Power pack for lapel mic: help needed please...

[huntgl]

Hello,

I want to install a lapel mic inside my acoustic guitar, but I need to insert some power before it goes to the the preamp. I believe I need to build a 9v power pack of sorts, and I was thinking building something into a small, guitar stomp box sized casing. I could then run the 3.5m jack (I could change this to a standard 1.4" jack) into the input of the stompbox, put the 9v power inside the box, and then a 1/4" jack out, which I could run to either the 1/4", or XLR input on my preamp. Although I am quite happy soldering things, I have no idea about circuitry; could anyone please show me a simple circuit that would make this work please?

Apologies for my ignorance and thanks for reading
Gary.

 

[canadaelk]

Google "Phantom Power" or use a dynamic microhone. E

 

[huntgl]

Thanks for that. I have phantom power on the preamp, but I read that 48v is way too much for these little lapel mics and will just damage it.

 

[canadaelk]

The supply to the condenser mic is current limited. What is the make/model of the mic you are using? Are you sure it is a balanced circuit? E

 

[mneary]

This article has a phantom power circuit for electret microphones. It's an old page, so the "schematic" is in ascii.
Powering microphones

 

[canadaelk]

Until we know what mic. you are using we are only guessing! E

 

[huntgl]

Thanks to both of you for the replies, I appreciate it. I had a look at the page with the powering circuits etc, but initially it's a bit over my head. I'll sit down over the next couple of days and have a few re-reads to see whether it falls into place. As far as the microphone make and model goes, it's a Sony stereo mic, but it has no model number on it at all. I actually found it tucked away in a box for a Sony Discman that belonged to my girlfriend's brother! It is a box type design with a silver sticker that says Sony and has a blue and red stripe and grill holes at each and of this face; it has STEREO marked on another face with grill holes at each end which are marked LEFT and RIGHT; there are grill holes on the 2 square faces at each end too (I think it was built in the 90s). The reason I say it is balanced is because of the mini jack, which has the two bands on it rather than the single band that you have on guitar leads. I thought that that signified it was balanced, but I may well be wrong?

Thanks again for your help and I'm sorry I can't be any more specific myself.

Gary.

 

[mneary]

Sony Discman tells us quite a lot. Someone may have one that they can measure.

 

[huntgl]

I could post a picture of it on here if that would help. Can you put images directly onto the forum or do they have to be on another site with a link posted here?

 

[audioguru]

I have never seen a person with two heads then they need a stereo microphone.
Maybe the mic is for recording an orchestra in stereo. Then it has a stereo plug.

You do not need a stereo mic to record only one guitar.

 

[canadaelk]

Audioguro:
Then you never watched those 2 awfull "Hitchiker's Guide to th Galaxy" movies. Beeblebrox had 2 heads. Now, that there are 2 movies, do we need quad mics?

huntgl:
On the bottom right of "Quick Reply" go to "Go Advanced" and you can do that.
E

 

[huntgl]

Here you go. I'm sorry about the image quality, but it's getting a bit dark here in gay paree.

I presume the 2 bands on the jack signify that it is stereo, and not balanced as I had presumed?

 

[huntgl]

I found this on ebay IN-LINE MICROPHONE BATTERY PHANTOM POWER SUPPLY MODULE on eBay (end time 18-Jun-10 16:10:48 BST), which will probably do the job, but I also see that some of the top end Sony mics have 'barrel' power packs that terminate in an xlr jack. They don't seem to be available seperately, and even if they were would probably cost deep in the purse.

 

[canadaelk]

OK: Back to square 1:
-What is the make/model of the mic? All modern commercial mics accept 9 to 50V phantom power, if wired correctly
-The pix you provided seem to be a gadget, for a video camera, cheap electrets plug into
-To mount a mic on an acoustical instrument may change the tonal quality of the instrument or not pick it up correctly
-As most acoustical guitars use metal strings maybe you should investigate an electro-magnetic pu
E

 

[huntgl]

I have read a number of sites that have said that 48v phantom power is too strong for these small lapel mics. I have to think that this is true if a company like Sony provides a barrel type battery power supply for its most expensive, professional lapel mics. These barrels terminate in an XLR connection, which would surely negate the need for a battery as they could run off of the board's phantom power, so why do they use a battery if that is the case? As I said at the start, I do not understand electronics, but these battery power packs seem to suggest that a lower voltage is preferable with a smaller lapel mic.

As far as the guitar goes, I have a piezo pickup already installed, but I want to try the internal mic (which is commonly used by many pros like Martin Carthy) too with a view to running a dual system (which are also commonly sold). I plan on making the mic removable, so that I can take it off when not playing live; it's also possible that it will not sound good enough, in which case I will remove it. The point is, is that I want to try it with a mic that I already have. As I said above, Martin Carthy has used this type of mic internally (a cheap Radio Shack model from what I have read) with really excellent, natural sounding results, so I am sure I will be able to get a decent result if I think it through and spend some time on it.

Thanks again,
Gary.

 

[audioguru]

An old or a professional microphone is a "condenser" mic. It needs a high polarizing voltage.
Modern, inexpensive "electret" mics also have excellent performance but have the high polarizing voltage built into the electret material. They have a Jfet inside to reduce the output impedance. The Jfet needs 0.5mA at 1V to 6V for power.

The Sony stereo mic probably has two electret mics inside. They are powered from the little battery.

 

[huntgl]

This mic doesn't seem to have a battery in it, hence my search for some external power. In the end I've decided to buy the power supply from ebay; it looks like it'll do the job to me, and if it sounds terrible then it won't have cost me an arm and a leg. The housing looks a bit weedy, but if it works well then I will try and re-house it in something a bit sturdier.

Thanks for all the replies, I appreciate you taking the time to send them

All the best,
Gary.

 

[audioguru]

The power supply for an ordinary electret mic is a 9V battery with perhaps a 47uF capacitor across it and a 10k current-limiting resistor. That's all that is needed.