Low volume / distortion with xlr microphone

Hi folks,

I have a Audix OM6 which sounds great when plugged into my mixer with an xlr cable, but when I connect it to the mod duo I get low volume, distortion, and noise. I’m using a xlr to 1/4’’ adapter. I could see this affecting the quality somewhat, but the results are drastically worse than when connected to my mixer.

I have tried playing with the input volumes on the mod but that hasn’t helped.

Any suggestions?

I don’t know if it could help, but using an XLR to stero 1/4" adapter allows you to use the balanced input on the Duo.

wouldn’t you also need an impedance-matching adapter for this situation?

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[Edit – I was wrong. Please see the discussion below: Low volume / distortion with xlr microphone ]

@plutek That’s my interpretation, too.

@dylanious you might try something like the Shure A85F transformer (mentioned here: https://www.shure.com/en-US/support/find-an-answer/connecting-microphones-to-guitar-amps). Note that I can’t vouch for this as I’ve never used one.

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Guys, can you give more explanation about the necessity for an impedence-matching adapter ?

[Edit – I was wrong. Please see the discussion below: Low volume / distortion with xlr microphone ]

Sure. Under fairly loud test conditions (94dB, 1kHz) the microphone @dylanious is using produces an output voltage of a millivolt or two, with an impedance of about 290 Ohms (see “Sensitivity” and “Output Impedance” specifications here: https://audixusa.com/docs_12/specs_pdf/OM6_v3.1_0719.pdf )

Most guitar amps are expecting an input voltage maybe 100 times bigger than that, but with a much higher impedance in the 100s of kiloOhm range. I’m making these number up, but they’re probably in the right ballpark. This is why, when plugged directly into the Duo, the microphone has such low volume.

If you put a transformer with, say, a 1:100 turns ratio in between the mic and the instrument-level input, you can step up the voltage from mic level so that it’s closer to what the Duo is expecting.

It turns out that a 1:100 transformer also raises the impedance of the signal by a factor of 10,000 (that is, 100 * 100). So this would take the 1.5mV, 290 Ohm signal and step it up to a 150mV, 2.9 megaOhm signal. This is just about in the right range for instrument level inputs like the ones on the Duo. You’re still better off using a good mic preamp like the ones on a mixer, but this will work in a pinch.

Does this make sense?

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I thought the Duo could select the input level according to what is plugged in and that Dynamic microphones would work out of the box.

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Hmm. That’s a good point, and something I totally missed. According to https://wiki.moddevices.com/wiki/MOD_Duo , the Duo has a 32dB gain stage for use with microphones.

I’m a little out of my depth here, but that’s never stopped me from wildly speculating before… 32 dB of preamp gain seems like enough to make the signal workable (32dB is about 40x voltage gain). I guess I’ll retract my previous comments and go over to being surprised that @dylanious is having problems. Perhaps the additional 32dB of gain isn’t switched on?

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Hello @dylanious,

before using a gain or compressor plugin inside your pedalboard, try to get enough gain before the analog to digital conversion. The corresponding input LED where you put your microphone in should lit yellow and green when singing into the mod duo.

From the wiki: The LED colors represent the following input levels:

  • Off: below -30dB
  • Green: between -30dB and -12dB
  • Yellow: between -12dB and -3dB
  • Red: above -3dB
  • Blinking strong red: 0dB (CLIPPING!)

Try to adjust your input following the wiki:
Input portion of the mod duo wiki

Feel free to ask anything further here in the forum.
Hope that helps.
Greetings and God bless, Marius

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