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Old 12th March 2009, 04:32 PM   #1
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Default diy microphone pre-amp

Hi, I'm building a microphone to calculate room correction for my sub. I am looking to use the Panasonic capsule from digikey and have seen varying reports as to whether a pre-amp is required.

http://www.vikash.info/audio/measurement_mic/

and

http://www.speakerbuilder.net/web_fi...diymicmain.htm

suggest that the leads from the capsule can be run right into the "mic in" on a computer.

I checked my laptop's (lenovo t61) manual, and the audio input on it is not a line in, but indeed a mic in with an (albeit probably lackluster) preamp.

So... do i need a separate pre?

Thanks

-Sadim
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Old 12th March 2009, 08:11 PM   #2
infinia is offline infinia  United States
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It depends what you are doing with it really... you can mod the capsule and then you would need a preamp. Search the site for either and/or linkwitz, microphone, mods, preamp, wm-60, wm-61, etc
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Old 13th March 2009, 12:10 AM   #3
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just want to make sure that i understand correctly...i only need the pre if I do the linkwitz mod?
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Old 13th March 2009, 12:22 AM   #4
cuibono is offline cuibono  United States
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You need a pre for two things - one, the capsule needs several volts to power it - some (few) soundcards actually have a voltage sitting on the input to power mics - but you'd have to figure out if its the proper polarity, voltage, which pin its on, etc (a pain). The other thing you need a preamp for is to control the volume of the signal coming into the computer (before it gets into the computer, software mixers won't do it). So I suggest you don't rely on your computer alone.

I highly recommend the linkwitz mic: http://linkwitzlab.com/sys_test.htm#Mic
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Old 13th March 2009, 02:56 AM   #5
infinia is offline infinia  United States
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Quote:
Originally posted by cuibono
You need a pre for two things.

I highly recommend the linkwitz mic:
right
More than 2 things actually,
3) Drives longer cables with less extraneous noise pickup.
4) Also allows the gain to be tailored to your application thus optimizing system S/N ratio.
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Old 13th March 2009, 01:10 PM   #6
bwaslo is offline bwaslo  United States
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Using the mic input on some computers or soundcards is not a good idea if you plan to do measurements. Very often the mic-in jack has a response that is rolled off toward lower frequencies, to minimize noise pickup and effects of mechanical noise -- mic input on most computers is intended for voice (VOIP, speech-to-text, chats) not music, so they are optimized for voice in situations where the mic is sitting on the table right next to the keyboard and noisy computer fan. Some also roll off the high frequencies for similar reasons.

So, you'd rather run the mic into a line input, which is flat over full-range. But mic capsules won't drive line inputs very well because of the small signal level, so you need the preamp to boost that (something like 20 to 30dB added gain is a good target).
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Old 13th March 2009, 03:21 PM   #7
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Default mic input

so in my case, with a laptop which only has a mic-in jack, I would need an external usb soundcard/DAC/pre to take measurements?
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Old 13th March 2009, 04:09 PM   #8
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because it seems like any input i put through the mic-in would be subject to that freq response you describe.
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Old 13th March 2009, 04:18 PM   #9
bwaslo is offline bwaslo  United States
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Probably so. Laptop mic inputs I've checked were always bandlimited, the built-in mic is mounted right in the housing with the drives and fans, a terrible place for a mic.
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Old 13th March 2009, 04:38 PM   #10
cuibono is offline cuibono  United States
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Quote:
Originally posted by sadim1223
because it seems like any input i put through the mic-in would be subject to that freq response you describe.

If your laptop has a headphone or line out, you could check the response of both in and out (together) with something like rightmark audio analyzer http://audio.rightmark.org/index_new.shtml

You're going to need something like a line out to test speakers anyway. And unfortunately, it seems like most built in computer audio jacks aren't worth much...
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