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Will this circuit work to connect an lapel mic mic (expecting ~2.5v plugin/bias power) to an XLR input providing phantom power (I have a choice of 12 to 48v).

I assume the lav mic has a typical electret capsule / JFET type arrangement. It's got a TRS plug, though T&R seem to be shorted.

EDIT: the mic claims to have an impedance of 2.2KΩ Max.

Question

EDIT: the circuit works, using 12v phantom, with a 100nF cap and a 10k resistor.

What disadvantages does it have compared to a more complex design such as those at http://www.epanorama.net/circuits/microphone_powering.html, and are there any simple alterations to improve it? Any rough suggestions for component values?

suggested schematic

I'm expecting the resistor to limit the current/voltage to the mic, and attenuate any audio signal going to the ground. The capacitor would prevent the phantom power on cold being shorted to ground, whilst allowing audio to pass.

Background

I'm trying to connect a cheap ($1 ebay special) electret mic to an XLR mic input, and power it with phantom power. I'm using a Zoom H5, so have a choice of 12-48v phantom. (I don't want to use a battery)

The mic works fine with the Zoom's plugin-power input (2.5v), giving a surprisingly good signal with very little noise, but I want to connect several to the H5, so need to use the XLR inputs (also want to use an H5 capsule, which the plugin-power socket disables). I'm aware I can buy XLR->Plugin Power adaptors, but they're pricy, as are lapel mics designed to plug into phantom. Also, I kinda want to see if there's a simple solution...!

Updated Circuit

In my final circuit, I've adjusted the resistor choices to account for the voltage drop on the phantom supply when under load. These values give about 3.5v on the mic, which gives a stronger signal than 2.5v, improving SNR, whilst keeping the DC resistance high enough that the supply doesn't drop too much.

50nF ceramic caps seemed to work best from the selection I had.

The circuit is still small enough that both the circuit and a 3.5mm socket will fit into the tail of an XLR plug.

The sound quality is great; it actually seems to out-perform the Zoom's built in plugin-power input, probably because the higher voltage increases the SNR.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Please ask specific questions relating to (hopefully) a design \$\endgroup\$ – laptop2d Sep 14 '16 at 13:56
  • \$\begingroup\$ @laptop2d - thanks - is that better worded now? \$\endgroup\$ – Dan W Sep 14 '16 at 14:54
  • \$\begingroup\$ Much better, by the way they have a circuit diagram tool when you ask questions. \$\endgroup\$ – laptop2d Sep 14 '16 at 15:00
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    \$\begingroup\$ I'm doing something very similar. Am I completely misunderstanding this thread? In your updated circuit the resistor values chosen will only give an impedance of 7.2ohms but you say your mic claims to have an impedance of 2.2kohms? The voltage given from the divider seems about right given voltage drop under load but I don't understand the impedance matching. Any help appreciated. Cheers, Marcus PS - sorry it's not a comment, apparently I need "50 reputation" to comment \$\endgroup\$ – Marcus Lee Jul 20 '17 at 9:32
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    \$\begingroup\$ I share @MarcusLee's confusion. Are your resistor values supposed to be in kΩ and not Ω? 18kΩ and 12kΩ would make more sense to me. \$\endgroup\$ – Dave Turner Oct 8 '17 at 14:22
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You have roughly the right idea, but you should use a resistor divider instead of a single resistor to bring down the voltage. With your setup, the microphone is still subjected to high voltage, depending on how much current it draws.

You say you want 2.5 V, but not at what impedance the mic wants to see. I'll use a target of 3 kΩ, which most electrets will be happy with. You need to check the datasheet of your specific mic.

The two relevant formulas are what voltage is applied to the mic and the impedance presented to the mic:

  Vmic = (R2 / (R1 + R2)) PWR = 2.5 V

  Vmic impedance = R1 // R2 = 3 kΩ

Now you have two unknowns and two independent equations. Solve for R1 and R2.

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  • \$\begingroup\$ Thanks! That's very helpful. I'd not considered impedance at all. The mic claims an impedance of 2.2kΩ. 2.5v seems to be a typical voltage, though a bit higher seems to produce a stronger signal. \$\endgroup\$ – Dan W Sep 14 '16 at 19:37
  • \$\begingroup\$ OK... the voltage divider equation I'm very familiar with, but I've never had to calculate impedance before. What's the '//' symbol in the impedance calculation? Is it R1.R2 / (R1 + R2)? \$\endgroup\$ – Dan W Sep 14 '16 at 20:03
  • \$\begingroup\$ @DanW -- yeah, it's "parallel connection" \$\endgroup\$ – ThreePhaseEel Sep 15 '16 at 0:31
  • \$\begingroup\$ @Dan: "R1 // R2" means the resistance of R1 in parallel with R2. \$\endgroup\$ – Olin Lathrop Sep 15 '16 at 10:33
  • \$\begingroup\$ Thanks for your help! I've now built and tested the circuit, and it works brilliantly, so I'm accepting the answer. \$\endgroup\$ – Dan W Sep 18 '16 at 15:08

protected by Community Aug 24 '18 at 10:33

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