As an audio geek and a bargain hunter, I’m often found trawling the internet for cheap/old/weird/cool stuff. I buy some of it too. After stumbling on some very nice old Lundahl SR-501 transformers and scooping them up pretty quickly I was left with the eternal question… what should I do with them? Apparently these are the first audio transformers demigod Lars Lundahl ever wound up and were commissioned by the national Swedish Radio group, hence the ‘SR’ suffix in the name. These are a 1:1 unit designed for either line input or line output use.
My Pathos mixer spits out an unbalanced signal yet is (currently, at least) plugged into a pair of active monitors happiest with a balanced signal. Suddenly a plan emerges.
I built my little balancing box inside of an Aliexpress knockoff 1590BB enclosure, which fit the transformers and input/output jacks nicely. I try to use Neutrik connectors everywhere, and this box was no exception. The Neutrik RCA jacks are a little luxurious pricewise, but should last longer than I will. I definitely fall into the ‘buy it once’ category of shopper.
I left the grounds from the mixer and XLR output disconnected so this unit effectively acts as a ground lift device too.
Any keen observer will notice that I royally stuffed up the alignment of one of the RCA jacks. So let this be a lesson to all: Do not get drunk and operate a drill press! Safety concerns aside, you will more than likely wind up with scrappy work and you will curse yourself every time you look at it. Nonetheless, the device works nicely and adds a great presence to the output of my mixer at the same time.
Schematic I used, will vary depending on your choice of transformers. I didn’t add any zobel network as a cursory look at squarewave performance on the scope looked fine. This was a pretty fast paced project and I didn’t overthink any of it.