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Back EMF? [message #67178] Sat, 23 April 2011 00:45 Go to next message
Frihed89 is currently offline  Frihed89
Messages: 30
Registered: June 2009
Location: Copenhagen, Denmark
Baron
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?
Re: Back EMF? [message #67182 is a reply to message #67178] Sat, 23 April 2011 03:05 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)

Back-EMF is the voltage produced when the mass of motor causes it to act as a generator after the drive voltage has shut off or reversed. It is a real concern in some cases, but not in high-efficiency speakers like these. The electro-mechanical properties that make it efficient also tend to reduce those kinds of side effects.

What you should watch for - an indirect measure of this - is the maximum impedance. You don't want a tube amplifier to have a wide fluctuation in impedance. But all models of π Speakers have very benign impedance curves. I've called them "tube friendly" speakers in the past, because they truly are. They offer high efficiency and gentle impedance curves.

Re: Back EMF? [message #67236 is a reply to message #67178] Tue, 26 April 2011 22:00 Go to previous messageGo to next message
Bill Epstein is currently offline  Bill Epstein
Messages: 1088
Registered: May 2009
Location: Smoky Mts. USA
Illuminati (2nd Degree)
My experience is limited to Parafeed amps but I have driven both Theatre 4s and the newer 4Pis with both 2A3 and 45 amps w/o bass driver anomalies.In the case of the Bottlehead 2A3 Paramours, they made Theatre 4s fill an auditorium with sound.

Amioutaline?
Re: Back EMF? [message #67274 is a reply to message #67178] Thu, 28 April 2011 11:38 Go to previous messageGo to next message
Matts is currently offline  Matts
Messages: 359
Registered: May 2009
Grand Master
the Pi 4's sound great with SET! I've used mine with parafeed 2A3s as well with great bass (both in quantity and quality) and no problems. Topic can get complex- with lots of factors, but not necessary here.
Re: Back EMF? [message #67296 is a reply to message #67178] Fri, 29 April 2011 18:05 Go to previous messageGo to next message
Frihed89 is currently offline  Frihed89
Messages: 30
Registered: June 2009
Location: Copenhagen, Denmark
Baron
Thanks. The guy who warned me is actually a very accomplished speaker designer, but tends not to like 2-way designs for high efficiency.
Re: Back EMF? [message #67318 is a reply to message #67296] Sat, 30 April 2011 16:24 Go to previous messageGo to next message
Matts is currently offline  Matts
Messages: 359
Registered: May 2009
Grand Master
that's why he's a "local speaker guru" and Wayne's an "international speaker guru"! Smile haha
Re: Back EMF? [message #67353 is a reply to message #67296] Sun, 01 May 2011 18:00 Go to previous messageGo to next message
Bill Epstein is currently offline  Bill Epstein
Messages: 1088
Registered: May 2009
Location: Smoky Mts. USA
Illuminati (2nd Degree)
Frihed89 wrote on Fri, 29 April 2011 19:05
Thanks. The guy who warned me is actually a very accomplished speaker designer, but tends not to like 2-way designs for high efficiency.


He'd love this:

index.php?t=getfile&id=379&private=0

... except the 2123s were "blown" and the "high efficiency" ribbons were anything but. Good intentions, high-quality drivers and a lot of plywood a good speaker does not make.

Remember these Bat-Speakers? And the giant horns attached to the XLH 1812s directly behind?
Re: Back EMF? [message #67357 is a reply to message #67353] Mon, 02 May 2011 03:34 Go to previous messageGo to next message
Frihed89 is currently offline  Frihed89
Messages: 30
Registered: June 2009
Location: Copenhagen, Denmark
Baron
Troels Gravesen: www.troelsgravesen.dk in english
Re: Back EMF? [message #67358 is a reply to message #67357] Mon, 02 May 2011 08:10 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)

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:
Re: Back EMF? [message #67413 is a reply to message #67358] Wed, 04 May 2011 11:57 Go to previous messageGo to previous message
spkrman57
Messages: 522
Registered: May 2009
Illuminati (1st Degree)
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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