ECM8000 microphone measuring techniques and usage discussion - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #31 of 170 Old 01-25-09, 06:35 PM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

Good insight. Thanks for the information.

And welcome to the Shack!
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post #32 of 170 Old 01-25-09, 09:19 PM
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Re: ECM8000 Tests (several pics)

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Okay, so BoomieMCT and I did a battery of tests on our ECM8000 mics.

The setup was my M-Audio MobilePre preamp, test amp, Magnepan MG10.1. ...

The Top trace is comparing his and mine horizontal. The Bottom traces are the vertical ones.

Anthony -

I came across your thread while trying to troubleshoot what I see as a problem with the HF response in my own measurements. I own speakers that use a Scan Speak Revelator 9500 tweeter that is supposed to have flat FR up to 30KHz, yet when I measure them, there is a sharp roll-off above roughly 15KHz. I see a similar problem with the graphs you published.

Here's what I get when I measure one of my speakers at 1M using the ECM and no cal file (vertical mic position).


Here is the response plot of a typical ECM mic as provided by Cross-Spectrum labs.



There is nothing that I can see in the above plot or in the generic cal files that would indicate that the ECM mic rolls off sharply before 20KHz. I also use the M-Audio Mobile Pre, so I wonder if this is contributing to the issue. I would love to get to the bottom of this.

Last edited by hifisponge; 07-19-09 at 05:44 PM.
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post #33 of 170 Old 01-26-09, 08:01 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

Hmm, I had not thought of that. I tell you what, I have another calibrated mic on loan to me right now. I will run a couple of sweeps with that in the mix and see if that rolloff is still there. If so -- an M-Audio problem. If not, still and ECM8000 problem, but at least it's narrowed down.
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post #34 of 170 Old 01-26-09, 08:37 AM
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re: ECM8000 microphone measuring techniques and usage discussion

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If so -- an M-Audio problem.
How could it be an M-Audio problem, if a full loopback soundcard calibration file was created and returned a flat line after a measure?

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post #35 of 170 Old 01-26-09, 08:47 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

The loopback is done on a 1/4" to 1/4" unbalanced connection. Not exactly the same circuit as the balanced mic and phantom power. So if that part of the circuit is causing problems it would be impossible to "cancel" it out with the loopback channel.

There is no balanced out of the MobilePre, so you can't even loopback XLR to XLR on the other channel.
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post #36 of 170 Old 01-26-09, 08:54 AM
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re: ECM8000 microphone measuring techniques and usage discussion

Quote:
The loopback is done on a 1/4" to 1/4" unbalanced connection. Not exactly the same circuit as the balanced mic and phantom power.
Loop unbalanced out, into the positive differential amp (pin 2) of the XLR mic-in using a pad.

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post #37 of 170 Old 01-26-09, 09:01 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

What about the phantom power? IIRC, that was the original concern that having phantom power on that circuit would be be a problem for the unbalanced input. I seem to remember asking M-Audio tech support and they never got back to me (which is odd because they usually got back to me with answers pretty fast).

I have the cables to do that with (XLR to RCA with RCA to 1/4 adapter). If I can verify that it's not putting 48V DC on the unbalanced connection I'll give that a shot.
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post #38 of 170 Old 01-26-09, 10:58 AM
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re: ECM8000 microphone measuring techniques and usage discussion

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What about the phantom power?
Just shut it off with the front panel switch.

You also need to make the little pad to lower the level from line to mic level... I can advise you on that.

When all is said and done, you'll probably find that the mic amp offers a small drop off in the lower end and the upper end will be fine. But, I suppose it never hurts to test it, and of course when you're done you'll have the file to use.

For example, here is my XENXY showing the line-out to line-in (yellow) and then compared against the line-out to mic-in through a pad (green). See how it drops a bit extra at the low end (inconsequential, really, since 10Hz is almost 0dB difference))) and then see no change at the upper end.



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post #39 of 170 Old 01-26-09, 12:11 PM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

I thought you meant to run the loop that way during measurements (in which case the phantom power had to be on).

what kind of divider? I have a test jig for speaker workshop that does a voltage step down while impedance matching. But now that I think about it, it matches the power amp and line out of the Mobile Pre. Hmm, I'll have to think about this more.
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post #40 of 170 Old 01-26-09, 12:33 PM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

I found this:
http://www.uneeda-audio.com/pads/

I think I can make the 100:1 pad at the bottom using a barrier strip and some open ended wires to suit. Then run a calibration sweep and check the results.

I'll still run that second mic just to be sure, but this should only take me about 10 minutes to gin up.
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