audio world, there is a solution to ground loops called "telescopic" ground and "Faraday shields." A telescopic ground works only with a cable which is a balanced line, that is, one which has two wires to carry the signal and a separate shield. In a telescopic ground, the shield is connected only at one end. This prevents the completion of the 'ground loop.'
Best telescoping shield cables (true telescoping shields) are constructed so that they have two shields which are insulted form each other. The idea is that you connect the inner shield to ground only at receiving end and the outer shield to ground only at the transimitting end.
1 ------------- ------------- 1 2 --------------- 2 3 --------------- 3In this way the ground loop is effectively broken but the RF shielding properties of the cable are still very good. Double shields and the capacitive coupling between (they are near each other in cable) form a good shield for radio frequencies. It has been claimed that telescoping a shield in cables with hybrid unbalanced/banaced audio ystems is often very successful at removing noise . The telescoping shield protects the inner conductors and drains away that unwanted noise to one location. Shield effectiveness gets less and less as you travel further from the grounded end.
Telescopic grounds cannot be used in unbalanced circuits, such as unbalanced audio interconnections and video coax, as the two conductors necessary to send the signal include the shield. That is, the shield is both the noise-reduction portion of the cable and a signal path as well. Unground that and, if the signal gets through at all, you will have the world's noisiest circuit as the other path (what was the shield) will be established through some other ground path through other equipment!
Some notes on using ground lifts
Disconnecting the screen in balanced audio cable will break the ground loop and possibly eliminate hum. But a word of warning before disconnecting the screen. If you are running equipment from two or more separate power receptacles, especially if they are widely separated and the building wiring is old or not up to spec, there may be residual 50 or 60 cycle voltages present between the two alleged ground connections. These will be small, of the order of millivolts, unless something is seriously wrong with the building system, but they may be large in terms of audio signals, and can do a lot of damage if fed into a high gain audio input. So it is important to try the effect of ground lift at low gain first.
Ground lift for unbalanced connections
Situations where two grounded equipments with unbalanced connections are connections have ground loop related humming problems and no other solution helps then you can try to use ground lift. Ground lifting in inbalanced connections works only efectivily when both of the equipments are properly grounded to same point. In some cases the humming problem may becomes worse if ground lift is used. So ground lift in unbalanced connections is not a foolproof method and it should be only used as temporary solution.
Here is a typical ground lift circuit for unbalanced connections:
Signal -------------------- Signal
Ground (not connected) Ground
Try this circuit only if you know that both equipments are properly grounded.
If the equipments are properly grounded this circuit will cause enormous
amounts of humming and potentially damage the input amplifier of the
receiving equipment because of the flowing stray currents on the ungrounded
equipment. The best solution for solving unblanced connection ground loops
is using audio line isolation transformer.
Note on using the circuit: Because this circuit cuts the shield of the audio cable this greatly reduces the RF shielding which the continuos cable shielding normally provides. If you are using the circuit above be sure that there are no severe RF interference sources like mobile phones or communication radios near your audio system. The RF shielding of the circuit can be made better by using the circuit aboe which provides cable shield continuity to RF signals but works as ground lift for audio frequencies.
If you happen to use RCA connectors in audio connections, you can try if this ground lift helps if you partially remove the RCA connectors, so that the center pin touches the jack, but the outer ground does not connect the jack side. If this solved the problem then you can do the ground lift circuit. In RCA connectors you can do the ground lift by inserting a rolled-up paper strip or plastic cut from plastic back between the RCA jack's ground shield and the receptacle. Another solution is cut the cable ground connection inside the jack.
If you are in buiness of modifying a cable to include groundlift option I would recommend adding a small capcitor to the place where you cut the cable to reduce the possibility of the ground lifted cable to pick up RF interference:
Signal -------------------- Signal
Ground -------||---------- Ground
Using ground lift circuits
Ground lift circuit would reduce possibilities of audible ground loops. A truly covers-all bases piece of equipment would have a switch to activate or deactivate the ground lift. To test if your equipment has ground lift, insert a balanced TRS or XLR cable into the equipment, and measure the resistance between the shield contact of the exposed connector and the casing of the equipment or the earth pin of the mains cable of the equipment, using a multimeter. If there's a 100ohm to 500ohm resistance, your equipment is ground lifted.
Ground lift is quite effective in balanced audio connections, but is much less useful in unbalanced connections, which is the connection type used in almost all consumer audio equipments. You might try this ground lift scheme with this connection type, but the results would be much worse. Even when you can limit the shield connector current to values which do not cause problems, there is still the ground potential difference between equipments which gets amplified (and you get still some 50 Hz noise). Even ground potential differerences much lower than 1 mV can cause serious noise problems with unbalanced circuits. If you have unblanced connections, I would advice you to use audio isolation transformer instead of ground lift when you are solving problems with nonbalanced audio connections.
Audio Wiring and Grounding article from Equitech web site provides more information how ground lift is wired and how it affects the audio system performance. Wires and connectors article at Multimedia Bluffer Guides.
You can easily test the effectiveness of ground lift in your system. urn the volume down, disconnect the shield at one end, and slowly bring the volume back up. It's easy if you've got phono connectors--just pull the plug partway out, so the pin makes a connection but the outer shell doesn't. Be warned: If other grounds aren't good enough or mose of the equipment are not grounded at all this will cause an even worse hum. If the PC is not properly grounded this kind of experimenting can even cause damages to audio equipments.
Tomi Engdahl <Tomi.Engdahl@iki.fi>