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I am rewiring a dynamic microphone that does not require any phantom power (also wicked simple no off / on), and I noticed, when I only wire the two wires to two of the three pins it still works and seems to sound great.

When I have taken apart other microphones and looked at how they are wired, they all seem to have this same configuration of having one wire go to what I think may be the ground pin and then the third XLR pin.

There is probably some logical reason but I don't know why.

Is it not safe / proper to wire only the two even if it works? What is the ground for and what is the purpose it is wired like this?

I had a friend mention something like if you use phantom power on a dynamic microphone it can mess it up, but I leave my phantom power on all the time and it doesn't do that. Is that why it is wired like this? (I attached the pictures to show.)

I have very little knowledge about electronics / I mostly have just hacked my way through in the past / mess with it until it works, and I don't really know what the ground is for / what the lead means and all the proper terminology. I want to make sure I am doing it properly to the standards of the industry.

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JRE
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Jeff
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    It'll work with 2 pins as you found but not in all scenarios. "Pin 1 is GND" so you have wired pins 1 and 3; this will work into an unbalanced input where pin 3 is the signal. Into a balanced input (pins 2 and 3) it'll sound faint and HF heavy, "one legged" as they say, with only the cable capacitance connecting to pin 2. Shorting pins 1 and 2 will "fix" that. It may also pick up hum; connecting signal to only pins 2 and 3 (into a balanced input) and screen to pin 1 will fix that; the fourth connection to case allows a safety ground to help avoid killing musicians... –  Jul 24 '21 at 12:47
  • @user_1818839 Thank you for the clarification! Now that I look closer at the XLR connector, I see there are labels 1, 2, and 3 on the pins, it looks like I am wired to what they are calling "1" and "2" and "3" is not wired... so what I should be doing, is connect the wires to 2 and 3.... and screen to pin 1 (I will need to google what screen to a pin means) and then that fourth thing that is like a rectangle with a circle and doesn't have a label I should connect to the aluminum / metal case. thank you! – Jeff Jul 24 '21 at 17:08

1 Answers1

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This may be most easily understood by looking at an old-fashioned transformer balanced configuration.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) An unbalanced microphone feeding a 600 Ω (Lo-Z) input. (b) A balanced version of the microphone feeding a high impedance unbalanced input using a transformer. (c) A phantom-powered microphone circuit using balanced / unbalanced transformers.

How they work:

  • Figure 1a shows a simple unbalanced configuration. One of the mic capsule's wires is connected to shield. The circuit is unbalanced and may be prone to hum pickup.

  • Figure 1b is balanced and will be much less susceptible to hum as the hum will be "common mode", appear on both inputs to the transformer and get cancelled out. This is likely the situation you are describing and it should be clear that disconnecting the screen doesn't affect the microphone but will likely result in more noise as the microphone case will not be grounded.

  • Figure 1c shows a phantom power configuration. The 48 V supply is fed in the centre-tap of XFMR2. Current splits both ways in the transformer with the result that the DC current is running opposite directions in each half of the winding so the flux in the core cancels out and the transformer doesn't saturate.

    • At the microphone end the reverse happens and the 48 V is collected, smoothed and used to power the microphone preamp.
    • Return current is through the screen or shield wire (pin 1 on an XLR).

I had a friend mention something like if you use phantom power on a dynamic mic it can mess it up, but I leave my phantom power on all the time and it doesn't do that is that why it is wired like this?

schematic

simulate this circuit

Figure 2. Connecting a balanced mic to a phantom-powered line.

As you can see from Figure 2, connecting your mic to a phantom-powered line will apply the +48 V to both terminals of the mic coil. Since there is no voltage difference between the two terminals no current will flow, there will be no degradation of performance and no damage will occur. This is not an accident; microphone phantom power was designed that way!

I've used transformers in all the examples above as it's very easy to see what's going on. Modern systems will use electronics in place of the transformers for reasons of cost and quality.

Transistor
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  • Cripes. I just learned some interesting stuff! (I've not looked into phantom power before and knew almost nothing about it.) Thanks! +1 – jonk Jul 24 '21 at 10:02
  • Thanks, @Jonk. I see you're on the night shift again. I hope all is well in your home. – Transistor Jul 24 '21 at 18:09
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    Our daughter, who is profoundly autistic, is showing long strides in increasing her understanding and skillsets. Things she has never done before, despite how hard we worked with her to teach, are now happening without our efforts. She's just learning on her own and advancing, almost by the day. It's almost like the story, 'flower for Algernon'. And I really hope that only half that story plays out here. (But I was awake, last night, because of pushing long hours into a product.) – jonk Jul 24 '21 at 20:59