The EXT MIC input on the Zoom H4N is specified as:

mini stereo phone jack
Input impedance = 2 kΩ 
Input level =  −7 dBm _ −47 dBm 
(0 dBm = 0.755 Vrms) so that's 0.346 Vrms
--> equals −7.23 dBu to −47.2 dBu; see discussion

The REC OUT of the Yamaha MG82CX mixing board is specified as:

dual RCA jacks
Output Impedance = 600 Ω 
Appropriate Impedance = 10 kΩ 
Nominal Level = Lines –10 dBV (0.316 V) 
Max before clipping = +10 dBV (3.16 V)
(where 0dBV = 1 Vrms)

Now −10 dBV is −7.78 dBu. I see the alternate labeling in some places in the documentation, too.

The dBm = 0.755 Vrms might be a typo: “dBm unloaded” commonly noted as dBu and used for this purpose is 0.775 Vrms. But if taken as written, the Input Level would be 0.337 Vrms to 0.00337 Vrms, which is −7.23 dBu to −47.2 dBu.

The nominal level means the highest level that's only occasionally exceeded. So I want to match the nominal levels. 0.316 is lower than either 0.346 or 0.337, so it ought to work.

In fact, before the performance my auto-level decided that “60” was a suitable value. But when someone started talking into the microphone, it pegged the scale and the auto-level turned it all the way down with no relief. The signal was too hot.

Why did it not work?

Related might be my understanding of the specifications.

On the input: does the Input list the available nominal levels I can achieve through using the Level control? Or is it the clipping point at the lowest setting to the quietest discernible sound at the highest setting? Or something else?

On the output: There is no volume control for the REC OUT, as it feeds directly from the Stereo Bus. If the nominal level is the loudest it normally handles with occasional excursions, the clipping level would be only slightly higher voltage. So why is it actually a 10× difference?

The input impedance is lower than the mixer wants, but that would draw more power and drop the voltage if it couldn't handle the current.

  • It is not considered good practice to connect a line output to a mic input. – audionuma Feb 26 '15 at 21:28
  • Sorry, commentary put online too fast. What I meant is that beside electrical level, there are impedance issues, as you have yourself mentioned. You might use something like epanorama.net/circuits/line_to_mic.html . You can find this type of adapter already made and enclosed in a single shell. – audionuma Feb 27 '15 at 6:25
  • 600 ohms on the transmission side and 2000 on the receiver, vs 10000 on the receiver, would draw more current but not change the voltage. Supposedly nothing about transmission line theory applies at audio freq and short runs. If the output can't handle the current the voltage will drop. Does something else happen if the two ends have impenances that are too close together? – JDługosz Feb 27 '15 at 7:31
  • Well I can't unfortunately provide electrical insights as I have no credit in this field but I can confirm to you, being a production sound mixer for around 20 years, that connecting a line output to a mic input doesn't provide good results if you don't use a level and impedance adapter. – audionuma Feb 27 '15 at 10:32
  • Interesting. I see attinuators for sale, which is labeled to drop the level by so many dB. There is no labeling concerning impedance, or which precice purpose it's meant for. If it works like the link you mentioned, that is a simple voltage divider, and 3 terminal "pots", what a knob goes on, is used exactly that way to make a volume control. In your experience, are some attenuators usable for somemthings but not others, though the dB pad is right? – JDługosz Feb 27 '15 at 18:38

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