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Recommend a good mic preamp for measurements


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#21 Ricci

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Posted 19 February 2016 - 03:09 PM

What are you trying to find out exactly? The maximum headroom of the recording chain?



#22 shadyJ

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Posted 20 February 2016 - 03:30 AM

Yes, basically. Trying to see what a $30 sound interface can and can't do.



#23 lukeamdman

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Posted 14 June 2016 - 11:51 PM

Dave, what mic preamp did you end up getting?

 

I'm looking at the M23 and need to find a decent mic preamp as well.



#24 dgage

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Posted 15 June 2016 - 01:35 AM

I usually use a Focusrite 2i2 and I really like it.  For $150 or less, I don't think it can be beat.  I also have a $1500 retail Motu 1248 for when I really need to make the measurements count but I don't think I've noticed any difference between the two but haven't really gone looking either.  If you want to step up from the Focusrite 2i2, I'd recommend the Motu Ultralite AVB since Motu has good drivers and have really been responsive and focused on constant improvement of their drivers.  But the Focusrite have been solid too.


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#25 lukeamdman

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Posted 15 June 2016 - 02:05 AM

I usually use a Focusrite 2i2 and I really like it.  For $150 or less, I don't think it can be beat.  I also have a $1500 retail Motu 1248 for when I really need to make the measurements count but I don't think I've noticed any difference between the two but haven't really gone looking either.  If you want to step up from the Focusrite 2i2, I'd recommend the Motu Ultralite AVB since Motu has good drivers and have really been responsive and focused on constant improvement of their drivers.  But the Focusrite have been solid too.

 

The 2i2 looks really nice for the price!  Looking at the chart mojave posted, since the 2i2 is good for +4 dBu it will clip at 126db? 

 

I want to be able to measure up to ~140db, so I might have to get the Motu. 



#26 dgage

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Posted 15 June 2016 - 02:49 AM

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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#27 lukeamdman

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Posted 15 June 2016 - 03:41 AM

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). 



#28 dgage

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Posted 15 June 2016 - 04:12 AM

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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#29 lukeamdman

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Posted 15 June 2016 - 04:36 AM

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!  :)



#30 dgage

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Posted 15 June 2016 - 06:04 AM

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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#31 lukeamdman

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Posted 15 June 2016 - 02:57 PM

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.n...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. 



#32 lukeamdman

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Posted 15 June 2016 - 03:10 PM

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

 

http://www.avsforum....ml#post24146003



#33 dgage

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Posted 15 June 2016 - 04:19 PM

Looks like I already measured the frequency response of the Shure A15AS:
 
http://www.avsforum....ml#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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#34 Ricci

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Posted 16 June 2016 - 11:03 AM

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

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Posted 16 June 2016 - 03:09 PM

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....e-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.hifizin...2-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.



#36 dgage

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Posted 16 June 2016 - 06:40 PM

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

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#37 lukeamdman

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Posted 16 June 2016 - 06:51 PM

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



#38 3ll3d00d

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Posted 16 June 2016 - 07:08 PM

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.



#39 dgage

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Posted 16 June 2016 - 07:37 PM

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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#40 lukeamdman

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Posted 16 June 2016 - 07:39 PM

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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