Building A Transformer-Balanced Mic Splitter

One of my latest DIY audio projects has been a simple transformer-balanced mic splitter for location work. While you can occasionally get away with splitting a mic signal with a Y-adapter, it can cause all sorts of noise and loading problems. Transformers… if they are high quality… do the job much better. Jensen transformers are famous for this application, but expensive, and cheap, lightweight transformers found in catalogs like Allied and DigiKey don’t handle high frequencies well. After much searching on the internet, I found Edcor audio transformers. This is a small US company that winds audio transformers… their WSM600/600 has a frequency response of 20hz-20kHz, and are affordable at around $12 each (I needed  a four-channel split for my Edirol R4Pro field recorder that I bought from Trew Audio… four signals to the recorder and four identical signals to the mixer that are then mixed and sent to camera.)

The splitter circuit is pretty simple. It's developed from the Jensen application notes, more transformer design info can be found at their website.

The splitter circuit is pretty simple. It’s developed from the Jensen application notes, more transformer design info can be found at their website.

The circuit is pretty simple.  The only components required besides the transformers and connectors are a resistor and capacitor for a radio frequency network. I used polyester film caps and metal foil resistors for low noise. A ground lift switch is optional… it isn’t a bad idea because it allows for potential ground loops, but it does complicate the build a bit.

The front and back panels for the 4 channel splitter. There's barely enough room for the connectors.

The front and back panels for the 4 channel splitter. There’s barely enough room for the connectors.

I first built a two-channel version, and I’m glad I did… I figured out where my screw-ups were. The case I selected was too tight for the two-channel version, really, so I was able to devise a better way to locate the components for the four-channel unit. When you build these, it’s best to wait to mount the transformers until after the connectors have been mounted and wired. And it’s difficult to machine the case sides for connectors or switches, you want to use only the smooth face plates for mounting hardware.

Wiring the unit was made easier by not bolting anything to the case before soldering… I made that mistake on my two-channel unit. And I saved a bunch of connectors by hard-wiring pigtails to the two-channel splitter, but it makes it cumbersome to store and vulnerable to damage. I decided to be conventional with the four-channel version and use connectors, but I’ll need to solder up eight short cables to go along with it. Soldering cables isn’t difficult, but rather time-consuming.

The case layout with the transformers in place. They'll be bolted into position after they're soldered.

The case layout with the transformers in place. They’ll be bolted into position after they’re soldered.

I’ll be adding more info on this project as it develops.

Update: Well, my splitter has been completed for some time now. It works great. In use, it’s a bit heavy… not something I can put in my bag, but it’s normally used on the cart anyway. The size isn’t inconsequential, either, though this is certainly the smallest case that would fit the unit. As it was, I didn’t have room for ground lift switches. Soldering the leads was a bear, because I was trying to keep the leads as short as possible. I used a mini shielded cable that I bought from Trew Audio that is normally used in lavalier mics.

14 responses to “Building A Transformer-Balanced Mic Splitter

  1. Could you tell me what modifications would be necessary to use one or two channels as line splitters? Such as for guitar or kybd.

    • Thanks for your comment. Bill Whitlock has written an excellent article for Jensen transformers about splitters. Take a look here, it should answer your questions far better than I could.

      Best, BG

  2. Pingback: BGilbertSound 2010 in review | Brian Gilbert Location Sound

  3. Hi Brian,
    I’m the owner of a recording studio in Italy, your project it’s very intresting… I will offer live recording service very soon and I want to try your schematic… where can I find it in a good resolution?

    Thank You!!!

    Enrico

    • bgilbertsound

      Enrico: Where in Italy? We’ve been twice… mi piace molto andiami in Italia (sorry, my Italian is not at all what I’d like). If I had a way to make money there, I’d move in a heartbeat.

      All you’ll ever want to know about transformer splitters is on the Jensen website… here’s the link. ciao, BG

  4. Hello Brian, thanks for reply!
    I found everything I need on Jensen website.

    I live and work in Forlì, an unknown town near the Adriatic coast, very sad place… ha ha ha!!!! But I’m happy with my family and my littel boy, my son, he’s just one year old.
    If you turn up again in Italy don’ forget to contact me!!! I will teach you Italian (?) and you will teach me how to keep the boom raised up for more that 10 minutes… he he he

    best wishes

    enrico

    • bgilbertsound

      Enrico- It’s a deal… though I think teaching you to put a boompole in a C-stand holder ;-) is a lot easier than teaching me Italian. It will probably be several years before I return again… it’s expensive to travel… but I’m certain I will come back, maybe with my sailboat, then I could stay longer. I’m curious, though… why is Forli a very sad place? I looked at some snapshots of Forli on Google, and doesn’t look like a bad place at all. But I’m biased… generally, in Italy, the smaller the town, the better I like it (except Venice, I loved it there. Perhaps no cars are the key… I love small towns that you can’t drive through the center!)

      Of course, if you looked at snapshots of Chattanooga, it seems at first like a wonderful town. And it is, unless you want to work in audio or video production… then it’s a real struggle. I often have to drive to Atlanta, Nashville, or Knoxvillle to work… (all three are larger, all are about 2-3 hrs drive away) Chattanooga is an affordable place to live, but there’s just not enough production going on here

  5. Brian, you’re right… and I think that we mean the same problems about our towns…
    In reality Forlì as become a dormitory suburb, no business venture, no money for culture and arts, just a fair place to walk sunday in center with family… Forlì has just a great position in history, from Roman Empire to second world war. But this fact I have idea that doesn’t intrest to the customers of my business… he he he!!!

    Chattanooga for example has a fine river, I’ve seen in some photos (me too on Google, I’m a copycat), great pic-nic in summer!
    I read on officially Chattanooga website that your city seems to be on of the 50 best places to live, this say tha National Geographic, NOOOOT BAAAAD!!!!!

    But, a question… do you really have intention to sail through the atlantic ocean with your boat??? You are not a sound engineer, but a skilled sailorman smoking pipe, confess! I’ll wait you standing on the dock of Cesenatico!!!
    I’d like to come in Tennesee, maybe with my family, by fly (no boat!). “…I’m goin’ to Graceland…” sang the great Paul Simon.

  6. I have absolutely to say you are a wellspring of learning Brian… I’m not jocking, I took a look at your books, very well done! You almos infect me the sail passion.
    Please always remember: dreams can really come true. In 37 years of my life nothing happened… In just 1 year I become dad and I’ve built the recording studios of my dreams… he he…
    Well, for this time you can come in Italy with plain, just to do a “reconnaissance mission”…
    you are right, live here in Italy for Italians it’s so hard, especially with the actual tax regime. Consider that you gain 1000$ for a recording job and you catch in your pocket less than the half of this amount.

    I hope to meet you in person one day…
    ciao

  7. Hi Brian,
    I’d like to build a portable version of your project to connect 2 Audio-Technica ATR-3350 to an iphone 5 mic input. What suggestions do you have? Can I get away only with an Y splitter?

    • bgilbertsound

      I’d try a plain Y-splitter. The transformer balanced mic splitter is an output device; in other words, it takes a single mic output and splits it to feed two inputs. You’re going the other way around… taking two outputs and feeding a single input. But I don’t know anything about an iPhone’s input impedance or level requirements, so I can’t say for sure. I don’t know what a 3350 is, either… if they are condenser mics, there’s a good chance it won’t work because of the power requirements. Sorry.

  8. Hi, Brian

    I’ve been looking for the transformer that you recommended, but the link is broken. I don’t understand anything about this device, so could you tell me which would be the possible substitution?

    Thank you

    Ricardo
    From Manizales, Colombia

    • bgilbertsound

      Ricardo: Do a google search for it… the Edcor site isn’t the clearest, but I was able to find the WSM 600/600 without too much trouble. Just in case, I replaced the link in the article. While there are other substitutes (Jensen makes some great transformers, for example, as do Cinemag and others), I like the Edcor transformers because they’re US- made, have good performance at a reasonable price point. Good luck!- BG

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