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|11th August 2004, 06:47 AM||#1|
Join Date: Aug 2004
Location: Santa Fe, NM
Microphone Element recommendations?
I am looking to build a microphone and possibly a pre-amp for my Mini Disc or MP3 recorder.
My Mini-Disc recorder has a pre-amp with phantom power. My MP3 recorder will need a pre-amp which I am working on.
However, for the microphone element I am having some troubles finding the specs I need. Basically I am building it to record band rehearsals, concerts, etc. So it needs to be able to handle loud dB. Maybe max dB around 120 or so.
Of course I would like it sound pretty good and be pretty compact. Any suggestions?
Panasonic, Radio Shack, others?
Here is an example of what I am kind of trying to do, just not with $209 pricetag.
|14th August 2004, 11:12 PM||#2|
Join Date: Jul 2004
this is exactly what I'm wanting/doing too!
Me, I am using Panasonic WM61a capsules which are without a doubt best for the money. They are noisier than other (much more expensive) mics though.
While noise isn't an issue anyway if the source is loud, also S/N can be reduced by making the mic a boundary mic, or using two capsules per channel, for example.
If you use that capsule on loud sources, you'd have to make a modification to decrease the high level distortion.
I'm currently working on Rev.2 of my tiny mic preamp as the current preamp was only a quick first try (but it already works great).
If you're further interested in the subject, you could also join the micdiy group in yahoogroups.
Here I'll copy the text file which goes together with my schematic:
Text to go with "mic_preamp2.gif"
This is an extremely simple mic preamp, yet not lacking performance.
Distortion and noise is low, while noise depends mainly on the mic impedance and on the OP-amp voltage noise and less on R5 (which sets the gain together with R2, as G = R2/R5 (approximately)).
Gain can be set as high as 1000 (60dB) which is usually not necessary.
To provide a certain range of gain, R5 can be realized as a resistor + a potentiometer, of course.
There may also be a ""upper"" limit of gain, depending on the OP-amp.
The OPA228 series for examlpe is designed for gains of at least 5, other op-amps may be "unity-gain stable" (though often allowing less maximum gain as a consequence).
My "real" counterpart is stereo, but for ease of understanding, the schematic contains only one channel.
In it's current state, it's meant to be used with an electret condensor mic, e.g. Panasonic WM60A Linkwitz 2-wire modified. To go with a standard capsule, you'd possibly have to use a smaller value resistor "R7", of course, you can connect your 3-wire modified capsule too if you know how
"Zin" stands for the impedance of the next stage, which mostly is around 47k, sometimes 20k (often with portable recorders) and may be as low as 10k for some devices' "line-in".
I use a polyester output coupling cap (C2), and there, 4.7uF is enough for impedances (of the next stage) of at least 10k. (Wima MKS2 is very small (considering it's a polyester cap) up to 10uF) You can still choose a smaller capacitance cap if you know what you do (but better not below 1uF).
The input coupling cap is selected with a bit more headroom, because I don't see the need to minimize it, given that the output coupling cap is bigger and considerably more expensive anyway. If you want to, you could still choose a smaller value than 1uF, but not less than 0.1uF in the given configuration.
C3 to C6 are meant to filter ripple from the power supply (here batteries) and possible other stray noise on the power supply lines. The bigger ones are sometimes recommended to be 10uF, sometimes 4.7uF, so it's up to you. The 0.1uF caps should be very close to the op-amp's power supply pins.
(I didn't even install those filter caps yet and couldn't notice any noise, as the op-amp has a good ripple rejection ratio and my circuit sits together with the mics in a metal grounded box, so barely picks up any hum)
As batteries, I use hearing-aid "zinc-air" batteries. They vary a lot in the price depending on the supplier, but where I get them, they give me nearly as much mAh for the money as a 9V alkaline block and pack a lot more mAh in small space, while it results in a much much smaller package if you need a splitted power supply.
(3x1.4V for + and 3x1.4V for -, and they are mostly delivered in a sixpack anyway, each 11.6mm(dia)x5.4mm)
Hope this is of any use!
Dominique Miagat, August 6, 2004
PS. I feel like I will rework this text soon as I'm still trying to optimize the mics and the preamp.
|14th August 2004, 11:23 PM||#3|
Join Date: Jul 2004
just to add:
capsule price: 1,50$ or so
preamp: not more than 15$
some soldering and tools required
just had a look at the link you posted:
Mine has about the specs of the high sensitivity model (they probably use the same capsule), but SOME dB more max SPL (due to the modification), is cheaper and the preamp is smaller
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