ECM8000 microphone measuring techniques and usage discussion - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 170 Old 04-03-08, 10:40 AM
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re: ECM8000 microphone measuring techniques and usage discussion

OK... so how much more info are you looking for? Would I be telling you things you already know if I said the SA transistors are forming a differential amplifier that provides the balanced output, and some gain, the BC transistor is an input stage for that amp, to provide impedance matching between the mic element and the diff stage (matched impedances make for the most efficient energy transfer) and also to provide some gain...
The resistors all set the bias for the transistors, but at the same time define the input/output impedances and gain of their particular stages? The 1uF caps allow the signal to pass from stage to stage while keeping the biases separate...
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post #22 of 170 Old 04-03-08, 10:48 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

That's good information. I always like learning more about circuits.

What I was really fishing for, was whether or not there is any sort of filter designed into this to compensate for anomalies in the frequency response. For instance, a notch filter at higher frequencies. It does not look like that is the case.
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post #23 of 170 Old 04-03-08, 02:56 PM
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re: ECM8000 microphone measuring techniques and usage discussion

Well, C6,7,9,10 certainly limit the high frequency response, but I wouldn't call it a notch filter, looks more like a lowpass... where's the cutoff, you ask? I dunno... have to review some textbooks to calculate that... frankly, easier to measure it... or even better, look at the cal file...
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post #24 of 170 Old 04-04-08, 07:43 AM Thread Starter
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re: ECM8000 microphone measuring techniques and usage discussion

That's the whole problem I was addressing, the cal files that are floating around the internet don't work for my mic in the top two octaves.

Also, even on the cal files, it is hard to differentiate between "baffle" (the enclosure and front face of the mic), capsule, and electronics effects in the mic.

At this point, this is more of a learning exercise. I am content to make my own cal files and revise them as I make more measurements. As of now, the top two octaves are based on a semi-nearfield (1') measurement of my Magnepan MG10.1 ribbon tweeter. Worst case, I have a cal file that matches its performance (and if its performance is flat, I have a good cal file )
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post #25 of 170 Old 04-05-08, 02:52 PM
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re: ECM8000 microphone measuring techniques and usage discussion

Quote:
I always like learning more about circuits.

What I was really fishing for, was whether or not there is any sort of filter designed into this to compensate for anomalies in the frequency response
No, there's no audio filter here. The small RC ladders on the outputs are way outside the audio range (those are pico-farad capacitors for HF noise).

The circuit is kinda interesting. It's purpose in the first single-ended stage (BC118) is mainly to provide high impedance buffering for the mic element. The second stage creates the out of phase balanced signal with a matched output impedance to reduce noise for long mic cables.

You can see the mic element and first buffer stage are biased from the simple zener circuit, while the output stage is turned on directly with the phantom voltage.

There's really no gain to speak of (it's all buffering and impedance matching). The first stage is a typical common emitter connection using feedback with no bypassing. The non-bypassed feedback provides for very high stability even if the temperature or beta of the transistor changes. There's a huge voltage/power gain loss in this configuration, but that's fine, since stability is more important here. They've used a high value for input impedance to the first stage so the element isn't loaded. My napkin calculation puts the gain of the first stage at less than 1.

The second stage is interesting. The single ended output transistor feeds an emitter follower configuration to drive the positive XLR 2 output that has been biased on by the phantom voltage. They take advantage of that stages collector lead signal (that is 180 degrees out of phase) to pass through the bottom transistor (that is also biased on by the phantom voltage), and then out the XLR 3 lead. This configuration allows them to use a matched set of pnp transistors to produce a balanced (differential signal). The matched components ensures a consistent output impedance for XLR 2 and XLR 3 to offer pretty good noise reduction.

This circuit makes the total mic itself light years better than simply using a stand alone element.

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post #26 of 170 Old 04-11-08, 11:36 PM
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re: ECM8000 microphone measuring techniques and usage discussion

Quote:
BoomieMCT wrote: View Post
Yeah, position errors aside, the delta between my mike and Anthony's stayed the same every time we did different runs. Also the difference between horizontal and vertical was very repeatable.
To add another data point: I've been experimenting with various real time analyzers running on my laptop. While measuring pink noise with my ECM8000 I can clearly see that the 10kHz+ part of the spectrum shifts up by at least 5dB when I point the mic at the speakers, and goes back down when I orient it vertically. The rest of the spectrum doesn't change when I re-orient the mic, and nothing changes much if I move it around without changing the orientation.
post #27 of 170 Old 07-07-08, 11:44 AM
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ECM8000 distortion

Hey guys,

I don't get over here often enough. This thread and the ECM schematic got linked over at AVS and I was a bit concerned with the schematic. Siegfried Linkwitz has done a lot of experimenting with the Panasonic capsules and has some DIY designs on his web page.

http://www.linkwitzlab.com/sys_test.htm#Mic

In part, he says:

Quote:
When the cartridge is connected as described by Panasonic it produces fairly high distortion at moderate SPL's and is marginally suitable for serious recording and measurement purposes. The microphone itself is extremely linear, but the built in FET amplifier stage is not configured optimally.

Fortunately, is it possible to modify the external connection to the FET. This involves some delicate work of cutting a trace on the tiny pcb in back of the cartridge and soldering thin, flexible wires to the standard two hookup points, and making a different connection to the capsule housing.
I noticed that the ECM has the capsule wired the conventional way. Obviously they aren't going to do surgery on every capsule. I wondered if their circuit might overcome SL's objections so I emailed him a copy of the schematic and asked. I received a prompt reply (the guy is a real gentleman) and here's what he says.

Quote:
Hi Dennis,

This mic will definitely suffer from distortion.

Too bad,

SL
FWIW.......
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post #28 of 170 Old 08-20-08, 12:55 PM
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re: ECM8000 microphone measuring techniques and usage discussion

The ECM8000 is not alone in this phenomenon. In recording orchestras for film sessions we use the omnidirectional Neumann M150 (or if you can find one the original M50). One of it's characteristics is that the high frequency response is very non omnidirectional past 2kHz. Besides it's amazing sound quality this is one of the reasons it's used so much for the Decca tree recording technique. It is not a cheap microphone costing around $6,000 - $7,000. In the Decca configuration there are always 3 of them (That's around $20k for 3 mics). The M150 was never meant to be a measurement microphone as the frequency response chart shows, but it is also not truly omnidirectional.

If the ECM8000 exhibits this type of polar pattern as well then wouldn't it be correct to say the the horizontal orientation would be the only position that would be correct to measure HF response?

I guess the correct question to ask is what is the orientation for the flattest frequency response?
My guess would be on-axis to the intended target. Not really what you want for a measurement microphone but at this cost, beggars can't be choosers!!

Vince
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post #29 of 170 Old 10-08-08, 10:15 PM
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re: ECM8000 microphone measuring techniques and usage discussion

I finally got around to checking out my ECM8000...the results are consistent with the data shown by Anthony in his first post. My mic has almost 5 db variation between horizontal and vertical and even in the best orientation still has several dB of peaking. The test results were entirely repeatable. Sadly, this mic is not suitable for full range speaker testing.

What is the least expensive *calibrated* mic available? Hopefully some resourceful person has found one that mere mortals can afford!
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post #30 of 170 Old 01-25-09, 01:50 PM
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re: ECM8000 microphone measuring techniques and usage discussion

I realize this thread is old, but I think many people may arrive here looking for info about the ecm8000. I can help with the explanation of the schematic posted and something more...
In order of the signal path:
- C1 (3n3) is a low pass filter, 6dB/octave, cutoff or -3dB point at about 16KHz. Changing it for 1n or less improves the high end response without any undesired side effects. If you're going to change this, use a ceramic 330p to 1n capacitor.
- The intended function of the first transistor, BC118 or BC331 (Behringer has used both), is to make the unbalanced signal a balanced one. At the emitter and collector we have the same signal as in the base (input), but at lower impedance and one inverted with respect to the other. The gain is 1 (0dB). But there is a flaw here: to work as intended C6 should be connected to the emitter of this transistor instead of to ground. I have not opened any of my ecm8000s to see if actual circuit has this mistake or is only the schematic that has the mistake, but no doubt it's a mistake. The effect of this flaw is a 6dB loss of level and SNR when connected to balanced preamps. As is in the schematic, this transistor would do nothing except adding some noise.
- The last two transistors are emiter followers for each of the two balanced signals. Their only but very convenient function is to lower the output impedance. As emitter followers, the gain is obviously 1 (0 dB). C6, C7, C9 and C10 make low pass filtering, the cutoff frequency is 200KHz, well above the audio band. R12 and R13 improve the slope of the low pass filter and provide provide stability and protection against highly capacitive or inductive loads at the expense of raising the output impedance a bit. Still the output impedance remains at an excellent low value. So everything right at this stage.
- So the total gain of the circuit is 1 (0 dB), so it does not have nor need any feedback loop. The design is very good except the flaw described above, that most probably is only in the schematic and not in the actual circuit (I may confirm this if I open my mic sometime).
--------------
Some people modify the capsule to use its FET as a drain follower instead of a common drain amplifier to raise the maximum SPL (sound pressure level) handling. This is completely unnecesary for any purpose other than close drum micing or measurements at levels above 115dB SPL (permanent ear damaging levels). If you intend to make this modification, note that you must also add an ultra low noise amplification stage immediately at the capsule output. When using the capsule's FET as a drain follower, we loose all the amplification we had as common drain amplifier, and because the very low level provided by the electret mic, we need to correctly amplify it very close to the capsule to avoid a severe degradation of SNR. Regardless of author's claims, every amplification circuitry I have seen designed by people that does such modification is simply lame and will convert your good balanced mic into a lot noisier, much higher output impedance and unbalanced ouput one, incompatible with pro mic preamplifiers, only with a bit higher SPL handling and that only in some designs. If you really need to improve the maximum SPL handling of this mic and modify the capsule as drain follower, keep using the original balancing buffer, that's pretty good, and simply add a common emiter amplifier with some feedback between the capsule and the balancing buffer. Use a bipolar transitor, not a FET: after the capsule's FET, we have a low enough impedance so we want here the smallest equivalent input voltage better than the smallest equivalent input current, so a bipolar is better here). Some modification is also needed to the power suply scheme: the added amplifier stage and the BC118 should be powered by more voltage than 6V, but can't be powered from 48 since most small signal transistors can't handle that, ideal would be around 24v given that most small signal transistors specify a maxVce of 30v. A simple zener and a capacitor would do it well. If done this way, the result would be a very similar balanced low impedance mic, with 12dB higher maximum SPL handling and most probably (depends on the added stage) only a bit noisier than the stock ecm8000. Even further you may add a switch to bypass the added amplifier stage for really high SPL handling above 130dB, supposing that the electret itself could handle that. Note that you increase the minimum phantom power requirement to 24v, and that the mic pre you connect it to must be able to handle very high levels. The maximum output of the mic this way would be near +20dBu, while even the highest end pro Yamaha preamps clip at -14dBu without padding or -4 with padding, but many other manufacturers allow near 0dBu levels at mic inputs without padding, and some even handle more than +10dBu. Anyway, all this is only is to use this mic for purposes very different for which it has been designed.
-------------
About the frequency response of this mic, what I have found is that there have been very different production series. My mics are all from the very first series, and sure don't match any of the responses I have seen reported by other owners in the net. I have not sent them to a lab for accurate calibration but I have a high end B&K 4007 measurement mic and by comparison I'm sure my ecm8000s don't show shuch an exagerated peak near 10KHz, but something between +1.5..+2.5dB. So what I got was cheap and reasonably good measurement mics, but you can not trust the high frequency part of the actual piece you get without calibration or at least comparison with another mic you can trust. And if you have to calibrate, you can use actually any small condenser or electret. I purchaed the ECM8000 to avoid moving my B&K (I drop it once and repair cost was $1200), and I was satisfied with its performance, but because I'm using a portable computer for measurement, now I find much more convenient one I made based in a subminiature $1 capsule mounted in a thermoretractile tube of only 5mm diameter. When I calibrated against the B&K, I found this $1 capsule was excellent, even better than my ECM8000s, and nicely fits in a small pocket of the portable's suitcase.
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