Hey everyone, first post here, just registered. I've been posting on the harmony central forums for a little bit, and also the homerecording forums for a little less time.
I posted this at both of those forums too, but only a very small percentage of the members there are likely to have enough knowledge of electronics to help me out, most of them just record. Fletcher told me on HC that one of the forums here would be a much better place to find this kind of advice, and I appreciate anything anybody can offer to help me.
Or if this would be more appropriate on another forum here, feel free to move it (just let me know so I can follow it) I'm more or less pasting it here, but I might add or change some things, who knows:
Hey guys, I'm seriously considering building an electret-based condenser.... this article here (which I'd seen before and thought about, but now it'd actually like to do it) got me wanting to do it:
http://www.prosoundweb.com/recordin...dmic_16_1.shtmlI dont like that that one has to have a batttery supply, so I'd be using a phantom-powered design based on the "Better electret microphone phantom powering circuit" most of the way down on this page:
http://www.hut.fi/Misc/Electronics/...e_powering.htmlAll the resistors would be 1% metal film resistors.... What is a good kind of high grade cap to use? I've been looking for film type ones, and having a hard time finding them that can get to 10uF... I fo und some metalyzed polyeser/polypropelene film caps that are 5uF, so I could run 2 in parallel, but they cost over 6 bucks a piece.... if that's what I'm going to have to do to get a good grade one, I'd be willing to, but I'd like to be sure that what I'm trying to do will even work before spending so much money! I'd prefer to use the best components I can reasonably get though.
There are some things I'm wondering about though, cuz I have some knowlege of electronics (I'm studying electrical engineering at cal poly, but I'm just a 2nd year, so I've taken hardly any EE classes, mostly calculus and physics.... lots of it)...
The 2k2 resistors biasing the capsule... it says that they should be as closely matched as possible to achieve optimum bias. I was thinking I could use trim pots in place of fixed resistors, to manually get them closer than even 1%. Would this be beneficial? Mainly I'm concerned that I might not be able to have as high a quality of trimpot. It seems to me that that resistor isn't really carrying the signal at all, but since it's biasing the capsule (cheap electret, the one reccomended in the first article, and possibly another with the same specs, I plan to make modular interchangeable capsules...), I'd think that noise from a lesser quality resistor could still find it's way into the signal chain by creating impurities in the bias entering the capsule's internal electronics.... If it probably wouldn't make much difference, I might as well just go with fixed resistors.
edit: any advice about different capsules that would be better, or just different, would be appreciated and useful. I only know that that's the one used in one particular project, but I'm willing to use anything that would work well.
Also, I was thinking about doing the same thing to make a variable bias for the two PNP transistors. It said they'd ideally be selected for closely matched gain, and I dont really have the option of picking through a bunch, nor do I have a suitable transistor tester, so I was thinking swapping some other R's for trimpots would make that adjustable.... but t hose would be more critical to the signal chain? Anybody have any advice/knowledge of such thigns / care to even think about?
Also, the cap in parallel with the zener, the page says should be a tantalum... I read on another site that I have more faith in, that the author of that site finds tantalum's to be the least reliable of all, and avoids them at all costs. Would I be able to swap this for a similarly specced electrolytic cap?
And in terms of the transistor... it lists the BC479 as an example of a model to use... Basically I just want to make sure I get a nice quality, low noise, great sounding, PNP transistor that appears will be running on 36V supply rails (is that because the zener draws off 12V from the 48V phantom voltage?) I dont want to skimp on the transistor.... I mean, a couple bucks for a super expensive transistor is nothing for one or two mics... Why put all the effort and use cheap components....
Finally, this should work right? So that schematic is pretty much complete, and the ground hot and cold outs will go right to my xlr jack? The one on the prosooundweb article is MUCH simpler, but I'd really prefer to put the extra time money and effort into making it phantom powered, and a true balanced design.... That would be much more rewarding.
FWIW, so far I think I'll try to make a mic body out of metal tubing.... I pucture a screw on end cap, with a big hole drilled in it, filed smooth, and a metal screen mounted inside. If I actually make this project happen, I'll surely do a writeup on it, sort of a step by step illustration of what I did, try to get pix and stuff, and put it up somewhere to share with you guys!
Or is there a better circuit I should be using? I'm by no means set on this particular one, it's just what I could find, and I dont have enough expertise to design my own from scratch. I'd like to make it as good quality as I reasonably can for such a project... Would using a circuit like that lose any quality over the simple battery design? I want to use the best quality components I can, of course. Any other concerns or things I should know or take into consideration? Anything will be usefull, basically I'm just trying to figure out how to make the only method I found work, but I'm willing to do anything differently for the better.
Thanks for any info anyone has! I know it's quite a long post, but I'd love any advice anybody has to offer, that's even remotely related to this.... or maybe even if it's completely unrelated but usefull advice!
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Topic: Questions about building an electret-based mic... (Read 5135 times)