Making a 3.5 mm (lavalier) to XLR (camera) cable at DVinfo.net

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Old January 23rd, 2011, 08:47 PM   #1
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Making a 3.5 mm (lavalier) to XLR (camera) cable

I need to make two cables that I haven't been able to find because they need to be two specific lengths and the 1/8" connectors have to be right-angled. I want to avoid using adapters because each is a potential point of failure. Here's what I need:

- one going from a handheld microphone (XLR) to my wireless belt pack (3.5 mm);
- one going from the wireless receiver (3.5 mm) to the camera (XLR).

Here are the microphone specs:

- Audio Technica hand-held unidirectional dynamic 600 ohm;
- Audio Technica Pro 88W/R wireless with condenser microphone (couldn't find impedance specs).

I have a surplus place where I can find pro-quality parts, I have the wiring diagrams and I know how to solder. But I need to know whether I need to match impedance. If so, what type of capacitor do I need and will it fit inside the XLR housing?

This will be for interviews, sometimes in noisy environments (music/crowds). Thanks for any advice you can provide.


J.
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Old January 24th, 2011, 03:55 AM   #2
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Neutrik produce a right angle stereo 3.5mm socket and should be available at better electronic stores.
I bought some the other day $5.95 [Aust].
Neutrik - Audio - 3.5 mm Tiny Plug - NTP3RC-B
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Old January 24th, 2011, 06:58 AM   #3
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Note that the transmitter might not have enough audio gain on its mic input to handle the low output level of a dynamic mic, Dynamics typically are considerably lower level than condensors and need more gain. What's the model number on the mic?
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Old January 24th, 2011, 10:41 AM   #4
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Quote:
Originally Posted by Jacques E. Bouchard View Post
...I have the wiring diagrams and I know how to solder. But I need to know whether I need to match impedance. If so, what type of capacitor do I need and will it fit inside the XLR housing?
I don't know much about the specifics of the AT wireless. But, in general, an output needs to be of lower impedance than the input it drives, on the order of a minimum of 10x, for linear performance across the audio spectrum. So, impedance "matching" is not the goal.

In the prosumer audio world, we have lo-z and hi-z. If there is adequate gain, a lo-z output may drive a lo or hi input. However, a hi-z output to a lo-z input is suspect.
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Old January 24th, 2011, 02:36 PM   #5
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So how do I make the cable work between the microphone and my wireless? I don't have a model number for the hand held, it just says 600 ohm on it.


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Old January 24th, 2011, 05:09 PM   #6
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Impedance shouldn't be a problem but you should use a blocking capacitor in the signal lead to prevent the transmitter's plug-on mic power from getting into the dynamic mic's coil. Insert a 220mfd 16v capacitor in the signal lead, poistive side connected to the TS connector. Wire XLR pin 2 through the cap to the TS tip, Jumper XLR pin 3 to XLR pin 1. Connect XLR pin 1 to TS sleeve. As I said before, levels might be an issue as the transmitter expects to see a signal level of about -45dB or so for full modulation and most dynamic mics only put out about -55dB or even less. Without knowing what mic you actually have there's no way to look up its specs so the only thing left to do is try it and see what happens. AT makes a pot-full of different handheld dynamic mics ranging from,$10 plastic toys up to some pretty decent broadcast mics and there's no way to tell just what you've got with only its impedance rating.
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Old January 24th, 2011, 05:27 PM   #7
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I found a model number, it's AT834 II. Google returns "AT834-2" as well, but none of the hits included specs. The Audio Technica site has no information. This microphone has to be at least 20 years old.

If it can help, the microphone is about 8" long from tip to tip. It's all metal and weighs a ton. It looks very much like this one (see attached).


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Old January 25th, 2011, 12:26 AM   #8
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" But I need to know whether I need to match impedance. If so, what type of capacitor do I need and will it fit inside the XLR housing?"

1) You probably don't need to worry about matching impedance. In all cases you cite, the output impedance of the source is much lower than the input impedance of the destination, and that is never a problem with audio signals.

2a ) You need a capacitor that will block whatever DC voltage is coming from the input. For the wireless transmitter, refer to the connection details. Some transmitters have the DC power on a separate pin and blocking this voltage would simply mean not connecting to the pin at all.

2b) You should not need a DC blocking capacitor for the XLR mic inputs. You should simply turn off the 48V phantom power.

3) For the kind of "plug-in power" that wireless transmitters commonly use, something like a 5~10 uF capacitor with a 5~10V rating would be fine.

4) Small-value electrolytic capacitors can be found in pretty small packages. Even conventional radial electrolytic capacitors can be found in sizes on the order of 4mm diameter by 8mm long, etc. And tantalums can be found almost the size of a wooden match head. And then there are also surface-mount devices. If you are very good, you can fit those even into a 3.5mm plug.
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Old January 25th, 2011, 05:54 AM   #9
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By the way, Audio Technica offers a purpose-built, transformer-balanced cable to connect the receiver's TRS output to a camera's XLR input. It's part number CP8306
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