I made some insert transformer balancing boxes for work about 6 months back. I work in live sound, and unfortunately at the company I work for we are stuck with desks with unbalanced inputs (mostly soundcraft) as we do not have the budget nor enough big jobs to justify buying midas desks or similar 'proper' heavy duty stuff. When your FOH rig gets large enough and you have lots of inserts etc, you sometimes get into trouble, and these boxes can be really useful. The boxes I made were just like NYD's but with an input and an output transformer. For money reasons, I used OEP's...forget the number, but they're the ones that can be strapped for 2k2 in and out, or thereabouts. I have no problem with the OEP's; for live sound they are transparent enough, but perhaps in more critical environments someone may say they are too 'coloured'. I do find them audible, but not at all in a bad way.
Whilst we're on balancing issues; I once found that I was unable to get rid of an insert hum with these boxes. It really confused me, as my natural instinct was to go with the 'but it's transformer floated...it MUST work!' Anyway, I had wired the sleeve of the 3-pole jack insert (desk ground) to the input XLR pin one, but left the output XLR pin one disconnected. This was in strict accordance with our policy of disconnecting all output pin 1's on all our gear. I theory this meant that the screen should never have been connected at both ends for any of our gear, but alas, a few rogue pieces of equipment were discovered that did not adhere to this policy...and on this particular gig a non-standard piece of gear meant that there was an additional ground path from the insert rack to an unbalanced insert point on the desk. Regardless of transformers and balancing and floating and all that, it means that there was a noise (hum) current going through that channel. That current actually makes the channel hum, and it's not really anything to do with with balancing, as it is introduced outside of the actual balanced lines.
Since then, I modded the boxes so that the pin ones on the input and output XLR's simply connect together, without connecting to the unbalanced insert ground (the sleeve of the 3-pole jack) at all. That ensures that the insert device ground simply cannot connect to the desk insert ground via the interconnect leads. The actual metal case itself is not connected to any ground, but the screens of the transformers do connect to the desk (3-pole jack) ground, but are insulated from the case.
Fun, innit?
Bjorn
