# Phantom +48v on loopback connection



## regiregi22 (Mar 13, 2014)

Hello,
I'm just learning how to use REW, pretty nice piece of software it is! There is just one thing; while doing the soundcard calibration with the loopback connection, I've noticed that frecuency response varies a lot depending if the phantom power +48v on the mic input is ON or OFF (good linear response is achieved by turning +48v off).

I know I don't have to use +48v when doing the loopback measurement, but if I am using a 2nd input/output for REW to track timming, it has to have +48v turned on, in order for the 1st channel to power the microphone. The problem is that my soundcard (Tascam US-322) doesn't allow me to turn ON +48v only on one channel. It's ON or OFF in both channels 1 and 2.

Do you have that weird response when using +48v on any input? There are not that many soundcards that allow you to selectively turn on +48v only on one mic input. Maybe I could just apply a calibration to the timming channel and call it a day, but something is telling that there is something very wrong with those +48v on the input. There's what looks like a HP filter from 100Hz at -3dB , and some visible oscillation that hits harder the higher the frecuency is.

I have read the forums rules about image size and scaling, but those two graphs being from a soundcard didn't look anywhere right on the standard scale (I haven't even calibrated SPL volume yet). My apologises for that if that breaks the rules. Red graph is without phantom power +48v, green one is with it.


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## EarlK (Jan 1, 2010)

You don't want to send 48 volts DC ( *:rolleyesno:* ) into the output electronics of a soundcard ( that'll stress the output components / possibly cooking them :spend: ).

Create your soundcard calibration ( using the microphone pre-amp ) with the 48 Volt Phantom "off" .

Connect/make your timing ( loopback channel ) using the *RCA out to the 1/4" ( line input )* .

Turn "phantom power" back on only when you are ready to plug a mic into the soundcard & take a measurement ( btw, that phantom power won't be present on the RCA to 1/4" loop-back cable ) .

:sn:


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## jtalden (Mar 12, 2009)

Welcome to HTS! 

EarlK beat me again, but I will post this anyway. :sad2: 

Your doing fine.  The scaling is fine for SC calibration where we are looking for small SPL values.

The 48V phantom power is just applied to the XLR portion of the input plug, not to the TRS portion of the input plug. The TRS portion of the plug is intended for mic connections and thus also has more gain.

The typical SC cal is to use a TS or TRS loopback cable for SC calibration with phantom power off (although it shouldn't matter). You can use an XLR cable, but then it is important to have phantom power off.

There is a bit of trade off in which to use. It is a more accurate and very low frequencies, but a bit obsessive, to use the XLR loopback. It is not really needed for adequate accuracy and since it is much more sensitive to gain settings it can result in a little more noise in the calibration. You can do it either way. 

After calibration is established on the mic channel then we are ready to measure by removing the loopback cable any using an XLR cable for the mic (phantom power then turned on to power the mic).

When measuring and using a loopback cable on the other channel for "loopback timing" either a TS or TRS cable should be used. The phantom power is not applied to that plug. There is no calibration of that channel as it is only used for timing reference of the signal. Calibration on that channel makes no difference to the timing accuracy.

The above is the normal case. Now to your results. I am surprised the green trace falls off to much more than the red trace. Is the green trace with XLR loopback cable and phantom power on? If so, I didn't know it would make that much difference with phantom power on, I never actually tried it. If that is the cause don't' worry about it and just follow the guidelines above. If not then there may be an issue that needs attention.


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## regiregi22 (Mar 13, 2014)

Thank you guys for your quick answers!




jtalden said:


> The 48V phantom power is just applied to the XLR portion of the input plug, not to the TRS portion of the input plug. The *TRS* portion of the plug is intended for mic connections and thus also has more gain.


I'd bet you were meant to say XLR.




jtalden said:


> The typical SC cal is to use a TS or TRS loopback cable for SC calibration with phantom power off (*although it shouldn't matter*). You can use an XLR cable, but then it is important to have phantom power off.


well, it somehow matters. I have tried soldering a pair of capacitors inline with + and - on the XLR (10uF, then bipolar 220uF), and the result is pretty much the same, no improvements. So I think that maybe those +48v are not to blame, there may be a mechanism to trigger that HP filter when phantom is on.





EarlK said:


> Connect/make your timing ( loopback channel ) using the *RCA out to the 1/4" ( line input )*


The difference between the selectable input XLR/jack is that the jack is line level (XLR is mic level otoh), has less gain and has a higher impedance. I have measured FR and it's even a bit broader in the jack input. Does any of these parameters affect the timming loop performance? What is sending that channel? is that just some kind of heartbeat, so amplitude and frecuency doesn't affect it significantly?




EarlK said:


> Turn "phantom power" back on only when you are ready to plug a mic into the soundcard & take a measurement ( btw, that phantom power won't be present on the RCA to 1/4" loop-back cable ) .


Checked, phantom is only present on the XLR input, not on TRS (anyway this is line input, so there's no point on powering it)




jtalden said:


> There is a bit of trade off in which to use. It is a more accurate and very low frequencies, but a bit obsessive, to use the XLR loopback. It is not really needed for adequate accuracy and since it is much more sensitive to gain settings it can result in a little more noise in the calibration. You can do it either way.


 As I said, LF response is even better (marginally) with the TRS line input. All tests are being done with balanced cables, btw. The thing is that I don't really have a problem calibrating it with either the XLR or the TRS, results are more or less the same. But what would be the FR response when phantoming a mic? I don't have any proof that phantom on a mic doesn't HP filter the signal.




jtalden said:


> When measuring and using a loopback cable on the other channel for "loopback timing" either a TS or TRS cable should be used. The phantom power is not applied to that plug. There is no calibration of that channel as it is only used for timing reference of the signal. Calibration on that channel makes no difference to the timing accuracy.


... and here's the answer to my timming signal related questions.




jtalden said:


> The above is the normal case. Now to your results. I am surprised the green trace falls off to much more than the red trace. Is the green trace with XLR loopback cable and phantom power on? If so, I didn't know it would make that much difference with phantom power on, I never actually tried it. *If that is the cause don't' worry about it and just follow the guidelines above.* If not then there may be an issue that needs attention.


The thing is that the cause for a calibration error can be at the same time the cause for a measurement error (or lack of FR response, which is the same). I mean, if in loopback I see those serious FR deviations, why can I expect them to dissapear when connecting a mic? any ideas on how to troubleshoot this?

Just out of curiosity; even with that deep frecuency deviation, if accurately measured in a repeatable manner, wouldn't REW be able to compensate it and provide a near flat response? In that sense, I'd be better of by making the measurement with phantom ON, so to accurately track it response.

On a different matter, jtalden, I've seen on your signature that you use a pair of DCX2496. I am going to order one of them soon (that's one of the main reasons for learning REW, going tri-amp). Currently looking around for all the mods that can be done (remove output board, etc). Any advice on that?


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## regiregi22 (Mar 13, 2014)

I forgot to say that I have some diy microphones using Panasonic WM-61A capsules and the Linkwitz mod, powered by batteries (no phantom power needed!!). I have just bought a Behringer ECM-8000 (to have a "reference" commercial phantom powered one) because they are dirt cheap, but FR should be more or less the same, if not better.

So tomorrow (it's 2am here in Spain, quite a bit late) I will test both mics doing the same measurement, in the same position and SPL, just to check and compare their FR, which should be more or less the same. Both mics are pretty linear in the bass frecuencies, and being a HP filter problem, I'm in the ballpark to quickly see if phantom is the culprit (or not phantom, but their implementation triggering somehow a HP filter).


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## jtalden (Mar 12, 2009)

regiregi22 said:


> Just out of curiosity; even with that deep frecuency deviation, if accurately measured in a repeatable manner, wouldn't REW be able to compensate it and provide a near flat response? In that sense, I'd be better of by making the measurement with phantom ON, so to accurately track it response.


I have never tried calibration with phantom power on. With it off there is only a minor difference between the TRS and XLR connections on my current interface; a Focusrite 2i2. I attribute that the additional preamp activated on the XLR to provide the needed additional mic gain. Possibly the additional preamp in your interface is set to roll off sooner, but is only engaged when the phantom power is on? You probably understand the possibilities better than I so I can not be much help. I can only confirm that REW will compensate for the soundcard response per the loopback calibration that was done. So if there is a difference in response with the mic attached verses the loopback cable then there will be a corresponding error in the charts.



> On a different matter, jtalden, I've seen on your signature that you use a pair of DCX2496. I am going to order one of them soon (that's one of the main reasons for learning REW, going tri-amp). Currently looking around for all the mods that can be done (remove output board, etc). Any advice on that?


I use the unmodified unit. I have no problems with it as is. That is I am not aware of any issues, but have nothing to compare it to for sound quality. I only have the measurements I made on the unit to provide confidence in it. All looked fine for my purposes. The only Issues I am aware of is; the DCX provides lots of headroom so a large EQ boost would not clip the unit, also the noise floor rises at higher freqs. As a result with consumer amps it is pretty easy to get into the DCX noise floor if gain structure is not carefully managed. I have seen several noise floor complaints particularly from those with high sensitivity horns using large consumer amps. I could see benefit to address those issues in those cases where the noise floor is a problem. Other than that, I am skeptical of the practical benefits in sonic performance for the available mods.


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## regiregi22 (Mar 13, 2014)

jtalden said:


> *I have never tried calibration with phantom power on. * With it off there is only a minor difference between the TRS and XLR connections on my current interface; a Focusrite 2i2. I attribute that the additional preamp activated on the XLR to provide the needed additional mic gain. Possibly the additional preamp in your interface is set to roll off sooner, but is only engaged when the phantom power is on? You probably understand the possibilities better than I so I can not be much help. I can only confirm that REW will compensate for the soundcard response per the loopback calibration that was done. So if there is a difference in response with the mic attached verses the loopback cable then there will be a corresponding error in the charts.


Me neither, it's just that I realized I have it turned on when calibrating one input and having a powered mic on the other



jtalden said:


> *Possibly the additional preamp in your interface is set to roll off sooner, but is only engaged when the phantom power is on?*


* That's my bet, but it ain't possible for an interface of this quality to do that roll-off, and not saying anything about it on the manual

As I said, I have some linear electret microphones that can be used with the XLR and TRS inputs, battery powered so they don't depend on phantom to being powered on.



jtalden said:



I use the unmodified unit. I have no problems with it as is. That is I am not aware of any issues, but have nothing to compare it to for sound quality. I only have the measurements I made on the unit to provide confidence in it. All looked fine for my purposes. The only Issues I am aware of is; the DCX provides lots of headroom so a large EQ boost would not clip the unit, also the noise floor rises at higher freqs. As a result with consumer amps it is pretty easy to get into the DCX noise floor if gain structure is not carefully managed. I have seen several noise floor complaints particularly from those with high sensitivity horns using large consumer amps. I could see benefit to address those issues in those cases where the noise floor is a problem. Other than that, I am skeptical of the practical benefits in sonic performance for the available mods.

Click to expand...

Well, there have been done decent analysis about it, regarding it performance upgrades, mainly involving the input and output boards. Being a diyer, I know those are critical steps for good sound. There are some more good threads, but this one from DIYAudio is highly recommended. I cannot copy the URL because the minimum 5 posts limit, but look on google for

"DCX2496 Upgrade Board - Objectively Tackling the Improvement of a Stock DCX2496"

Sorry for the offtopic, get back to the thread*


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## regiregi22 (Mar 13, 2014)

I finally got around trying both the panasonic wm-61A self-powered in the line in and the Behringer in the mic input + +48vdc phantom. Results are more or less the same after applying calibration files, so the problem was due to applying phantom on the soundcard output.

Everything looks pretty good!

Thank you all for your help


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## jtalden (Mar 12, 2009)

I'm glad you were able to confirm the calibration is good. Is very frustrating when there is lack of confidence in the measurements.

I agree on the practical value of the DCX I/O board mods as the SNR would be greatly improved. That will make gain structure a non issue for that unit. It is just that I am able to work just fine within my measured ~80dB SNR available at my signal level. My AVR is able to provide a pretty strong signal. The digital mods don't apply for my use and the other mods seem only to address non issues from a practical viewpoint.

As you proceed with your plan to convert your passive speaker to active using the DCX, you will find that there are many opportunities improve the measurements and change the sound markedly. Enjoy the process.


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