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I think I can make the 100:1 pad
Here's a very suitable -30dB attenuator that will lower line to mic level from unbalanced to a balanced XLR mic.

I drew it for you in Visio below. Very simple.

Hook it up and create a soundcard cal file and save it.

Then measure to be sure of your flat line.

Remove the loopback and use that channel for measuring.



line to mic pad.jpg

When you're done, tell us how much different the soundcard file is from the previous line-out to line-in file.

brucek
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #42
Alright, I'll give that a try tonight and post the calibration results (1/4 to 1/4 and 1/4 attenuated to mic). thanks.
 

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I'll give that a try tonight and post the calibration results
Great, whenever you get the time, I think it will be a benefit to others who will ask if they should be concerned about the mic preamp portion of the M-Audio Mobile Pre. I suspect that the mic preamp will not be significant in relation to the standard line-in to line-out calibration that most do.

I've proven that the XENYX802 preamp is not worthwhile including in the soundcard cal routine, and this will add another to the list (or not). Now we'll know for sure.

Many users aren't really capable of this fabrication (as you are), so this will be a help... If possible, it would be good to see the graph in the same scale as mine above, with vertical = +/-30dB and horizontal of 2Hz-25KHz.

brucek
 

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Alright, I'll give that a try tonight and post the calibration results (1/4 to 1/4 and 1/4 attenuated to mic). thanks.
Thanks for taking this project on Anthony. I certainly don't have the experience needed to construct the attenuation circuit to test the mic input.

All the best,

- Tim
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #45
Okay, some results:
Dashed is the Line to Line (1/4" to 1/4" unbalanced), Red is Line to Mic with an attenuator circuit to match the levels.

The only difference in the two is the scale. I was expecting differences in the high end and found none, but the low end was pretty interesting. Not a huge difference, but measurable for 20Hz up a bit.
 

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Anthony -

So I guess that rules out the M-Audio sound card / preamp. Must be user-error on my part somewhere.

What do you make of the HF roll-off in the graphs you displayed in your first post? What might be the cause?

- Tim
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #47
Nope, see the new thread I've started. I have more results that just add to the confusion. I split this thread into a new one to discuss just the MobilePre and the ECM8000 so we can figure out what's going on.
 

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Re: MobilePre ECM8000 oddities

Okay, some results:
Nice work Anthony. So, the results are as expected. The low end is a bit off and the upper end is much the same. Even down to 10Hz, it wouldn't matter too much which file you use, so I think we're safe in advising members that a line-out to line-in for external soundcards is suffice for the soundcard calibrate loopback test.

brucek
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #49
Re: MobilePre ECM8000 oddities

Okay, more weirdness. Below is a series of measurements I made with the new soundcard cal file (which seemed to only affect the low end anyways).

Speaker is a Magnepan MG10.1. Measurements were at the listening position. Red is the current ECM8k cal file, Blue is the old cal file, and Green is the Rat Shack meter used as a mic (with appropriate cal file and C weighting)
micsandcals.jpg

So with 2 different mics, I get the same fast rolloff in the top octave. It looks like the new mic file fixes the baffle "bump".

To see if it's a vertical/horizontal thing, I ran a semi-nearfield measurement of the speaker both horizontal and vertical using the new "split" ECM8k cal file.
nearhorvert.jpg

Purple is vertical, yellow is horizontal. Both have the rolloff at 15kHz.

So I believe we've ruled out the mic, and via nearfield measurements ruled out the speaker. It's not the calibration or loop, but it seems to be the soundcard.

Any thoughts folks?
 

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Re: MobilePre ECM8000 oddities

Any thoughts folks?
Well, it's as expected. The old ecm8000 cal file did not account for baffle step, and the new one does. The plots of the cal files themselves easily reveals why the two ecm graphs of the speaker are different. I do have more confidence in the newer ecm file (as far as generic files go).

The RS meter reveals what we already know about the RS meter, in that somewhere between 3KHz and 10KHz they all demonstrate a peak. Simply not usable above 3KHz.

brucek
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #51
Re: MobilePre ECM8000 oddities

But still, you figure the ECM would show a better top octave than the RS meter and yet they still both show that steep drop above 15kHz.

Hmm, I think my next project may be to build one of those condenser mics from scratch and see how it performs. If for no other reason, to learn more about what goes into all this. I've had enough tweeter crossover problems that I want to make sure I have something I trust up there (and I don't have $500 for a EarthWorks :( )
 

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Re: MobilePre ECM8000 oddities

So I believe we've ruled out the mic, and via nearfield measurements ruled out the speaker. It's not the calibration or loop, but it seems to be the soundcard.
I don't see how it could be the soundcard if your loopback measures flat to 20KHz. That completely eliminates the soundcard. It's the only thing you can be sure of...

I don't think you can be 100% sure of the mic until you have it calibrated.

How have you ruled out the speaker?


brucek
 

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Re: MobilePre ECM8000 oddities

I just find it to be too much of a coincedence that both anthony and I are showing a steep roll-off in the HF above 15KHz. I show it when using both an ECM Mic and DBX mic.

Here are a series of nearfield plots I took using the DBX mic. The different plots represent on-axis, slightly above, slightly below, and to the left and right. (I was trying to find some axis, any axis, that I would get flatish treble response above 15KHz).



I'll be getting my calibration files for my DBX mic this week, but Cross Spectrum has already sent me a preview of the plot, and there is no roll-off in the HF, so I don't expect the cal file to fix this problem. In fact the DBX mic's response looks very similar to the ECM, just with less of a hump at 10KHz.
 

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Re: MobilePre ECM8000 oddities

Anthony -

I'm not sure that the Maggies are a good reference for this test. I've read in the past that it is difficult to get an accurate FR reading of panel speakers because of the way they project sound.

For reference, I looked for FR measurements done by AV mags on the maggies and found these:

Magnepan MMG W (measured by Ultimate AV Mag)


Magnepan MG 3.6 (measured by Ultimate AV Mag)


As you can see, they are showing a fairly steep roll-off in the upper HF. Do you have any other more conventional speakers around to test?

Here's what my speakers are supposed to be doing in the HF.

Wilson-Benesch speakers (measured by Hi-Fi News UK)
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #55
Re: MobilePre ECM8000 oddities

Heh, I wonder if Ultimate AV is using our mic :)

That's a good point, though, about testing other speakers.

I have some Adire HE10.1's (don't know why that number is in both of my speaker sets :) ) in the tube system upstairs. I can bring one downstairs and remeasure.

Panel speakers radiate in a "middle finger" lobe pattern (for lack of a better image). The on-axis is very strong but narrow then flanked by weaker lobes around the 15 to 30 degree mark, then another null and another set of even weaker nodes around the 45 degree mark. Overall it looks like the classic figure 8 pattern of a dipole, but with nulls inbetween the fingers. A neat trick to getting a great center image is to aim the speaker so the second lobe is aimed at the listening position (about a 30 degree toe-in). It's weird but it keeps a pretty stable center image across the stage without the dreaded "sweet spot".

Anyway, I was measuring about 8 inches from the panel, which should be enough for the ribbon and panel to integrate, but you are right, all it takes is for the mic to be in the wrong null and the FR goes all wacky.

I'll try the other speakers tomorrow night. I can also try a raw BG Neo3PDR and Neo8PDR tweeter/panels to see how they fare.

Thanks for the input guys, it'll be great to get to the bottom of this.
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #56
Re: MobilePre ECM8000 oddities

Well, I just looked through some old measurements and this is not apparent in all of them -- just the ones of the Magnepans. Some random measurements don't seem to have it (like almost touching the panel nearfield), but almost all of the 1m to 3m measurements do. Hmm, I guess I started with a bad assumption, since the speaker was supposed to be flat to 24kHz. But dipoles are tough to measure, tweeters are tough to measure, and in-room is tough to measure. So dipole tweeters in room must be really tough to measure :)

My guess is it's some sort of cancellation effect from position or frame/panel. Sorta like a broad comb filter effect. It's too high to be dipole cancellation and the likelihood of me putting the mic in an exact null every time is slim. I'm just thinking out loud here to try and find a solution that fits your problem as well, Tim.

I'll test those horn tweeters tomorrow and check back. Seems we're at least a bit closer.
 

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Re: MobilePre ECM8000 oddities

Hmmm ....

I ran some more measurements, and I don't know what I did differently but I'm able to get respectable HF extension when I use the mixed cal file for the ECM mic, BUT only when I point the mic at the speaker .

Yellow plot is horizontal mic position, blue is vertical.
 

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I have been prototyping a new setup, which is why I found this forum. I have never had a tool like REW before! All I can say is WOW. Anything else I said would ensure that the moderators would delete it.
Anyway, I would not be in a position to comment without REW, So...
I couldn't get the ECM8000, However the local shop did have a JBL re-badged DBX TRA M for a similar price. So I bought it and a small mixer to supply power and set about measuring my new speaker. (singular)
I have been extremely happy with the JBL as my Shiva X measures (close field) to what was modelled in WinISD for a 120 litre cabinet. ie ~7 Db down at 20Hz.
As for the top end, this mic happily did a good job of following the Raven 3.2's response with no major anomolies.
If you cannot find the ECM then I can quite happily suggest using the JBL.
 

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Elite Shackster , HTS Moderator Emeritus
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Discussion Starter #60
Re: MobilePre ECM8000 oddities

Nice measurements there. Interesting that the JBL tweeter has that top octave falloff as well -- not exactly a cheapo tweeter in its own right.

I'll post mine tonight in this exact same format. You said 1 foot, 6ms gate (from zero, or from first impulse received?). The candidates will be the MG10.1 ribbon tweeter and the compression tweeter used in the Adire speaker. I do not remember the brand name on it, but it screwed into the back of the Eminence woofer.
 
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