Mic level vs. Line level FIGHT!

January 25, 2009 7:50 AM

How do I build a microphone preamp for a Macbook Pro?

Oh hai. I just recently learned that the 3.5mm input on my MBP (core duo, 2.16ghz, 17in) is not actually a 3.5mm mic in, but a 3.5mm stereo line-in. What's the simplest circuit I can build to remedy this? What about more complex ones?

I bought a Zalman ZM-MIC1 and assumed that it would work out of box, not knowing that I would need some kind of preamp. I have soldering skills and a decent understanding of what electronic components do, how do I get the mic output up to line level for like six bucks at Radio Shack?

If you're so inclined, explain what is making the loudness happen and suggest cool modifications for this circuit as well.

posted by knowles to technology (6 comments total)

This will work. So will this. The second link explains how they work.

Snark.

You'll probably be better off getting a cheap USB mic.
posted by onedarkride at 8:48 AM on January 25 [1 favorite has favorites]

I was thinking more like this or this, which I found by previous googling. Maybe a way to fit in a gain knob, or something from someone with practical experience saying, "no, don't do it this way, use this widget so you don't fry your laptop."
posted by knowles at 10:09 AM on January 25

You've basically answered your own question because you aren't going to find anything simpler than that. Assuming you've got a condenser mic go for it. That should take minutes to assemble on a breadboard for testing. You don't even need a breadboard. Just some alligator leads.

If that's beyond your ability you should really start by reading some electronics tutorials.

If you want a real preamp here's a simple opamp preamp. The latter is the one I'd build.

Or just use a USB mic. Or get a FireWire box if you want to do more pro-level recording.
posted by 6550 at 10:59 AM on January 25

It is likely that the microphone has a built-in preamp. The problem is that the stereo line-in jack provides no power to the microphone preamp. Typically the microphone will need a 2V to 5V supply. You need to check the pinouts of the microphone plug and the line-in jack. The microphone is single channel and the line-in is stereo. On the microphone the connections are power, ground and signal. On the stereo jack the connections are left signal, right signal and ground -- there is no power out.

So you probably just need a patch board to wire up the connections properly. Ground to ground, mic signal to either left or right line-in, and then use a battery for mic power. A couple of AAA batteries should work. Do not connect mic power to line-in.
posted by JackFlash at 10:59 AM on January 25

I see its just a little crappy Gaming Headset/Microphone.
Zalman ZM-MIC1 High End Gaming Microphone.

ie a Dynamic Consumer Microphone.

ignore Jackflash
posted by mary8nne at 12:10 AM on January 26

Mary8nne, obviously you do not know what you are talking about. If you look a the specs for the Zalman mic you can determine that it is a condenser mic, not dynamic. Almost all condenser mics have a built-in one-transistor preamp/impedance converter that provides line-in voltages.

You will also see in the specs that the mic requires 2 volts DC as input power (2 to 4 volts will probably work). The problem with plugging the mic into a line-in jack is that line-in does not supply mic power. Only a mic input jack supplies power. So if the OP wants to use the mic with a line-in jack he will have to supply power externally, most easily by a couple of batteries.

The basic problem is not lack of a preamp. The problem is that the line-in does not provide the required operating power for the microphone.

If you don't want to use a USB mic and want to use line-in, then the easiest thing is to get a battery power mic like this.
posted by JackFlash at 9:10 AM on January 26