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I have:

  • a Boya BY-MM1 microphone which has a nominal impedance of 300 ohms
  • a Steinberg UR-12 with a balanced microphone input that has a nominal impedance of 4 kOhm

I connect the microphone to the audio interface (via a 3.5mm cable and a 3.5mm->XLR adapter, if that's important) and I hear no sound. Another microphone, which is instrumental and for which I don't know the exact impedance or model, works perfectly (it is connected via an XLR cable, if that's important.)

I am suspecting that the impedance ratio is too off. Am I right or have I missed something else? If I'm right, what should the ratio be? I've read it should be about 10, but it doesn't seem like the case here.

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  • \$\begingroup\$ When dealing with audio signals, if the source impedance (mic) is lower than the input impedance, which is the case here as 300 ohm < 4 k Ohm. Then there usually is no issue regarding the impedances. I'm saying that there is a 99.9% chance that the issue is something else and not the impedances. \$\endgroup\$ – Bimpelrekkie Oct 14 '20 at 14:22
  • \$\begingroup\$ This is a device usage question and not electronics design question. And this is not about impedances but supplied power. The mic needs supply voltage (sometimes called bias) and the audio interface does not provide bias, it can only provide 48V phantom power for XLR mics. Unless the XLR to 3.5mm adaptor contains phantom power to mic bias voltage converter, the mic does not work. \$\endgroup\$ – Justme Oct 14 '20 at 14:24
  • \$\begingroup\$ Need the datasheet for the mic. If it's connected to a 3.5mm jack it may be a cheapo that expects the bias voltage for an electret capsule. Otherwise, you need to check its connections against that adapter. Is it unbalanced (tip+sleeve). balanced (tip+ring, sleeve grounded) or one of the TRRS connection schemes? \$\endgroup\$ – Brian Drummond Oct 14 '20 at 14:25
  • \$\begingroup\$ You are connecting a microphone with line-out audio levels to a microphone amp. Why? Does the XLR adapter have a matching transformer built-in? \$\endgroup\$ – ocrdu Oct 14 '20 at 14:27
  • \$\begingroup\$ @ocrdu no, it doesn't \$\endgroup\$ – feakuru Oct 14 '20 at 14:28
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The microphone has an electret capsule and needs supply voltage (sometimes called bias voltage) of about 2 to 5 volts to work.

Actual microphone inputs on equipment such as mobile phones, PCs, cameras and audio recorders can provide this bias voltage, and sometimes 3.5mm line inputs can be configured to provide mic bias, but XLR ports on equipment like the audio interface simply do not provide mic bias.

XLR inputs can only usually provide phantom power voltage, which is typically 48V, and it is used for powering condenser mics so it is not compatible with electret mics. The phantom power is not needed by dynamic coil mics.

If the XLR to 3.5mm adapter is an ordinary adapter made for simply connecting a line level audio to XLR input, it will not work with the microphone which needs biasing.

A special XLR to 3.5mm adaptor is needed, which contains circuitry to convert the 48V phantom power down to few volts to be suitable for electret mic biasing. And the phantom power must be turned on for the XLR port.

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