DIY mic pre-amp kit

zcream

zcream

Well-known member
First a background. I wanted to mic my actors for my film using a lav on each actor and some portable recorder. Some scenes had 5-6 people talking.
I first started with iriver ifp recorder with the Giant Squid mics - about 35 bucks per setup. This gave me pre-amp noise as the iriver uses cheap pre-amps. It was recorded as 160kpbs mp3 mono 44.1khz sampling.
Then I tried a bunch of used Sony Hi-MD recorders, about 50-75 each. These default to AGC, and I had to buy a MZ-RH1 (M200 actually) in order to transfer the recording using USB2.0 (the older Hi-MDs come with USB1.1). The pre-amps on the older NetMd and MD were crap.
Now I have the Tascam DR-05, which fixes all the problems related to pre-amps and gives 96khz sampling. However, it is still too big to carry in a pocket.


I found that the biggest problem for human conversational voice is not sampling (44.1khz vs 48khz) or compression (PCM vs 160kbps mono mp3) but pre-amp noise. Resampling can be done easily via command-line using soX.


I also found the form factor of the iriver ifp series to be most convenient for micing actors.


So I looked around for circuits for a simple battery powered electret mono pre-amp for a iriver ifp. BTW the iriver was found to have very low distortion using the line-in setting. http://bartus.org/akustyk/ifp-899/index.html


I found these DIY battery powered mic pre-amp kits.


http://www.canakit.com/electret-microphone-pre-amp-kit-ck495-uk495.html - 11.95/-


http://www.quasarelectronics.com/preamps-mixers.htm#Mono-Preamplifiers-General-Purpose ~ 10/- each

http://www.vakits.com/product_info.php?cPath=43&products_id=260


Has anyone used these, any comments ?? BTW I am looking for something to be powered from a 9V or a AA battery, a portable device.
 
I am rather dubious about your scheme.
1) None of those small utility preamp kits will have signal-to-noise ratio (SNR) performance any better than the mic preamps in the little voice recorder gadgets you are using. In fact, they are probably even worse (based on the components they are using).
2) Then SNR performance is also highly dependent on the quality of microphones you are using, and their placement (not too far from the mouth). A poor SNR performance is to be expected with low-end mics and low-end recorders.
3) Those iRiver IFP series don't appear to be made anymore. There is no mention of them on the iRiver website, and Amazon has only a few used ones avaiable.
4) Creating a very small, battery powered mic preamp circuit for your particular application is not a simple engineering task. It seems quite possible that the cost/benefit factor is negative.
5) I question your confidence in re-sampling. It is quite common to end up with undesirable sound artifacts in the final mix when you are un-compressing, mixing, and then re-compressing (for the delivery format).

You didn't mention why you think your low-end separate recorder scheme is the best solution for your production? You have probably re-discovered why people aren't using that method.
 
The plan has issues like lack of monitoring and probable sync drift with those recorders.

Your right that 44.1 VS 48 is not a significant difference. And actually that MiniDisk can sound like DAT if the mics and preamps are good.

Unfortunately none of the ones you linked are going to be very good. These are cheesy low end designs for DIY projects.

You might instead want to get a copy of "Home Recording for Musicians" by Craig Anderton. He has some pretty decent mic preamps in there.

You can also buy a decent one from Radio Design Labs . It's small and runs around $100.

Sample rate conversion is not a big problem but you should be using s program that is designed for audio because it's not that simple.

And BTW the kits you linked are all for use with raw PCB capsules. I'm not sure they would work with an actual mic. And they are going to get bulky when you add batteries and a shielded case.
 
"Actual mics" are electronically the same as those 80-cent capsules on the boards. So the interface wouldn't be a problem. However the quality (including SNR) of those kit circuits is quite primitive as they were not designed for high-quality audio applications.

It sounds to me like the microphones are nowhere close enough to the actors' mouths and that is why the SNR is so poor.
 
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