Lapel Mic / Recording Units.

[Tony Fogarty]

Hi

I'm considering buying a reasonably cheap digital audio recorder to use with a wired lapel mic I already own. The idea is to cheaply bypass the need for a wireless mic set up on a particular shoot.

I'm not very technical when it comes to ohms and powered inputs etc, so wanted advice on whether it would work.

I have an Audio Technic AT831. It comes with an AT8531 power unit. (When I use this with my cam, I use phantom power from the cam - but the 8531 unit can take a battery - which I assume is for when it needs to provide phantom power?).

I was considering getting an XLR to 3.5mm jack cable, to plug from the AT power unit into the "mic input" on a Tascam DR07 or similar.

Has anyone any advice about whether this would work or if I might encounter problems with compatibility between equipment?

Thanks in advance.

Tony

[Greg Miller]

Hi Tony,

I was hoping someone with specific experience would chime in. Since not, I'll try to offer a bit of helpful theory.

There are various types of "balanced" mic outputs. For example, many dynamic mics have their voice coil connected directly to the XLR p2 and p3 and to nothing else. These are truly balanced, but are also "floating." They can be used with an unbalanced recorder, simply by connecting p3 to ground on the recorder's input. (p2 would go to the recorder's "hot" input, of course.)

Some older "balanced" condenser mics had an audio transformer in their output stage. If the transformer secondary was floating (same as the above description) then they, too, could be wired to work with an unbalanced recorder input. (This assumes that the mic had its own power supply and was not depending on phantom from the recorder.)

On the other hand, some "balanced" outputs do not have the transformer; they are more correctly called "active balanced." A transistor, op-amp, or some other electronic components (other than a transformer) are connected directly to the XLR p2 and p3. Mics with an "active balanced" output most likely will not work if wired directly to an unbalanced input.

I'm sorry that I don't really know which type of output configuration applies to your 8531 module.

So the safest and most reliable answer is as follows. Use a separate audio isolation transformer between the 8531 and the recorder. That will avoid any possible damage to the 8531 circuitry that might be caused by grounding one of its outputs (if they are, indeed, active outputs). It will also prevent DC "plug in power," coming out of the recorder, from getting into the 8531 and possibly causing problems there.

The transformer's primary impedance should be close to, or slightly greater than, the output impedance of the 8531 module. A 1:1 transformer (where the secondary impedance is the same as the primary) should work. You might even benefit from a transformer that has a higher secondary impedance: the voltage going into the recorder will be higher than the voltage coming out of the 8531, so you may see an improved S/N ratio in the recordings. (On the other hand, in an especially loud environment, this may cause problems with overloading the recorder input stage.) Be sure your transformer is designed for mic-level use, and has good magnetic shielding; otherwise it will be susceptible to hum pickup. And check the specs: even the best transformer will degrade the audio slightly (but perhaps not audibly); a cheap transformer can do bad things to your audio.

I hope that gives you a starting point for your further investigation. Hopefully someone who has used the 831/8531 combination will let us know any specific solutions that have been helpful.

[Tony Fogarty]

Cheers Greg,

Thanks for being kind enough to post quite a detailed response. I've passed it onto a more "techie" friend to help me as I'm lost when it comes to the wiring side of things...

Tony