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Discussion Starter · #1 ·
I would like to measure the electrical response of a pre-amp's subwoofer/.1 LFE output to examine the following..

1. When a room correction mechanism like Audyssey or Anthem ARC is used, is it consistent across sources, for example what happens when the player decodes vs when it bitstreams? This thread indicates a bug that was discovered in a Denon receiver but it only alludes to how it was done with REW with no specifics.
2. I would like to use an external source like this DIY Audio test dvd or a movie clip with challenging LFE like WOTW pod emerges, and look at the electrical response in REW.
3. I would like THD and THD+N measurements for just the subwoofer output of the pre-amp with no actual subwoofer.

Basically, I want to measure the pre-amp LFE separately and then measure the actual room response with a subwoofer and Galaxy Mic.

There are several conflicting reports (maybe I am misunderstanding) that kind of address if the above is possible or not with REW. For example
this says external source is not possible
this says you can use the internal tones to measure THD and THD+N which is also what I am interested in
While some have accomplised an external source like this post, is it accurate?

Please clarify.

I can think of a few ways of measuring the pre-amp LFE output electrical response. First if I want to use the internal tones in REW, it is pretty straightforward when the analog output of the sound card is used. Analog output of the card to analog input of the receiver, use a listening mode like stereo, set the Xover appropriately and then take the .1 LFE output from the pre-amp and connect it to the soundcard line-in, use REW internal tone, examine and have fun.

However, if I want to use a DD 5.1 source, bitstreaming from a player like the PS3 vs Decoding to PCM in the PS3 transfer via HDMI to the pre-amp and then measure the .1 LFE of the preamp, is that possible? I would to turn of Audyssey on/off, Dynamic EQ on/off, Dynamic Volume On/Off, and measure the electrical response and possibly examine the frequency response, THD, THD+N etc.

Is the above possible. I am pretty familiar with ETF so all that I need now is some direction. I plan on buying and external sound card like the turtle beach SRM. Maybe I need all it's capabilities like the optical SPDIF output in that to use DD 5.1 sources played on the PC. If one of you knowledgeable folks can give some direction, it would be much appreciated. I am asking for a lot, I promise in return, I will faithfully post all my findings so it woudl help a wider audience.
Thanks in advance,
-Jai
 

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Lots of questions here, I'll try and step through them 1 by 1...
First, yes, you can make a loopback connection from your soundcard out, through your pre-pro, to the soundcard in, and measure the response of your equipment. For this you don't need an external source.
Second, if you're just doing comparisons, there's nothing wrong with using a spdif output from the soundcard into the prepro, just understand that because the calibration will be off, you're not looking at an absolute FR, you can only compare FRs from mode to mode.
Third, yes you can use an external source with REW IN RTA MODE. For instance, there's nothing wrong with using an external source to generate pink noise with REW in RTA mode to get a picture of the response. You CANNOT, to the best of my knowledge, use an external source to run REW's sweep to get the response used to calculate the IRs and Waterfalls. REW gates the response it measures, and syncing the routine with the sweep is critical.
For THD or THD+N, feed REW a sine wave and use in RTA mode. The sine wave can be internal or externally generated.
The post you linked to regarding external sources, I'm not really sure what the poster was trying to do, how, or why.
Feel free to read up on REW in the sticky at the top of this forum, and also the manual.
 

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Discussion Starter · #3 ·
Lots of questions here, I'll try and step through them 1 by 1...
First, yes, you can make a loopback connection from your soundcard out, through your pre-pro, to the soundcard in, and measure the response of your equipment. For this you don't need an external source.
Second, if you're just doing comparisons, there's nothing wrong with using a spdif output from the soundcard into the prepro, just understand that because the calibration will be off, you're not looking at an absolute FR, you can only compare FRs from mode to mode.
Third, yes you can use an external source with REW IN RTA MODE. For instance, there's nothing wrong with using an external source to generate pink noise with REW in RTA mode to get a picture of the response. You CANNOT, to the best of my knowledge, use an external source to run REW's sweep to get the response used to calculate the IRs and Waterfalls. REW gates the response it measures, and syncing the routine with the sweep is critical.
For THD or THD+N, feed REW a sine wave and use in RTA mode. The sine wave can be internal or externally generated.
The post you linked to regarding external sources, I'm not really sure what the poster was trying to do, how, or why.
Feel free to read up on REW in the sticky at the top of this forum, and also the manual.
Greg,
Thanks a lot for the answers.
While using an external source with REW in RTA mode, I think you mean a source not in the REW software, but some other source material on the computer residing as a media file that can be loaded into REW, is that right?
Is it possible to use an external source that is a different program on the same computer, for example play a DVD through theatertek, use the SPDIF output into the preamp, loopback from the preamp to the sound card and sample the .1 LFE while your scene/track of interest is playing?
Secondly, is it possible to use an external hardware, like a PS3 to play a source and then use the .1 LFE from the preamp to the sound card while your scene/track of interest is playing, same as in the previous case?
From the above two, I would like FRs and as you indicated, since I would be comparing only mode vs mode (Example "preamp1 vs preamp2" or "mode 1 in preamp1 vs mode 2 in preamp1" etc) the relative difference is what I am interested in rather than absolute values.
Thanks again :bigsmile:
 

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While using an external source with REW in RTA mode, I think you mean a source not in the REW software,
Yes.
but some other source material on the computer residing as a media file that can be loaded into REW, is that right?
Not really. What I meant was a CD/DVD/BD/other connected to your system, playing test tones, and only using REW (in RTA mode of course) to record the sound produced in the room.
Is it possible to use an external source that is a different program on the same computer, for example play a DVD through theatertek,
Hmmm... maybe, you'll have to try it, or someone more knowledgable than I will have to answer... it's possible it could create a conflict, but I'd go ahead and try it. The downside is that you'll never know for sure exactly what dB anything SHOULD be at, because it won't be calibrated, but for comparing one setting to another it should be fine if it works.
use the SPDIF output into the preamp, loopback from the preamp to the sound card and sample the .1 LFE while your scene/track of interest is playing?
Should be ok as long as you accept that it won't be "calibrated". But for before/after comparisons, should be fine given the above caveat on conflicts.
Secondly, is it possible to use an external hardware, like a PS3 to play a source and then use the .1 LFE from the preamp to the sound card while your scene/track of interest is playing, same as in the previous case?
That should work, but again you have to realize that what you read won't be calibrated. For the record, once it comes out of the preamp, the signal you mention shouldn't be LFE any more, it should be SUB, but that's mostly semantics.
From the above two, I would like FRs and as you indicated, since I would be comparing only mode vs mode (Example "preamp1 vs preamp2" or "mode 1 in preamp1 vs mode 2 in preamp1" etc) the relative difference is what I am interested in rather than absolute values.
Thanks again :bigsmile:[/QUOTE]

Hope it works, please post your results so we can all learn from them :nerd::T:wave::D:bigsmile:
 

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Discussion Starter · #5 · (Edited)
Yes.

Not really. What I meant was a CD/DVD/BD/other connected to your system, playing test tones, and only using REW (in RTA mode of course) to record the sound produced in the room.

Hmmm... maybe, you'll have to try it, or someone more knowledgable than I will have to answer... it's possible it could create a conflict, but I'd go ahead and try it. The downside is that you'll never know for sure exactly what dB anything SHOULD be at, because it won't be calibrated, but for comparing one setting to another it should be fine if it works.

Should be ok as long as you accept that it won't be "calibrated". But for before/after comparisons, should be fine given the above caveat on conflicts.

That should work, but again you have to realize that what you read won't be calibrated. For the record, once it comes out of the preamp, the signal you mention shouldn't be LFE any more, it should be SUB, but that's mostly semantics.
From the above two, I would like FRs and as you indicated, since I would be comparing only mode vs mode (Example "preamp1 vs preamp2" or "mode 1 in preamp1 vs mode 2 in preamp1" etc) the relative difference is what I am interested in rather than absolute values.
Thanks again :bigsmile:
Hope it works, please post your results so we can all learn from them :nerd::T:wave::D:bigsmile:[/QUOTE]

Greg,
I sure will post as I learn more. For now, the sound card has been the showstopper since the RME did not work. I had another desktop that I moved to my HT room that has Sigmatel HD Audio with Line in/out and digital coax out on the motherboard. This should work right? I just have to deal with the inconvenience of the desktop for now. I should be soon up and running with REW if all goes well.

Regarding THD/THD+N measurements, I want to be able to measure the pre-amp subwoofer output's distortion when approaching reference levels and maybe reference and beyond. I want to try this in the following configurations...
a) Audyssey on and off
b) Audyssey DEQ on/off
c) Audyssey DVOL on/off
d) Bitstreaming DD from player and let preamp decode and repeat a) b) c)
e) Decode DD to PCM from player and repeat a) b) c)

I have a suspicion that EQ is introducing mild distortion. Don't ask me why. Just by ear. I want to measure and find out if it is just imagination. Once I get the hang of REW I want to compare pre-amps like the Integra 40.1 to the 80.1 and maybe the Statement D2/v w/ARC

Is this possible only with the internally (from REW) generated sine wave or can I use an external BD/DVD player that plays a sine wave and then measure from within REW?
My desktop has an SPDIF output, so if I want to do uncalibrated mode to mode THD and THD+N comparisons, I either would like to use the SPDIF output on the desktop. This would not faciliate e) above. For e) Ideally I would like to play a sine wave in an external DVD/BD player and measure the THD/THD+N in REW while decoding in the player vs decoding in the preamp. Am I on the right track?.:scratch:

BTW, the reason I want to use the digital output from the sound card or the external player is to avoid the A->D conversion that would happen in the pre-amp if I were to use the analog line out of the sound card. Will such a test that I am contemplating be valid for THD/THD-N when uncalibrated but done mode vs mode?

Hope you understand what I am trying to quantify. I am tired of wondering, and asking and guessing. This time I want to measure and find out for myself. With help from folks like you here, I should be able to make progress.
Thanks a lot!
Here is some context about where I am coming from and why I have all these questions...
Anthem ARC and Audyssey MultEQ Pro
Why does Audyssey in some cases sound too bassy/not tight etc..I have a theory..
post# 21655 and 21659 showing Audyssey electrical response
 

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Jai,

I've done some testing of random receiver and processor sub pre-outs that I own and they are surprisingly bad (Pioneer, Onkyo, NAD). Nothing fancy and just for my own information about my units.

At a REF volume setting and the pre-out trim at -10db they are all distorting already with a cd level output. Movies have much hotter output levels with the LFE track so who knows how much is there. I simply watched it with Spectrum Labs waterfall display so i didn't calculate the THD % but you can clearly see it increase with the level. There are people who run their pre-out trim well above 0db or even cranked. They are probably clipping the snot out of them with any sort of movie playback volume.

I'd be interested in seeing what you come up with.
 

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Discussion Starter · #7 ·
Jai,

I've done some testing of random receiver and processor sub pre-outs that I own and they are surprisingly bad (Pioneer, Onkyo, NAD). Nothing fancy and just for my own information about my units.

At a REF volume setting and the pre-out trim at -10db they are all distorting already with a cd level output. Movies have much hotter output levels with the LFE track so who knows how much is there. I simply watched it with Spectrum Labs waterfall display so i didn't calculate the THD % but you can clearly see it increase with the level. There are people who run their pre-out trim well above 0db or even cranked. They are probably clipping the snot out of them with any sort of movie playback volume.

I'd be interested in seeing what you come up with.
As much as I appreciate your input, I am disappointed as this is the first time I get clear confirmation that my gut feelings were right. :unbelievable:. I was hoping to be wrong.
The sad thing is most folks like those distortions as it gets pretty loud, walls shake, paint peels, their subwoofer cooks them dinner and all those things happen. For someone who is used to good quality sound this becomes a nightmare experience. I really would like to learn from your exeperience Ricci.Can you elaborate a bit more on how you saw distortion visually? Were you able to use digital inputs?
Thanks!
 

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I've done some testing of random receiver and processor sub pre-outs that I own and they are surprisingly bad (Pioneer, Onkyo, NAD). Nothing fancy and just for my own information about my units.

At a REF volume setting and the pre-out trim at -10db they are all distorting already with a cd level output. <etc>
Does your measurement INPUT chain have enough distortion headroom to measure this?

My soundcard input, for example, cannot handle the full output of my sub-out signal without distorting. This distortion occurs well before clipping and it doesn't help to reduce the input volume control as it is the input stage prior to the volume control that is overloading.

One needs to verify this when making electronic loopback measurements (and also when using a mic as the source if measuring very high SPL levels).
 

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You can use the REW Spectrum/RTA view to show and calculate THD for tones played on other sources. However, you need to take care that there is no clipping in the soundcard, a CD level output will easily clip a typical line level input. It would be unusual for just about any processor to show significant levels of clipping or distortion on its line outputs.
 

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Discussion Starter · #10 ·
Does your measurement INPUT chain have enough distortion headroom to measure this?

My soundcard input, for example, cannot handle the full output of my sub-out signal without distorting. This distortion occurs well before clipping and it doesn't help to reduce the input volume control as it is the input stage prior to the volume control that is overloading.

One needs to verify this when making electronic loopback measurements (and also when using a mic as the source if measuring very high SPL levels).
bjs,
Ignoring SPL levels, how does one guarantee that distorton does not occur at the soundcard input?
Also, how did you know that such a distortion was happening at the line input in your sound card? Did it show up in REW?
Thanks,
-Jai
 

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Discussion Starter · #11 ·
You can use the REW Spectrum/RTA view to show and calculate THD for tones played on other sources. However, you need to take care that there is no clipping in the soundcard, a CD level output will easily clip a typical line level input. It would be unusual for just about any processor to show significant levels of clipping or distortion on its line outputs.
Again,
what are the best known ways to avoid this type of clipping in the sound card? Are there special high quality ones that are recommended?
 

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It is not a sound card quality issue, just signal levels - driving the input with a larger signal than it is designed to accept. The audio spec for soundcard inputs is typically 1.0V rms full scale, whereas a CD player has 2.0V rms full scale output. AV processor subwoofer outputs can often generate even higher signal levels. You get a good indication by looking at the VU meters in REW to make sure the peaks on the input never reach 0dB FS. At levels close to full scale the inputs are more prone to distortion, but clipping in the input stage generates high levels of odd order distortion.
 

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Does your measurement INPUT chain have enough distortion headroom to measure this?

My soundcard input, for example, cannot handle the full output of my sub-out signal without distorting. This distortion occurs well before clipping and it doesn't help to reduce the input volume control as it is the input stage prior to the volume control that is overloading.

One needs to verify this when making electronic loopback measurements (and also when using a mic as the source if measuring very high SPL levels).
I'm fairly certain that it does, but I cannot absolutely rule it out. The levels are at well below the level of clipping the SC input when I see the distortion appear and start to increase. The SC I'm using is a Turtle Beach AA SRM. Nothing special to be sure, but I run the input sensitivity at minimum and use Spectrum Labs to look at the output of sine waves (you could use REW ). However the fact that the different units (even 2 different Onkyo's 806 and 886) all exhibit different spectra and at different levels does give me some confidence that I'm seeing THD from the receiver output. Could be a combo of both the SC input and the pre out signal....

Just for example in my 3 systems I run my amplifiers with input levels full and my SW pre-out is all of the way down at -10 or -15db from the processor or receiver. I use DCX's or a Rane PE-17 in between, both of which are pro units and allow for a wide range of signal boost or cut. I use those to get the level to the subs to where it needs to be for calibration. Interestingly in none of the cases is a signal boost needed of the receiver sub signal into the amps. It's in the pass through to -3db range. All of my amps have either adjustable input sensitivity or a modified one though.

When I was checking the SW pre-out I just used a sine wave out of REW like 30hz -3db into the cd input, while monitoring with Spectrum Labs (or REW) and run the pre-out into the sc with the input sensitivity at it's lowest setting and the sub pre-out at it's lowest level setting too. I set the main volume to whatever REF level would be and from there increase the sw pre out level until you start to see distortion show-up. Continue increasing the level and monitoring, paying attention to the input level. Try a few different frequency of sines like 10, 40, 63, etc. You may want to disconnect the mains BTW or turn them off.

Let me see if I saved any of the screenshots...
 

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...The levels are at well below the level of clipping the SC input when I see the distortion appear and start to increase.
Yes, as I mentioned soundcard distortion can begin to rise well below clipping. That's why people need to check the actual distortion at the levels being used.

ricci said:
I run the input sensitivity at minimum
As I mentioned previously reducing input sensitivity doesn't necessarily help as the problems occur prior to that stage. On my soundcards I leave the input volume on full for distortion testing since if the signal is too strong for full volume then I know that it will have excess distortion regardless of the input level setting.

Anyway, the test you describe in your last paragraph isn't valid for these reasons. You really do need to independantly verify the soundcards distortion at the levels you are using. Feel free to prove me wrong, but I'm fairly certain you are mostly measuring soundcard distortion.
 

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...You get a good indication by looking at the VU meters in REW to make sure the peaks on the input never reach 0dB FS...
Unfortunately it is not sufficient to look at the VU meters *if* you've had to reduce the input sensitivity to achieve a decent reading.

This is a critical point for anyone trying to measure high (ie more than 1v RMS) voltages with their soundcards.
 

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Discussion Starter · #16 ·
Unfortunately it is not sufficient to look at the VU meters *if* you've had to reduce the input sensitivity to achieve a decent reading.

This is a critical point for anyone trying to measure high (ie more than 1v RMS) voltages with their soundcards.
I did a quick measurement with a voltmeter on the subwoofer pre-out. A 30Hz sine wave at reference level (played from an external player, PS3 in this case) was well below 1v. It was around .5 in the integra 40.1 with subwoofer trim level at 0. However playing pod emerges WOTW DTS, I saw spikes of about 3v. Even my Rane PE-17 had it's overload LED light up bright. With a 30Hz sine wave at least if I am measuring correctly, chances of overload at the sound card line in should be very low even at reference levels.
Since Ricci used sine waves in the 30Hz range, I don't think he must have encountered clipping and distortion at the line it at around .5 v unless his receivers are very different from the Integra.
 

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Discussion Starter · #17 ·
I'm fairly certain that it does, but I cannot absolutely rule it out. The levels are at well below the level of clipping the SC input when I see the distortion appear and start to increase. The SC I'm using is a Turtle Beach AA SRM. Nothing special to be sure, but I run the input sensitivity at minimum and use Spectrum Labs to look at the output of sine waves (you could use REW ). However the fact that the different units (even 2 different Onkyo's 806 and 886) all exhibit different spectra and at different levels does give me some confidence that I'm seeing THD from the receiver output. Could be a combo of both the SC input and the pre out signal....

Just for example in my 3 systems I run my amplifiers with input levels full and my SW pre-out is all of the way down at -10 or -15db from the processor or receiver. I use DCX's or a Rane PE-17 in between, both of which are pro units and allow for a wide range of signal boost or cut. I use those to get the level to the subs to where it needs to be for calibration. Interestingly in none of the cases is a signal boost needed of the receiver sub signal into the amps. It's in the pass through to -3db range. All of my amps have either adjustable input sensitivity or a modified one though.

When I was checking the SW pre-out I just used a sine wave out of REW like 30hz -3db into the cd input, while monitoring with Spectrum Labs (or REW) and run the pre-out into the sc with the input sensitivity at it's lowest setting and the sub pre-out at it's lowest level setting too. I set the main volume to whatever REF level would be and from there increase the sw pre out level until you start to see distortion show-up. Continue increasing the level and monitoring, paying attention to the input level. Try a few different frequency of sines like 10, 40, 63, etc. You may want to disconnect the mains BTW or turn them off.

Let me see if I saved any of the screenshots...
I have the Rane PE-17 too which is a very nice unit.
Ricci, where were the sliders set at? (input gain and output gain)?
Are both all the way up? That is what is recommended to reduce noise. To avoid overload,
I have them set at unity gain, two dashes below the top. My crown CE4000s are set at input sensitivity
at 1.4 (33.8dB gain) and the gain knob is at 3/4th in bridged mode. The subwoofer output is at -12 trim level. Even with the most intense movie bass (WOTW example) the overload on the rane never lights up.
It looks like at approx 2v the PE-17 begins to overload.


Excerpt from RANE PE-17 manual...

For optimum noise performance, you should take as much
gain at the Input as you can, with a corresponding reduction at
the Output. This is done by moving both Gain sliders upward.
Note that the In control is calibrated top-to-bottom +12 dB to
-12 dB, while the Out control is calibrated just opposite. Thus,
by moving both sliders together, you always maintain unity gain
through the Equalizer, but you vary how much gain/attenuation
occurs at the Input or Output of the unit. Therefore, by operating
the unit with both sliders as high as possible, you take a lot
of gain up-front, while simultaneously reducing the signal (and
noise) at the Output. Use the Overload light as a guide: occasionally
lighting is set just right. Continuously lighting is too
much gain, while never lighting is too little gain.
 

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Yes, as I mentioned soundcard distortion can begin to rise well below clipping. That's why people need to check the actual distortion at the levels being used.



As I mentioned previously reducing input sensitivity doesn't necessarily help as the problems occur prior to that stage. On my soundcards I leave the input volume on full for distortion testing since if the signal is too strong for full volume then I know that it will have excess distortion regardless of the input level setting.

Anyway, the test you describe in your last paragraph isn't valid for these reasons. You really do need to independantly verify the soundcards distortion at the levels you are using. Feel free to prove me wrong, but I'm fairly certain you are mostly measuring soundcard distortion.
You are probably right. I was not trying to measure voltage actually at the time. I was measuring the pre-out FR and trying to set the gain structure up to avoid clipping the inputs and just happened to try what I described above as a side test. If I was to set my sc's input sensitivity at maximum it would be clipping all over the place with even a modest level. I do distinctly recall that I got different results with different units though.:scratch:

How can you possibly get a valid distortion test with a mic if your SC is limiting you to a very low level input before it adds distortion of its own? (seems like from what you've described this is the case with yours as well?) I guess this limits to keeping the level low with very low output from the mic mixer? This before even getting to possible microphone distortion issues.

How would you recommend measuring SC input/ output distortion?
 

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I have the Rane PE-17 too which is a very nice unit.
Ricci, where were the sliders set at? (input gain and output gain)?
Are both all the way up? That is what is recommended to reduce noise. To avoid overload,
I have them set at unity gain, two dashes below the top. My crown CE4000s are set at input sensitivity
at 1.4 (33.8dB gain) and the gain knob is at 3/4th in bridged mode. The subwoofer output is at -12 trim level. Even with the most intense movie bass (WOTW example) the overload on the rane never lights up.
It looks like at approx 2v the PE-17 begins to overload.


Excerpt from RANE PE-17 manual...

For optimum noise performance, you should take as much
gain at the Input as you can, with a corresponding reduction at
the Output. This is done by moving both Gain sliders upward.
Note that the In control is calibrated top-to-bottom +12 dB to
-12 dB, while the Out control is calibrated just opposite. Thus,
by moving both sliders together, you always maintain unity gain
through the Equalizer, but you vary how much gain/attenuation
occurs at the Input or Output of the unit. Therefore, by operating
the unit with both sliders as high as possible, you take a lot
of gain up-front, while simultaneously reducing the signal (and
noise) at the Output. Use the Overload light as a guide: occasionally
lighting is set just right. Continuously lighting is too
much gain, while never lighting is too little gain.
I use my Crown CE4000's and Crest 8002's at the highest sensitivity input settings. The Rane is in use with one of the CE4000's and is set to attenuate about -3db total if I remember right. It's been a while since I set that up. The input is about at 2 marks down and the output attenuation is up a little higher.
 

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Discussion Starter · #20 ·
:scratch:
You are probably right. I was not trying to measure voltage actually at the time. I was measuring the pre-out FR and trying to set the gain structure up to avoid clipping the inputs and just happened to try what I described above as a side test. If I was to set my sc's input sensitivity at maximum it would be clipping all over the place with even a modest level. I do distinctly recall that I got different results with different units though.:scratch:

How can you possibly get a valid distortion test with a mic if your SC is limiting you to a very low level input before it adds distortion of its own? (seems like from what you've described this is the case with yours as well?) I guess this limits to keeping the level low with very low output from the mic mixer? This before even getting to possible microphone distortion issues.

How would you recommend measuring SC input/ output distortion?
Ricci,
I was wondering about the same thing...:scratch: but then came up with the following...
Couldn't you use the Rane PE-17 in the path to the sound card line-in to attenuate the signal to be less than 1v (use a voltmeter) ? This would avoid clipping in the line-in.
 
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