DIY Microphone circuit w/phantom power?? - diyAudio
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Old 4th January 2012, 02:20 AM   #1
diyAudio Member
 
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Default DIY Microphone circuit w/phantom power??

Woo hoo. First post!
Getting to the point,
I have some parts I am recycling into a microphone ...
-Old steel Mic body (with XLR jack output)
-Electret Capsule from old cassette recorder (nice tone)
-SPST push-button switch
-Blue LED diode (1.5-2v drop at 80mah)
-box of various cap's and resistors
Goal ...
-A very simple, low parts count vocal microphone.
-All the components must fit inside mic body (hence low part #)
-When SPST switch is thrown, phantom power runs both the
capsule and LED (which indicates 'power on').
-No clipping with power up/down of switch!

Where I am at?
Well, after playing rubix cube with the parts I have got, I have arrived at something I don't understand at all (*cough* newb here *cough*)... The diagram below provides me with clear quality audio, with absolutely no clip when shorting/switching the SPST.
Any clues how this supply's good audio with no connection to ground?
Would it be possible to add the LED in somewhere (have tried a few different routes only to also add the 'clip' - which must be avoided)?
Click the image to open in full size.
Layout your harshest critiques ... That is what I came here for!

Thanks.
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Old 4th January 2012, 04:31 PM   #2
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Well. a small addition.
Click the image to open in full size.
I can power the LED with this circuit above with no 'pop'.
Yet the switch still only activates the mic, and the LED is on constant (as long as phantom is supplied).

right way to go?

(also, moderator - if there is a better forum for this post glad to have it moved)

Last edited by krienert; 4th January 2012 at 04:44 PM.
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Old 4th January 2012, 06:13 PM   #3
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
Why don't you look up "phantom power" for microphones. It is a well defined method of powering mics that need it.

How can this work with no connection to ground? But there IS a connection to ground - pin 1 of the microphone. And +48v is applied equally to pins 2 and 3. SO there is no voltage between 2 and 3, but they both have voltage with respect to ground. Shorting pins 2 and 3 kills the signal, and doesn't bother that "common mode" 48v on them.
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Old 4th January 2012, 06:22 PM   #4
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taking that into consideration,
Is it possible to switch the LED when shorting pins 2 and 3 without pop?

(and yes, I have done much reading on the subject.)

Last edited by krienert; 4th January 2012 at 06:45 PM.
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Old 5th January 2012, 02:54 AM   #5
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
But due to the way the phantom voltage is fed to the circuit, closing the switch doesn;t change that voltage.

SInce this is one stand alone thing, consider using a switch with an extra pole just for the LED. ANything connected to the signal lines and causing a step change in current is likely to pop. Keeping it off the signal lines reduces that risk. A small battery could power the LED for complete separation.

80ma sounds extremely high for the LED current. LEDs are still easily visible at much lower currents than their max. "80mah" I am not familiar with with respect to LEDs. mah sounds like milliamp-hours, some sort of small battery rating. Of course it wouldn;t be the first thing I hadn;t heard of that everyone else knows.
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Old 5th January 2012, 02:13 PM   #6
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Enzo,
That was a typo.
It is Ma, and actually the led was running 80 (max) and I have ran it as low as 3 with some dim output.
The application calls for a medium amount of light, but not bright at all.

Is there any way to create an unbalanced voltage between pins 2 and 3 so as to provide a path for the led across? Like a significantly bigger resistance on one over the other creating a voltage path (without unbalancing the overall output) ?

I might be stepping on thin ice here with all this in respect to the mixer and its on-board power.

No luck on the battery inside the microphone... not for lack of room, but for lack of anything but this spst switch.
Is there any way to branch diodes off the switch making a separate circuit, while still being controlled(if so, the battery idea might work)?

"...it wouldn;t be the first thing I hadn;t heard of that everyone else knows..."
Much of the plot line to my life man. Your not alone.

Thanks.

Last edited by krienert; 5th January 2012 at 02:26 PM.
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Old 6th January 2012, 12:48 AM   #7
Enzo is offline Enzo  United States
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Location: Lansing, Michigan
The whole point of phantom is that it applies a voltage in a balanced way so the mic never "sees" it. If you upset the balance, not your mic has DC across itself.

No way to mount a double pole switch?
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Old 6th January 2012, 01:03 AM   #8
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double pole single throw right?
Wont that still result in a pop upon closing gate?
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Old 6th January 2012, 04:42 AM   #9
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Location: Los Angeles
Quote:
Originally Posted by krienert View Post
taking that into consideration,
Is it possible to switch the LED when shorting pins 2 and 3 without pop?

(and yes, I have done much reading on the subject.)
It appears you're re-hashing this thread from last August.

Power LED from 48 volt phantom power

G
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Old 6th January 2012, 07:36 PM   #10
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whoa my friend.
hash or no hash, I felt this project was not the same as that posting.
While that thread has been a huge reference in learning how to go about this - it seemed right to seek knowledge on the differences in goals.
If it really is a bother to have this posting here - I implore the moderator(s) to remove it with no dread from me.

I should be more thorough in the future I suppose.
Thanks.
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