# Strange soundcard measurements...Thoughts?



## Ricci (May 23, 2007)

Check this out. 

I'm measuring a Presonus Firebox audio interface yesterday and one result seems particularily strange. I was measuring the various input and output connectors to verify the response flatness. Most of them are very close to each other and quite flat. However one result doesn't seem to make sense. 

This unit has line level 1/4" inputs and outputs on the back, a headphone output, and combination Neutrik 1/4'/xlr microphone inputs. All of the line outputs and inputs measure very flat and extended including the headphone output. All graphs are very similar and will basically lay one on top of the other regardless of which combo of inputs and outputs is used, with only minor variation at the extremes. Like so. 














One thing that I've noted of late is that running the loopback through the microphone preamp via xlr often has the effect of dramatically rolling off the low end response. This effect is not there with a 1/4" cable more often than not. Cut to yesterday. Using a rca to xlr cable from Monoprice I get the usual sharp roll off of the extreme low end frequencies from both the main left and right 1/4" outputs and from the other line outs into the mic xlr input. However when I switched to the headphone output into the xlr mic input the response went back to a flat extended response like the normal line level tests. I repeated this test multiple times just to confirm. The only change between the 2 tests was to move the same cable from 1 output to the other and repeat. :scratch:

This measurement shows what I'm talking about. The one that rolls off the bass dramatically is representative of any of the regular 1/4" line outs routed back through either of the 2 mic xlr inputs. The much more extended trace is from simply switching to the headphone output instead. 















It is not a difference in the flatness of the headphone output compared to the other line outs as confirmed by measurement of all of them back into the regular 1/4" line inputs being virtually identical. Only when running them into the mic preamp xlr inputs does this difference show up.

Now I do not think that the XLR inputs are as rolled off as indicated by the measurements with the line outputs looped back through. I believe the response with the headphone output is the correct one, as the manufacturer rates this unit at -0.5db at 10hz which is right in line with what I've measured. The question is why is there a dramatic roll off introduced when I use the xlr input for loopback? Is it a difference in the wiring for the xlr inputs compared to the 1/4" lines? Or maybe it's got something to do with an impedance mismatch? Why does the headphone output looped back through the XLR with the exact same cable not behave in the same manner? :scratch:


Thoughts? I know there is probably some simple explanation so please help explain it to me.


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## JohnM (Apr 11, 2006)

Headphone out probably has a lower output impedance than the line outs, maybe the XLR inputs are (or can be) low impedance?


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## Ricci (May 23, 2007)

This is basically what I'm thinking too and the response that I got at another site. It appears that the headphone is low impedance like the XLR inputs and the line outs/ins are all high impedance. The impedance mis match is likely causing the roll off of the signal on the loopback measurement. I suspect that the measurement using the headphone out is the correct response of the mic preamp section. 

How to go about measuring the low impedance XLR preamp then?:scratch: I have a matching transformer adapter that I tried but it made the roll off even worse!


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## ZX14DREAM (Jan 26, 2011)

I'm a newbie here and have been really impressed with the way you all exchange information at Home Theater.lddude:
I have been on an Audio Improvement mission and found this site.
Im trying to use REW or TrueRTA with a firebox and this post is as close as I've come in searching.

What is the proper way to setup the loop on the firebox?

I 'm using the SPDIF out for my signal to system. It works great so that part is OK. My DOD RTA mic is not working so I know I will buy a CALMIC of some sort.

Thanks


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