Recently I managed to pick up a radio mic set from eBay (which, as you'll probably know, is my favourite source for film gear, but perhaps not for much longer since I discovered BBList, a Craigslist for broadcast equipment).
They were the Audio Ltd. RMS 2000 set, which were about 10 years old but still in great condition and full working order. The newer versions (RMS 2040) were used on Skyfall, the Harry Potter films and the new Star Wars.
The problem was that, unlike the Sennheiser or Sony equivalents, these use the 6-pin LEMO FFA.2C connector. The set came with an XLR-output cable and a mic with a LEMO connector, so for production use they were ready to go.
However, I had a couple more uses for them than just a simple lav-mic-to-XLR-recorder scenario. I thought that it might be useful to also be able to use them as a wireless-IEM set (essentially wireless headphones), and I also wanted to be able to plug the receiver directly into my 5D or another similar camera.
And so I asked the nice soundman from Hampshire who I bought the radio mic from if I could buy a couple of these strange LEMO connectors and he happened to have a couple that he could spare.
I then figured out that the cables that I'd need to make to do what I wanted were the following:
- 2x LEMO to female minijack
- 1x Male mini-jack to male mini-jack
- 1x Female XLR to male mini-jack
This way I can use the female XLR to transmit the output of a sound board, for instance, or any other non-phantom-powered XLR mic, as well as plug my Shure IEMs into the receiver.
My initial attempt at making the mini-jack cables was to buy a couple of pre-made mini-jack extension cables and cut them up and use the ends. While this did work, the cables themselves were a bit horrible and they didn't coil properly.
So instead I had a look on eBay to see if I could find the parts I needed. I ended up with these:
It took me a while to figure out the pinouts for the LEMO connectors because there's literally no documentation anywhere. I ended up opening the male-XLR to LEMO cable that came with the mic and here's what I discovered:
(Diagram made with Paper)
The mapping of the three cores (2-core + shield) is nothing too special, but the connecting of pins 3 and 4 on the LEMO connector are actually part of the RMS2000's clever functionality -- the transmitter and receiver can't be switched on unless the input/output cable is connected. This means that you can't accidentally leave them switched on in a bag, draining battery.
I then used this mapping along with this guide to find out the mapping of LEMO to TRS.
Then it was time to make the cables themselves. I am somewhat of an electronics enthusiast, and so I already had a soldering iron and solder lying around, but even then the one I use cost me £8 from Amazon, and so it's not exactly much of an investment.
Here's what the inside of a LEMO connector looks like:
And here's me nearly done soldering a mini-jack (TRS):
There is a reason that one of the mini-jack-to-LEMO cables is slightly smaller than the other one. Something I learnt relatively recently is that there are balanced cables and there are unbalanced cables and there are stereo cables. I'm not entirely sure how the "balanced" part works, but I know this: when I plug my stereo IEMs or headphones into a balanced cable, the audio only comes out of one ear. To solve this, I had to solder the ring and the tip parts of the TRS together. This, however, makes the cable unbalanced because there's only a positive and negative, and no third (because the third is connected to the positive).
As such, when I made the two adapters, I made one of them "stereo unbalanced" and the other "mono balanced". The stereo one is meant for headphone output, the the mono balanced is meant for microphone input. I made the stereo one shorter because that'll be the one that's in my pocket as opposed to the other one which will be on the camera or in the sound bag.
You can see the completed cables in the banner image of this post. On the whole, I'm very pleased with how they turned out -- and I saved quite a bit of money compared to buying the cables ready-made. The total cost of the components (excluding LEMO connectors which were £12 each) was £17, (averaging £4.25 per cable) which is about half of the price of buying pre-made ones.
I'm not claiming to be some kind of sound wizard but I thought this might be helpful to anyone who wanted to try their hand at making their own cables. Feel free to drop me an email or comment if you have any questions/queries about making some for yourself.