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Full range sweep - odd graph

1984 Views 7 Replies 4 Participants Last post by  davidbrn
I've just been having a go at doing some full range sweeps, with a view to producing some convolver filters for use in J River. The graphs though look really odd - there's a 25db drop off from 7k to around 250. My speaker should roll off at around 55, so I can only assume I've done something wrong.

I'm measuring with a Behringer ECM8000 mic connected to a Behringer MIC100 valve pre-amp, and an M-Audio Audiophile 2496 soundcard. The sound card calibration looks 'normal' i.e similar to others on here, no mic calibration file.

Any suggestions?

Text Blue Green Line Pattern
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> Maybe a , "Flipped" polarity switch on the MIC1000 compared to your original SC calibration ?

> Run another sweep without a soundcard calibration file .

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Thanks for the suggestions. Tried both, no appreciable difference. I've also tried my on-board sound card instead of the M-Audio, again, no difference. I'm going to get hold of another mic preamp to see if that makes a difference, and if not, another mic.
The slope looks to me like there's a 6 dB/octave high pass filter with -3 dB at somewhere around 6-7 kHz.
This is a first-order filter, i.e. probably a normal RC high pass filter.

My guess is you have a significant impedance mismatch of some sort in the signal chain, and capacitor-coupled outputs.

f = 1 / (2 pi R C)

Where R is roughly equal to the input impedance, and C is the output capacitor.
Assume perhaps a cutoff frequency intended at 20 Hz, and we'll have a mismatch of about 300x.
Pro XLR is 600 ohm, and consumer RCA usually 10k-20k ohm so this is 10x higher than you'd expect. Odd indeed.
Well spotted ipse and welcome to HTS.


I have the XLR output of the MIC100 connected to the RCA in of the sound card with an XLR-RCA adapter, which is what your describing. I'll report back when I've got a 1/4" - RCA adapter to use the preamps other output.
I looked up the specifics on your gear.

Well, yes and no.
An unbalanced (like mono headphone plug) 1/4" to RCA converter should work for you, with some caveats...

TRS signal levels are higher than RCA, but since you have an adjustable limiter on the amp you can set it to not overpower the input.

Low-impedance (XLR/TRS) output into high-impedance (RCA) input shouldn't cause this as such, it should cause the opposite, no blocking of infrasound.
The other way around, high into low would cause something like this slope, but generally up to more like 600 Hz than 6000...

So yes, the culprit is most likely the XLR converter.
Without a power supply, there's no elegant way to convert between XLR and RCA that works with every type of gear.
Simply taking the positive pin as signal and grounding the negative side would work fine on some gear and damage others. This would be "unbalanced XLR".
(It would work fine on your amp according to the data sheet.)

In this case I think your converter might be using a transformer, which is impossible to get right with unknown variables... but it is less likely to damage the gear.
If it is creating a series-resonant bandpass filter (R L C in series), it would also drop off at 6 dB/oct after the peak, and it looks like that could be what's happening. Your microphone is supposed to be flat to 20 kHz, and 7kHz seems early for speakers to start rolling off. (Decent trebles are fairly cheap.)
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Update on what I’ve found.

I ditched the XLR-RCA converter and got a mono ¼” plug to RCA socket converter. This was plugged into the MIC100 and then an RCA cable to the PC. Disappointingly the result was no different to the first graph.

I did note that using this method the “Gain” and “Output” dials on the MIC100 both had to be turned to their maximum setting to get a usable reading, which I suspect is not normal.

Next attempt was to swap the MIC100 for an ART Micromix, again using the ¼” plug to RCA socket converter. Instant success. I only did a quick uncalibrated test to see what the shape of the resulting graph would be, and it looks much like expected, with a largely flat response. The massive drop off has gone.

I think that either
a. There is an incompatibility somewhere with the MIC100, but I have no idea what it could be.
b. I have a faulty MIC100

Thanks for your suggestions, all I need now is to get the house to myself for a morning to do some proper measuring.
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