I think Mr. Gravesen is completely wrong here.
The problem is not the low resistance of the coil. It's rather the C-Coil itself. When you get a huge inductance of 18 mH with just a few windings around a core, that means that there's absolutely no air gap in the magnetic path. Hence the inductance of these coils is rather random and every measurement gives another result...
In the old days such coils only where used for applications where a high inductance was required but the absolute value was of little interest. 🙂
The problem is not the low resistance of the coil. It's rather the C-Coil itself. When you get a huge inductance of 18 mH with just a few windings around a core, that means that there's absolutely no air gap in the magnetic path. Hence the inductance of these coils is rather random and every measurement gives another result...
In the old days such coils only where used for applications where a high inductance was required but the absolute value was of little interest. 🙂
It's not totally random but very dependent on the flowing current. If you throw enough power trough the wire, they shows their proper values.
When the measurements don't have enough electrical power, they shows less inductance than they really have.
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The LC circuit being a resonant filter has zero response at DC. However, the LR method, being lowpass, does have the same and therefore is capable of allowing a steady DC through the inductor.
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So, I know DCR is critically important to driver modelling, but let's talk about crossover components.
I don't care about actual R at DC so much as I want to know the effective L and R which will appear through the useful range of frequencies this coil will see. Say, 16 Hz to 3000 for instance.
The LR method, using tones in that range with some reasonable current values (250 mA?) seems the place where I would use as my standard.
Hopefully, LR at DC will yield the same results, but if there were a difference, I care at AC, not DC.
Now I have to ask though, with very large inductors is the L even constant with single cycles as it is with test tones??
Hope I've confused everyone. 🙂
I don't care about actual R at DC so much as I want to know the effective L and R which will appear through the useful range of frequencies this coil will see. Say, 16 Hz to 3000 for instance.
The LR method, using tones in that range with some reasonable current values (250 mA?) seems the place where I would use as my standard.
Hopefully, LR at DC will yield the same results, but if there were a difference, I care at AC, not DC.
Now I have to ask though, with very large inductors is the L even constant with single cycles as it is with test tones??
Hope I've confused everyone. 🙂
Well, there seems to be a difference brought about by the complex permeability of the magnetic material.
If DC & HF are both possible with the LR method, then why not something in between, like a low frequency AC (to simulate bass etc.)?
However, the choice of waveform (DC, tone, burst, white noise etc.) to be superimposed during the measurement needs to come from the following people:
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Any halfway decent coil will have a value and a loss. They can be expressed in different ways but we usually like our inductance value expressed in henries, be they millihenries, microhenries or whatever's convenient. Loss can be expressed as the inductor's Q, or sometimes in ohms (at the frequency of interest), or phase angle or a couple other schemes.
The measurement problem happens when the impedance of the coil is very low and the signal generator can't drive it. Adding resistance, either because the coil has very high losses, or just adding a series resistors, lets the signal generator do its thing without being asked for too much current and having the output collapse or burn up.
Measurement bridges are designed for low impedance loads. Some, called incremental inductance bridges, are made for high current and can drive crazy low impedance loads. They exist for the purpose of making measurements are various AC and DC current combinations so you can see what the core properties are. Almost any meter designed to measure inductance should have little trouble because it will either measure at a very low voltage, or at a high enough frequency that the reactance of the inductor is large enough not to be a problem.
Sound cards and similar things aren't designed for low impedance inductive loads. You can add a resistor, but what you really need to do is add a high current buffer designed for the load in question.
Bottom line is to measure at a high enough frequency, and hopefully in the frequency band where the thing will be used, such that the impedance is comfortably high.
The measurement problem happens when the impedance of the coil is very low and the signal generator can't drive it. Adding resistance, either because the coil has very high losses, or just adding a series resistors, lets the signal generator do its thing without being asked for too much current and having the output collapse or burn up.
Measurement bridges are designed for low impedance loads. Some, called incremental inductance bridges, are made for high current and can drive crazy low impedance loads. They exist for the purpose of making measurements are various AC and DC current combinations so you can see what the core properties are. Almost any meter designed to measure inductance should have little trouble because it will either measure at a very low voltage, or at a high enough frequency that the reactance of the inductor is large enough not to be a problem.
Sound cards and similar things aren't designed for low impedance inductive loads. You can add a resistor, but what you really need to do is add a high current buffer designed for the load in question.
Bottom line is to measure at a high enough frequency, and hopefully in the frequency band where the thing will be used, such that the impedance is comfortably high.
I agree. In fact there was another very similar thread (below) where the OP was looking for a method to measure his/her crossover inductor, to which I had replied similarly.
SEAS driver datasheets
And Conrad Hoffman: It's nice to see you here. I thought you were mostly in the solid state forum.
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I used to be really into speaker building, but it's been a couple decades! Wound and measured a boatload of crossover coils. My core interest is metrology and I collect GR bridges and other things. A friend has one of the GR incremental inductance bridges, 1633-A, plus the big tube-based generator and power amp (1308-A). A beast of a thing, but beyond my price and space limitations. The manual for the 1633-A however, is easily available and almost required reading for anybody interested in cored inductors and measurement methods.
Sir, you're very lucky to have access to such professional equipment. All I have now is a cheap multimeter that is over 15 years old and has probably already lost its calibration. The situation is similar for many other DIY people, though their struggles are sometimes with measuring SPL and at some other times, with something else.
This results in people coming up with crude measurement methods that use makeshift instruments. Since measurement and instrumentation is your speciality, it would be really helpful if you could please share a few measurement tips/tricks that are useful for speaker building.
This results in people coming up with crude measurement methods that use makeshift instruments. Since measurement and instrumentation is your speciality, it would be really helpful if you could please share a few measurement tips/tricks that are useful for speaker building.
Decades ago I was extremely lucky to acquire a GR acoustic measurement system consisting of a rack cabinet with a sound and vibration analyzer, driven by a stepper motor, coupled to a big GR high speed chart recorder. It could do 1/3 octave, 1/10 octave, continuous sweeps and the recorder was fast enough to record things like room decay. It was huge and heavy! It's the gadget to the right in this photo- http://www.conradhoffman.com/gr01.jpg
Today you can get way more capability for free, using nothing more than your PC and sound card. Or, yet more than that if you're willing to pay a small amount. It's the same with driver measurements. I used to hook them up to an amp and signal generator to measure the impedance, and built a small test box for each one. That same software can do the same, though you may have to build a simple interface. We've got way more capability today than a commercial speaker company might have had "back in the day", and for peanuts. For components, you can get the Peak or similar testers for next to nothing, compared to traditional LCR meters.
Need a signal source? Burn a CD using Audacity. Pink noise, white noise, tones, tone bursts or whatever you need.
When I first started building speakers I bought crossover parts and assumed the values printed on them were reasonably correct. Rarely measured a thing and it worked out fine. The trick was mostly choosing good drivers. I never built a good speaker with a bad driver and tried to stick with well known names. 90% of my effort was simple box simulation, and then tweaking the crossover for SQ. As much as I like measurement, I have to note that the worst speaker I ever built was the one with the most beautiful flat response curve you've ever seen!
Always ask yourself, "If I measure this, what am I going to do with the information?" Speaker measurements can lead you down a deep rabbit hole if you don't combine them with an educated ear.
I don't really have any secret tips/tricks, though I can usually come up with a measurement method using next to nothing if somebody tells me what they've got and what they want to measure.
Today you can get way more capability for free, using nothing more than your PC and sound card. Or, yet more than that if you're willing to pay a small amount. It's the same with driver measurements. I used to hook them up to an amp and signal generator to measure the impedance, and built a small test box for each one. That same software can do the same, though you may have to build a simple interface. We've got way more capability today than a commercial speaker company might have had "back in the day", and for peanuts. For components, you can get the Peak or similar testers for next to nothing, compared to traditional LCR meters.
Need a signal source? Burn a CD using Audacity. Pink noise, white noise, tones, tone bursts or whatever you need.
When I first started building speakers I bought crossover parts and assumed the values printed on them were reasonably correct. Rarely measured a thing and it worked out fine. The trick was mostly choosing good drivers. I never built a good speaker with a bad driver and tried to stick with well known names. 90% of my effort was simple box simulation, and then tweaking the crossover for SQ. As much as I like measurement, I have to note that the worst speaker I ever built was the one with the most beautiful flat response curve you've ever seen!
Always ask yourself, "If I measure this, what am I going to do with the information?" Speaker measurements can lead you down a deep rabbit hole if you don't combine them with an educated ear.
I don't really have any secret tips/tricks, though I can usually come up with a measurement method using next to nothing if somebody tells me what they've got and what they want to measure.