PDA

View Full Version : Electronics Help: Build Electret Omni Mic using 48V Phantom Power


insub
11-30-2015, 08:48 PM
**EDIT: This discussion continued in the Any Electronics DIYers Here? (http://forum.cockos.com/showthread.php?t=133420&page=42) thread. Starting on page 42 post #1675. When I get back to this project I will post what I learned & made here and there.**

Calling all electronics experts!
I have minimal electronics knowledge. I can read a schematic and solder a through-pin board, but my education level on the subject is introductory. Especially in regards to audio. So please, forgive my ignorance.

Goal: Use commercially available circuit boards etc to build a custom electret omni microphone powered via 48v phantom power from a mixer.

I've found schematics online that show a circuit, but I'd prefer to solder as little as possible. I suppose if there's no way around it I can create the entire circuit from scratch, but I'd rather not.

Can I hack these items together to make a functional microphone?
Electret Microphone Amplifier - MAX4466 with Adjustable Gain. (http://www.adafruit.com/products/1063) This circuit requires 2.4-5VDC to operate, so I'll need to knock the voltage down. Also, I'm concerned that the signal will be too hot. I believe it's designed to output line level signals for a computer/rasberry pi, etc. But the gain is adjustable from 25-125x. So, I wonder if the lowest setting might produce a mic level signal?

DC-DC converter 5-60V to 1.25-26V. (http://www.ebay.com/itm/DC-DC-LM2596HV-Buck-Converter-5V-60V-to-1-25V-26V-Power-Module-48V-to-3V-5V-12V-/221503434920?hash=item3392a230a8:g:U2oAAOSwGvhT0MB 9) This converter is the right voltage, but I don't know how big it is. No dimensions are given, but in the photo the corners of the board look like the in and out through-pin holes. So, I'm guessing that this is small enough.

1. Will the circuit work using the converter to obtain proper voltage for the electret board?
2. But, then the output signal will have to be connected back to the 48V conductor, so I'm guessing there's a problem here that needs to be addressed?

3. How long can the cable be from the electret capsule to the capsule amp? The Peavey VCM 3 Choir Mic has a 33' long wire from the capsule to the output module. So, I'm guessing it can be at least that long. I'm assuming that mic is an electret based on its size.
4. Would this cable need to be shielded? And if so, do you connect the drain to the capsule amp ground connection?

Most of the online schematics and DIY methods call for powering the capsule amp with battery power. But, I want to power them using 48V phantom power because the mic placement will not be readily accessible for changing batteries.

If there's some simpler method or other recommended products, please, share.
I know it's a lot of questions. Thanks for helping an electronics noob out!

Reason:
My church has zero budget for music equipment right now. I want to make some inexpensive mics to capture the room sound in an inconspicuous way. Due to the architecture, traditional hanging choir mics would require an enormous amount of cable to be run. Drastically increasing the installation cost. Electret capsules are super-cheap and have flattish frequency response. Omni pickup pattern is perfect for capturing the room sound. I'm hoping to build mics for less than $30 USD each. It looks like typical hanging choir mics start at around $100 and go up from there. The Audix MicroBoom system looks awesome, but one mic is $550!

One more question:
5. All the hanging choir mics seem like they'd be electret, but they are all listed as having a cardiod pickup pattern. How is this possible?

karbomusic
11-30-2015, 08:55 PM
Maybe post over in the DIY thread too... I'm sure someone can advise...

http://forum.cockos.com/showthread.php?t=133420

insub
11-30-2015, 08:57 PM
Thanks, Karbo. I didn't know about that thread.

clepsydrae
11-30-2015, 11:35 PM
For anyone else finding this thread I have posted a response that i hope will be helpful here (http://forum.cockos.com/showpost.php?p=1602895&postcount=1678).

clepsydrae
11-30-2015, 11:40 PM
5. All the hanging choir mics seem like they'd be electret, but they are all listed as having a cardiod pickup pattern. How is this possible?

Plenty of electrets are cardiod; that's what the little holes are for. See e.g. the EM184 (http://micbooster.com/primo-microphone-capsules/37-primo-em184-cardioid-capsule.html).

Omni is a fine pattern for recording in situations like you describe, but so is cardioid. But the enclosure for the capsules is much more complicated to design (requiring the grills and vents and attention to resonant spaces and all that) than with omni.

insub
12-01-2015, 06:39 AM
Wow, thanks a lot, Clepsydrae!

I never found those resources in all my searching (which was a lot before posting here). I really appreciate the links too!

I'm guessing that those instrument mics were later builds... very professional looking!

The capsules you linked have much better SNR, but the price is a big difference from the really cheap $1 ones I've been reading about. Have you ever made a mic with one of those, like the Panasonic WM-61A?

I see the omni EM172 ($10) is a lot cheaper than the cardiod capsule ($31), but still a pretty big jump from the WM-61A.

karbomusic
12-01-2015, 07:32 AM
Have you ever made a mic with one of those, like the Panasonic WM-61A?


I 'think' the pics beginwhereur posted may be WM-61A. If not, I know that he, ReaDave and I bought a handful of these around that time and I think at least ReaDave built one if not both beginwhereur and ReaDave; all of which exist in that thread somehwere. I still have my three WM-61As but haven't built anything using them yet; coming soon. Here is fine, but post what you can over in the DIY thread since we have all kept up with it for nearly two years now and we don't want to miss it. :)

moribund
12-01-2015, 09:03 AM
Cleps, are you connecting this directly to 48v phantom power? If so, how are you not destroying the mic (which is spec'd at a maximum of 10v?)

insub, I'll try to address some of your questions:

Can I hack these items together to make a functional microphone?
Electret Microphone Amplifier - MAX4466 with Adjustable Gain. This circuit requires 2.4-5VDC to operate, so I'll need to knock the voltage down. Also, I'm concerned that the signal will be too hot. I believe it's designed to output line level signals for a computer/rasberry pi, etc. But the gain is adjustable from 25-125x. So, I wonder if the lowest setting might produce a mic level signal?

These little electret mics already have a preamp in them that send out a healthy mic level, so you don't need to build a mic pre.

DC-DC converter 5-60V to 1.25-26V. This converter is the right voltage, but I don't know how big it is. No dimensions are given, but in the photo the corners of the board look like the in and out through-pin holes. So, I'm guessing that this is small enough.

Most of these DC-DC converters use switching circuits that will emit an audible whine when you use them with audio, so you probably want to avoid them, unless you design one yourself that sets the switching frequency well out of the audio range. This is kind of irrelevant, since you don't actually need the mic pre anyway.


3. How long can the cable be from the electret capsule to the capsule amp? The Peavey VCM 3 Choir Mic has a 33' long wire from the capsule to the output module. So, I'm guessing it can be at least that long. I'm assuming that mic is an electret based on its size.

This is dependent on how you wire the electret (i.e., what size resistor you use). The electrets you're looking at for DIY are not balanced (although you can set up a balancing circuit for them with a few more parts), but they are fairly low impedance (typically 2.2K)so you should be able to get away with a longer run than a guitar cable, but I'm not sure how much more.

4. Would this cable need to be shielded? And if so, do you connect the drain to the capsule amp ground connection?
Yes and yes (although the latter may not be necessary).

Unless you really want to got the DIY route here, I would suggest you just pick up some of these: http://www.karmamics.com/shop/K-Micro-Matched-Pair.html They're probably going to be as good as you can make yourself with a lot less hassle. Ive got a couple and they perform quite well. They're not likely to sound as good as the Audix, but they're no where near $500 either.

insub
12-01-2015, 09:15 AM
Unless you really want to got the DIY route here, I would suggest you just pick up some of these: http://www.karmamics.com/shop/K-Micro-Matched-Pair.html They're probably going to be as good as you can make yourself with a lot less hassle. Ive got a couple and they perform quite well. They're not likely to sound as good as the Audix, but they're no where near $500 either.

Thanks, for the responses!

Wow, a pair of mics with clips for $40! How can they do that?

Awesome find! Looks like they're out of stock at the moment. At that price there's no reason not to get them it seems.

clepsydrae
12-01-2015, 12:14 PM
Maybe we should consolidate all this discussion to one thread or the other?... I for one am going to respond in the DIY thread (http://forum.cockos.com/showthread.php?t=133420) from here on out, after this post:

I see the omni EM172 ($10) is a lot cheaper than the cardiod capsule ($31), but still a pretty big jump from the WM-61A.

It's 10 pounds, ~= $15 dollars, so yeah. The EM184 is 13 pounds = $20. (The 31 you see on the site is actually for a matched pair, which they now offer, awesomely.)

Cleps, are you connecting this directly to 48v phantom power? If so, how are you not destroying the mic (which is spec'd at a maximum of 10v?)

The schematic (http://lacinato.com/pub/electrets/simplest_schematic_v3.png) (discussed more in the DIY thread) takes 48V, knocks it down to an appropriate range for the capsule, and outputs a balanced mic signal.

Unless you really want to got the DIY route here, I would suggest you just pick up some of these: http://www.karmamics.com/shop/K-Micro-Matched-Pair.html They're probably going to be as good as you can make yourself with a lot less hassle. Ive got a couple and they perform quite well. They're not likely to sound as good as the Audix, but they're no where near $500 either.

Lookin' good, there. Sensitivity is not as good as the primo capsules (-32 vs -28, assuming karma is using the same scale which they didn't specify on the site). But they are a matched pair, which could be nice, especially if they do more than just level match (which is probably unlikely, i guess).

I did a head-to-head with a $600+ dpa instrument mic and the instrument mic i made from a primo capsule and preferred (narrowly) the primo (i did some blind ABX'ing to help determine this). The mic i made is full-range, so it picks up too much body noise on, say, a mandolin, and the pickup pattern may not be as good as the dpa for live use (not sure yet) but the point is: i am personally convinced that the "cheap and awesome electret" era is upon us.

karbomusic
12-01-2015, 12:20 PM
The schematic (http://lacinato.com/pub/electrets/simplest_schematic_v3.png) (discussed more in the DIY thread) takes 48V, knocks it down to an appropriate range for the capsule, and outputs a balanced mic signal.



Before we head to the other thread.. That schemo looks like Eagle. :)

clepsydrae
12-01-2015, 12:24 PM
Before we head to the other thread.. That schemo looks like Eagle. :)

Is there anything else? :-)

insub
12-01-2015, 12:31 PM
Ok, future discussion in the DIY thread.

I'll come back here with duplicate details if/when I commence my own project. I probably will try the DIY now since my interest is peaked. Even though it looks like a couple of mics are commercially available for not much more than the DIY method.

karbomusic
12-01-2015, 12:35 PM
Is there anything else? :-)

Not in my book. :D A lot of people like DipTrace but I've grown so fond of Eagle for both schematics and PCBs that I find it hard using anything else at this point.