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From: keithe@tekgvs.UUCP (Keith Ericson)
Newsgroups: net.audio
Subject: on crossovers - active vs. passive
Message-ID: <1259@tekgvs.UUCP>
Date: Fri, 18-Oct-85 14:19:57 EDT
Article-I.D.: tekgvs.1259
Posted: Fri Oct 18 14:19:57 1985
Date-Received: Sun, 20-Oct-85 05:18:28 EDT
Organization: Tektronix, Beaverton OR
Lines: 78

Gentlefolk -
	The discussion of passive crossover design and the problems
involved in modelling the driver(s) is getting lots of net.audio space
lately. But, friends, it's *old technology* and not really current in
todays audio market-place, especially for those capable - as I believe
lots of net.audio reader are - of some relatively simple electronic
construction. (Right now you're saying 'what IS he talking about! -
bear with me...)
	OK - the problem in a conventional (i.e., passive-crossover)
system is that you've got this monstrous power amplifier, with a
fraction of an ohm output impedance, trying to control an
electro-mechanical motor. The motor - the speaker - has a "mind of it's
own" as to what it wants to do with the signal thrown at it and it
will, if left alone, proceed to do whatever it thinks best with the
signal. One way you can start to "leave it alone" is to interpose a
crossover network between the amplifier and the driver. This prevents
the amplifier from damping out the extraneous motion that the driver
wants to make. It's like this:
	Put a long-duration pulse to the speaker (long duration being
several times longer than 1 over the resonant frequency). The cone
moves out, then moves  back. But when it moved out, it probably
over-shot the "correct" location and oscillated into place, finally
getting to where it was supposed to be. Thes oscillations generate a
"back emf" (remember that from your power engineering classes?) that
tries to drive the amplifier. But if the amplifier (1) has a low
output impedance and (2) is connected pretty close (i.e., through
a very low impedance connection) then the back-emf will have to generate
a lot of current to exist. Instead, the back emf "gives up" and the
speaker doesn't oscillate around the resting location (yeah, I know
that's really simplified, but it gets the point across). With the
passive crossover there's lots of circuitry that can allow the current
from the back-emf to flow and not be damped out by the low output
impedance of the power amplifier. I's like trying to drive your car
with a spring between your hands and the steering wheel. You can steer,
but bumps will throw the car all over the place 'cause you can't correct
for them with the springs in the way.
	The best way around this is to use a power amplifier for each
speaker, and put active crossovers between the pre-amp and the power
amplifiers. The advantages include the following:
	1) The crossovers are in a well controlled impedance. (You
	   could even use a passive implementation - if you really
	   wanted to for some reason.)
	2) The crossovers work with low-level signals instead of
	   having to be capable of handling tens (hundreds?) of
	   watts of power. 
	3) The speakers are driven directly by the (low output-
	   impedance) power amplifier.
	4) Active crossovers completely do away with inductors, the
	   hardest part of any passive, high-level crossover to
	   fabricate.

	In the "old days" when good power amplifiers were prohibitively
expensive it was important to reduce their number. That just isn't the
case now days. Today almost anyone can afford a multiple power amplifier
system. Except that to *buy* the active crossovers is prohibitively
expensive. But they are quite simple to build, and there is really no
"magic" involved. Just check into National Semiconductor's "Audio/Radio
Handbook" publication (circa 1980). (But use the Signetics 5534 op-amp.)
	Well this has gone on long enuff. If you haven't hit the 'n' key
by now you must !really! be interested in this stuff...

keith

Keith Ericson  at TekLabs (resident factious factotum)
Tektronix, PO 500, MS 58-383
Beaverton OR 97077
(503)627-6042
uucp:	 [ucbvax|decvax|ihnp4|(and_many_others)]!tektronix!tekgvs!keithe
CSnet:	 keithe@tek
ARPAnet: keithe.tek@rand-relay
-- 
Keith Ericson  at TekLabs (resident factious factotum)
Tektronix, PO 500, MS 58-383
Beaverton OR 97077
(503)627-6042
uucp:	 [ucbvax|decvax|ihnp4|(and_many_others)]!tektronix!tekgvs!keithe
CSnet:	 keithe@tek
ARPAnet: keithe.tek@rand-relay