ECM8000 microphone measuring techniques and usage discussion

[brucek]

I think I can make the 100:1 pad

Here's a very suitable -30dB attenuator that will lower line to mic level from unbalanced to a balanced XLR mic.

I drew it for you in Visio below. Very simple.

Hook it up and create a soundcard cal file and save it.

Then measure to be sure of your flat line.

Remove the loopback and use that channel for measuring.





When you're done, tell us how much different the soundcard file is from the previous line-out to line-in file.

brucek

[Anthony]

Alright, I'll give that a try tonight and post the calibration results (1/4 to 1/4 and 1/4 attenuated to mic). thanks.

[brucek]

I'll give that a try tonight and post the calibration results

Great, whenever you get the time, I think it will be a benefit to others who will ask if they should be concerned about the mic preamp portion of the M-Audio Mobile Pre. I suspect that the mic preamp will not be significant in relation to the standard line-in to line-out calibration that most do.

I've proven that the XENYX802 preamp is not worthwhile including in the soundcard cal routine, and this will add another to the list (or not). Now we'll know for sure.

Many users aren't really capable of this fabrication (as you are), so this will be a help... If possible, it would be good to see the graph in the same scale as mine above, with vertical = +/-30dB and horizontal of 2Hz-25KHz.

brucek

[hifisponge]

Anthony wrote:

Alright, I'll give that a try tonight and post the calibration results (1/4 to 1/4 and 1/4 attenuated to mic). thanks.

Thanks for taking this project on Anthony. I certainly don't have the experience needed to construct the attenuation circuit to test the mic input.

All the best,

- Tim

[Anthony]

Okay, some results:
Dashed is the Line to Line (1/4" to 1/4" unbalanced), Red is Line to Mic with an attenuator circuit to match the levels.

The only difference in the two is the scale. I was expecting differences in the high end and found none, but the low end was pretty interesting. Not a huge difference, but measurable for 20Hz up a bit.

[hifisponge]

Anthony -

So I guess that rules out the M-Audio sound card / preamp. Must be user-error on my part somewhere.

What do you make of the HF roll-off in the graphs you displayed in your first post? What might be the cause?

- Tim

[Anthony]

Nope, see the new thread I've started. I have more results that just add to the confusion. I split this thread into a new one to discuss just the MobilePre and the ECM8000 so we can figure out what's going on.

[brucek]

Okay, some results:

Nice work Anthony. So, the results are as expected. The low end is a bit off and the upper end is much the same. Even down to 10Hz, it wouldn't matter too much which file you use, so I think we're safe in advising members that a line-out to line-in for external soundcards is suffice for the soundcard calibrate loopback test.

brucek

[Anthony]

Okay, more weirdness. Below is a series of measurements I made with the new soundcard cal file (which seemed to only affect the low end anyways).

Speaker is a Magnepan MG10.1. Measurements were at the listening position. Red is the current ECM8k cal file, Blue is the old cal file, and Green is the Rat Shack meter used as a mic (with appropriate cal file and C weighting)


So with 2 different mics, I get the same fast rolloff in the top octave. It looks like the new mic file fixes the baffle "bump".

To see if it's a vertical/horizontal thing, I ran a semi-nearfield measurement of the speaker both horizontal and vertical using the new "split" ECM8k cal file.


Purple is vertical, yellow is horizontal. Both have the rolloff at 15kHz.

So I believe we've ruled out the mic, and via nearfield measurements ruled out the speaker. It's not the calibration or loop, but it seems to be the soundcard.

Any thoughts folks?

[brucek]

Any thoughts folks?

Well, it's as expected. The old ecm8000 cal file did not account for baffle step, and the new one does. The plots of the cal files themselves easily reveals why the two ecm graphs of the speaker are different. I do have more confidence in the newer ecm file (as far as generic files go).

The RS meter reveals what we already know about the RS meter, in that somewhere between 3KHz and 10KHz they all demonstrate a peak. Simply not usable above 3KHz.

brucek