Headphones and Microphone into Cell Phone?

August 28, 2005 4:13 PM

I want to make a box with parts from RadioShack that will allow me to plug a microphone and headphones into my cellphone handsfree jack.

Idealy, I would like to be able to plug the box directly into the handsfree jack and then plug the microphone (via XLR) and headphones (via 1/4" stereo jack) into the box. Googling doesn't turn up much. Any electronics experts have any advice? Schematics? I think that I should be able to do this with relatively inexpensive parts from RadioShack and a little bit of soldering

posted by TurkishGolds to technology (9 comments total)

i have no idea, but im curious as to why youre looking to do this? a big fat xlr mic and quarter inch headphones?
posted by Satapher at 4:37 PM on August 28, 2005

especially since the sound quality would still be squashed by the phone
posted by Satapher at 4:40 PM on August 28, 2005

The impedence and line value of the XLR mic is probably going to be different than your standard cellphone headset, plus, it'll also probably require a mic amp. XLR audio mics require 'phantom power' to operate.


Beyond that, I'll let someone with more pro audio knowledge answer in detail.
posted by loquacious at 4:52 PM on August 28, 2005

You're not going to believe this, but I was just googling for this last night! this is about all I found:
http://www.expansys-usa.com/product.asp?code=113967.
Plus one on ebay.co.uk that was cheaper but not, obviously, after shipping to the states...

oh, on preview, what is XLR? my link is for 3.5mm mic and headphones to 2.5mm single (combined) plug. It is easy to go from 1/4" to 3.5mm with an adapter, but i dunno about XLR.
posted by misterbrandt at 5:34 PM on August 28, 2005

loquacious:

Phantom power does not depend on the connector. Many dynamic mics have XLR connectors, but only condensor mics need phantom power. XLR is almost always balanced. That means there are two signal leads and one ground lead. Both signal leads carry the same signal, but with opposite polarity and the difference in voltage between the two leads is the sound signal. Noise and interference adds nearly equally in both leads, not affecting the difference, so you get a cleaner signal that way. To utilize this, you need a differential amplifier for the mic (one that amplifies the difference in inputs and neglects any common offset). The phone might not be able to drive the headphones without intermediate amplification.

Cut up an old handsfree, trace the wires and try, but don't maim it so badly you can't use the connector. Look in a book about op-amp circuits for the amplifiers.
posted by springload at 6:07 PM on August 28, 2005

I believe a cell's handsfree jack is different from the handsfree on a cordless phone, can anyone confirm that? And is it just a pin difference?

I always thought the mics in handsfree headsets were piezo electric, but apparently they are electret. Both piezo and electret mics need power, but niether are the same as the phantom power used on condenser mics. Here is a guy who moded his cell with an old telephone handset. Here is an article on powering electret mics. Finally, here is the wikipedia article on phantom power. Finally, here is a very extensive discussion of various microphone technologies, electret is discussed in the section on condensor (capacitor) microphones.

Those might help. It shouldn't be hard to do at all, I'll check in later to see if I can help more.
posted by Chuckles at 6:39 PM on August 28, 2005

Sorry about the two finallys... Also, somebody should try searching slashdot for projects about moding cell phones (I might get around to this later)...
posted by Chuckles at 6:42 PM on August 28, 2005

I want it becuase I do remote broadcasts for a radio station, and I'm often in situations where our standard broadcast equipment won't work for one reason or another. In that situation, I usually have to just call the studio on my cell phone, but that looks bad to people watching. If I have a microphone and headphones, it looks cool and people don't care that it's really just a cell phone call. Basically, it will be a backup. Kind of like this (scroll down to "Matrix TelCell Module," but I want to build it myself ... I don't want to spend $500.
posted by TurkishGolds at 6:47 PM on August 28, 2005

ah, and XLR has nothing to do with Phantom Power, both dynamic and condenser mics can plug in with XLR, its the condenser mics which require PP.
posted by Satapher at 8:58 PM on August 29, 2005