DIY-Central.com - Go Create something! - PCB Design, Electronics

I've seen the new Zoom H4 Handy Recorder. I think it's a neat little thing! And best of all, it's "cheap" at $299. Of course, "cheap" is a relative word, but compared to other gear out there, this seems to be one of the low priced new entry. It looks good too!

While reading the H4 specs, this caught my eye... "The H4 features 2 studio-quality electret condenser microphones configured in an X/Y pattern for true stereo recording."  The keyword here is "electret microphone." And the picture seems to confirm that it's really just an electret mic.

So I'm thinking, maybe I can DIY my own stereo mic.

I've used an electret microphone on a non-audio project. I made a sound-triggered flash sync for my Canon EOS30D camera. In this case, I just used the audio output from the mic to trigger an SCR, which then triggered the camera flash.

Can we used the same electret mic to create a good enough sounding stereo mic? I think we can.

So I begin adapting my sound-triggered flash sync and while doing some Googling around, I came across this product.
Stereo Super Ear Amplifier Kit

It's a kit made by Velleman and it uses 2 electret mics, and a headphone output jack. Hmmmm.... interesting. So I ordered a kit (hey, it's less than $10 so if it didn't work, it's not too much of a loss). You can use the link above to order your own kit.

The circuit is simple... the Left and Right channel is the same, so the operation of the circuit is identical. Basically, an electret condenser microphone is biased for operation using a single resistor and capacitor. The output of the microphone goes to a potentiometer which acts as the volume control. This is then amplified by the NE5532 opamp, which is then routed to a headphone jack. Since the whole circuit operates from a single supply voltage (4.5Volts), the opamp uses a virtual ground by the use of 2 resistors in it's input pin. This of course, means that each leg of the split power supply will be only Vcc/2, where Vcc=4.5Volts.

I built this kit in less than an hour... while watching TV. And construction isn't hard and it's very simple. So if you're ready to jump into this Electronics hobby and looking for your first audio project, why not try this stereo mic kit? It's only less than $10.

Here are some pics during construction... enjoy.



There are assembly instructions and schematics on the box. Very easy to follow.



Schematic diagram... just an electret microphone going into an opamp for amplification.


Simple to follow assembly instructions.



All the components are included in the kit. Even the battery holder!



First, I inserted all resistors and capacitors in the PCB.



RED LED to indicate power ON. (I used my own square LED instead of the supplied LED.)


Here's a TIP: When soldering components that fall out easily from the PCB (in this example, the 8-pin IC socket), use a masking tape to hold it in place. Make sure it's flushed to the board. When you're done soldering, you can remove the masking tape.



Next, I soldered the electrolytic capacitors, switches and headphone jack.



Here's a closeup shot of the stereo potentiometer which will act as the "volume" or "gain control" for the stereo mic.



Next, I soldered the pins which is used as attachment terminal posts for the electret condenser mics.



Here, you can see the two electret mics attached to the terminal posts. I also soldered the wires (red and black) from the battery holder. You may want to cut the wires short. The battery holder is behind hte PCB and is attached to the PCB using 2 screws.



Everything is assembled, add some fresh batteries, plug in a headphone, and turn it on... adjust potentiometer for a comfortable volume... IT WORKS! 

Frankly, I'm surprised at how well it works and it's sound quality.

PROS: Easy to assemble, cheap, good quality

CONS: Unbalanced output, the microphones are splayed out instead of toed-in like the Zooom H4. This can cause a "hole" in the middle of the sound image. No 1/4" jack.

But the CONS of this project can easily be fixed. For the mic, just have them point to the opposite side. So the left mic will be pointing towards the right direction, and the right mic will be pointing towards the left direction.

To convert it to balanced output, you can use a transformer, or just a solid-state unbalanced to balanced converter. You can visit the Fivefish.net/diy site for a couple of examples of an unbalanced to balanced converter. Then use a 1/4" jack for the outputs instead of the mini-stereo jack.

And now, for some sound clips.  (Pardon the humming and swishing sound... that's the sound of my server fans. I'll try to do a proper recording setup. But I just recorded this clip quickly in my office.) StereoMic.mp3 (563.67 KB)