The Bunk wrote:Can anybody recommend a suitable University course for me to go on to understand that?
No need for the uni course. Galvanic isolation means 'no direct electrical connection' -- something that is often achieved in a DI box by using an output transformer. The input and output sides of the transformer are completely separate insulated coils of wire, so electrically isolated from each other, while the audio signal passes from one to the other in the form of an alternating magnetic field.
That electrical isolation is very useful in a DI box because it means that if there's a major electrical fault in the guitarist's gear, for example (eg, a missing safety earth meaning the gear potentialy becomes live), it won't be able to reach and damage the PA equipment -- and vice versa. It can also help in avoiding ground loops.
So, if I'm running the PA at a gig and I'm not 100% certain of the quality of the band's gear (or if it's an outdoor gig where a spontaneous cloudburst might occur), then the extra safety factor of isolating transformers in the DI boxes separating their and my gear gives welcome peace of mind. The same applies if I'm gigging with m keyboards and I'm not convinced of the PA system or those operating it! ;-)
All passive, and most active, commercial DI boxes have an output isolation transformer, although the Radial J48 and the Orchid boxes are amongst the exceptions -- they use active electronic outputs instead. The benefit is less weight, lower cost and better quality because good-sounding transformers are heavy and expensive, and cheap ones introduce audible distortion... But the downside is a direct electrical connection between source and destination equipment, so a major fault on one side can potentially reach the other..
Consequently, I am very happy to use the excellent (and very attractively priced) Orchid boxes on my own projects where I know everything I'm using them with is well maintained and safe... And I use boxes with transformers for their electrical isolation if I have any concerns.
I hope that helps.