From: DPierce@world.std.com (Richard D Pierce)
Subject: Re: Wanted to know bi-amp speakers advantages
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 6 Jan 1993 00:46:05 GMT

BUNK ALERT! BUNK ALERT!

In article <1993Jan5.174202.1533@eafs000.ca.boeing.com> Mark J. Vandiver writes:
>>     I have a pair of m-1 mirage speakers. It bi-amp.
>>Can anyone tell me what's the advantage of a bi-am speakr?
>
>Bi-Amping has many advantages, although hard to describe.

I should say, since you got enough of it wrong!

>
>Since the crossover is prior to the amplifiers, they do not
>consume inordinate ammounts of power wasted as heat. you pay alot
>for those watts, don't burn them up in a crossover.

With the exception of those few resistive components that are necessary to
match levels, the crossover components, being reactive, disipate precisely
zero power and generate no heat whatsoever. Even given non-ideal
components, such as an inductor with a few tenths of an ohm resistance,
the actual power disipation is very low. Say, for example, you're trying
to stuff a couple of amps through an inductor with .2 ohms DC resistance
that's then feeding a woofer with 7 ohms DC resistance. The total power
dissipated in the inductor will be 2^2*.2 or 0.8 watts, while the poor
woofer will have to dissipate 2^2*7 or 28 watts, all as waste heat. So the
big loss compnent in either case (passive or active) is the driver's DC
resistance, not the crossover components

>Crossover impeadance for a speaker crossovers has to be low, resulting
>in high power loss. Bi-Amp crossover impeadance is high.

I would be curious to hear precisely what you mean by "high power loss." A
recent project I completed for a client used pretty cheap components in a
fairly complex 3-way unsymmetrical passive network (2nd order LP on
woofer, 2nd order HP on midrange at 500 Hz, 2nd order LP on midrange, 3rd
order LP on tweeter at 4000 Hz, includes conjugate matching networks on
woofer and midrange, has 16 passive components) that exhibits overall
about 0.7 dB broadband loss. Looking at this in its most conservative
view, that corresponds to about 92% efficiency. Compare this to the 1%
efficiency typical of most electro-dynamic drivers, and I hope you see why
I have a hard time swallowing your assertion about "high power losses."

>Amplifiers are not working as hard. The bandwidth the amps are
>seeing is much smaller, resulting in less phase distortion and THD.

The assertion that narrower bandwidth on the input to an amplifier reduces
phase distortion is hogwash. The "phase" distortion is independent of the
input signal, and dependent upon the actual bandwidth of the amplifier. To
further show the flaws in the argument, consider, if you will, the "phase
distortion" added inherently by the crossover itself. This "distortion" is
going to be the same whether a given response function is implemented
actively or passively.

>Gives greater control over equalization of speakers.

How?

There ARE good reasons for active crossovers, but you've failed to mention
any of them.

For example, there are classes of response functions that are
unachievable with passive circuits. This is probably the only real
advantage of active networks, but one that no commercial system takes
advantage of, to the best of my knowledge. It's not clear if these classes
of functions are useful to a particular loudspeaker application or not.

They are easier to design (but not necessarily easier to implement) than
passive networks for a variety of reasons. Chief among the reasons:  you
don't have to worry about non-resistive loads, although with the use of
properly design conjugate networks, this is not an issue with passive
networks either. But for a novice that has not a clue how to compensate
for non-resistive load impedance (and for many highly regarded experts who
haven't a clue either), it's one headache they don't have to worry about.

It makes the speakers cheaper (but the increased costs of extra
amplification more than often enough completely counteracts any advantages
there. You might argue the case that a few milligrams of silicon are far
ceaper than a few pounds of copper, aluminum and polypropylene (yeah, but
what about the extra copper and iron in the transformer for the xover 
power supply).



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