flyback voltage--good, bad, or indifferent?

[Bill F.]

I was just reading Bob Carver's white paper on the principles behind his True Sub. Fascinating stuff--a recommended read.

http://www.nortek.net/learning_center/audio-video/sunfire_sunwoofer_white_paper.htm

His strategy involves maximizing the back-EMF from the driver's voice coil to limit the amount of current flow and resulting heat. To overcome the back-EMF, he specs a (theoretical) 2,700W amp (that is, capable of 104V which would push 2,700W through 4 ohms). However, the large amount of back-EMF boosts the reactive component of the impedence to the point that the amp almost always sees a much higher impedence. He cites this as the basis for the high relative efficiency of his True Sub (An 11" cube sub that puts out 112dB @ 20Hz while drawing all its power through a 6A slow-blow fuse must be doing something very right!)

Anyhow, this got me wondering if giant back-emf numbers relative to the voltage capability of an amp cause most amps problems. As long as there is voltage to drive an impedence, Are there any practical concerns that would put a cap on how much back-EMF you can expect an amp to handle with grace? Are there any special provisions that should be built into such an amp?

Bill

 

[Nelson Pass]

A lot of words which describe the rise in speaker impedance
at resonance which makes the speaker draw less power
in that range. As far as I can make out, every cone woofer
carries this feature, and all amplifiers enjoy it.

 

[Bill F.]

Thanks for rubbing my nose in the obvious.

What got my attention was the idea of using flyback, usually the enemy of bass, to create more efficient bass output via a higher voltage amp. Being an amp newbie, I didn't know what the implications were when a very large percentage of impedence was reactive. I take from your post that there are none.

What I'm still wondering is this (and I solicit your patience if this seems equally obvious): In a subwoofer system, could an amp with large voltage swing but only average current capability, electronically equalized (low pass), effectively drive a Carveresque transducer below, say, 50Hz? In other words, would the reactive impedence continually keep the current demand low, or are there forseeable instances when the amp would see moments of low impedence and current-clip? Does the reactive impedence ever lag or does it keep pace with the low-frequency voltage waveform?

As I said, this may be patently obvious to many, but indulge me--I'm in school here.

Bill