To allow for greater movement while dancing and singing, Madonna was one of the earliest adopters of hands-free headset microphones, with the headset fastened over the ear, and the microphone capsule on a boom arm that extends to the mouth. Because of her prominent usage, the microphone design came to be known as the “Madonna microphone”.
It turns out that if you shop for an inexpensive Madonna microphone (example at right), it will invariably have a peculiar plug at the end of its cable. The plug won’t fit into any of the ports on your notebook computer. And it turns out that for no amount of money can you get an adapter that adapts this peculiar plug to work with any port on your notebook computer. This article describes how I managed to overcome this challenge.
Why might one want to use a Madonna microphone? There are three situations where I would find a Madonna microphone to be helpful. A first situation is when presenting a webinar or attending a videoconference. In this situation, by far the worst way to provide my voice to attendees would be to use the “speakerphone” microphones that are built into a notebook computer. The built-in notebook computer microphones pick up ambient noise and are likely to yield a booming or echo-prone or hollow sound for the attendees. The second-best way to provide voice to a webinar or videoconference is a traditional headset with a boom microphone. This approach does provide very consistent and high quality sound for the attendees, but has the drawback of being in prominent view and thus visually distracting for the attendees. My preference is the Madonna microphone which can be unobtrusive, indeed perhaps almost unnoticeable to the attendees.
A second situation where a Madonna microphone can be very helpful is when I am presenting in-person to a large number of people at a location organized by someone else. The organizer will have hired an AV person for the event, and the way it usually works out is that the only microphones provided by the AV person are handheld microphones and lavalier microphones. For my presentation style, handheld microphones are very inconvenient. And, as mentioned above, with a lavalier microphone, if one were to turn one’s head to the left or right, the sound drops out. So my habit is to bring my own Madonna microphone with me to such an event. I use the transmitter belt pack provided by the AV person, but I decline the lavalier microphone. I plug my own Madonna microphone into the transmitter belt pack.
A third situation where a Madonna microphone is very helpful is when I use my own public-address equipment at a location where I get to organize the event. I use Line-6 digital transmitters and receivers, connected to a mixer which in turn connects to powered speakers.
Let’s start by collecting in one place all that we know about inexpensive Madonna microphones. (A set of two microphones, shown at right, can be gotten for $21 on Amazon.) The plug is called a female 4-pin mini XLR plug, also called a TA4F plug (Wikipedia article).
The maker of such an inexpensive Madonna microphone provides almost no documentation, other than to say that the peculiar plug (shown at right) is designed to plug into an RF transmitter belt pack, and perhaps to mention that the microphone capsule contains an electret-condenser microphone. The idea is that somewhere in the same room with the RF transmitter is an RF receiver, which connects to the equipment that makes use of the voice of the human user. In a typical musical performance or public-address application, the RF receiver connects to an XLR microphone input of a mixer, which in turn connects to amplified speakers.
As mentioned above, what I use are Line-6 digital transmitter belt packs (one is shown at right with a Madonna microphone that hooks over both ears) and matching receivers. Common sense tells you that the transmitter belt pack must necessarily have a matching male 4-pin mini XLR jack, also called a TA4M jack. And indeed it does.
If you click around enough on the Internet, you can eventually find out what each of the four pins does:
-
- pin 1 – shield (ground, optionally also bonded to the connector body),
- pin 2 – a DC power supply, often said to be +5 volts, and
- pins 3 and 4 – audio.
Yes, it turns out that it was not really necessary to incur the expense and complexity of a four-pin connector. Decades ago, when Shure established this standard microphone interface, it could have been defined with a three-pin connector. But that is not what happened.
Why exactly is this DC power supply needed? If you click around enough on the Internet, you will find out that every electret-condenser capsule contains an electret condenser microphone (“EC” at right in Figure 1) and also contains a field-effect transistor (“FET”). The capsule connects to the outside world with two wires — a ground wire and a second wire. Any web page discussing such a capsule explains that the second wire connects to two places:
-
- a connection through a capacitor (C in Figure 1) to some device that makes use of the audio signal (shown as “Output” in Figure 1), and
- a connection through a resistor (R in Figure 1) to some source of DC power (shown as “V+” in Figure 1).
The purpose of the capacitor is, of course, to block any of the DC from reaching the output. The purpose of the resistor is to limit the DC current to some safe level, protecting the FET and protecting the external source of DC power. (In the schematic of Figure 1, the capacitor is shown as an electrolytic, but I think nowadays most engineers would prefer a tantalum capacitor.)
We can then match up these connections with the four pins of the mini XLR connector mentioned above. The ground line must go to pin 1, the resistor-coupled line must go to pin 2, and the capacitive-coupled output must go to pins 3 and 4. To try to see this, we can disassemble the mini XLR plug (as shown at right) and we see a shell A, a female 4-pin socket B, a miniscule circuit board C, a plastic sleeve D, a strain relief E, and a plastic shell F.
Close inspection of a first side of the circuit board C shows two conductors from the capsule entering this view at left. There is a white wire and a shield. (This white wire turns out to be the “second wire” mentioned above in connection with Figure 1.) The white wire is soldered to a pad on the near side of the circuit board in this view. The pad has a feed-through and then reaches a resistor marked E01. (An ohmmeter tells us that this is a 10KΩ resistor.) The other end of the resistor is connected at right to pin 2 (top right in the photograph).
The uninsulated ground (shield) wire also enters this view at left and is soldered to a pad on the back side of the circuit board in this view. On the near side, we see a feed-through at center bottom of the circuit board, with a trace extending to the right to connect to pin 1 (bottom right in the photograph). As will later become clear, this connects the shield wire to this pin 1.
Close inspection of the second side of the circuit board C once again shows the white signal wire from the capsule entering this view at left. In this view we see the white wire soldered to a pad on the far side of the circuit board. As was noted above, that pad has a feed-through to this side. We see that this feed-through reaches the left end of a tantalum capacitor marked A106. (A106 means that this is a 10µF capacitor.) The right end of the capacitor is connected to pins 3 and 4 (top right in the photograph).
We can clearly once again see the uninsulated ground (shield) wire which enters this view at left and is soldered to a pad that we now see. It has a feed-through to the other side. On the far side, as previously discussed, the feed-through reached a trace extending to the right to connect to pin 1.
In this way, we see that the actual connections inside the mini XLR plug match Figure 1 (shown a second time at right) in all respects.
Let’s continue by collecting in one place all that we know about the 3.5-mm port on the notebook computer. We know that the port has four contacts, matching the plug shown at right. The contacts are called “tip”, “ring 1”, “ring 2”, and “shell”. (Everybody calls this a “TRRS” plug.) We know that we can plug a headset into this computer port, using the plug shown at right, and the headset will provide stereo sound (a “left” speaker and a “right” speaker) and the headset can have a microphone. Common sense tells us that this explains the four connectors (ground, left, right, and microphone). The natural next question is which contact provides which function? We presume that “shell” must be the ground and shield. We presume that the remaining three functions (left, right, and microphone) are somehow allocated among the remaining three contacts (T, R1 and R2).
It turns out that the maker of the notebook computer carefully avoids ever actually documenting the 3.5-mm port other than to specify the diameter of the opening.
If you click around enough on the Internet, you will find that there are two competing standards for the allocation of these four contacts to these four functions:
| tip | ring 1 | ring 2 | shell | |
| CTIA | left | right | ground | microphone |
| OMTP | left | right | microphone | ground |
As shown in the table, the Cellular Telecommunications Industry Association (CTIA, Wikipedia article) felt that “ground” should be on ring 2, and the Open Mobile Terminal Platform (OMTP, Wikipedia article) felt that “ground” should be on “shell”. I am not making this up! If you have a headset with an OMTP plug, it won’t work right if it is plugged into a device with a CTIA port, and vice versa.
At this point we wonder what kind of microphone capsule is used in a TRRS headset (ignoring whether its 3.5-mm plug is an OMTP plug or a CTIA plug). We figure that barring some big surprise, it is also an electret-condenser capsule with an FET inside, just like the capsule in our Madonna microphone. If so, then we wonder, how does the FET in the TRRS headset get its DC power?
Again keep in mind that the maker of the notebook computer goes to some lengths to avoid answering any of these questions. But if you click around enough, you can find a rare web page that will let slip that the notebook computer probably provides something called “PiP” or “plug-in power” (Wikipedia article).
Plug-in power, it turns out, is exactly the resistor and capacitor shown in Figure 1 above, reproduced a third time at right. I must emphasize that the maker of the notebook computer keeps mum about this. But in the four-terminal 3.5-mm TRRS port of the notebook computer, the “microphone” terminal comes from the FET of the microphone capsule of the connected device. In this port, inside the computer, this “microphone” terminal connects with a resistor R to a source of DC power, and also connects with a capacitor C to audio circuitry inside the computer.
Nowhere is it ever documented what the value of the resistor is in the notebook computer, or the value of the capacitor, or the number of volts of DC potential provided to the resistor.
At this point we have collected in one place all that we know about the 3.5-mm port on the notebook computer, which is not very much information. So now we must resort to testing and experimentation to learn more.
We sacrifice the cable shown at right, cutting the cable, working away its blue braided jacket, and stripping off a white plastic jacket to expose four wires (yellow, green, red and blue). Continuity testing reveals that the yellow wire goes to the tip, the green wire goes to ring 1, the red wire goes to ring 2, and the blue wire goes to shell.
We note, to our horror, that this blue-braided cable is unshielded. So we resolve that in the soldering activity that follows, we will try to minimize the length of the blue-braided cable.
At this point we do not know whether the notebook computer follows the CTIA standard or the OMTP standard. We are able to say with some confidence, however, that the yellow and green wires are for left and right speakers, and thus that we do not care about them at all, so we can cut them short.
We now plug this 3.5-mm TRRS plug into the 3.5-mm TRRS port on the notebook computer. We power up the computer. We put a voltmeter onto the red and blue wires. We discover that the blue wire (shell) has a positive voltage (about 3.5 volts) relative to the red wire (ring 2). From this we conclude that the computer probably follows the CTIA standard. And we realize that we were completely wrong to presume that the shell terminal is used for shield.
The alert reader will have already noticed that the number 3.5 (the voltage provided by the PiP function of the computer) is not as big as the number 5 (the voltage said to be provided by the Shure-style transmitter belt pack). Not only that, but we realize that we cannot assume that the value of R inside the computer matches the 10KΩ that was inside the mini XLR plug. We also realize that we cannot assume that the value of C (the capacitor) inside the computer matches the 10µF value for the capacitor that was inside the mini XLR plug. We hope and trust that these things will not be a problem.
The alert reader realizes that we left a question outstanding — what DC voltage does the Line-6 digital transmitter belt pack provide on its pin 2? A voltmeter tells us that it provides about 5.7 volts.
Having learned as much as we can about the Madonna microphone and its four-pin mini XLR plug, and having learned as much as we can about the 3.5-mm TRRS port of the computer, we pick a plan of action. We will cut off and discard the four-pin mini XLR plug with its circuit board and resistor and capacitor. We will make two soldered connections as shown in the table below. Basically we will solder the white wire from the microphone capsule to the blue TRRS wire, thus connecting it to shell. And we will solder the shield from the microphone capsule to the red TRRS wire, thus connecting it to ring 2.
| TRRS position |
TRRS cable wire color |
TRRS computer function (CTIA) | microphone wire color |
| tip | yellow | left speaker | none (no connection) |
| ring 1 | green | right speaker | none (no connection) |
| ring 2 | red | ground | shield |
| shell | blue | mike | white |
Having made our soldering plans, we select some pieces of heat-shrink tubing (“HST”) and cut them to desired lengths. We pick two very small-diameter lengths of HST and slip them onto the white and shield conductors. We pick a medium-diameter length of HST and slip it onto the microphone cable. We pick a larger-diameter length of HST and slip it onto the blue braided cable. We carry out the two solder joints. We slide the small-diameter HST over the two solder joints and heat it to shrink it. We slide the medium-diameter HST over the two small-diameter HST lengths and heat it to shrink it. Finally, we slide the larger-diameter length of HST so that it laps over the blue braid and laps over the medium-diameter HST, and heat it to shrink it.
We end up with a finished product shown in the photograph. Counterclockwise from top left, we have the 3.5-mm TRRS plug, some blue braid, the heat-shrink tubing (HST), the microphone cable (about 3 feet in length), and the over-the-ear microphone boom, ending in the electret-condenser microphone (ECM) capsule. The capsule has a small yellow foam windscreen.
We test it on the notebook computer, and we are happy to find that it works. Apparently the resistor and capacitor values are okay and the DC voltage is okay.