dgage

Recommend a good mic preamp for measurements

82 posts in this topic

If you step up to the Motu Ultralite AVB, you might also want to step up to the M30 since I remember reading it being a little better than the M23.  Having both the M30 and M50, I haven't noticed a difference.  Then again, I haven't tried measuring 140 dB either. :)

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I'm going to give the EMM-7101-CHTB a try.  140db max with 3% distortion at 145db. 

 

Sensitivity is 6mv/PA, so even with the +4 dBu limit on the 2i2 I can hit 140db.  However, I have some high quality Shure XLR attenuators, so with -15db of attenuation the 2i2 will be good for 150db+ (of course the 7101 will compress well before that, but at least the preamp won't be the weak link). 

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I like the theory, now it's up to execution. Lol. Let us know how it works out. And use ear plugs and ear muffs. :)

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I like the theory, now it's up to execution. Lol. Let us know how it works out. And use ear plugs and ear muffs. :)

 

Anything in particular that sounds a little hairy/off about what I have planned?  So far I've stuck to basic USB mics (UMIK-1 from CSL and a SPL Lab USB meter), and since you've already used very similar stuff please speak up if something sounds off!  :)

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I'm not familiar with the EMM mic so no thoughts there but I haven't used an attenuator with a mic.  Your logic is sound in theory but I'll be interested to see how well it works.  And I'll be interested in seeing those crazy numbers in a home theater and how loud you'll be down low.  :wacko:

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I'm not familiar with the EMM mic so no thoughts there but I haven't used an attenuator with a mic.  Your logic is sound in theory but I'll be interested to see how well it works.  And I'll be interested in seeing those crazy numbers in a home theater and how loud you'll be down low.  :wacko:

 

The 7101 is an interesting mic:

 

http://www.isemcon.net/ashop/datasheets/iSEMcon-IEPE-CCPmic.pdf

 

Look at page 4.  There's also two versions, the CSTB (30mv/PA) and the CHTB (6mv/PA).  It's DC coupled and apparently nearly flat to DC.  mojave (buying it from him) says it's only down 0.25db at 5hz!

 

Before he sends it over he's going to compare it to a new ACO Pacific mic he's getting that's flat to 3hz to verify the 7101's calibration file. 

 

I'll definitely compare the results with and without the XLR attenuator, as that will add some LF roll-off as will the 2i2. 

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Looks like I already measured the frequency response of the Shure A15AS:

 

http://www.avsforum.com/forum/155-diy-speakers-subs/1281290-minidsp-69.html#post24146003

Well there you go...I was wondering how the attenuator would affect the signal. With that info you could measure up to 130-140 without the attenuator to get an idea of actual response and then go above 140 to get max output above say 15-20 Hz below which you could extrapolate.

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Luke my advice is use the KISS principle.The least amount of components and calibration files the better.

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Luke my advice is use the KISS principle.The least amount of components and calibration files the better.

 

Yeah that's what I've been thinking about the last few days. 

 

In order to avoid the XLR attenuator (has measured LF roll-off), I'm going to spend the extra $100 for the Focusrite 6i6 which has a max mic input of +8.5 dBu vs. +4 dBu of the 2i2.  That would allow enough headroom for ~144db where the EMM-7101 is also staring to distort/compress.

 

I've researched the roll-off of the 2i2 and I think this statement makes sense:

 

http://www.hifizine.com/2012/12/focusrite-scarlett-2i2/

 

"While it’s often recommended that loopback tests used to “calibrate” an interface such as this are performed with the microphone input, I’m not sure that is always correct. If the circuit has an output coupling capacitor, then the lower input impedance of the microphone input may affect the low-frequency rolloff (in a way that it will not be affected if a microphone is used as the source)."

 

 

 

Here's a frequency response strictly on the line/mic in:

 

http://johnr.hifizine.com/2012/09/focusrite-scarlett-2i2-measurements/

 

2i2-Frequency-Response-Line-In.png

 

 

Looks like it's down less than 0.5db at 5hz.  If that's the case there's no need for a calibration file.

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My understanding is the calibration files are usually for the upper frequencies, not lower.

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My understanding is the calibration files are usually for the upper frequencies, not lower.

 

The UMIK definitely has some roll-off on the low end.

 

After comparing the calibration files for the 7101 vs. UMIK, I'm not even going to use the cal file with the 7101:

 

7101%20vs.%20UMIK_zps3bmw4qr3.jpg

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I would repeat that loopback with your device to be sure. I have a 6i6 and it is 2dB down at 5Hz and about 7dB at 2Hz on that test, different device of course so it may behave differently or they may have revised firmware/hardware along the way.

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The UMIK definitely has some roll-off on the low end.

 

After comparing the calibration files for the 7101 vs. UMIK, I'm not even going to use the cal file with the 7101:

 

 

Let me rephrase, when I looked at the calibration files for my Earthworks M30 and M50 mics, there wasn't any correction at the bottom end.  I think I remember reading that was pretty common for most good mics.

 

And I didn't find the UMIK to be very accurate at 90Hz so I'm not surprised that it has roll-off on the low end.

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I would repeat that loopback with your device to be sure. I have a 6i6 and it is 2dB down at 5Hz and about 7dB at 2Hz on that test, different device of course so it may behave differently or they may have revised firmware/hardware along the way.

 

As one of the articles I linked above mentioned, I'm not sure a loopback test from a line output in the 6i6 to its own mic input is the best way to determine the roll-off for just the mic input (my signal source won't be the 6i6 but rather my processors HDMI output).  There's going to be roll-off on the output that's then combined with the input's roll-off, not to mention the impedance differences from the line out vs. an actual microphone which also affects the frequency response.

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Let me rephrase, when I looked at the calibration files for my Earthworks M30 and M50 mics, there wasn't any correction at the bottom end.  I think I remember reading that was pretty common for most good mics.

 

And I didn't find the UMIK to be very accurate at 90Hz so I'm not surprised that it has roll-off on the low end.

 

Ahhh gotcha!

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I've researched the roll-off of the 2i2 and I think this statement makes sense:

 

http://www.hifizine.com/2012/12/focusrite-scarlett-2i2/

 

"While it’s often recommended that loopback tests used to “calibrate” an interface such as this are performed with the microphone input, I’m not sure that is always correct. If the circuit has an output coupling capacitor, then the lower input impedance of the microphone input may affect the low-frequency rolloff (in a way that it will not be affected if a microphone is used as the source)."

 

 

 

Here's a frequency response strictly on the line/mic in:

 

http://johnr.hifizine.com/2012/09/focusrite-scarlett-2i2-measurements/

 

 

 

That quote is absolutely true and often missed. There is often an impedance mismatch. The headphone output is usually better for the loopback though it may still introduce it's own effects.

 

Also Dave is right with the cheaper mics it is often the HF that really needs corrected most. The more expensive and trusted measurement mics are good enough to use without a cal file on the low end and often up to 18-20Khz. My rig is down a bit at 5Hz. I don't bother with a cal file at all. I know it's there but it's not enough to be worth worrying about. A lot of the cal files I've seen are questionable to say the least.

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As one of the articles I linked above mentioned, I'm not sure a loopback test from a line output in the 6i6 to its own mic input is the best way to determine the roll-off for just the mic input (my signal source won't be the 6i6 but rather my processors HDMI output).  There's going to be roll-off on the output that's then combined with the input's roll-off, not to mention the impedance differences from the line out vs. an actual microphone which also affects the frequency response.

Yes I know, there is certainly a difference in the measured result when you do mic input vs line input. However I said "on that test" in my post for a reason :) he uses a loopback from line output to line input and that's what I am describing in my post. So if 2dB at 5Hz vs 0.5dB at 5Hz is important to you then I recommend verifying your own signal chain with your actual equipment.

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Yes I know, there is certainly a difference in the measured result when you do mic input vs line input. However I said "on that test" in my post for a reason :) he uses a loopback from line output to line input and that's what I am describing in my post. So if 2dB at 5Hz vs 0.5dB at 5Hz is important to you then I recommend verifying your own signal chain with your actual equipment.

 

Sorry man!  My reading comprehension is tanking because of a lack of sleep (newborn in the house).  I can't imagine how the wife feels!

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Sorry man!  My reading comprehension is tanking because of a lack of sleep (newborn in the house).  I can't imagine how the wife feels!

 

I find kids are able to sleep better when they aren't trying to sleep next to 140dB in the next room though I understand how you might think the tactile feel might lull them to sleep anyway. :D

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I just pre-ordered the 2nd gen 6i6 from Musicians Friend for $229.99 shipped.  Should ship around 7/1.

 

The specs say the max on the mic input is +8.5dBu but I also noticed it has a 10db pad that can be turned on.  Does that means it's really good for +18.5dBu, or is the +8.5dBu spec is with the pad already on?  I'll email them about it.

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Focusrite confirmed the mic input of +8.5dBu on the 6i6 is without the 10db pad engaged.  They're going to update their site/doco. 

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Focusrite confirmed the mic input of +8.5dBu on the 6i6 is without the 10db pad engaged.  They're going to update their site/doco. 

 

I kinda figured that would be the case.

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I'm getting some pretty funky loopback results from the 6i6.

 

First, a loopback from line out to line in with a TRS to TRS cable:

 

cap1_zpssnqecw0a.png

 

 

That looks fine to me.  A little more LF roll-off than I'd like but nothing glaringly wrong. 

 

Second, another loopback from line out to line in, but this time using a "combo" jack input on the front that accepts both TRS for line level inputs and XLR for mic inputs.  Still using the same TRS to TRS cable:

 

cap2_zpsklozpyeo.png

 

 

 

Uhhh....what the heck is that? 

 

Third, line out to mic in using a TRS to XLR cable:

 

cap3_zpsq8xi9tfs.png

 

Whoa....

 

 

All 3:

 

cap4_zpsudci29bt.png

 

 

 

I'm using ASIO in REW and a 48khz sampling.  The higher the sampling rate the worse it gets.

 

Any ideas?

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your line in-out loopback looks like a mic input

 

here's my 6i6

 

cyan is line out to mic in (front combo jack)

red is line out to line in (rear TRS)

purple is line out to inst in (front combo jack)

 

post-1440-0-84141600-1469223604_thumb.png

 

the other measurements look like some sort of feedback. Can you show a pic of mixcontrol?

 

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