Intro: Build the MS Alice Stereo Microphone
Typically this requires the use of two mics: one to pick up the middle (this is usually omnidirectional or cardioid) and one to pick up the ambience from the sides (that has to be a figure-8 mic). In the studio these are typically high end condenser microphones or ribbon microphones. The basic gist of this technique is that you have a “Mid” microphone pointing directly at the sound source you want to record and a “Side” microphone that faces to either side of the source. Then in mixdown you can do a little shuffling of the side signal and can get a very nice stereo image. The other key benefit is that is collapses into mono without any artifacts or phase nuances. I originally designed and built one of these in the 1990's using little panasonic WM60 capsules. I wrote it up as a DIY article in Recording Magazine and released it as a kit with PAiA electronics.
At the time I was really impressed with how well this worked with inexpensive capsules and top quality opamps. It wasn't quite on par with studio mics but it was a really great project for its day. A couple decades later many things have changed. My skills as a designer and understanding of analog electronics are better. The availability of parts has improved, specifically microphone capsules. And finally, digital recording software has become ubiquitous – even open source software. It was time to revisit building an MS microphone!
Based on my previous instructable I already had the building blocks needed for this project. The “Aha Moment” came when I built four identical microphones and one was out of phase with the other three. Since I built them at the same time this one really bothered me. Could it be the capsules had poor quality control? It wasn't until I disassembled the microphones that I found I had wired the capsule backwards in the errant microphone. I had wired the ground to the center pin and the input to the FET to the case of the capsule. Because the capsule was in a plastic holder and insulated, the only thing that happened was reversing the phase of the signal compared to the other three microphones. This lead me to pondering and thinking... What if I wired two capsules in series with one set of electronics? Would it work? If so, I could face the capsules front to back and create a figure eight pattern. And sure enough, it does work. Now, if I could fit those two capsules into one case with a third capsule facing front, I had the makings of a complete MS mic in one body. I also needed two circuits in the case and the ability to bring both signals out of the microphone at the same time and I was all set. This required a smaller capsule than the TSB2555B that was in my other microphones. The same company makes one called the TSB-165. It is pretty renown in the mic building community and is the one used in the “Alice” microphone that originally got me started with building my own microphones.
So I ordered another BM-800 microphone from Amazon, fired up my soldering iron and turned the idea into reality. In honor of Scott and his design, I am naming this microphone the “MS-Alice”.
Here is what you will need to build this project:
Three TSB-165 capsules
One BM-800 donor body: Go to amazon or ebay and search “BM-800” then sort on price.
One five Pin Male XLR
One five Pin Female XLR
Two male three Pin XLR cable mount connectors
30-40 feet of Mogami balanced small microphone cable
Two sets of electronics from the original instructable
a small portion of 1/2” PVC pipe for the microphone capsule holders.
Assorted hand tools, soldering iron, hot glue gun (of course, this is an instructable) and hook up wire.
UPDATE** I added two audio files. One with this mic and the other with a pair of my TSB2555B mics in ORTF layout (spaced about 8 inches apart angles about 110 degrees.) edited 5 Oct 2016.
Step 1: Step One: Disassemble the Donor Body
Disassemble the BM-800 donor body. The three bodies in the picture show you the variations in what you get. But for the price, I am not complaining!
Remove the PC board, the top section (head basket), the XLR connector, and break it down completely. Keep all the little screws, you will need them. We are only using the mechanical housing. It still blows me away that you can buy one of these for under $20. You can not build the equivalent for anywhere near the same price.
Step 2: Make and Mount the Capsule Holders
Make and mount the capsule holders
This one is pretty easy but took me a bit to figure out. Cut three small pieces of 1/2” PVC pipe about 3/16” to 1/4” rings. See the photo's. Then using a small saw, cut a section of the ring out. This will let us expand it to put the capsules into the rings. They hold really nicely. Originally I tried to use epoxy to assemble the rings together and the epoxy didn't stick to the PVC. I settled on Hot glue as it is both structurally sound for what we are doing and allowed me to work with it easily while aligning the capsules. Juggling how they mount is also necessary to make sure they will all fit.
Mount two capsules such that they face opposite each other and that the front of the capsules are in the same plane as much as possible. This is the one thing my Project r MS mic couldn't do. The Side capsules were about 1.5” apart. Acoustically, not optimal. This implementation is much better. Once the hot glue has cooled for the side capsules hot glue them to the microphone shell. See the photos. There is a tab on the top of the shell. We need to glue the side mic assembly so that one of the capsules faces the tab. I screwed this up the first time... This will be our reference when the whole thing is assembled.
Once the side capsules are mounted and cool, mount the mid capsule on top of the side capsules. Center it as much as possible in the microphone shell.
Wiring The Capsules
Prepare some short lengths of hook up wire. I used 24gauge type EE because I have a bunch of it but almost any thin wire will work. The Mid capsule is wired just like the original Modified Chinese Microphone instructable. Solder a wire to the center of the capsule and one to the housing (ground) of the capsule and fish the wires through the holes in the mic base.
The side capsule wiring is where all the magic happens. They are wired in series but with the phase reversed between them. See the photos. I connected the two center leads together and then ran wires form the shell or housing down into the mic body. This makes them pickup sound from front and back but cancels out sound coming in form the side as it arrives at the diaphragm of the capsules at the same time but wired out of phase causes the signals cancel each other out. It isn't flawless but it works surprisingly well. Watch the video for a demo of me talking while rotating the microphone. After you have inspected the capsule mounting and run the wires carefully put the basket back on and make sure that capsules don't touch it and that there is clearance at the top and sides. You have to weave it a bit to get it over the capsules.
Step 3: The Internal Electronics
You will need to build two sets of mic circuits identical to the “Pimped Alice” circuit from my original instructable. Thanks to Homer Leal, who laid out the PCB from the original schematic, I had actual PCB's to use in this build. I have attached the schematic and layout here. They are also in the original instructable.
The big mod here is how we get the signals out of the microphone. What made this easy is the fact that the BM-800 body uses a standard 3-pin XLR connector. That can easily be replaced with a 5-pin, allowing both microphone signals to be carried through it using a common ground. We will have to make a custom breakout cable but that is pretty easy too. I bought a Switchcraft 5-pin male XLR and disassembled it to just have the 5 pin connector portion. For EMI and RF immunity (translates into noise!) I soldered .0022uF caps to the connector pins 2,3,4 and 5 to ground (pin one) What is really nice is that the ground connection is also carried to the housing. This is exactly what you need to get great EMI and RF protection. This is called the “Pin-1” problem, and Pin-1 is ground for XLR audio connections. See this app note for more information.
The BM-800 allows you to mount a PCB on either side of the rail on the shell. I found it easiest to mount the PCB's then solder the wires to the pads. See the photos for details. I cut the leads to length and then carefully stripped them prior to soldering.
After double checking your wiring, reassemble the rest of the microphone body.
Step 4: The Breakout Cable
The Breakout Cable
I used thin shielded twisted pair Mogami-W3031 that is thin enough to put two cables into one connector. I used two pieces about 15 feet long. It is available from Redco. See the parts list. You should make the cable at least 15ft long. That way the microphone can mount high up on a microphone stand if needed. For the break out cable we will use a five pin female XLR connector with the both shields wired to Pin-1 the Mid mic output wired to pin-2(+) and pin-3(-) and the Side mic to pin-4(+) and pin-5(-). The other end of each wire terminates into the male XLR with standard audio pin outs. Shield to pin-1 pin-2 (+) and pin-3(-). I also put some heat shrink on the connector for the Mid microphone so that I can tell easily which one it is.
Step 5: Testing and Use
Testing and use:
Connect the break out cables to your mic preamp and turn on phantom power. You should hear the Mid mic loudest when talking directly into the front. Turn the Mid level down and turn up the Side level. Speak into the side and rotate the mike body. When you are directly in front of the metal strip on the basket you should hear a noticeable drop in level. The tonal quality should be the same on both sides. Once you are satisfied lets see how to record with it!
The key to MS recording is having the front or Mid microphone facing the sound source with normal positioning. The Side microphone elements should face outward on either side. Now the magic is what we do with the side signals during mixdown. See the diagram. This used to take a mixing board and multiple patch cables. Then matching levels carefully. Ahhh... I miss those days! In fact one of my other projects from the 90's was a DIY MS-decoder.
Now with recording software it is easy to do all internally. We are going to use Audacity you can download it here The concept is applicable to any recording program.
To record, connect the MS microphone with the Mid microphone in channel one and the Side microphone in channel two. Adjust levels and record as you normally would. Once the recording is complete we have to do a few things to achieve the MS effect.
Step one: Split the stereo channels into two mono channels (if you recorded one stereo channel)
Step two: Make sure the Mid channel is panned center and set for nominal level
Step three: Duplicate the Side channel and name one “Side Right” and the other “Side Left”
Step four: Invert the duplicate you made. For Audacity, select the track, go to effects menu and select “invert”.
Step five: Pan “Side Right” all the way to the right and “Side Left” all the way to the left. Or in Audacity's track menu, assign the left track to left and right track to right.
Step six: Adjust the levels of the right and left side levels to get a good balance of the stereo image. You can also merge these into one stereo track to make it easier.
The correct level of Mid to Side is more of a taste thing. It will also depend on the ambience level of the recording. My demo video using the audio from the MS mic in this instructable was recorded in my game room with my kid on cello. Not the optimal venue, but works to demo the microphone.
The absolute beauty of this technique is that if you collapse the stereo back into mono or it gets mixed into mono while listening on a phone, the Side channel cancels itself out leaving just the Mid. No other stereo technique does this.
As of my writing this my son has just started 11th grade and his school orchestra is just starting up again. I am looking forward to using the with a couple quartets and ensembles. I will also try and get the whole orchestra at their fall concert. I will post links here when I get them.
I am very impressed with this microphone and have no qualms about calling it studio grade. It is as quiet as the rest of my mic collection and provides something that is just not readily available commercially. Not that there is a huge market for MS microphones, but having one in your arsenal that is plug and play with minimal setup is one more chance to get that perfect recording. The MS Alice pairs nicely with my Zoom H4N (or any other portable stereo recorder). In fact the Zoom has a built in MS decoder although I like to do this myself in post. After all, I am a DIY guy.
I hope you enjoy this instructable and build a microphone of your own. Thanks to Scott Helmke for starting my DIY microphone experience, the guys on the mic builders forum on Yahoo, (There is a wealth of knowledge there) and Homero Leal for taking the time to layout and build the PCB blanks I used for this project. And my wife who can't figure out why I have built my own mic locker... but lets me cover the kitchen table with my stuff and solder while she knits.
TomB366 made it!