-=> Quoting Hung Cao to All <=-
 HC> I need some information.  I have a reciever that I can hook up to
 HC> 4 spkrs to in twos sets (A & B).  It says "A or B, 4ohms minimum
 HC> per speaker"  "A AND B, 8ohms minimum per spker."  I have a pair
        This means the amp only has so much current it can deliver
into a given load.  Loudspeaker loads are given in ohms.  An ohm is a
measure of impedance.  Impedance is an electrical term consisting of
three types which are resistance, capacitance, and inductance.
Without getting into details, the receiver is basically saying its
amplifier section can drive either one set of speakers that average as
low as 4 ohms OR it can drive two sets of speakers with a combined
impedance of not less than 8 ohms.
 HC> of Polks in there right now which are 8ohms but I also have a set
 HC> of car spkrs i would like to add.  I think the car spkrs are
 HC> 4ohms though although it doesn't say so, i heard somewhere that
        Most car speakers are 4 ohms because it's a cheap and dirty
way to get more power (power = Voltage x Current) from a car battery
(which is about 12 volts) since the lower the impedance, the more
current that can be drawn.  Without a special converter that can
increase the voltage of the battery by changing it into AC and then
back into DC (which adds expense), 4 ohm speakers allow twice the
power to be delivered to the speakers by requiring twice the current
to be drawn.  (You can deliver power either by increasing voltage or by
increasing current.)  So, *most*, but not all car speakers are 4 ohms.
Some may be less, some may be more.  My original factory car speakers
were 12 ohms!  The replacement speakers I put in were 4 ohms.
        That's really simplifying it because impedance can vary (a
lot) over frequency.  You can have speaker that are 26 ohms at 10kHz
and 2 ohms at 100Hz.  The manual is usually talking about the average
"load" the speaker represents.  But dips below that can cause problems
for some amps.
 HC> most car spkrs are 4 ohms (correct me if i'm wrong).  Can someone
        Yes, "most" are.
 HC> explain to me about what ohms are and what would happen if I
 HC> connect my car spkers to the reciever with the Polks?
        If you connect a pair of 4 ohms speakers and a pair of 8 ohms
speakers, the resultant load would be:
         (z1xz2)/(z1+z2) = (4x8)/(4+8) = 2.67 ohms
        What they're saying is that the TOTAL load can not be lower
than 4 ohms.  If you combine two 8 ohm loads, the resultant combined
load is 4 ohms and that's why 8 ohms is the minimum the amp recommends
for TWO sets of speakers.
        As for what would happen, if you overdrive the amp it will
"clip," which means that the AC audio siganl, which looks like a sine
wave will "hit the rails" (which is the upper and lower maximum
voltage swing).  It can go any higher, so the wave turns into DC at
the upper and lower limits (flat line) or a square wave.  This can
damage your speakers over time for short bursts.  Constantly
overdriving your speakers can destroy the drivers and burn out the
amp.  You may or may not be able to drive such a load with that amp at
low volume, but to be safe, don't do it.
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