Ground loop is a common problem when connecting multiple audio-visual system components together, there is a good change of making a nasty ground loops. Ground loop problems are one of the most common noise problems in audio systems. Typical indication of the ground loop problem is audible 50 Hz (or it's harmonics 100Hz, 150 Hz etc.) noise in sound. Most common situation where you meet ground loop problems are when your system includes equipment connected to earthed elecric outlet and antenna network or equipments connected to different grounded outlets around the room.
When you find a ground loop problem, you have to analyze the problem. If you have equipments connected to many outlets, then try to connect them all to one grounded extension cord and then plug this cord to one grounded outlet. This solves usually many problems. The most effective way to stop the 50 Hz noise is to break the galvanic connection which causes the ground loop. This is the only solution to problems which deal with grounded outlets and radio/TV cable/antenna connection. Breaking of ground loop can be done in many ways, some which are better and some are less good. Some people try to cut the connection between aquipment case and safety ground connection, but this is dangerous and not a good idea.
If you want to do the ground loop elimination in audio path, you have to cut the galvanic connection but pass the whole audio range. The simplest and most common way to do the isolation is use audio transformer which is ment for audio use. Transformers for audio use have some problems like distorted bass response and attenuating in high-frequency response. High-quality audio transformers cover whole audio band with good response, but those are quite expensive.
There are ready made circuits available from shops selling car audio stuff (ground loops are usually problem also in car environment). If you live in USA, take a look in Radio Shack's catalog on car electronics or check the Radio Shack Product Support pages which have specifications of Ground LP Isolator (270-0054) which cost about $12US.
If you want to build one yourself, you have to get two audio transformers which have 1:1 transformation ratio and greater than 1 kohm impedance. There are high quality audio transformers in the markes that meet those specs, but those can be quite expensive. Another option it to use 600:600 ohm isoltation transformers widely available for telecommunications and other uses. Those are not that high quality thatn good audio transformers, but can be well adequate for many used if good transformer is selected.
I built my isolatiors usign two high quality telephone line coupling transformers which have 600 ohm impedance. This is the most commonly transformer type used in high-speed modems. Best of those are quite wideband devices (far more bandwidth than usual 300-3400 Hz as used in telephone). Using two of those transformers and few RCA connectors made quite satisfactory (but not really hifi) audio isolator. The connetion is easy: connect primary side of the transformer to one audio connector and secondary to other.
I used EOP Z1612 transformers in my test circuit and got quite acceptable frequency response of +-1 dB from 40 Hz to 20 kHz as you can see in figure below. The bass frequency below 40 Hz is not good.
The frequency measurements were made with Nacamichi T-100 Audio Analyzer and the isolator circuit was connected between it's 600 ohm output and 50 kohm input. I don't know if EOP Z1612 transformers are still available at Farnell, but you can try. If you are looking for other high quality transformers which could be a used, I would try ETAL P2001. I haven't been able to test their performance in this application, but they have proven to good transformers in other laboratory test and applications. Avoid cheapest telephone and audio transformers, because their performance is very poor at frequencies over about 5 kHz (for example Radio Shack (273-1374)).
This circuit is a simple isolator for TV and Radio antenna connection. This circuit passes radio frequency signals nicely, but does not pass significantly 50 Hz signals, so the ground loop is eliminated. The circuit can be easily built into antenna connector or to a small box. I would recommend to use small metal box, where you connect one of the antenna connectors to the metal box and isolate other connector from box. Metal box allows mechanically strong contruction and provides good shielding against radio interference. The capacitors in the circuit should be rated at least 250VAC to make sure that the adapter withstands situation when antenna network ot television/radio is floating at mains live potential.
There is one disadvantage of this (and other isolation circuits): the circuit breaks the continuous shielding of the antenna cable which makes you antenna cable pick up radio interference more easily. Usually this is no big problem, but if you notice severe interference then you might have to stop using this isolator. The beast place tu put this isolator (to keep possibility of interference minimun) is just between TV receiver and antenna cable going to wall.
There is an alternative approach to antenna isolation problem: use transformer as in audio lines. The problem is to how to make a good transformer for antenna signals. If the transformer causes impedance mismaches, this can cause signal reflections which disturb you and maybe your neighbours also.
One way to do it is to use readily availabe 75 ohm to 300 ohm transformers (called "baluns" or "matching transformers") used when connecting old televisions with 300 ohm antenna input to modern antenna network with 75 ohm connectors. Those units are readily available from TV shops. Be sure to buy models which have no galvanic connection between 75 ohm input and 300 ohm output (so there is isolation between input and output), because some models only do impedance matching but galvanic no isolation (they are useless in this project). The circuit is easy to build. Just take two of those 75 to 300 ohm antenna transformers and connect their 300 ohm sides together. Now you have antenna isolator you need.
____________ ____________
__| |------X------| |__
75 Ohm |__ | 300 Ohm | __| 75 Ohm
|____________|------X------|____________|
This circuit has been suggested by Paul Grohe (grohe@galaxy.nsc.com), who
suggested that suitable tranformers are available from Radio Shack
(Radio Shack cat. #'s 15-1140 and 15-1253 or MCM #33-050 and #33-010).
If you experience radio interference picked by this circuit, you can can try the following method to make shilding ogh the circuit better: You can wrap the whole little assembly in aluminium foil and ground the foil to the "cable" cable shield. But don't let it touch the other ground.
Aluminium Foil Shield
+--------------------------------+
| +--------+ +--------+
--Cable Co Coax-O----| Balun |======| Balun |-------- To TV
| +--------+ +--------+
+--------------------------------+
connect to shield of incoming coax
I have seen another transformer circuit posted in the usenet. kari Hautanen (kari.hautanen@online.tietokone.fi) wrote an article to sfnet.harrastus.elektroniikka newsgroup about antenna isolator. The article says that you can built a suitable transformer using following method: Promary and secondary are three turns of 0.2 mm Cu-wire wrapped around small magenta coloured toroidal ferrite core. Seems quite simple, but the article did not mention the exact core size. It mentions the source of the circuit to be a book Osmo A Wiio, Reijo Laine, Radioamatöörin käsikirja, EU Harrastekirjat 1978, 1980, page 164. I have not been able to locate the book to check the circuit, but the circuit in one ready-made isoltor had very similar circuit: It had three turns of thin wire in promary and secondary wrapped around small ferrite core. The wiring inside the isolator is very similar to the audio isolator circuit, the only difference is that now connectors are antenna connectors and the transformer is the one designed for antenna signals (described above). It is recommended to build this circuit to a small metal box, but be sure that one of the antenna connectors is isolated from then metal box.
FOr more ideas how to fight against ground loop caused by cable TV connection, check the Fixing Cable-TV Hum in Audio Systems article by Jay Rose at http://www.tiac.net/users/jcrose/cablehum.html.
Ground loop isolation can be done also in mains connection by using mains isolation transformer which safelyt isolates the connection between equipment and mains ground. This kinds of isolation transformers are sometimes called "safety isolation transformers", because they are used in dangerous environments (electronics repair shops, construction sites, electronics laboratorios) to provide more protection against electric shock. Connecting your grounded equipment through this type of isolation transformer to mains can isolate the ground loop. Usually this type of transformers are quite big and expensive, because they must be cabable of handling the power which your equipment takes.
Isolatation transformers are not good in all situations but sometimes they are very useful (like in laboratories and some temporary audio system installetions) for avoiding harmful ground loop problems. There are isolation transformer made for different uses and with different construction and connections. It is a good idea to go to a good electonics shop and ask help for selecting suitable isolation transformer for your application. In some countries you are not allowed to break the ground connection even if you are using isolation transformer and in some coutries you are allowed to have only one equipment connected to one isolation transformer. Remeber to use the transformer according the instructions and local electric code to keep your system safe.