Over the years I have trouble shot hum issues in recording studios, TV studios, radio stations, I-Max, war game rooms for the Feds, video rides, data centers out the ying yang and a slew of Mega-churches. In each case all issues with hum were associated with improper wiring methods.
Iso transformers were helpful when the end user wanted a quick fix and did not want to spend the bucks to locate the issues.
IG as I said before were of little use and in most cases complicated the issues.
That was always my find also, using a transformer to isolate DC and noises?
DC I can see but what can DC do to sound equipment? I have never seen sound equipment that didn't have power transformers somewhere in the line of supply. even tube type amplifiers still have a transformer, so how is DC getting passed that? as for line pops and noise on the AC system, well a transformer will not stop them, and can even amplify it.
Like I said this is where a good quality line filter comes in, and even one with voltage regulation would be a great plus, and as for stopping 60hz hums, it will not if wired as per the NEC, current on the grounding is what causes ground loop hums and as long as there is a primary to secondary neutral connection, you will not get rid of ground loops.
Active UPS systems (ones that are always on battery power and only use the POCO to recharge) would be another plus as then no interruption in the recording and the fact your always off the POCO grid.
I have heard these same statements made by many old time sound guys and they are in most part myths, you have to understand the circuit at hand before you can solve the causes.
