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In Reply to: RE: JBL 16 ohm dirver that measerus 6.7 posted by Paully on July 20, 2016 at 18:59:49
Paully
The DC resistance of the voice coil you measure with a VOM will not be the same as the operational resistance of the voice coil with an AC test signal, or music for that matter. If the driver is to be horn loaded, the acoustical resistance of the horn will add to the resistance of the voice coil in actual operation. So for a horn loaded driver, round up the DC resistance to the next higher standard: i.e. 8 or 16 Ohms. Sometimes a blown voice coil will measure a very low DC resistance, with 2 or 3 Ohms for instance on a 16 Ohm driver, because the voice coil is shorted. Playing a test signal or music through this driver would likely cause it to go open and reveal that it's non operational, though the driver may revert to the shorted condition with a further DC measurement.
Hope this helps
Paul
Follow Ups:
Beyma appears to disagree:
CP385N8 Nominal impedance 8Ω, minimum impedance 5.2Ω, DC resistance 4.7Ω.
I see similar figures from other manufacturers but most only list nominal and minimum but not DC.
b.l.
The T.S. specification Re is the impedance of the driver at 1 Hz, which is generally assumed to be virtually the same as the impedance at 0 Hz which is the DC impedance. If the driver specs are plus or minus 10% tolerance (like the JBL driver Bill linked to here) there seems little practical reason to look for a more accurate Re spec., though anyone is welcome to explore that path. The nominal impedance represents what the driver is doing at work, and you need an accurate figure to design a crossover for the driver. The ubiquitous Electro-Voice T35 horn tweeter will measure 6+ Ohms DCR in it's 8 Ohm version, and about 12+ Ohms DCR for the 16 Ohm version, while the nominal impedances of 8 and 16 Ohms will give accurate results when used to design crossover points for them in my experience. I'm unfamiliar with the Beyma driver you mention here, and a google search turned up this post (!). so I can't speak to the accuracy of their specs. In general, T.S. specs tend to be fairly accurate when comparing different drivers within a manufacturers line for the big guys like EV or JBL, but somewhat less so when comparing drivers by different manufacturers to each other.
Paul
The usual DCR/Re is between 0.7 and 0.8 the rated impedance, although it's not a hard and fast rule. Where that Beyma is concerned I'd call it a 6 ohm driver based on DCR alone. DCR/Re typically is the same as minimum impedance, but in this case they list the minimum impedance at 4.5kHz, so the coil inductance is reflected in that figure. Lacking an impedance chart the assumption would be that it would dip below that below the suggested crossover frequency, reaching DRC/Re at some point.
As I mentioned above somewhere the problem is that JBL call the driver with 6.5Ohm DC resistance '16 Ohm' and the version with 3.3Ohm resistance '8 Ohm' which appears out of kilter with every other manufacturer who would call those 8 and 4 Ohm respectively.
The drivers appear to be fine and measure well within spec, it is this manufacturers nomenclature which is confusing until one reads the minimum impedance spec (9 and 5 Ohm at 5kHz).
Larger voice coils, like those used by JBL, will have high inductance, which pushes impedance up, as will relatively large diaphragms compared to the throat size, and for that matter the horn itself. I agree that calling the 2426H 8 ohms is a bit of a stretch, 6 ohms would be more like it. It's a bit difficult to make out the impedance value on the data chart because of the odd vertical scale, but it appears to be in the vicinity of 8 ohms at the recommended 800Hz crossover frequency, so that might be what they're basing it on.
44mm (1.75") voice coil and diaphragm are pretty much standard for a 1" driver and is the same as the Beyma driver I mentioned elsewhere and all other Beyma 1" bolt-on cds.
They were also measured on a horn but the difference between Re and minimum impedance is far less: 4.7 and 5.2Ohm giving 0.5Ohm difference while the very similar JBL gives 1.7Ohm for the 8Ohm version and 2.5 for the 16.
That said one of theirs (SMC-225Nd) has an Re that is actually higher then its minimum impedance. Not sure what I should make of that, some kind of resonance may be.
Not that it would really matter much to me since I'd run them active and then the exact impedance matters very little except for roughly working out headroom/SPL with any given amp. ;-)
Some of the specs for the JBL 2426H/J linked by DJK here were derived with the driver driving a 25 mm (1 in.) diameter terminated "plane wave" tube, which would have a different acoustical resistance (as seen by the driver) compared to some of the other specs derived with the driver on a horn. The plane wave tube standard was set so that different drivers performance could be compared with a standard load separate from whatever horn the driver could be attached to which was not necessarily standard. "It pays to read the instructions" as my wife sometimes reminds me.
Paul
That does help, it lets me know that I am correct that the low measuring drivers aren't worth the risk in purchasing. I get that the drivers will measure below the stated impedance, but they shouldn't measure that low. I think you are right, the really low one (4.7) is probably blown. The other (6.7) probably has a swapped 8 ohm diaphragm or is blown. So I think I will wait for one to come along that measures higher and closer to 16 even if they won't measure 16 exactly.
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