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Author Topic: Can I power a lavalier mic with a battery powered circuit?  (Read 4827 times)

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Offline nolken

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Can I power a lavalier mic with a battery powered circuit?
« on: January 31, 2013, 06:40:02 PM »
So I have a countryman emw lavalier microphone. it is the version designed to work with wireless systems, but I dont have the wireless system. I would like to be able to hard wire the microphone to my preamp, but I do not have a power supply for it. I have a direct box with a 50k to 600 ohm impedance matching transformer in it that I would like to use to balance the signal. my mic is rated for 1.5-24V and an impedance of 1500 ohms.

Can i just power it with a 3V battery and a couple of electrolytic capacitors, using the direct box in reverse to balance the mic? I've heard that it could but it didn't work for me. could the problem be that it is dropping the impedance down too low? since the transformer has an 1:83 winding ratio, it'll drop the impedance down to about 18 ohms?

If this isn't supposed to work, is there another option? countryman wants to charge me $140 for the xlr connector by itself or else i would just go with that option. I really need the mic this weekend so i'm hoping to get this figured out soon.

thanks.

Offline nolken

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Re: Can I power a lavalier mic with a battery powered circuit?
« Reply #1 on: February 01, 2013, 03:00:53 AM »
thank you for responding!

So I followed your advice and just took the direct box out. I got it to "work" set up as in the image, but the levels are extremely low. I have to either talk directly into the mic or turn the gain up all the way. It is very faint. Any ideas?

Offline nolken

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Re: Can I power a lavalier mic with a battery powered circuit?
« Reply #2 on: February 01, 2013, 08:24:49 PM »
okay i got it to work very nicely now. The image shows exactly what I ended up with. I put the resistor in the wrong spot. fixed that and it sounds perfect. if i put a resistor between black and shield it has a ton of noise.

how do you determine the proper capacitor value?
what kind of capacitor is best for this application?
Is there a way to protect the mic from accidentally powering on the phantom power?

as far as question 1 goes, I calculated a value of .416 uF. the smallest capacitor that works and still sounds good is .1 uF and the largest is 1uF so my calculation seems accurate. I basically just did the resistance of the cap over the input impedance of the pre-amp, plugged in the desired dB loss (12 dBu) at 8 Hz and solved for C. i figure that is pretty close to a low frequency roll-off of 12 dB/octave. I just need confirmation that that is correct since I've never done this type of thing before.

for question 2, all I have are mylar and electrolytic caps, 1uF being the smallest electrolytic, .1uF and .47uF being mylar. I've heard before molars aren't to great.

and question 3. I tried diodes, and they do not work at all. I don't know what other options there are. I really don't want to accidentally plug the mic in with phantom power on and fry the mic.

also, if I wanted to actually run this off of phantom power, would i just put in a resistor of say 4.6k (mic is 1.5k) in series with the mic and maybe a 12v zener diode as extra protection. of course i would put a cap in also. could i do that, or would i need to use a step down transformer?

EDIT: oops, i didn't add the image
« Last Edit: February 01, 2013, 09:16:57 PM by nolken »

Offline nolken

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Re: Can I power a lavalier mic with a battery powered circuit?
« Reply #3 on: February 02, 2013, 12:04:54 AM »

I think you probably still have something not right based on your diagram, but if it works, OK.  What you have drawn should have almost no signal because the battery is connected directly to the output, and the battery is a low impedance load on the mic.

what exactly do you mean by the battery is connected directly to the output? the capacitor blocks out the dc voltage to the output, but allows the AC signal from the mic to still pass. do i need to put something between ground and the output? The system didn't work without the limiting resistor (R1). as soon as i added one it started working again. I don't know the reason for this.

f3 = 1 / (F * 2 * pi * R)

Where f3 is the -3dB frequency corner, F is capacitance (in farads, microfarads are 1/1000000) and R is the *load*, not source impedance.  Below f3, you have a rolloff of 6dB/octave (this is a first-order filter).

So the rolloff depends on the load impedance of the preamp, assuming that is significantly larger than the source impedance of your microphone (which varies according to how it is wired; I'm not sure how you have yours wired).

cool, that is exactly what i had, but my R was the input impedance of the preamp. is input/load/preamp impedance one-in-the-same

Quote
also, if I wanted to actually run this off of phantom power, would i just put in a resistor of say 4.6k (mic is 1.5k) in series with the mic

No, because as the signal swings from high to low you will eventually get excess drain to source voltage

Is this true even if i do not choose to balance the signal, only use phantom as a voltage source?

 

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