Tabitha, an OPA MS Microphone

Mid-Side or “M/S” recording was first described by Alan Blumlein in the early 1930’s. It consists of two microphones. A front facing microphone for the “Mid” or “M”, and a side facing microphone for the “Side” or “S”. The Mid mic is usually a cardioid pattern mic although an omni will work. The side has to be a figure-8 microphone for this to work. Typically this is done with two separate microphones. Today's project will build a self contained mic that accomplishes this with excellent audio properties. I named her Tabitha for her magical powers of hearing what is around her in interesting ways.

Let’s see how M/S works and then how we can use it. The math behind it is simple and deceptive at the same time. You point Tabitha at the sound source with the Side Capsule “+” side facing left. By “+” I mean that it is in phase with the Front or “Mid” capsule. To decode this, we mix the Front with the Side and send that to the left channel. When combined, a virtual microphone is created that faces to the left. Simultaneously, we mix the Front with the Side inverted in phase and send that to the right channel. By inverting it, the side mic right side is now in phase with the Front capsule and we create a virtual microphone facing right.

There is an AES paper from 1982 that goes into all the math. The cool thing is by varying the side levels, we shift the virtual mic pattern and thus, how the resulting stereo sounds. If you mix in .707 of the side, or about -3dB compared to the front, you create a virtual XY microphone. The really cool thing about M/S is that it collapses perfectly into Mono. We will come back to that one as this is really useful for taking MS microphone techniques to the next level.

Tabitha is available as a Kit from JLI Electronics. It can be found here: Tabitha

Additionally there is a breakout cable kit available as well. That consists of two 3-Pin Male XLR jacks and one 5-Pin Female jack and some some two channel microphone cable.

Tabitha consists of a JLI2555B capsule and two smaller JLI165 capsules. These are both really good electret capsules that do not have an internal FET. Thus we can use my existing dual OPA board from the Endora microphone with them. For those of you unfamiliar with that board and circuit, see this: Tabitha uses a state of the art FET input operational amplifier as a high impedance buffer to extract our audio signal from the capsules.

I presented this at the 153rd AES in a paper. This circuit has 40dB lower distortion than the typical FET circuit currently used by most microphones making it very transparent with no sonic coloring. We are using two of the circuits, all on one PCB. It uses a common ground at Pin 1 and a five Pin XLR. The first circuit uses Pin 2 and 3 for its connections and the second circuit uses Pins 4 and 5.

The trick to getting the figure 8 side mic is combining the two TSB165’s. These are cardioid capsules. One will face to the left and the other to the right. We wire them in series but out of phase. The left facing one will have its ground connected to the ground of the right facing one. Then the right facing one has circuit ground connected to its center terminal. It is key to keep track of which capsule is the left facing one. Marking the capsule holder makes this easier. 

A 3D printed saddle holds all the capsules in place and a little E6000 glue ensures they are fixed in place. The Mid capsule faces to the front and wires normally. The Mid capsule uses Channel “A” on the dual OPA board. This gives us the standard “phase” for a microphone. An increase in air pressure to the capsule results in an increase in voltage out of the mic. To create the figure-8 pattern for the side we are adding two capsules together. We need to do that in a certain way for this to work. We have two capsules, one facing left, one facing right. To meet the normal standard for M/S we need the Left capsule to cause an increase in voltage when an increase in air pressure reaches it. So we connect the center terminal of the left to Signal “B” of the dual OPA board and then connect the ground of the left capsule to the ground of the right capsule. Then we connect the center terminal of the right capsule to ground. The Side capsules use channel “B” of the dual OPA board. The Saddle has a few holes in it to allow for wire routing between sides.

The output of the OPA board feeds a five pin XLR connector so now we need a 5 pin to dual 3 pin XLR breakout cable to connect to a recorder or mic preamps. 

The board is powered by Phantom power and works with both P48 and P24. 

Capsule and Saddle Preparation

1. Solder two 3” wires to the JLI2555B capsule. It is much easier to do this now than after it is mounted in the saddle. Red to the center terminal and green to either of the side ground tabs.  
2. Use a sharpie or similar and mark an “L” for left, on one side of the capsule saddle.
3. Use a small amount of E6000 glue to glue the two JLI165 capsules in the saddle. Hold the side marked “L” and make sure that the front of the JLI165 faces left as you insert it. The other one will face right while the saddle is in this position.
4. Put a small amount of E6000 on each mounting point for the JLI2555B capsule. Place that on the top of the saddle. Hold it in place with a rubber band as the glue dries. Let this dry for several hours.
5. Solder a 3” blue wire to the Left JLI165 Capsule.
6. Solder a 3” green wire to one of the ground connections of the capsule. 
7. Feed the green wire through the holes in the saddle so that you can connect it to the ground terminal of the Right capsule. 
8. Solder a 3” green wire to the center terminal of the Right JLI165 capsule. 
9. Push the Side capsule wires through the hole that lets them come through the center of the saddle.
10. Slide the four rubber grommets into the capsule mounting slots.
11.  Inspect the frame, specifically the four holes on the top and identify the orientation 

Note: The Left capsule will face the Tab

1. Use two M2.5 8mm long screws with flat washers to mount the saddle to the frame. Snug them up but don't crush the rubber bumper.
2. Feed the Mid capsule wires through the two rubber bumpers that do not have screws in them. 
3. Carefully place the headbasket over the capsules. There are two slot areas to help.
4. Mount the headbasket with two small M2.5 screws

XLR connector preparation

1. Bend the leads of capacitors so you have one like the one on the left and three on the right.

1. Tin the five solder cups on the XLR connector and connect the capacitors as shown. 
2. Slider the first capacitor so that bent lead fold goes to Pin 1 and the long tail connects to the XLR ground tab. 

Note: The goal here is that Pins, 2,3,4, and 5 all have a 22nF capacitor between them and ground. 

1. Connect the other three capacitors between Pins 2,3,4,and 5 to ground. 
2. Tin the five 1.5” wires on both ends
3. Solder a Green 1.5” wire to Pin 1 
4. Solder a Red 1.5” wire to Pin 2
5. Solder a Black 1.5” wire to Pin 3
6. Solder a Blue 1.5” wire to Pin 4
7. Solder a White 1.5” wire to Pin 5   
8. Insert the slightly rounded M2.5 screw into the XLR connector. This will back out to lock the connector to the frame.
9. Insert the XLR connector into the frame wires first. There is an alignment tab on the XLR and a groove on the frame.
10. Align the screw with the small hole and then unscrew the screw to lock it to the frame.

Connect and mount the Dual OPA PCB

Note: It is easier to connect the XLR wires before mounting the PCB

1.  Solder the five XLR wires to the Dual OPA Board. 
2. Pin 1 to Common Ground X1, The center connection on the Board
3. Pin 2 to X2A
4. Pin 3 to X3A
5. Pin 4 to X2A
6. Pin 5 to X2B
7. Mount the PCB to the Frame with two M2.5 screws.
8. Connect the Capsule wires
9. Mid Capsule Signal to SGNL_A
10. Mid Capsule Ground To GND_A
11. Side Capsules Signal (Left Capsule Center) to SGNL_B
12. Mid Capsule Ground (Right Capsule Center) To GND_B

Final Assembly 

1. Slide the body cylinder over the frame. It has a cutout that matches the tab on the frame.
2. Screw the base on.

Mark the Left Side

Use a sticker or piece of tape to mark the left side of the microphone. You will need to know the orientation of the mic when recording. Another thing you can do is use puff paint and put a drop of white on the headbasket area

Congrats! You have built Tabitha, a world class M/S microphone. 

Testing and Use:

Connect Tabitha to a recorder or Audio Interface . You will need a breakout cable. 

You should hear clean audio through both channels. Along with checking your wiring, there is a troubleshooting guide here.

Using Tabitha.

To ensure the Stereo field is maintained throughout you need to record with the Mid to Channel 1 and the Side to Channel 2. You can record either one stereo file or two mono files. Like this:

I am not a fan of using internal MS encoding that many field recorders provide. This limits what you can do in Post. Decoding back into stereo is quite easy without plugins on any DAW. Here is what you do. Bring the files into your DAW, if it is a stereo file, split it into two mono files. Left is Channel 1 and RIght Channel 2. Pan the left center and send it to both left and right. Now take the Side signal and send it to two tracks, or duplicate the side channel track. Pan one hard left. Pan the second hard right and invert the phase.  

I use Reaper and will group the side tracks and link their faders. That gives me one adjustment for evenly mixing in the side signals. Here is one more thing that decoding it this way allows you to do. Depending on the sound source and how far the mic is away from it you may want to adjust the panning of the mid slightly to the right. Or, unlink the side channels and adjust right/left by the level of the side signal. Experiment, you will get some very interesting results. Thanks to some really good capsules and the excellent OPA circuit, Tabitha is a fantastic Mid Side microphone that you can build yourself. Here are two sample recordings. One is Tabitha in front of a Leslie speaker on a Hammond B3 and the second is my backyard at night. There is traffic in the distance and biologics.

Tabitha is a perfect microphone for use in immersive audio. She makes a great stereo spot microphone when recording. And, if you rotate Tabitha so the side capsules face up and down instead of left and right, you can capture height information. Multiple Tabitha's can be arranged in an array or as room mics in multiple locations. I am exploring this further and plan on presenting my results this spring.