| Back EMF? [message #67178] |
Sat, 23 April 2011 00:45  |
Frihed89
Messages: 30
Registered: June 2009
Location: Copenhagen, Denmark
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Baron |
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A local speaker guru has warned me that a 12-15" driver may produce a lot of back-EMF that amps with very poor damping factor (2A3 SET) have a hard time coping with, regardless of sensitivity.
First of all, what is back-EMF? Second, is it a problem with the 2A3 on the Four Pi?
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| Re: Back EMF? [message #67358 is a reply to message #67357] |
Mon, 02 May 2011 08:10  |
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Wayne Parham
Messages: 18787
Registered: January 2001
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Illuminati (33rd Degree) |
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I appreciate the nod and I hope nobody was offended.
Putting personalities aside, just looking at the facts, it really isn't accurate to say a 12" or 15" woofer is more or less likely to generate greater back-EMF than a woofer with smaller cone diameter. It isn't even the diameter that matters, it's the mass. And that's not the only thing that comes into play. There's the amount of mechanical damping verses electrical damping, and there's the strength of the motor, the magnetic strength and the voice coil impedance.
I've measured woofers that created such strong back-EMF they actually induced current sufficient to make sound in adjacent drivers connected through a passive crossover network. This is unusual, of course, but it is an extreme example of back-EMF. A driver like this has to be connected directly to an amplifier output, and the amplifier would need to have good damping factor, i.e. low output impedance. No tube amp would work well with a driver like this. Ironically, it wasn't even that large - it had a 6" diameter cone.
The kind of woofer I expect to have the most trouble with is the one with a fairly heavy cone and very loose suspension. It's designed to be used as a subwoofer. That kind of woofer often generates a lot of back-EMF. The truth is, that kind of tuning is popular in car subwoofers, and there are a lot of them with cones from 6" to 10". The problem isn't limited to drivers larger than 12", in fact, I'd say the ones most likely to be a problem are the little ones with real heavy cones. Nothing to damp them but the amp.
High-efficiency drivers tend to have lighter cones and stronger motors. That's not to say they are all immune to back-EMF - I've seen a few with fairly high impedance at resonance (Zmax), which is an indirect indicator of back-EMF. And that's really the problem, in most cases, because you're not usually dealing with back-EMF at the extreme of the example I gave above. Usually, it's the impedance fluctuation interacting with the tube amplifier's output impedance that is the problem. This creates a voltage divider that varies with frequency, which is another way of saying it creates a filter. The amplitude response is adversely affected as a result.
More information:
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| Re: Back EMF? [message #67413 is a reply to message #67358] |
Wed, 04 May 2011 11:57 |
spkrman57
Messages: 522
Registered: May 2009
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Illuminati (1st Degree) |
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I have used the following small tubed amps on my 4-Pi spkrs utilizing 8 and 16 ohm 2226 drivers with no problems:
45/2A3/300B/Triode strapped EL34
I will re-iterate what Wayne has said in the past concerning the 2226 liking more wattage than what the little amps put out for max performance (10 % of max or 60 wpc).
I agree with that, but I will say that the dropoff in performance using little tubed amps under 10 wpc is not really that drastic. I don't know too many other 15" drivers that would work as well for me!!!
Just my 2 cents worth of course!
Regards, Ron
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