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> electret microphone and phantom power, powering and electret with phantom power
mathieujohnson
Posted: May 26, 2005 12:32 am
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hi,
for my art projet that i did with the 1000 led circuit on the other thread, i had to pick up the sound signal through microphones, so i tried electret capsules wich cost really cheap (i didn't want my good mics to be stolen).
anyway, i powered them with a really basic circuit, being a 9v battery in series with the capsule.
But i was wondering if, with a different circuit, phantom power could be used.
Not that have a battery is so hard, but it would make my microphones smaller and be easier to use from system to system.
I have like 14 capsules that i have bought, so, i can experiment a little, and since their dirt cheap, i could easily replace them, hay, 1.50$ canadian is nothing compared to the 1k price on my studio mics.
So since the sound is pretty decent, they definitly could get some use, at least as a talk back mic.
so what about phantom power on electrets?
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Geek
Posted: May 26, 2005 02:39 am
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Hi Mathieu,

Phantom power is +48VDC through a 6.8K resistor. If you know the current of your electret, you can figure the added resistor from there smile.gif


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draget
Posted: May 26, 2005 12:38 pm
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Cant you run those little electet things off 9V, isnt it just proper recording ones that are 48V?
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Geek
Posted: May 26, 2005 06:12 pm
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Yes, but phantom power is very common in recording equipment.

Electret's actually are optimum at 4.5VDC, but ones with built-in supplies require 9 or 10 VDC, but aren't usually harmed with an automotive 13VDC.

Mathieu wants to power an electrt off of phantom power and let's assume he will need 9VDC.

Let's assume his mic element draws 350uA

R=E/I or (48-9)/0.00035 = 111.489K

Phantom power already has a 6.8K resistor inline, so a standard 5% resistor of 100Kis close enough.

How to apply the circuit? For two-wire electrets, bridge the 100K resistor with a 4.7uF capacitor, solder that to the mic with the cap "+" pointing towards the equipment. For three wire electrets, tie one side of the 100K resistor to the "+" side of the capacitor and add that to the equipment line. Tie the free end of the resistor to the element power, with another (1uF will do) to ground as a bypass, and the "-" end of the 4.7uF capacitor to the audio output.

The above formula should be used to determine exact resistor value.


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Søren
Posted: May 27, 2005 01:35 pm
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Hi,

QUOTE (mathieujohnson)
But i was wondering if, with a different circuit, phantom power could be used.
Phantom power, Like Greg mentions, is specified to 48V (although it might often be found down to 24V or even lower in some equipment) and is used for powering preamplifiers in balanced systems.
What you need is bias voltage (don't worry, even a lot of professionels don't know the difference between phantom and bias), in an unbalanced system.

QUOTE
Not that have a battery is so hard, but it would make my microphones smaller and be easier to use from system to system.
If you're talking about real phantom power in a balanced system, you would need to use an unbalanced to balanced coupling at the mike end, as pseudo balancing wouldn't work (since there would be no ground reference for the voltage - Phantom power has no potential difference on the signal lines, which is the whole idea in it).

If you just want to feed a bias voltage ower the signal cable in an unbalanced system, that's pretty easy, since you just isolate the audio with capacitors at each end and use a resistor, zener diode, capacitor, resistor combo to give a steady bias at the mike end.

But to device a circuit, we need to know which kind you're talking about - expensive mikes point to a balanced system, but I have witneesed horribly unbalanced setups in places where I would have expected a balanced setup to be mandatory.

QUOTE
So since the sound is pretty decent, they definitly could get some use, at least as a talk back mic.
Actually, some of the little bastards are of so high quality that, with the right preamplifier, they outperform even half decent studio mikes.

QUOTE
so what about phantom power on electrets?
For equipment with real phantom conforming to specs, this will give a bias voltaeg from the phantom supply:
user posted image
For a 2-terminal capsule, just connect BIAS and SIGNAL


Regards,

Soeren


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mathieujohnson
Posted: May 28, 2005 07:49 pm
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cool
thanx a lot for the diagram
thats pretty much what i needed.
I powered my mike with a 3v but tried a 9v and sound was softer, better wounding, also tried a 12v and sound was even better, i don't know if the capsule could take up to 48v, i could try it out, since its really inexpensive.
So i'll try the diagram and see what happens, they might become some great studio mics for a really inexpensive price WOW haha
thanx again
Mathieu
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Søren
Posted: May 29, 2005 03:21 am
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Hi,

QUOTE (mathieujohnson)
I powered my mike with a 3v but tried a 9v and sound was softer, better wounding, also tried a 12v and sound was even better, i don't know if the capsule could take up to 48v, i could try it out, since its really inexpensive.
Did you use the same resistor ?
Normally the actual voltage at the capsule terminal is not very high (a couple of volts or less).

Ask where you got the capsules, for their required current (most likely somewhere between 0.5mA and 3mA) and their running voltage (if they know). Then the appropriate resistor can be calculated for any given (higher) voltage (within reason).

If you like to experiment a bit, the resistor could be replaced with a smaller one, to just keep it alive and a variable resistor could be series connected to allow changes to the bias on the fly.

QUOTE
So i'll try the diagram and see what happens, they might become some great studio mics for a really inexpensive price WOW haha
Yes, ECMs do come in various qualities, but I have used fairly cheap ECMs as measuring mikes with really good results (compared to the real deal) - admiddetly it ain't Bruel&Kjaer quality, but even as a local to them, I would have to pay through the nose to get anywhere near their equipment.

As a side note, Bruel&Kjaer produce the measuring equipment that Harley Davidson use to tune each bike to the patented HD sound (Yes... They really has a patent/trademark/whatever it's called on the motor sound - even though old VW beetles sounds much the same wink.gif)


Regards,

Soeren


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Soeren

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mathieujohnson
Posted: May 29, 2005 05:04 pm
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actually, i didn't even put any resistors in place. it was just the power supply and the mic. in series.
though i did put 3 mics on the same power supply, they were in series with the capsule but the 3 mics were in parallel to the power source.....
anyway, the mics don't seem to be harmed by the voltage, as 12volts didn't kill them at all,i will definitly experiment with the capsules to get different sounds.
thanx a lot for the help and the diagram!
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