inductor measurements

[henrylrjr]

Hi all,

I am planning to build outboard crossovers for my b&w n803 speakers. I had good results with outboard xovers I built, with upgraded components, about a year ago for different speakers. I could not find any specs for the xover inductors so removed them from the PCBs to measure them so I could order new ones. At that time I had 2 LCR meters. One measured at 15kHz and the other allowed measuring at either 120Hz or 1kHz. The measurements varied between the meters. Measurements of the same inductor was .990mH at 120Hz, .991mH at 1kHz and .880mH at 15kHz.

I have all the specs for the b&w xover, but don't know the measurement frequency they used for the inductors, but don't want to remove them until the new xovers are built.

There are two reasons for this post. One is a heads up about different meter frequencies, and effect on results, although I am probably the only one that didn't know about that. The other is a question. Does anyone know if there is a standard frequency used by speaker builders? If not I will send an email to B&W tech support. They were very helpful with questions in the past.

Thanks,
henrylrjr

 

[Conrad Hoffman]

You would almost always measure at a low frequency so internal capacitance wouldn't affect the reading. 1 kHz is pretty much standard. One should also be aware of measurements made at the frequency of interest since that's how you'll use it.

 

[KatieandDad]

All the B&W schematics and parts lists are readilly available.

B&W Group North America Service & Support - Home

The 803N comes under the Nautilus range heading.

 

[henrylrjr]

Thanks for the reply. Your remark, frequency of interest, also crossed my mind. I was thinking of using 120Hz for the woofer inductors, 1kHz for the midrange inductors and 15kHz for the tweeter inductor. Then thought that, if B&W didn't do something like that, I could end up with a completely different sounding speaker. I think I will send them an email just to be sure.

Thanks,
henrylrjr

 

[henrylrjr]

inductor measurement frequency

I have the schematics but couldn't find anything about measurement frequencies.

Thanks,
henrylrjr

 

[Conrad Hoffman]

Thanks for the reply. Your remark, frequency of interest, also crossed my mind. I was thinking of using 120Hz for the woofer inductors, 1kHz for the midrange inductors and 15kHz for the tweeter inductor. Then thought that, if B&W didn't do something like that, I could end up with a completely different sounding speaker. I think I will send them an email just to be sure.

Thanks,
henrylrjr

Though they were probably aware of the actual performance, measurements on a schematic would almost always be stated at the lower frequencies.

 

[speaker dave]

I would use your lower frequency meter. Although I can see your logic, I don't know of any company that measured L at a variety of frequencies based on use in the speaker (loss perhaps, but not L).

As a note, air core inductors should measure about the same for a wide range of levels and frequencies (away from self resonance). Not so sure about iron core and it probably varies with particular core type.

David

 

[henrylrjr]

The inductor measurements were for the air core type and it happend on all the inductors I checked about a year ago. The difference between 120Hz and 1kHz were negligible. The real difference was checking at 15kHz. FWIW, I spoke with an engineer at Erse, when I got the different measurements at 15kHz, and he said he would expect that and they do all their checking at 1kHz. I'll post B&W's answer when I get it.

 

[KatieandDad]

Air cored is the way to go. Any other core will saturate at some point.

 

[wolf_teeth]

Air cored is the way to go. Any other core will saturate at some point.

But that point will likely be much higher than most listeners can tolerate on a continuous basis.

Wolf

 

[AndrewT]

Measure the inductance at a frequency where the inductive reactance is the same as the speaker nominal impedance.

I do this by series connecting an 8r0 resistor to the inductor. Apply a sinewave from my generator (50ohm can feed an 8ohm + inductance reasonably well).
Adjust frequency until the Vdrop measured across the resistor exactly matches the Vdrop across the inductor. You now have the F-3dB frequency for the inductor feeding 8ohm and the phase is at 45degrees. Read the frequency with your frequency meter.

I don't know how well 50ohm generator feeds a 4ohm load.

A bass frequency inductor is probably going to result in an LF test frequency.
A tweeter frequency inductor is probably going to result in an HF test frequency. Yes, it could be >3kHz