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Old 6th February 2008, 06:24 PM   #11
Davey is offline Davey  United States
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Location: Bremerton, WA.
Kevin,

Not much.

The basic concept is pretty simple. You perform a two-channel (differential) measurement and have the program you're using plot the difference between the two. The reference "probe" would at the input of the device-under-test and the measurement "probe" at the output. You can use the same basic setup on line-level gear, a speaker-level setup, (like we're talking about here) or just about anything else. You just have to make sure the voltage levels do not blast the soundcard inputs.

I like to use SpectraPlus with a white noise excitation and average over a minute or so. You'll end up with a totally smooth curve that shows the exact electrical response of the device-under-test.

ARTA works well too. You can use the STEPS application to sine sweep slowly over a period of time, or the ARTA application which uses an MLS stimulus.

Hope that helps.

Cheers,

Dave.
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Old 6th February 2008, 07:05 PM   #12
hahfran is offline hahfran  Germany
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Somehow I fail to get this. With a differential measure you get the impedance of the respective crossover - hipass or lowpass -
under its real load, the driver ( built in the enclosure, with dampening etc. ) only if the driver itself were a real, frequency independent load.
But in your case, you measure the voltage drop across the crossover filter and that depends on the driver's impedance which is around 20 ohms at resonance of the total system and tends to rise with frequency.
Wouldn't it make more sense to measure the voltage across the driver terminals? As the amp is considered a voltage source, the impedance variations of the driver do not apply.

I just made such a measurement and I see a lowpass slope that differs considerably from the filter loaded with 8 ohms.
The slope of the 12 dB design is actually 8 to 9 db per oct but this isn't even constant.
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Old 6th February 2008, 08:37 PM   #13
Davey is offline Davey  United States
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Yep, that is essentially what you're doing. You're taking a reading across the driver terminals and simultaneously across the input to the crossover. The comparison (difference) of those two measurements yields the exact response of the crossover itself.

If the amplifier is indeed functioning as a true voltage source and you're sure the input signal is flat then the reference measurement should be flat and...in theory...a single measurement would suffice. But, what if the amplifier is not a true voltage source. Many are not. Or what if your signal generator is not flat across it's response? In those cases a two-channel measurement will exclude all the other variations (elsewhere) in response that may sneak into the measurement.

If your measurement differs considerably from what you expected then there may be another setup factor that you don't have set correctly yet. Or maybe it is correct and this technique is highlighting (very well) the response variations of the complex load of driver and crossover. The electrical response of crossover networks seldom looks "generic" and usually reflects quite a bit of tweaking by the system designer to get the ultimate acoustic slopes correct. That is the essence of crossover design.

Anyways, this differential technique is a really powerful measurement scenario in most conditions.

Cheers,

Dave.
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Old 6th February 2008, 09:25 PM   #14
hahfran is offline hahfran  Germany
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Thanks yes I get it now. The complex or vector sum across the filter and the driver must be equal to the amp output voltage.
Since I have just one pair of ground-free inputs I have to derive the diference manually from two separate measurements.

The design of the active crossover is a perfect task for a sim.
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Old 7th February 2008, 03:10 PM   #15
Davey is offline Davey  United States
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I'm not totally familiar with LAUD, but I thought a dual-channel measurement like this should be straightforward. You have both a "Cal" probe and "Measurement" probe, yes?

I'm not sure why you would need "ground-free" inputs. The ground circuit within the crossover network is most likely common to both input and output so that would be the reference for both probes.

The basic process should be the same as a normal measurement in LAUD except your measurement probe is attached directly to the driver terminal vice being used with a microphone/preamp for an acoustic measurement.

Cheers,

Dave.
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Old 7th February 2008, 04:03 PM   #16
hahfran is offline hahfran  Germany
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LAUD is restricted to a rather antique Pinnacle soundcard but as you mention CAL probe I think my even older IMP can do that.
I'll try IMP. If I find a PC running DOS native.
Thanks.
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Old 10th February 2008, 07:16 PM   #17
hahfran is offline hahfran  Germany
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I have finally managed to derive the passive filter characteristics.
The high pass is relatively easy as it exhibits a constant slope of 16 dB / oct beyond the -3dB.
That can be realized with an active filter which depends largely on the open loop gain , such as a Rauch filter. One has to design
a discrete op amp with a well defined open loop gain to achieve the 16 db characteristics.
The lo pass however looks weird especially in the frequency range that matters most for music, between 800 and 2000 Hz.
Only beyond the crossover frequency it exhibits an almost constant slope of 10 dB.
An approximation are 3 directly coupled RC with a small amount of positive feedback.
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