Intro: The Paint Can "Blumlein Pair" Stereo Microphone
If you have never heard of a Blumlein Pair, it is named after Alan Blumlein. He invented it and patented the idea in1931. At the time microphones usable for this method were not in existence. Read more here. This recording method uses two figure eight microphones spaced 90 degrees apart. They face the main sound source pointing left and right in an “X”. The front mics pick up a great stereo image of the source and you get a nice spread out stereo image. But the beauty comes from also catching the ambiance of the room in the rear capsules also. This works really well for orchestras and live music in a great sounding room. I have always wanted to try this method. Now that my kid is in the high school orchestra (And it is a good one too!), what better time could there be? To achieve this we need two figure eight microphones, of which I only own one. I figured I would just build one microphone that has everything internally all set up in it to do this. All we need to do is create two figure eight mics place them 90 degrees apart, and put them in one enclosure. I recently found some very good microphone capsules, the TSB2555B , which I used in my “build a cheap LDC Instructable”. They are very flat with a slight 3db bump around 10K hz. These should be perfect for this project. I could have just built four of the single mics with electronics into one enclosure but then you have to use a mixer to combine everything correctly. I wanted one enclosure and one stereo output, not four individual outputs. So I designed a circuit that combined two capsules into one signal but inverted one of them thus creating a figure eight microphone. Using two of the circuits, one for left and one for right completes the project.
Step 1: How It All Works
Here is how this all works We start with a J305 FET similar to most condenser microphone circuits. This time we are just using it as an impedance converter. Most inexpensive electret condenser capsules have the FET built in but finding a really good one is difficult. (And... I already had nine more of the TSB2555B capsules.) Here is how the capsule converts sound to a signal. The capsule contains an internal charge because it is an Electret capsule. Sound waves hitting the capsule cause it move with the sound. The movement changes the capacitance of the capsule which causes a small voltage to be developed. That voltage shows up on the 1 gig ohm resistor and the gate of the FET. The FET acts as an impedance converter and allows us to generate a usable signal. Then we couple the signal into a differential amplifier based around one half of an OPA2314 Operational Amplifier (op amp). I like this one as it is very low noise and has great specs for audio use. You could use a 5532 dual op amp also but don't use one that isn't meant for audio use. One capsule feeds the inverting input, one feeds the non inverting input. If you put the two capsules back to back, you just created a figure 8 response pattern. From the first op amp stage we feed both the pin 2(+) of the XLR via 200 ohm resistor (50 to 330 ohms will work) and also a second op amp stage setup as unity gain buffer. The buffer then feed pin 3(-) of the XLR via another 200 ohm resistor. Now we have a balanced output that can go right into a pre-amp or mixing board just like any other microphone. I built two of these on a little perf board. They share a common power supply and all four of the FETs use the Vfet that is basically a filtered DC voltage. Filtering is done by 47uF capacitor and the resistor coming from the +15 volts. One output is panned hard right and one hard left to generate our stereo signal.
Step 2: Powering the Microphone
To power the whole thing I built a small +/- 15Volt supply using a 24v center tapped transformer, a bridge rectifier, some filter capacitors and three terminal voltage regulators. See the Power supply schematic for details. Note: The 7815 and 7915 are wired differently! Get the data sheet for your regulator. That all went into a dual gang J-box from the local hardware super store, Lowes.
Step 3: Physical Construction (Or at Least an Attempt!)
Now that the electronics portion is figured out, we need to mount and house everything. One of the challenges here is that the capsule and FET are such high impedance devices that the wire between them acts as an antenna. If you power this up outside of a case all you will hear is noise, humm and 60 cycle buzz. One of the challenges of Condenser microphone is to let sound in and airflow but block EMI. Metal screen is what is needed here. This can be purchased from a couple places: Mcnichols and McMaster-Carr The screen is quoted in “Mesh” size which means number of openings per inch. 12 to 16 both work well. Brass is easiest to work with and cleans up nicely. To make an enclosure I settled on a metal paint can that I could cutout sections of and then solder on the screen to allow sound in. I started with a one quart can. But there were problems... Mainly with getting everything inside and not touching. Plus closing the lid... Lets just say I had to cut slots in the can to get the PC board to go in. Then I bent the thing trying put the lid on. Not a good day for me. But, I did prove the concept! And decided to buy a gallon paint can. Who builds a microphone that big? I did. Thinking about it, for what it was going to be used for it wasn’t like it needed to be right above a snare drum or having a harmonica player cup it. It was going to be on a stand above and in front of the Orchestra. Why not a bigger housing?
Step 4: The Capsule Holder
The next thing I had to sort out was mounting the capsules. Not just mounting them but positioning them too. These are pretty delicate and required some thought on my part. Normally they are mounted in a saddle or gently pressed into a holder. I thought about this and realized the capsules are 25mm in diameter and that 1” PVC pipe is nominally one inch in diameter internally. So I cut a small piece and then cut out a small section. Basically making big letter “C”. The piece closed up slightly when the small cut section was removed. And, a capsule fit in snuggly. This was a huge win for me as I can see using this capsule in a multiple designs. (yea! more instrucables!) Now to hold four of them in the correct position, I cut a 1/2” piece of 2” PVC making a ring that I could glue the four capsule holders too. I actually bought just a 2” PVC fitting and cut the ring from it. I used a hand saw and a miter box to make a clean straight cut both for the 2” piece and the capsule holders. After cutting them I cleaned them up bit with sandpaper and washed any dust off with soap and water. Then I used PVC glue to mount the holders to the base ring. The glue is pretty quick acting. I pressed the two pieces together for about a minute then moved on to the next. Then let the whole thing cure over night.
Step 5: Build the Capsule Assembly
Place the four capsules face down on a towel or some other soft surface. Solder the 1 gig resistor between the center terminal and ground. When you have done this with all four capsules carefully press them into the holder. I then used a couple drops of Hot glue to lock them in place. Now take 8 or more rubber bands and place them diagonally across the capsule holder. We will use these to hold the assembly to the long bolt on the paint can cover
Step 6: Mounting the Capsule Assembly
Now how to mount the whole thing... As mentioned before, I started with a 1 quart paint can. And, it didn’t quite work out. It did however prove the concept and provide me with housing for testing other designs. Not all was lost. One of the challenges of the original paint can was positioning the capsules without touching anything. We want to isolate them from bumps and thumps. To get it all to fit the capsule assembly touched the PC board. That picked up too much rumble and thumps. Using the larger paint can did work.
I took off the lid and drilled a hole right in the middle, two other holes for the cables (one for audio and one for power) and two to mount the PC board. For the wire entry, I used rubber grommets to protect the wire. Lowes sells those too. To mount the capsules I used a ¼ -20 eye bolt. I could only find one that was 2” long. So I then added a ¼ - 20 standoff as an “extender”. This with a 4” carriage bolt allowed me to put the capsule assembly above the PC board and isolate it with a bunch of rubber bands. Then using three nuts and a couple of washers I formed a spot to retain all the rubber bands and position the capsule assembly.
Step 7: Build the Circuit
I built the circuit on a radio shack PC board Hopefully they will still make this for a while! The key to this is to align the FET's all on one side that will face upward when the board is mounted. We want to float the gate leads so that we can connect it to the capsule without touching the PC board. Please double check the data sheet for your FET. I got mine here:
I misread this the first time when using the FET and got the leads backwards For mounting to the can I used a couple of these 4-40 angle brackets Once you have the board finished take a break and check your work. I typically use water based flux solder. It says “No Clean” but I wash the bottom of the board with a toothbrush, soap and water. Then let the board air dry. A really good method for drying is to set your oven to 220 degrees. When it reaches that temperature and beeps, turn it off an put your PC board in. When it is dry, mount it on the paint can lid.
Step 8: Mount the Capsule Assembly and Connect Everything
Carefully attach the Capsule Assembly to the paint can lid. I used a chop stick to move the rubber bands around the washers and get them in the groove. Holding the eye bolt in a small vise helps. Mark the two capsules that will be “Right Front” and “Left Front” Sharpies are great for this. I connected the grounds together on the capsule and ran one ground wire to the PC board. There is virtually no current flow so ground loops here are not going to be an issue. While keeping track of the Front and Back, use small pieces of wire to connect the capsules to the correct FET's. The schematic shows the op-amps going to an XLR connector. What I actually did was wire the board directly to 40 feet of Belden dual twisted pair wire.
This I broke out into to Male XLR connectors to directly connect to my mic preamp. For powering it I used a separate twisted pair shielded wire to carry the +/-15VDC to the board. A little clunky to travel with, but easier on stage.
Step 9: Build the Paint Can Case
With the capsules mounted on the paint can lid we can easily see where the slots for sound need to be. We need to leave some metal of the can or it wont be structurally strong enough. We also need to have the sound reach each capsule. Use a sharpie and mark the can so that there will be about 2” of space for the screen and leave an inch or so of metal for support around the four cutouts. I used a Dremel with an abrasive cutting wheel to cut the rectangles out of the can. Then cut the brass screen so that there is about ½ of overlap to the cutouts on the can. Now comes the hard part. Soldering the screen to the can. I used a small propane torch. The kind plumbers use. Probably not the best choice, but it is what I had. I did get some practice in on the smaller can. It is important to solder these so that there is metal to metal contact. We are actually making a Faraday Cage to hold everything in so electrical connections are a must. After the can cooled I washed the flux off and then proceeded to use Brasso to shine up the screen. Then I washed the whole thing again and dried it. I covered the outside and edges of the screen with black felt. Hobby Lobby carries peel and stick felt that is perfect for this. I also put felt inside the can to dampen any weird resonance or echos in the can. It looks much better after the Brasso and Felt treatment. Although it does look like the Knight from Monty Python and the Holy Grail. Prior to putting the Lid on solder a wire (about 12-14inches) to one of the screens so we can ground the whole assembly when the lid is on. I put a couple stripes of red and yellow electrical tape on the can so that I could tell where the "Right Front" and "Left Front" are.
Step 10: Final Assembly
Prior to putting the lid on, power the microphone up and connect the XLR connectors to your mixer or Audio Interface. You should get a bunch of buzz and noise. Carefully put the lid over the microphone capsules. The noise and buzz should fade as you cover it all up. This means everything is working! Disconnect power and let's permanently mount the lid. Carefully align the front capsules with their respective screen openings. As you seal the lid in place, be careful. I almost crushed the one quart can doing this. To enable mounting on a microphone stand, I took a mic holder apart and reassembled it on the eye bolt. Then using a goose neck adapter and a clamp mount I can mount the complete assembly on a stand. I have a 9 foot light stand that is perfect for this.
Step 11: Lets Record!
I arranged to record a practice session with the Guyer high school orchestra. I used the Blumlein Pair as just built here with a matched pair of microphones I built from Microphone Parts using their CK-12 capsules and their electronics. I fed the microphones into a Focusrite USB interface and recorded into Audacity which lets me record all four tracks simultaneously. The pre-amp gains were set very close to each other which means the gain structure internal to my design is good. The noise floor of my design is about 3-4db louder than my reference microphones but still very acceptable.
The kids rehearsed in the school auditorium which unfortunately has a very noisy HVAC system, which the microphones piced up rather nicely :-(
And both pieces I recorded have very quiet sections to them. I did two mixes of each, one that is the Blumlein pair only and one that has the other mics mixed in panned hard right hard left at -9db in the mix. I really like the stereo spread of the Blumlein pair but it seems to miss some of the center. Using all the mics sounds pretty amazing considering the room noise and the fact that this is a high school orchestra!
Taking it further
I picked up too much thumping when people walked near the Blumlein Pair. Way more than the other two microphones, which were not even in shock mounts. I think I need to replace the rubber band suspension with some other form of vibration dampening. I also think I need a better solution to the case than a paint can, although it does start some conversations.
A special shout out to Mrs Hanlon and the Guyer Rhapsody Orchestra. You guys Rock! And thanks to my wife for putting up with my strange obsession with sound and DIY projects. If you like this project, please give me some feedback.