ECM8000 microphone measuring techniques and usage discussion

**Terry McGee** · **10-12-09, 05:52 PM**

Hmmm, not so easy to get inside. I whipped the three tiny screws off the base of mine and successfully withdrew the preamp board to the limits of the capsule wiring - only about 2" (50mm). The PCB is covered in shrinkwrap (a good precaution against anything touching the inside of the case). So, presumably unless you're willing to remove the capsule you can't get the preamp out, and even then you have to remove and replace the shrinkwrap to see anything. I think I'll take the other approach and see if there are levels on both pins 2 and 3. I have some work on a preamp coming up - that would be a good time to answer such a question.

Terry

**Terry McGee** · **11-07-09, 09:35 PM**

Good grief - I seem to be right! Unheard of!

A couple of posts back, I conceded that brucek was probably right in assuming a drawing error in the ECM8000 circuit drawing shown previously.

My attempts to look inside were thwarted by the PCB being shrouded in heatshrink. I finally got around to slapping a CRO across the output from my ECM8000, and could only find a signal on pin 2 of the XLR. There was no audio at all on pin 3.

Just to prove the CRO and probes were working OK, I put both probes on pin 2, both traces showed equal signal. Then put them both on pin 3. Neither trace showed a signal. Whichever probe was put on pin 2 showed signal, the other didn't. Not much scope for doubt there.

To be sure that the signal was real, I played flute, and could see the wavelengths change as I changed note. Definitely real.

Both pins show the presence of phantom power, slightly different at 35V and 30V. That implies currents around 2mA in each lead, so there's no question of the wiring being wrong, and 4mA total sounds pretty typical.

Can't see anywhere where I'm going wrong. Always possible I guess that there's something "wrong" with my mic. Perhaps someone else with an ECM8000 and a CRO would like to check their mic?

If it's true, it's not a big deal of course, for reasons I mentioned earlier - we don't need a balanced output voltage, as long as we have a balanced output impedance, so that a balanced mic preamp will phase out any hum pickup on the cable.

So, until someone finds that they are getting signal on both pins 2 and 3, we can probably assume the circuit as originally written is correct! And that the careful duplication of the output stage is just for impedance balancing.

Terry

**Anthony** · **11-09-09, 08:41 AM**

What is a CRO? I could check mine if I knew what kind of tester that was. I have access to o-scopes, LCR's, and a really good Fluke multimeter at work.

**brucek** · **11-09-09, 08:50 AM**

Quote:

What is a CRO?

Cathode Ray oscilloscope.

Quote:

Not much scope for doubt there.

Pardon the pun....

Quote:

And that the careful duplication of the output stage is just for impedance balancing.

And the most important part of any balanced setup. It's responsible for the CMR - (as you know).

brucek

**Terry McGee** · **11-09-09, 03:09 PM**

Hi Anthony

Sorry about the "CRO" - as brucek has mentioned, it stands for Cathode Ray Oscilloscope. Shows you how old I am - in my days in electronics all CRO's were based on "cathode ray tubes" - essentially a small-screen TV tube with electrostatic steering rather than the magnetic steering used on TVs. Indeed in my early days, they were all tube. So I guess I should get with the times and call it a Scope.

Now, keep in mind if you take your mic to work, you'll need to give it some phantom power to operate. And you'll need to set the Scope channels to AC to avoid the traces being pushed off screen by the phantom voltage. Be careful not to short either pin to earth (um, ground!) - the sudden spike can be very hard on the preamp input devices. (They're designed to operate on a signal around 1mV and shorting a pin to earth will give it around 30,000 times that!)

But if you can, it would be great to be able to put the matter beyond doubt!

brucek mentions the CMR, or CMRR as we used to call it. In case that's not a familiar expression to some, it's the Common Mode Rejection Ratio - the ability of the system to reject common mode signals (such as hum picked up equally on both wires) in favour of differential mode signals (such as the audio which is transmitted on only one of the wires, or which is transmitted normally on one wire and in anti-phase, or "upside down", on the other).

So the question we're investigating is which of those two options the ECM8000 uses.

Terry

**WopOnTour** · **11-09-09, 08:16 PM**

Hi 1st post (well besides the 5 pads)

I just purchased one of these mics with a MIC-MATE USB adapter for phantom power
I wish to use it with TrueRTA to help setup some speakers
My question is - should keep the mic close to the USB adapter (with a short XLR patch cable) and run a long USB cable OR useshort USB and run a long XLR to the mic??
Thanks to any help
Regards
WopOnTour

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