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With SPL using DVM do I measure the input 2 or the mic (input 1)?
The mic input is used for SPL measurements. The gain for input 2 is set arbitrarily, so while that input serves a critical purpose for timing measurement, it does not for amplitude measurement.

Maybe quicker if you could post a photo of the setup (if you can of course)?

Thanks again
I wish it was quicker. Posting photos is never "quicker" with forum software.
 

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Hi Wayne thanks I wondered about the lower frequencies.
Please can you clarify for XLR mics, I have a Steinberg CI-1 pre-soundcard
• For loopback Input 2 to output 2 TRS > TRS or XLR > TRS
• For SPL calibration Input 2 to output 2 TRS > TRS
• For mic cable should be XLR > XLR
timing loopoback: TRS - TRS
SPL calibration: TRS - TRS
mic: XLR - XLR

For the loopback should the input level (2 or B can't remember) be the same as the mic input or does it not matter?
It does not matter.

With SPL using DVM do I measure the input 2 or the mic (input 1)?
The mic input is used for SPL measurements. The gain for input 2 is set arbitrarily, so while that input serves a critical purpose for timing measurement, it does not for amplitude measurement.
Just wanted to stress that the TRS-TRS connection for loopback timing is crucial for the Focusrite 2i2, which routes phantom power to both XLR inputs simultaneously. AFAIK, The 2i2 will suffer damage if an XLR-TRS connection is used for that purpose, because phantom power will be applied to the output. I learned this the hard way, but it's no longer an issue now that I'm aware.
 

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I didn't realize that the calibration process I suggested could damage the audio interface if the phantom power was not disabled. I think that thread should be deleted as it is very easy to make that mistake. I am surprised I have not made that mistake already.

Will the Staff please remove it? Link
 

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Just wanted to stress that the TRS-TRS connection for loopback timing is crucial for the Focusrite 2i2, which routes phantom power to both XLR inputs simultaneously. AFAIK, The 2i2 will suffer damage if an XLR-TRS connection is used for that purpose, because phantom power will be applied to the output. I learned this the hard way, but it's no longer an issue now that I'm aware.
I didn't realize that the calibration process I suggested could damage the audio interface if the phantom power was not disabled. I think that thread should be deleted as it is very easy to make that mistake. I am surprised I have not made that mistake already.

Will the Staff please remove it? Link
Maybe it's just me, but that sounds like your taking responsibility. I believe your suggestion is still valid. The user just needs to use the correct non-XLR cable for the loopback connection. In that case, it shouldn't matter whether or not phantom power is disabled. Just to verify I understand correctly: the loopback connection on one channel is needed for a timing reference during measurements using the microphone on a different channel, correct? So that means phantom power MUST be on, and an XLR cable should NOT be used for the timing reference (at least in the case of the 2i2), correct?

@OP
You may want to avoid the 2i2 if this issue makes you nervous. There are more details here in my thread here
 

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If my 2i2 can be damaged if phantom power on while the loopback cable is connected that is not good. I presume the situation is similar for many other audio interface boxes. While I am willing to take this risk myself, I am not ready to suggest that someone else to use this method. A simple mistake in the process can be too costly.
 

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Just wanted to stress that the TRS-TRS connection for loopback timing is crucial for the Focusrite 2i2, which routes phantom power to both XLR inputs simultaneously. AFAIK, The 2i2 will suffer damage if an XLR-TRS connection is used for that purpose, because phantom power will be applied to the output. I learned this the hard way, but it's no longer an issue now that I'm aware.
The phantom power feed circuit looks like this:

phantom.jpg

or this

phamtom2.GIF

The circuit is balanced, DC-coupled with a 6800 ohm source resistance. Any audio interface output circuit is going to have series coupling capacitors so I suppose it becomes a matter of the breakdown voltage of those caps vs the actual voltage applied across them with those series resistors. Seems like risk of damage is minimal. There will be a thump when Phantom Power is turned on/off, but that alone is not damaging to circuit components.

Edit:

The second diagram above shows the mic preamp input with phantom power feeding the mic itself. Replace the mic in the diagram with the output stage of an audio interface, AC-coupled (probably) and designed to drive high output levels (always). If each of the two balanced output drive devices can handle the 7 mA current (if not AC-coupled), then there should be no problem.

I am not saying it is impossible to fry an audio interface that way. IF you say you did, then you probably did. I would say it sounds like the output stage was under-engineered if you did.
 

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Hi please can someone tell me which is the better choice?
Thanks in advance
If you're still watching it's probably in amusement. I'm sorry for the sidebar that's not really contributing to your decision except to steer you toward the Roland.
 

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I am not saying it is impossible to fry an audio interface that way. IF you say you did, then you probably did. I would say it sounds like the output stage was under-engineered if you did.
Your engineering analysis is a relief in the face of what few circuit skills I have. :T

I honestly cannot remember the exact conditions of the mishap, but I am certain it happened while trying to take a loopback measurement. Give a newbie a few instructions and look what happens! A couple years later, I'm ready for some more punishment in time-domain land; so I'll report back if I manage to destroy another one... this time paying closer attention. :R
 

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I didn't realize that the calibration process I suggested could damage the audio interface if the phantom power was not disabled. I think that thread should be deleted as it is very easy to make that mistake. I am surprised I have not made that mistake already.

Will the Staff please remove it? Link
I removed it.
 
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