How to build a Stereo Microphone and Battery Box

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How to build a high quality stereo / binaural miniature microphone:

In the US, you can get most of the parts at www.digikey.com - in Europe, consider www.conrad.de (cable, all other parts) and http://www.schlotzhauer-versand.de/ for mic capsules.

Required parts:

  • 2 miniature electret microphone capsules (get the best ones you can find - your selection will have a major influence on the sound quality - recommendation: Panasonic WM-60/61, Monacor MCE-2000/4000/4500). (article about directional vs. omni-directional mics)
  • 3.5 mm (1/8") stereo plug with cable, preferably angled and shielded
  • a few cm / inches of heat shrinkable tubing, slightly larger diameter than the capsules
Additional requirements:
  • soldering iron + solder
  • hot melt glue + gun
  • scissors
Warning: Be very careful when applying heat to microphones, it could irreversibly damage the diaphragm. Keep the procedure(s) as short as possible.

1) Skin and solder the cable to the capsule, the shield to the outer shell of the capsule, keep soldering time as short as possible. Use some sort of heat sink where available.

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2) Bend the cable and add some hot melt glue. (heat)

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3) Cut suitable pieces off the heat shrink tube (about 1/2 inch) and put it over the capsules.

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4) Heat it with a lighter or similar tool for a short time just until it doesn't shrink any further, remove leaking glue as long as it is fluid. This process requires some experience. You may want to practice first on other objects before ruining your capsules. Never apply excessive heat to a single spot, keep the heat source moving. Constantly cool the capsule by blowing against it for a few seconds directly after heat has been applied. Do not touch the hot glue with your fingers.

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When using omni-directional microphones, for a realistic stereo image it is important to separate the mic elements about 6-8 inches (15-20 cm) and preferably have a separator in between, which is ideally your head. You can attach the microphones to the rim of eyeglasses using the same heat shrink tube method, put them into eye glass retainers (croakies) or just use black electrical tape. Don't mount them too far up front or you'll loose a great part of the head's separating characteristic. Having them as close as possible to your ears is ideal.

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Matching: (advanced)

If you want to make sure to have no significant channel differences, you need to match the capsules before assembly. As with any electronic part, no two capsules are exactly the same, but you can minimize the differences by picking a pair with close to identical characteristics.

To do so, solder a random pair of mic capsules to the plug with cable. Download the test tones here: test_tones.zip

Play the tones through a Hi-Fi loudspeaker at a moderate volume. Hold the mic capsules very close together and monitor the level indicator on your recorder while slowly moving the capsules back and forth from the loudspeaker. Both channels should read about the same, otherwise try different capsules. The test tones are 100Hz and 1000Hz (1kHz) sines in mp3 format.

Edited by greenmachine

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How to build a miniature battery module / box:

What do you need it for?

The battery module is used to power electret (pre-polarized conenser) microphones. Electret microphones need a small bias voltage of approx. 2-10 Volt for operation (with a tendency to work cleaner close to the upper limit). MiniDisc- and other recorders often provide such a voltage at the microphone input ('plug in power'), but not at the line input. If you want to record high Sound Pressure Levels (like amplified music/instruments, concerts, everything you would call 'loud') without noticeable distortion, you should use the line-in of your recorder. The line-in uses a significantly lower pre-amplification than the mic-in, which would overload soon at high SPLs. The line-in provides no bias voltage ('plug in power') for the microphones though, thus the microphones need to be powered externally, which is the task of this module.

Usage:

The module is operated by a commonly used 9 Volt battery (MN1604 / 6LR61). The microphones' connector belongs to the battery module's jack - the module's connector belongs to the recorder's input jack. No harmful voltage will enter the recorder. The current draw is usually very low (about 0.5-1 mA), thus the long battery life of about 500 - 1000 hours with an alkaline battery (recommended). it's not necessary to unplug the battery after use, just disconnect the microphones and there will be no current flow. It is advisable to wrap some tape around the gap between battery and module to avoid accidental touching of the battery poles with metallic objects like keys. Clean the stereo connector from time to time with a damp cloth.

How to do it:

Find a 9V battery (MN1604 / 6LR61), no matter what condition, open it as seen in the pictures. All we need are two outer plastic shells. Recycle the batteries.

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Other parts needed:

  • A small piece of circuit board
  • Two polarized or non-polarized capacitors (doesn't really matter, though some audiophiles swear on non-polarized ones, which are usually significantly larger). The capacity should be at least 1µF (micro Farad), otherwise you will get an audible low frequency roll-off. The voltage rating needs to be at least as high as the battery voltage.
  • Two resistors, preferably 1% metal film for low tolerance and thus better channel matching. Carbon resistors tend to have tolerances of 5%. If you have a multimeter, you can match them yourself. Most electret mic capsules work best with resistors in the 2-10kΩ (kilo Ohm) range. A lower value (close to 2k) will deliver more power to the mics, but the channel separation will suffer. With a higher value (close to 10k) you will have a good channel separation, but the mics will get a bit less voltage. The difference is hard to notice though - if you have the choice, use something from the middle, otherwise use anything that's available between 2-10kΩ.
  • One 1/8" / 3.5mm stereo jack
  • One cable with a 1/8" / 3.5mm stereo plug
Solder the parts to the circuit board, as seen in the schematic, the stereo jack belongs to the side of the mic input, the cable with connector to the output side. If the jack is not a closed type, wrap some tape around it to prevent glue entering the inside. Solder the board to the battery connector. Fill the gap with hot melt glue. Cut a piece off the top plastic shell so that there's place for the jack. Loop the cable around to the opposite side of the jack so that there's no stress on the connections when in use. Put the top shell on it and fill the gap with hot melt glue.

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Because of its (lack of) size, this is a rather advanced design. If you don't feel handy with the soldering iron, you should probably head for a larger design, but you'll get the idea.

Update:

Alternative design - no need to disassemble batteries, which might not contain required parts, all you need is a (disassembled) standard 9V connector from an electronics store and some more hot melt glue. It perfectly covers the battery connectors:

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Update 2008-03:

Here's a new idea: battery box built into the lead of the microphone. Uses diodes to forward the plug-in power from the recorder's mic-in jack. Can be used with or without battery. The battery clip should not be touched if used without battery.

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Edited by greenmachine

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Posted · Report post

I have to pin this in Live Recording; it'll get more visibility there. I think I'll try to make one of these soon, but I imagine the result will be pretty sloppy. Thanks for the contribution. :D

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So that's what they look like inside! As the proud owner of both of these gems it would be quite remiss of me if I didn't share some of the spoils. As soon as the gallery is back up and running I'll post some samples.

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Added a short introduction to the battery module instructions per request. Hope it's clearer now. Enjoy. ;)

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rather excellent thread g/m & i have rated it as such.

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Clear as water. Thanks Greenmachine

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First of all - great thank you to greenmachine!

Really great DIY! Anyway it gave me inspiration as long as i'm waiting for my new unit (MZ-RH910). After that i get my unit i can start live recording and do my first bootlegs at all. ;)

Thank you all guys!

The thing i built. I had to improvise and go without mic-jack, just cord inside the box and out. And i also had to use 11kOhm resistors instead of 10kOhms, but i think it won't matter so much.

[attachmentid=908]

I'll let you know, when i get my first recordings done.

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Hehe, nice to see someone actually doing it, i'd like to see more people giving it a try. Looks pretty good for a first attempt. Actually, the resistors should usually be somewhere in the 2-10kOhm range, but you're right - it doesn't matter much.

A few months ago when i joined here, i had no idea about all this - what a good microphone should look like, how it should be placed - i didn't even have the slightest clue what a battery box is or what you could need it for. But i knew about Minidisc's high quality recording capabilities and wanted to use it for live recording. Music is an important part of my life, why not conserve some of it?

I started experimenting with some older microphone capsules i had lying around from disassembled tape/radio recorders soon. The first results were promising, but the recording came out clipped above a certain loudness. So i started researching if there are possibilities to avoid it. What i found was the (simple) headphone attenuator and the battery box. I went for the simpler solution first which gave a slight improvement at high SPLs but realized soon that this design has some serious limitations. So i gave the battery box a try. I had some serious problems to find a small yet durable design, the first try was really awful, rather large and too fragile. But i realized that this kind of external powering of the microphones is the way to go for undistorted recording at high SPLs. At the same time i discovered the very welcomed source follower mod which i use for serious recording ever since. Spending lots of thoughts about optimized designs, some less successful attempts and testing various microphone elements finally lead me to these two designs, as illustrated above. I'm a happy recordist ever since.

Thank you all for your kind words, i'm glad to help.

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Thanks a lot, greenmachine. I just have one question. Can I use the battery module with the Microphone Madness MM-MCSM-4 mic?

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Yes, it should work for all electret mics without a rather uncommon certain way of implementing the source follower mod, which would require a reversed voltage.

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Yeah, DIY is just wonderful, because i have boxfull of capacitors, resistors and much more like this from old soviet union, when there was alot of selfbuilding stuff. I'm glad that i have this box, because we in Estonia (country, where i live) don't have Radioshacks or what it was. And it's real hard to get anything. It even seems that MD is dying here (actualy anywhere?), but i couldn't get RH910 over here, wich i wanted. There is only on store selling MDs and they only had RH710, i think. So i ordered mine from amazon. But that's important at all.

About the mics i can't say anything right now, because i don't have a MD recorder YET! I have only tested it with computer mic jack and i know it is working.

EDIT: I got my MZ-RH910 and got first bootleging done. Mics attached to my glasses i recorded hour-lenght concert. I got sample from that concert under my album. Feel free to judge and say what seems bad and so on.

Edited by pisialvar

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Hello all, I've got a few concerts coming up soon. I've made two battery boxes in the past few years and have been through a few designs. I built one with an altoids box a long time ago, amazing that site is still around to show those pictures.

I'm mostly going to rock concerts with very high spl's. with my new box, i'd like to adjust the bass roll off ever so slightly, but i'm not sure how exactly. I will be using some self made panasonic cap mics. and a new rh10 if it gets here in time. I am just fascinated at how small you guys have made this thing, i never thought to have the battery just suspended by the attachment like that. think i still might run some duct tape around it to be safe, but i like the look and simplicity.

couldnt' find these answers while searching.

so, now on with my questions..

does anyone know how to adjust the capacitor to lower the bass roll off, how does it affect the amount (dB) of drop and the starting point (kHz)?? i know there are equations but i couldn't find the line-in impedence on the rh10.

what exactly happens with a lower and a higher resistor?

i know 1% metal film are recommended. i'm not sure if i have any local dealers in central illinois that i can drive to to get those, and dont have time to order online. should i just buy several of the 5% and get two with about equal resistances on my multimeter? isnt' the 1% and 5% really all about tolerances?

i am totally hobby on this and only have my fading gen physics class to guide me. I hope someone can help me out with this soon, my big concert is on saturday and i just found this the other day or i would have started sooner.

thanks for the ideas and any help. please email, but i think it woudl be beneficial to get everything her for people as well.

EDIT:::

added some pics of what i did, there are a few more of my older ones in the gallery section[attachmentid=931][attachmentid=932][attachmentid=933]

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Edited by orangezero

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does anyone know how to adjust the capacitor to lower the bass roll off, how does it affect the amount (dB) of drop and the starting point (kHz)?? i know there are equations but i couldn't find the line-in impedence on the rh10.

The main job of the capacitor is to block DC voltage from entering the line input. If you want to use it as a bass roll-off filter, the formula for calculating the roll-off frequency or capacitor value of a 1st order (6dB/oct) filter is given by:

fc = 1 / (2 * Pi * R * C)

C = 1 / (2 * Pi * R * fc)

fc = cutoff / rolloff frequency in Hz

Pi = 3.1415927...

R = Input resistance in Ohm

C = capacitor value in F (Farad), 1 uF = 0.000001 F

Assuming that fc should be 150 Hz and the line input impedance is 47 kOhm, your cap would have to have about 0.0226 µF (22.6nF).

A first order filter lowers the volume by 6dB each octave below the cut-off frequency (fc). Having fc at 160 Hz would equal: 0 dB at 160 Hz and above, -6 dB at 80 Hz, -12 dB at 40 Hz, -18 dB at 20 Hz and so on. Each doubling / bisection of the frequency equals one octave.

Capacitors usually have huge tolerances of 20 % or more, so they should be picked in pairs for a precise roll-off.

I don't have specific values for the RH10, but line-in impedances seem to be around 47 kOhm average.

I usually prefer to set the roll-off point well below the audible range (20 Hz) for several reasons:

- why alter the frequency response of close to perfectly linear mics? I rather would roll off afterwards via software if really necessary

- you won't know how a roll-off will sound in the end. If you have filtered too much , you need to raise low frequencies afterwards again.

- tolerances of capacitors and unknown line in impedances

- mic and line input have different impedances

- only 6dB/oct.

what exactly happens with a lower and a higher resistor?

The resistor determines the maximum current available for the mic caps. Electret mics usually need up to about 1 mA, often less. At 10k (resistor) and 2k (mic impedance) the maximum current at 9V would be 0.75 mA (9/12000) per channel. If you use a value significantly above 10k, there wouldn't be enough current available to power the mic caps adequately -> lower output, higher distortion. A lower value would allow for a higher current, but lowers the output signal because of a parallel connection to the mic. The art is to find a good average value (usually in the 2-10k range, although i tend to prefer higher values between approx. 5-10k). Don't worry too much about it, something in the aforementioned range should fit well for various capsules.

i know 1% metal film are recommended. i'm not sure if i have any local dealers in central illinois that i can drive to to get those, and dont have time to order online. should i just buy several of the 5% and get two with about equal resistances on my multimeter? isnt' the 1% and 5% really all about tolerances?

Don't worry about it, two matched carbon film resistors should perform equally well. I used such in my first BB with equally good results. Even if you don't match them, there shouldn't be a significant channel difference. You'll have more problems to find two matching mic elements anyway, there are rather huge tolerances.

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thanks for the explanation of all that. i had heard several people (probably you at some point) mention to avoid bass roll off as much as possible and rely on post processing if necessary. the problem is almost everything i record is way too bass-y. and its a pain for me to adjust the eq settings all the time if i go from studio songs to live recordings. i know i could do it in a program like soundforge as well.

sometimes its hard getting an accurate recording that doesn't overload (even with a battery box) when the band you're recording takes pride in being the loudest around. i've had one or two shows that have had clipping solely because of the bass and the acoustics of the small bar. thought this might be a way to alleviate some of that.

however, looking at radioshack and what i have available, looks like i'm going with 10k and 2.2 because of availability.

thanks again for the help and ideas.

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sometimes its hard getting an accurate recording that doesn't overload (even with a battery box) when the band you're recording takes pride in being the loudest around. i've had one or two shows that have had clipping solely because of the bass and the acoustics of the small bar. thought this might be a way to alleviate some of that.

You should take a closer look into the source follower modification if your mics are overloading at high SPLs. This involves some delicate work of cutting a trace on the tiny pcb in back of the microphone cartridge and making a different connection to the capsule housing. I'm using it successfully for a while now. Never had noticable clipping caused by my equipment, no matter how awfully loud the source has been. Further details per request.

The problem of overloading microphones can't be avoided by using a more aggressive bass roll-off as it's just a filter between mic and recorder. You don't relieve the mic from low frequencies this way.

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yeah, if you could let me know how its done, or provide some pictures that would be great. i do have some leftover caps somewhere. it didnt' happen too much, and there are certainly a lot of other factors to consider also, may have had a poor 9V battery as well. i just thought that i could lower the bass input a bit and that would help the clipping somewhat, or at least even it out. most shows are way over the top on bass because of the environment. after looking around, it seems as if i was probably using the 1 uF capacitor, so perhaps 2.2 uF will make a little diffence.

I was thinking as i wrote the last message, that its amazing how clear these mics can be. they do a lot better than my own ears when i'm there. i learned early on it is just plain stupid to not have earplugs in if you are at a show that needs a battery box. I wish more people would point that out. i'd like to be able to hear these recordings several years down the road.

anyway, i'm off to radioshack to get some parts, and will be soldering tonight. thanks again.

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The source follower mod:

I can't explain why it works, all i know is that it does work. It improves Panasonic (and maybe other) electret mics' performance at high SPLs and lowers their sensitivity by approx. 5-6 dB. There are at least three different methods to implement it, this one is the most practical in my view, no reversed polarity or additional wires required. But you need to isolate the outer shell properly to avoid touching it directly since it becomes the new (+).

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I was thinking as i wrote the last message, that its amazing how clear these mics can be. they do a lot better than my own ears when i'm there. i learned early on it is just plain stupid to not have earplugs in if you are at a show that needs a battery box. I wish more people would point that out. i'd like to be able to hear these recordings several years down the road.

I'd never go without.

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EDIT:::

added some pics of what i did, there are a few more of my older ones in the gallery section

[attachmentid=931][attachmentid=932][attachmentid=933]

Does the pic on the left show the finished design? If so, it looks rather fragile. For a better seal maybe you should add more of that gray material (cement? :lol: ) and loop the cable around in order not to have stress on the connections when used in rough conditions.

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Does the pic on the left show the finished design? If so, it looks rather fragile. For a better seal maybe you should add more of that gray material (cement? :lol: ) and loop the cable around in order not to have stress on the connections when used in rough conditions.

no, i actually added two more layers of jb weld to it (the grey stuff). it looks like a huge wad of dried gum with a hole, a 9v and a wire sticking out of. i'll try an add a finished one later. my friend was over tonight and he was laughing at it. i'll probably end up taping the cord to the 9v as well, i have it curved down the bottom of it now. just to be safer

thanks,

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no, i actually added two more layers of jb weld to it (the grey stuff). it looks like a huge wad of dried gum with a hole, a 9v and a wire sticking out of. i'll try an add a finished one later. my friend was over tonight and he was laughing at it. i'll probably end up taping the cord to the 9v as well, i have it curved down the bottom of it now. just to be safer

thanks,

Ok, added two more pics of the finished product. i'm not usually too rough on my stuff, think it should be okay now. the cord is wraped around inside a little bit and stuck on my quite a bit of that jb weld. its small enough now that i could build another with a slight roll off and take it with as well and it would still be less stuff to carry.

thanks again guys, (gm especially)

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my friend was over tonight and he was laughing at it.

He'll stop laughing as soon as you present him the results. Good job.

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He'll stop laughing as soon as you present him the results. Good job.

yeah, they always like a copy, after they are done laughing. i'm the one with all the wires and parts running across my body and hidden in my clothes at shows and bars. anything to make me less mechanical is a blessing. i think, despite all the irritating things, this rh10 will be about the best recorder yet. too bad they can't make a decent line-in recorder with levels and a memory card for storage. simple usb attachment and recognized as a disc drive in windows. (oh, and thats under what i paid for my rh10, forgot about the microtrack and others)

i'll stop getting off topic.

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Do you guys have a preferred store (online or otherwise) for scoring a pair of decent electret capsules to use in this DIY? After looking at the prices of Core and Reactive for their mic sets this looks like a nice alternative, if I can find the components. :D That battery box is slick too, nice miniaturization.

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Most of them seem to use Panasonic WM-60 or WM-61 omni capsules for their 'binaurals'. Unfortunately, the less sensitive WM-60 seems to be discontinued, but http://www.digikey.com/ (US located) has a large quantity of WM-61 available.

In europe these seem not available without importing, but we have a variety of Monacor elements instead, which partially seem to be re-labled Panasonics as well: http://www.monacor.de/en/produkt_aufl_gesa...I&spr=EN&spr=EN

The MCE-2000 and MCE-4000 (omnis) perform pretty well IMO. I have no idea about cardioids, never felt the need for using such yet. There are various online stores which carry them, google for it.

Since they're not that expensive and need to be matched, it's advisable to buy more than you need.

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