Audio isolation solves groundloop problem
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.
Usually when you install an audio galvanic isolation (any type) to almost any place in the audio cable connection is solves the ground loop caused humming problem nicely.
Possible isolation components
The most suitable component for audio line galvanic isolation is audio transformer. It is very compact, completely passive and widely available. For audio signal isolation transformers are the way to go and they have been used for this application in audio studios successfully for test of years. In audio studios commonly used technique very traditional is to balance the lines between the systems using audio transformers that do the balancing and galvanic isolation.
While optoisolators can be used to isolate grounds they have insufficient linearity and dynamic range for analog audio. Optoisolators have also the problem that circuit using them will always need some external poer source to operate. I do not recommend using optoisolators for any high quality audio isolation application.
Cheapest transformers for audio use have some problems on the sound quality, typically distorted bass response and attenuating in high-frequency response. Basically a poor transformer slows down high frequencies more than low frequencies (because of transformer leakage inductance). This allows the low frequencies to pass first, creating what we perceive as a "fat/warm" tone. Inadequate frequency response on the low end (roll off at like 20Hz), causes low frequencies to be "slowed", allowing the upper frequencies to be heard first, this is perceived as "barky/ brittle".
High-quality audio transformers cover whole audio band with good response, but those are quite expensive. Depending the application you might have to do some weighting against the price of the transformer and the sound quality.
Whenever possible, an input transformer should be used at the destination end of an interconnect cable. Input transformers offer much better Common Mode and Radio Frequency Rejection than is possible with output transformers.
Audio isolation transformer product information
There are ready made circuits available from shops selling car audio stuff (ground loops are usually problem also in car environment). The price is typically in $10-40US / 10-40 Euros range (there are more expensive professional and "hifi" versions). The cheapest versions not be the highest quality solution though. If you are looking for very good sound quality, you might want to invest to some better quality (and much mor eexpensive) audio transformers.
For more professional product check JK Audio Pureformer Stereo Isolation Transformer. Those products seem to be quite suitable for solving ground loop problems in consumer audio systems, but I have not tested them myself.
Europeans should take a look at their nearest dealer which carriers Monacor products, because Monacor's new catalogue lists FGA-40 (Best.-Nr. 06.4370) Ground Isolators which are 1:1 audio isolation with 10 kohms impedance (cost around $20). I have used this isolator successfully in some audio installations and I am quite satisfied with that as a problem solver. I have found that the transformers in this product will attenuate the low frequency signals if the transformer is driven by high impedance output, because the impedance gets quite low on lowest frequencies.
For professional audio use buy high quality commercial audio isolation/balancing transformers (those are very handy to keep around to solve unexpected ground loop problems).
Balancing the line and removing of humming in PA systems instrument connection can be achieve too by using passive DI boxes, available at your local music stores. Many DI boxes are active and can be almost useless in getting rid of earth loops and stopping buzzes & hums etc. A GOOD passive DI which provides isolation is proven only way to go. (Active ones work also if they are properly designed and provide necessary ground isolation).
- Xitel Ground Loop Isolator - RCA isolator
- Biltema 31-441 Galvaaninen erotin - RCA 1:1 audio isolator
- Cordial CLT 1 - 600/600 Ohm line-transformer for the galvanic separation of the sound signal - XRL connectors
- Monacor FGA-40 (Best.-Nr. 06.4370) Ground Isolators - RCA 1:1 audio isolation with 10 kohm impedance
- Jensen Transformers - company which makes audio isolation transformer and DI boxes - XLR and RCA connectors
- Leon Audio Active Direct Box - 6.3 mm jack to XLR with isolation
- Leon Audio Passive Direct Box - 6.3 mm jack to XLR with isolation
- Lundahl Transformers XLR inline transformer units for breaking up ground loop in audio systems - XLR isolation and XRL-RCA adaptation, good quality professional devices
- Radio Shack Ground LP Isolator (270-0054) - cost about $16US, RCA plugs, for car audio
Tomi Engdahl <Tomi.Engdahl@iki.fi>