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Re: "passive" vs "active"

Posted by Presto (A) on November 21, 2006 at 23:48:21

In Reply to: Re: "passive" vs "active" posted by Christine Tham on November 21, 2006 at 20:11:47:

Christine:

About multi-amping...

Yes, I do see how the crossover and the driver "add up" (vectorally) to provide a net impedance as seen by the amp. Let's look at three examples, all being different from one another.

1) Your speakers - each amp gets it's own unique leg of a parallel network, lets pick the woofer
2) A full range speaker - an amp gets the impedance of a speaker but no x-over
3) An amp in an active system driving a midrange (crossovers in preamp stage)

In your case, the amp is seeing the net (vectoral) sum of the impedances of the crossover network and the driver. In the case of the fullrange, the amp is simply seeing the impedance of the driver. In the third case, the amp is also seeing the impedance of the midrange, but **only at those frequencies that are within the pass-band and crossover regions**. In the stop band, there is no "information" being passed, therefore there are no "voltages" being amplified at those frequencies, therefore there is no current drawn at those frequencies, therefore the amp does NOT see the impedance at those frequencies. A specific example would be an active bandpass filter so steep that it only allows one frequency to pass through, say 1000Hz. In this case, the amp would only see impedance at ONE frequency - 1000Hz. The impedance graph is now just a "dot" representing the vectoral sum of the reactance and the resistance at that particular frequency. The impedance at 100hz and 10,000 hz of a driver connected to such a filter would matter not one single bit, since no signal at those frequencies would ever be amplified in the first place.

So, to compare the amp in an active system to that of a passive system, in a passive system the amp or amps always see the sum of all parallel networks, both drivers and filters - even in the case of passive multiamping - where each amp gets its own unique filter and driver. This impedance "seen" encompases the entire audible bandwidth in either case. An amp in an active system only "sees" the impedance of the connected driver that are NOT in the stop-band (aka only the pass-band and crossover regions.)

I guess you could say that although a load has an "impedance" from DC to light, the only impedance that MATTERS to the amp are for those frequencies that it is called upon to reproduce. An amp (or any voltage source) only can "see" impedances at frequencies that it is driving voltage at. Impedances at other frequencies are there - they're just "not in use".

If what you are saying is true, then one could not use a ribbon tweeter with a near-zero impedance below its usable frequency range. The amp would "see" this almost-dead-short and go into overload immediately. This is not the case at all. You CAN connect this driver directly to an amp - so long as your ACTIVE crossover prevents your amp from ever GENEERATING a voltage that is sufficiently low enough in frequency to drive a current at those frequencies. No low frequency voltage, no low frequency current, no low impedance is seen by the amp, and nothing is overloaded. Try running that tweeter WITHOUT the active filter, and the amp will fry as fast as the tweeter will!!

Cheers,
Presto

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