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Field Coil Speakers + OP Transformers??


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9/27/2000 3:45 AM
dave
Field Coil Speakers + OP Transformers??
I've been waiting for the Weber field coil speakers to come out for a while now, & just had a disturbing thought. Would they work with a conventional amp with an output transformer or not? Thanks for any input..
 
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9/27/2000 2:20 PM
ken gilbert
yes, of course they would work.  
 
the field coil just emulates a permanent magnet. it really has little to do with the output circuit of the amplifier itself.  
 
kg
 
9/27/2000 3:06 PM
Ray Ivers

Ken,  
 
Your post got me thinking (as usual) ...  
 
When the field coil voltage is derived from the amp B+, I wonder what kind of interactions occur during power amp overdrive? "Speaker sag" when the B+ drops down? Weird 'bounce' effects as the choke field collapses?  
 
Oh, no! - another ingredient in the 'tone soup'. :)  
 
Ray
 
9/27/2000 4:05 PM
ken gilbert
well, ray, thinking about it...  
 
usually the field coil is used in place of a choke in the power supply. that means it's got the full B+ current flowing through it. that also means that the magnetic field strength (B) in the gap will INCREASE as you pull more current through the field coil, which is exactly what happens as you increase the output power of the amplifier. the coil, and therefore, the magnetic circuit, doesn't care what absolute level of voltage is on the rail--it's only concerned with ampere-turns... CURRENT.  
 
it seems to me that this may offer a slight degree of compensation for the compression effect of the sag in B+ voltage, since as the output circuit began to conduct heavier currents, the efficiency of the speaker system would INCREASE.  
 
now, i DON'T think the weber product will have such a design, with the field coil being placed in the B+ rail. or maybe it will? in a lot of other ways, that's not the best way to run the coil... as i've just described, the REAL way to set the magnetic circuit strength is with a CURRENT source, preferably variable, so as to adjust the acoustic efficiency. with the B+ derived coil current, that's also a good way to get hum in the output... although it COULD be cancelled out to some extent with a careful attention to phase.  
 
kg
 
9/27/2000 4:42 PM
Ray Ivers

Ken,  
 
The only way I'd ever seen them run was powered off the B+, but I wholeheartedly agree; build a power supply just for the coil! Might be a damn cool attenuator, Ken. I wondered also if any 'sag' effect would be in reverse, so to speak, and your post answered that for me.  
 
I wonder if it would it be possible to build a speaker without a magnet, with two closely-coupled concentric opposite-wound voice coils both driven from the amp? I wouldn't mind having a 30 lb. 4-12 cabinet.  
 
Ray
 
9/27/2000 8:38 PM
Don Symes

One fixed and one moving coil, right?  
 
I 'spect you'd give away a great wacking load of effieciency - which is why FC speakers faded from view (I 'spect).  
 
I'm under the (marketing) impression that the coil movement for a given current is proportional to the strength of the magnet.  
 
Now, as ways to get power-amp overdrive without ear-crushing volume go, using wildly inefficient speakers seems like a promising way to go.
 
9/27/2000 9:04 PM
tommy

Here's one to think about:  
 
On a DC motor, if you increase the field current, and keep the armature voltage the same, the motor actually slows down. Since there is more flux, the armature can turn slower and generate the same counter-emf.  
 
How does this relate to this speaker talk, since it operates on the same principal?  
 
Chris
 

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